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7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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F u n d a m e n t a l s o f C o m p u t a t i o n a l F l u i d
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H a r v a r d L o m a x a n d T h o m a s H . P u l l i a m
N A S A A m e s R e s e a r c h C e n t e r
D a v i d W . Z i n g g
U n i v e r s i t y o f T o r o n t o I n s t i t u t e f o r A e r o s p a c e S t u d i e s
A u g u s t 2 6 , 1 9 9 9
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C o n t e n t s
1 I N T R O D U C T I O N 1
1 . 1 M o t i v a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1 . 2 B a c k g r o u n d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1 . 2 . 1 P r o b l e m S p e c i c a t i o n a n d G e o m e t r y P r e p a r a t i o n . . . . . . . 2
1 . 2 . 2 S e l e c t i o n o f G o v e r n i n g E q u a t i o n s a n d B o u n d a r y C o n d i t i o n s . 3
1 . 2 . 3 S e l e c t i o n o f G r i d d i n g S t r a t e g y a n d N u m e r i c a l M e t h o d . . . . 3
1 . 2 . 4 A s s e s s m e n t a n d I n t e r p r e t a t i o n o f R e s u l t s . . . . . . . . . . . . 4
1 . 3 O v e r v i e w . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1 . 4 N o t a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 C O N S E R V A T I O N L A W S A N D T H E M O D E L E Q U A T I O N S 7
2 . 1 C o n s e r v a t i o n L a w s . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 . 2 T h e N a v i e r - S t o k e s a n d E u l e r E q u a t i o n s . . . . . . . . . . . . . . . . . 8
2 . 3 T h e L i n e a r C o n v e c t i o n E q u a t i o n . . . . . . . . . . . . . . . . . . . . 1 2
2 . 3 . 1 D i e r e n t i a l F o r m . . . . . . . . . . . . . . . . . . . . . . . . . 1 2
2 . 3 . 2 S o l u t i o n i n W a v e S p a c e . . . . . . . . . . . . . . . . . . . . . . 1 3
2 . 4 T h e D i u s i o n E q u a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4
2 . 4 . 1 D i e r e n t i a l F o r m . . . . . . . . . . . . . . . . . . . . . . . . . 1 4
2 . 4 . 2 S o l u t i o n i n W a v e S p a c e . . . . . . . . . . . . . . . . . . . . . . 1 5
2 . 5 L i n e a r H y p e r b o l i c S y s t e m s . . . . . . . . . . . . . . . . . . . . . . . . 1 6
2 . 6 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 7
3 F I N I T E - D I F F E R E N C E A P P R O X I M A T I O N S 2 1
3 . 1 M e s h e s a n d F i n i t e - D i e r e n c e N o t a t i o n . . . . . . . . . . . . . . . . . 2 1
3 . 2 S p a c e D e r i v a t i v e A p p r o x i m a t i o n s . . . . . . . . . . . . . . . . . . . . 2 4
3 . 3 F i n i t e - D i e r e n c e O p e r a t o r s . . . . . . . . . . . . . . . . . . . . . . . 2 5
3 . 3 . 1 P o i n t D i e r e n c e O p e r a t o r s . . . . . . . . . . . . . . . . . . . . 2 5
3 . 3 . 2 M a t r i x D i e r e n c e O p e r a t o r s . . . . . . . . . . . . . . . . . . . 2 5
3 . 3 . 3 P e r i o d i c M a t r i c e s . . . . . . . . . . . . . . . . . . . . . . . . . 2 9
3 . 3 . 4 C i r c u l a n t M a t r i c e s . . . . . . . . . . . . . . . . . . . . . . . . 3 0
i i i
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3 . 4 C o n s t r u c t i n g D i e r e n c i n g S c h e m e s o f A n y O r d e r . . . . . . . . . . . . 3 1
3 . 4 . 1 T a y l o r T a b l e s . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1
3 . 4 . 2 G e n e r a l i z a t i o n o f D i e r e n c e F o r m u l a s . . . . . . . . . . . . . . 3 4
3 . 4 . 3 L a g r a n g e a n d H e r m i t e I n t e r p o l a t i o n P o l y n o m i a l s . . . . . . . 3 5
3 . 4 . 4 P r a c t i c a l A p p l i c a t i o n o f P a d e F o r m u l a s . . . . . . . . . . . . . 3 7
3 . 4 . 5 O t h e r H i g h e r - O r d e r S c h e m e s . . . . . . . . . . . . . . . . . . . 3 8
3 . 5 F o u r i e r E r r o r A n a l y s i s . . . . . . . . . . . . . . . . . . . . . . . . . . 3 9
3 . 5 . 1 A p p l i c a t i o n t o a S p a t i a l O p e r a t o r . . . . . . . . . . . . . . . . 3 9
3 . 6 D i e r e n c e O p e r a t o r s a t B o u n d a r i e s . . . . . . . . . . . . . . . . . . . 4 3
3 . 6 . 1 T h e L i n e a r C o n v e c t i o n E q u a t i o n . . . . . . . . . . . . . . . . 4 4
3 . 6 . 2 T h e D i u s i o n E q u a t i o n . . . . . . . . . . . . . . . . . . . . . . 4 6
3 . 7 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 7
4 T H E S E M I - D I S C R E T E A P P R O A C H 5 1
4 . 1 R e d u c t i o n o f P D E ' s t o O D E ' s . . . . . . . . . . . . . . . . . . . . . . 5 2
4 . 1 . 1 T h e M o d e l O D E ' s . . . . . . . . . . . . . . . . . . . . . . . . 5 2
4 . 1 . 2 T h e G e n e r i c M a t r i x F o r m . . . . . . . . . . . . . . . . . . . . 5 3
4 . 2 E x a c t S o l u t i o n s o f L i n e a r O D E ' s . . . . . . . . . . . . . . . . . . . . 5 4
4 . 2 . 1 E i g e n s y s t e m s o f S e m i - D i s c r e t e L i n e a r F o r m s . . . . . . . . . . 5 4
4 . 2 . 2 S i n g l e O D E ' s o f F i r s t - a n d S e c o n d - O r d e r . . . . . . . . . . . . 5 5
4 . 2 . 3 C o u p l e d F i r s t - O r d e r O D E ' s . . . . . . . . . . . . . . . . . . . 5 7
4 . 2 . 4 G e n e r a l S o l u t i o n o f C o u p l e d O D E ' s w i t h C o m p l e t e E i g e n s y s t e m s 5 9
4 . 3 R e a l S p a c e a n d E i g e n s p a c e . . . . . . . . . . . . . . . . . . . . . . . . 6 1
4 . 3 . 1 D e n i t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1
4 . 3 . 2 E i g e n v a l u e S p e c t r u m s f o r M o d e l O D E ' s . . . . . . . . . . . . . 6 2
4 . 3 . 3 E i g e n v e c t o r s o f t h e M o d e l E q u a t i o n s . . . . . . . . . . . . . . 6 3
4 . 3 . 4 S o l u t i o n s o f t h e M o d e l O D E ' s . . . . . . . . . . . . . . . . . . 6 5
4 . 4 T h e R e p r e s e n t a t i v e E q u a t i o n . . . . . . . . . . . . . . . . . . . . . . 6 7
4 . 5 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8
5 F I N I T E - V O L U M E M E T H O D S 7 1
5 . 1 B a s i c C o n c e p t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2
5 . 2 M o d e l E q u a t i o n s i n I n t e g r a l F o r m . . . . . . . . . . . . . . . . . . . . 7 3
5 . 2 . 1 T h e L i n e a r C o n v e c t i o n E q u a t i o n . . . . . . . . . . . . . . . . 7 3
5 . 2 . 2 T h e D i u s i o n E q u a t i o n . . . . . . . . . . . . . . . . . . . . . . 7 4
5 . 3 O n e - D i m e n s i o n a l E x a m p l e s . . . . . . . . . . . . . . . . . . . . . . . 7 4
5 . 3 . 1 A S e c o n d - O r d e r A p p r o x i m a t i o n t o t h e C o n v e c t i o n E q u a t i o n . 7 5
5 . 3 . 2 A F o u r t h - O r d e r A p p r o x i m a t i o n t o t h e C o n v e c t i o n E q u a t i o n . 7 7
5 . 3 . 3 A S e c o n d - O r d e r A p p r o x i m a t i o n t o t h e D i u s i o n E q u a t i o n . . 7 8
5 . 4 A T w o - D i m e n s i o n a l E x a m p l e . . . . . . . . . . . . . . . . . . . . . . 8 0
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5 . 5 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3
6 T I M E - M A R C H I N G M E T H O D S F O R O D E ' S 8 5
6 . 1 N o t a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6
6 . 2 C o n v e r t i n g T i m e - M a r c h i n g M e t h o d s t o O E ' s . . . . . . . . . . . . . 8 7
6 . 3 S o l u t i o n o f L i n e a r O E ' s W i t h C o n s t a n t C o e c i e n t s . . . . . . . . . 8 8
6 . 3 . 1 F i r s t - a n d S e c o n d - O r d e r D i e r e n c e E q u a t i o n s . . . . . . . . . 8 9
6 . 3 . 2 S p e c i a l C a s e s o f C o u p l e d F i r s t - O r d e r E q u a t i o n s . . . . . . . . 9 0
6 . 4 S o l u t i o n o f t h e R e p r e s e n t a t i v e O E ' s . . . . . . . . . . . . . . . . . 9 1
6 . 4 . 1 T h e O p e r a t i o n a l F o r m a n d i t s S o l u t i o n . . . . . . . . . . . . . 9 1
6 . 4 . 2 E x a m p l e s o f S o l u t i o n s t o T i m e - M a r c h i n g O E ' s . . . . . . . . 9 2
6 . 5 T h e ; R e l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3
6 . 5 . 1 E s t a b l i s h i n g t h e R e l a t i o n . . . . . . . . . . . . . . . . . . . . . 9 3
6 . 5 . 2 T h e P r i n c i p a l - R o o t . . . . . . . . . . . . . . . . . . . . . . . 9 5
6 . 5 . 3 S p u r i o u s - R o o t s . . . . . . . . . . . . . . . . . . . . . . . . . 9 5
6 . 5 . 4 O n e - R o o t T i m e - M a r c h i n g M e t h o d s . . . . . . . . . . . . . . . 9 6
6 . 6 A c c u r a c y M e a s u r e s o f T i m e - M a r c h i n g M e t h o d s . . . . . . . . . . . . 9 7
6 . 6 . 1 L o c a l a n d G l o b a l E r r o r M e a s u r e s . . . . . . . . . . . . . . . . 9 7
6 . 6 . 2 L o c a l A c c u r a c y o f t h e T r a n s i e n t S o l u t i o n ( e r
j j e r
!
) . . . . 9 8
6 . 6 . 3 L o c a l A c c u r a c y o f t h e P a r t i c u l a r S o l u t i o n ( e r
) . . . . . . . . 9 9
6 . 6 . 4 T i m e A c c u r a c y F o r N o n l i n e a r A p p l i c a t i o n s . . . . . . . . . . . 1 0 0
6 . 6 . 5 G l o b a l A c c u r a c y . . . . . . . . . . . . . . . . . . . . . . . . . 1 0 1
6 . 7 L i n e a r M u l t i s t e p M e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . 1 0 2
6 . 7 . 1 T h e G e n e r a l F o r m u l a t i o n . . . . . . . . . . . . . . . . . . . . . 1 0 2
6 . 7 . 2 E x a m p l e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0 3
6 . 7 . 3 T w o - S t e p L i n e a r M u l t i s t e p M e t h o d s . . . . . . . . . . . . . . 1 0 5
6 . 8 P r e d i c t o r - C o r r e c t o r M e t h o d s . . . . . . . . . . . . . . . . . . . . . . . 1 0 6
6 . 9 R u n g e - K u t t a M e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0 7
6 . 1 0 I m p l e m e n t a t i o n o f I m p l i c i t M e t h o d s . . . . . . . . . . . . . . . . . . . 1 1 0
6 . 1 0 . 1 A p p l i c a t i o n t o S y s t e m s o f E q u a t i o n s . . . . . . . . . . . . . . 1 1 0
6 . 1 0 . 2 A p p l i c a t i o n t o N o n l i n e a r E q u a t i o n s . . . . . . . . . . . . . . . 1 1 1
6 . 1 0 . 3 L o c a l L i n e a r i z a t i o n f o r S c a l a r E q u a t i o n s . . . . . . . . . . . . 1 1 2
6 . 1 0 . 4 L o c a l L i n e a r i z a t i o n f o r C o u p l e d S e t s o f N o n l i n e a r E q u a t i o n s . 1 1 5
6 . 1 1 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 7
7 S T A B I L I T Y O F L I N E A R S Y S T E M S 1 2 1
7 . 1 D e p e n d e n c e o n t h e E i g e n s y s t e m . . . . . . . . . . . . . . . . . . . . . 1 2 2
7 . 2 I n h e r e n t S t a b i l i t y o f O D E ' s . . . . . . . . . . . . . . . . . . . . . . . 1 2 3
7 . 2 . 1 T h e C r i t e r i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 3
7 . 2 . 2 C o m p l e t e E i g e n s y s t e m s . . . . . . . . . . . . . . . . . . . . . . 1 2 3
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7 . 2 . 3 D e f e c t i v e E i g e n s y s t e m s . . . . . . . . . . . . . . . . . . . . . . 1 2 3
7 . 3 N u m e r i c a l S t a b i l i t y o f O E ' s . . . . . . . . . . . . . . . . . . . . . . 1 2 4
7 . 3 . 1 T h e C r i t e r i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 4
7 . 3 . 2 C o m p l e t e E i g e n s y s t e m s . . . . . . . . . . . . . . . . . . . . . . 1 2 5
7 . 3 . 3 D e f e c t i v e E i g e n s y s t e m s . . . . . . . . . . . . . . . . . . . . . . 1 2 5
7 . 4 T i m e - S p a c e S t a b i l i t y a n d C o n v e r g e n c e o f O E ' s . . . . . . . . . . . . 1 2 5
7 . 5 N u m e r i c a l S t a b i l i t y C o n c e p t s i n t h e C o m p l e x - P l a n e . . . . . . . . . 1 2 8
7 . 5 . 1 - R o o t T r a c e s R e l a t i v e t o t h e U n i t C i r c l e . . . . . . . . . . . 1 2 8
7 . 5 . 2 S t a b i l i t y f o r S m a l l t . . . . . . . . . . . . . . . . . . . . . . . 1 3 2
7 . 6 N u m e r i c a l S t a b i l i t y C o n c e p t s i n t h e C o m p l e x h P l a n e . . . . . . . . 1 3 5
7 . 6 . 1 S t a b i l i t y f o r L a r g e h . . . . . . . . . . . . . . . . . . . . . . . . 1 3 5
7 . 6 . 2 U n c o n d i t i o n a l S t a b i l i t y , A - S t a b l e M e t h o d s . . . . . . . . . . . 1 3 6
7 . 6 . 3 S t a b i l i t y C o n t o u r s i n t h e C o m p l e x h P l a n e . . . . . . . . . . . 1 3 7
7 . 7 F o u r i e r S t a b i l i t y A n a l y s i s . . . . . . . . . . . . . . . . . . . . . . . . 1 4 1
7 . 7 . 1 T h e B a s i c P r o c e d u r e . . . . . . . . . . . . . . . . . . . . . . . 1 4 1
7 . 7 . 2 S o m e E x a m p l e s . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 2
7 . 7 . 3 R e l a t i o n t o C i r c u l a n t M a t r i c e s . . . . . . . . . . . . . . . . . . 1 4 3
7 . 8 C o n s i s t e n c y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 3
7 . 9 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 6
8 C H O I C E O F T I M E - M A R C H I N G M E T H O D S 1 4 9
8 . 1 S t i n e s s D e n i t i o n f o r O D E ' s . . . . . . . . . . . . . . . . . . . . . . 1 4 9
8 . 1 . 1 R e l a t i o n t o - E i g e n v a l u e s . . . . . . . . . . . . . . . . . . . . 1 4 9
8 . 1 . 2 D r i v i n g a n d P a r a s i t i c E i g e n v a l u e s . . . . . . . . . . . . . . . . 1 5 1
8 . 1 . 3 S t i n e s s C l a s s i c a t i o n s . . . . . . . . . . . . . . . . . . . . . . 1 5 1
8 . 2 R e l a t i o n o f S t i n e s s t o S p a c e M e s h S i z e . . . . . . . . . . . . . . . . 1 5 2
8 . 3 P r a c t i c a l C o n s i d e r a t i o n s f o r C o m p a r i n g M e t h o d s . . . . . . . . . . . 1 5 3
8 . 4 C o m p a r i n g t h e E c i e n c y o f E x p l i c i t M e t h o d s . . . . . . . . . . . . . 1 5 4
8 . 4 . 1 I m p o s e d C o n s t r a i n t s . . . . . . . . . . . . . . . . . . . . . . . 1 5 4
8 . 4 . 2 A n E x a m p l e I n v o l v i n g D i u s i o n . . . . . . . . . . . . . . . . . 1 5 4
8 . 4 . 3 A n E x a m p l e I n v o l v i n g P e r i o d i c C o n v e c t i o n . . . . . . . . . . . 1 5 5
8 . 5 C o p i n g W i t h S t i n e s s . . . . . . . . . . . . . . . . . . . . . . . . . . 1 5 8
8 . 5 . 1 E x p l i c i t M e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . . 1 5 8
8 . 5 . 2 I m p l i c i t M e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . . 1 5 9
8 . 5 . 3 A P e r s p e c t i v e . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 0
8 . 6 S t e a d y P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 0
8 . 7 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 1
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9 R E L A X A T I O N M E T H O D S 1 6 3
9 . 1 F o r m u l a t i o n o f t h e M o d e l P r o b l e m . . . . . . . . . . . . . . . . . . . 1 6 4
9 . 1 . 1 P r e c o n d i t i o n i n g t h e B a s i c M a t r i x . . . . . . . . . . . . . . . . 1 6 4
9 . 1 . 2 T h e M o d e l E q u a t i o n s . . . . . . . . . . . . . . . . . . . . . . . 1 6 6
9 . 2 C l a s s i c a l R e l a x a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 8
9 . 2 . 1 T h e D e l t a F o r m o f a n I t e r a t i v e S c h e m e . . . . . . . . . . . . . 1 6 8
9 . 2 . 2 T h e C o n v e r g e d S o l u t i o n , t h e R e s i d u a l , a n d t h e E r r o r . . . . . 1 6 8
9 . 2 . 3 T h e C l a s s i c a l M e t h o d s . . . . . . . . . . . . . . . . . . . . . . 1 6 9
9 . 3 T h e O D E A p p r o a c h t o C l a s s i c a l R e l a x a t i o n . . . . . . . . . . . . . . 1 7 0
9 . 3 . 1 T h e O r d i n a r y D i e r e n t i a l E q u a t i o n F o r m u l a t i o n . . . . . . . . 1 7 0
9 . 3 . 2 O D E F o r m o f t h e C l a s s i c a l M e t h o d s . . . . . . . . . . . . . . 1 7 2
9 . 4 E i g e n s y s t e m s o f t h e C l a s s i c a l M e t h o d s . . . . . . . . . . . . . . . . . 1 7 3
9 . 4 . 1 T h e P o i n t - J a c o b i S y s t e m . . . . . . . . . . . . . . . . . . . . . 1 7 4
9 . 4 . 2 T h e G a u s s - S e i d e l S y s t e m . . . . . . . . . . . . . . . . . . . . . 1 7 6
9 . 4 . 3 T h e S O R S y s t e m . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 0
9 . 5 N o n s t a t i o n a r y P r o c e s s e s . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 2
9 . 6 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 7
1 0 M U L T I G R I D 1 9 1
1 0 . 1 M o t i v a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9 1
1 0 . 1 . 1 E i g e n v e c t o r a n d E i g e n v a l u e I d e n t i c a t i o n w i t h S p a c e F r e q u e n c i e s 1 9 1
1 0 . 1 . 2 P r o p e r t i e s o f t h e I t e r a t i v e M e t h o d . . . . . . . . . . . . . . . 1 9 2
1 0 . 2 T h e B a s i c P r o c e s s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9 2
1 0 . 3 A T w o - G r i d P r o c e s s . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 0 0
1 0 . 4 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 0 2
1 1 N U M E R I C A L D I S S I P A T I O N 2 0 3
1 1 . 1 O n e - S i d e d F i r s t - D e r i v a t i v e S p a c e D i e r e n c i n g . . . . . . . . . . . . . 2 0 4
1 1 . 2 T h e M o d i e d P a r t i a l D i e r e n t i a l E q u a t i o n . . . . . . . . . . . . . . . 2 0 5
1 1 . 3 T h e L a x - W e n d r o M e t h o d . . . . . . . . . . . . . . . . . . . . . . . . 2 0 7
1 1 . 4 U p w i n d S c h e m e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 0 9
1 1 . 4 . 1 F l u x - V e c t o r S p l i t t i n g . . . . . . . . . . . . . . . . . . . . . . . 2 1 0
1 1 . 4 . 2 F l u x - D i e r e n c e S p l i t t i n g . . . . . . . . . . . . . . . . . . . . . 2 1 2
1 1 . 5 A r t i c i a l D i s s i p a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 3
1 1 . 6 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 4
1 2 S P L I T A N D F A C T O R E D F O R M S 2 1 7
1 2 . 1 T h e C o n c e p t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 7
1 2 . 2 F a c t o r i n g P h y s i c a l R e p r e s e n t a t i o n s | T i m e S p l i t t i n g . . . . . . . . . 2 1 8
1 2 . 3 F a c t o r i n g S p a c e M a t r i x O p e r a t o r s i n 2 { D . . . . . . . . . . . . . . . . 2 2 0
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1 2 . 3 . 1 M e s h I n d e x i n g C o n v e n t i o n . . . . . . . . . . . . . . . . . . . . 2 2 0
1 2 . 3 . 2 D a t a B a s e s a n d S p a c e V e c t o r s . . . . . . . . . . . . . . . . . . 2 2 1
1 2 . 3 . 3 D a t a B a s e P e r m u t a t i o n s . . . . . . . . . . . . . . . . . . . . . 2 2 1
1 2 . 3 . 4 S p a c e S p l i t t i n g a n d F a c t o r i n g . . . . . . . . . . . . . . . . . . 2 2 3
1 2 . 4 S e c o n d - O r d e r F a c t o r e d I m p l i c i t M e t h o d s . . . . . . . . . . . . . . . . 2 2 6
1 2 . 5 I m p o r t a n c e o f F a c t o r e d F o r m s i n 2 a n d 3 D i m e n s i o n s . . . . . . . . . 2 2 6
1 2 . 6 T h e D e l t a F o r m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 8
1 2 . 7 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 9
1 3 L I N E A R A N A L Y S I S O F S P L I T A N D F A C T O R E D F O R M S 2 3 3
1 3 . 1 T h e R e p r e s e n t a t i v e E q u a t i o n f o r C i r c u l a n t O p e r a t o r s . . . . . . . . . 2 3 3
1 3 . 2 E x a m p l e A n a l y s i s o f C i r c u l a n t S y s t e m s . . . . . . . . . . . . . . . . . 2 3 4
1 3 . 2 . 1 S t a b i l i t y C o m p a r i s o n s o f T i m e - S p l i t M e t h o d s . . . . . . . . . 2 3 4
1 3 . 2 . 2 A n a l y s i s o f a S e c o n d - O r d e r T i m e - S p l i t M e t h o d . . . . . . . . 2 3 7
1 3 . 3 T h e R e p r e s e n t a t i v e E q u a t i o n f o r S p a c e - S p l i t O p e r a t o r s . . . . . . . . 2 3 8
1 3 . 4 E x a m p l e A n a l y s i s o f 2 - D M o d e l E q u a t i o n s . . . . . . . . . . . . . . . 2 4 2
1 3 . 4 . 1 T h e U n f a c t o r e d I m p l i c i t E u l e r M e t h o d . . . . . . . . . . . . . 2 4 2
1 3 . 4 . 2 T h e F a c t o r e d N o n d e l t a F o r m o f t h e I m p l i c i t E u l e r M e t h o d . . 2 4 3
1 3 . 4 . 3 T h e F a c t o r e d D e l t a F o r m o f t h e I m p l i c i t E u l e r M e t h o d . . . . 2 4 3
1 3 . 4 . 4 T h e F a c t o r e d D e l t a F o r m o f t h e T r a p e z o i d a l M e t h o d . . . . . 2 4 4
1 3 . 5 E x a m p l e A n a l y s i s o f t h e 3 - D M o d e l E q u a t i o n . . . . . . . . . . . . . 2 4 5
1 3 . 6 P r o b l e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 7
A U S E F U L R E L A T I O N S A N D D E F I N I T I O N S F R O M L I N E A R A L -
G E B R A 2 4 9
A . 1 N o t a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 9
A . 2 D e n i t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 5 0
A . 3 A l g e b r a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 5 1
A . 4 E i g e n s y s t e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 5 1
A . 5 V e c t o r a n d M a t r i x N o r m s . . . . . . . . . . . . . . . . . . . . . . . . 2 5 4
B S O M E P R O P E R T I E S O F T R I D I A G O N A L M A T R I C E S 2 5 7
B . 1 S t a n d a r d E i g e n s y s t e m f o r S i m p l e T r i d i a g o n a l s . . . . . . . . . . . . . 2 5 7
B . 2 G e n e r a l i z e d E i g e n s y s t e m f o r S i m p l e T r i d i a g o n a l s . . . . . . . . . . . . 2 5 8
B . 3 T h e I n v e r s e o f a S i m p l e T r i d i a g o n a l . . . . . . . . . . . . . . . . . . . 2 5 9
B . 4 E i g e n s y s t e m s o f C i r c u l a n t M a t r i c e s . . . . . . . . . . . . . . . . . . . 2 6 0
B . 4 . 1 S t a n d a r d T r i d i a g o n a l s . . . . . . . . . . . . . . . . . . . . . . 2 6 0
B . 4 . 2 G e n e r a l C i r c u l a n t S y s t e m s . . . . . . . . . . . . . . . . . . . . 2 6 1
B . 5 S p e c i a l C a s e s F o u n d F r o m S y m m e t r i e s . . . . . . . . . . . . . . . . . 2 6 2
B . 6 S p e c i a l C a s e s I n v o l v i n g B o u n d a r y C o n d i t i o n s . . . . . . . . . . . . . 2 6 3
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C T H E H O M O G E N E O U S P R O P E R T Y O F T H E E U L E R E Q U A T I O N S 2 6 5
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C h a p t e r 1
I N T R O D U C T I O N
1 . 1 M o t i v a t i o n
T h e m a t e r i a l i n t h i s b o o k o r i g i n a t e d f r o m a t t e m p t s t o u n d e r s t a n d a n d s y s t e m i z e n u -
m e r i c a l s o l u t i o n t e c h n i q u e s f o r t h e p a r t i a l d i e r e n t i a l e q u a t i o n s g o v e r n i n g t h e p h y s i c s
o f u i d o w . A s t i m e w e n t o n a n d t h e s e a t t e m p t s b e g a n t o c r y s t a l l i z e , u n d e r l y i n g
c o n s t r a i n t s o n t h e n a t u r e o f t h e m a t e r i a l b e g a n t o f o r m . T h e p r i n c i p a l s u c h c o n s t r a i n t
w a s t h e d e m a n d f o r u n i c a t i o n . W a s t h e r e o n e m a t h e m a t i c a l s t r u c t u r e w h i c h c o u l d
b e u s e d t o d e s c r i b e t h e b e h a v i o r a n d r e s u l t s o f m o s t n u m e r i c a l m e t h o d s i n c o m m o n
u s e i n t h e e l d o f u i d d y n a m i c s ? P e r h a p s t h e a n s w e r i s a r g u a b l e , b u t t h e a u t h o r s
b e l i e v e t h e a n s w e r i s a r m a t i v e a n d p r e s e n t t h i s b o o k a s j u s t i c a t i o n f o r t h a t b e -
l i e f . T h e m a t h e m a t i c a l s t r u c t u r e i s t h e t h e o r y o f l i n e a r a l g e b r a a n d t h e a t t e n d a n t
e i g e n a n a l y s i s o f l i n e a r s y s t e m s .
T h e u l t i m a t e g o a l o f t h e e l d o f c o m p u t a t i o n a l u i d d y n a m i c s ( C F D ) i s t o u n d e r -
s t a n d t h e p h y s i c a l e v e n t s t h a t o c c u r i n t h e o w o f u i d s a r o u n d a n d w i t h i n d e s i g n a t e d
o b j e c t s . T h e s e e v e n t s a r e r e l a t e d t o t h e a c t i o n a n d i n t e r a c t i o n o f p h e n o m e n a s u c h
a s d i s s i p a t i o n , d i u s i o n , c o n v e c t i o n , s h o c k w a v e s , s l i p s u r f a c e s , b o u n d a r y l a y e r s , a n d
t u r b u l e n c e . I n t h e e l d o f a e r o d y n a m i c s , a l l o f t h e s e p h e n o m e n a a r e g o v e r n e d b y
t h e c o m p r e s s i b l e N a v i e r - S t o k e s e q u a t i o n s . M a n y o f t h e m o s t i m p o r t a n t a s p e c t s o f
t h e s e r e l a t i o n s a r e n o n l i n e a r a n d , a s a c o n s e q u e n c e , o f t e n h a v e n o a n a l y t i c s o l u t i o n .
T h i s , o f c o u r s e , m o t i v a t e s t h e n u m e r i c a l s o l u t i o n o f t h e a s s o c i a t e d p a r t i a l d i e r e n t i a l
e q u a t i o n s . A t t h e s a m e t i m e i t w o u l d s e e m t o i n v a l i d a t e t h e u s e o f l i n e a r a l g e b r a f o r
t h e c l a s s i c a t i o n o f t h e n u m e r i c a l m e t h o d s . E x p e r i e n c e h a s s h o w n t h a t s u c h i s n o t
t h e c a s e .
A s w e s h a l l s e e i n a l a t e r c h a p t e r , t h e u s e o f n u m e r i c a l m e t h o d s t o s o l v e p a r t i a l
d i e r e n t i a l e q u a t i o n s i n t r o d u c e s a n a p p r o x i m a t i o n t h a t , i n e e c t , c a n c h a n g e t h e
f o r m o f t h e b a s i c p a r t i a l d i e r e n t i a l e q u a t i o n s t h e m s e l v e s . T h e n e w e q u a t i o n s , w h i c h
1
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2 C H A P T E R 1 . I N T R O D U C T I O N
a r e t h e o n e s a c t u a l l y b e i n g s o l v e d b y t h e n u m e r i c a l p r o c e s s , a r e o f t e n r e f e r r e d t o a s
t h e m o d i e d p a r t i a l d i e r e n t i a l e q u a t i o n s . S i n c e t h e y a r e n o t p r e c i s e l y t h e s a m e a s
t h e o r i g i n a l e q u a t i o n s , t h e y c a n , a n d p r o b a b l y w i l l , s i m u l a t e t h e p h y s i c a l p h e n o m e n a
l i s t e d a b o v e i n w a y s t h a t a r e n o t e x a c t l y t h e s a m e a s a n e x a c t s o l u t i o n t o t h e b a s i c
p a r t i a l d i e r e n t i a l e q u a t i o n . M a t h e m a t i c a l l y , t h e s e d i e r e n c e s a r e u s u a l l y r e f e r r e d t o
a s t r u n c a t i o n e r r o r s . H o w e v e r , t h e t h e o r y a s s o c i a t e d w i t h t h e n u m e r i c a l a n a l y s i s o f
u i d m e c h a n i c s w a s d e v e l o p e d p r e d o m i n a n t l y b y s c i e n t i s t s d e e p l y i n t e r e s t e d i n t h e
p h y s i c s o f u i d o w a n d , a s a c o n s e q u e n c e , t h e s e e r r o r s a r e o f t e n i d e n t i e d w i t h a
p a r t i c u l a r p h y s i c a l p h e n o m e n o n o n w h i c h t h e y h a v e a s t r o n g e e c t . T h u s m e t h o d s a r e
s a i d t o h a v e a l o t o f \ a r t i c i a l v i s c o s i t y " o r s a i d t o b e h i g h l y d i s p e r s i v e . T h i s m e a n s
t h a t t h e e r r o r s c a u s e d b y t h e n u m e r i c a l a p p r o x i m a t i o n r e s u l t i n a m o d i e d p a r t i a l
d i e r e n t i a l e q u a t i o n h a v i n g a d d i t i o n a l t e r m s t h a t c a n b e i d e n t i e d w i t h t h e p h y s i c s
o f d i s s i p a t i o n i n t h e r s t c a s e a n d d i s p e r s i o n i n t h e s e c o n d . T h e r e i s n o t h i n g w r o n g ,
o f c o u r s e , w i t h i d e n t i f y i n g a n e r r o r w i t h a p h y s i c a l p r o c e s s , n o r w i t h d e l i b e r a t e l y
d i r e c t i n g a n e r r o r t o a s p e c i c p h y s i c a l p r o c e s s , a s l o n g a s t h e e r r o r r e m a i n s i n s o m e
e n g i n e e r i n g s e n s e \ s m a l l " . I t i s s a f e t o s a y , f o r e x a m p l e , t h a t m o s t n u m e r i c a l m e t h o d s
i n p r a c t i c a l u s e f o r s o l v i n g t h e n o n d i s s i p a t i v e E u l e r e q u a t i o n s c r e a t e a m o d i e d p a r t i a l
d i e r e n t i a l e q u a t i o n t h a t p r o d u c e s s o m e f o r m o f d i s s i p a t i o n . H o w e v e r , i f u s e d a n d
i n t e r p r e t e d p r o p e r l y , t h e s e m e t h o d s g i v e v e r y u s e f u l i n f o r m a t i o n .
R e g a r d l e s s o f w h a t t h e n u m e r i c a l e r r o r s a r e c a l l e d , i f t h e i r e e c t s a r e n o t t h o r -
o u g h l y u n d e r s t o o d a n d c o n t r o l l e d , t h e y c a n l e a d t o s e r i o u s d i c u l t i e s , p r o d u c i n g
a n s w e r s t h a t r e p r e s e n t l i t t l e , i f a n y , p h y s i c a l r e a l i t y . T h i s m o t i v a t e s s t u d y i n g t h e
c o n c e p t s o f s t a b i l i t y , c o n v e r g e n c e , a n d c o n s i s t e n c y . O n t h e o t h e r h a n d , e v e n i f t h e
e r r o r s a r e k e p t s m a l l e n o u g h t h a t t h e y c a n b e n e g l e c t e d ( f o r e n g i n e e r i n g p u r p o s e s ) ,
t h e r e s u l t i n g s i m u l a t i o n c a n s t i l l b e o f l i t t l e p r a c t i c a l u s e i f i n e c i e n t o r i n a p p r o p r i a t e
a l g o r i t h m s a r e u s e d . T h i s m o t i v a t e s s t u d y i n g t h e c o n c e p t s o f s t i n e s s , f a c t o r i z a t i o n ,
a n d a l g o r i t h m d e v e l o p m e n t i n g e n e r a l . A l l o f t h e s e c o n c e p t s w e h o p e t o c l a r i f y i n
t h i s b o o k .
1 . 2 B a c k g r o u n d
T h e e l d o f c o m p u t a t i o n a l u i d d y n a m i c s h a s a b r o a d r a n g e o f a p p l i c a b i l i t y . I n d e p e n -
d e n t o f t h e s p e c i c a p p l i c a t i o n u n d e r s t u d y , t h e f o l l o w i n g s e q u e n c e o f s t e p s g e n e r a l l y
m u s t b e f o l l o w e d i n o r d e r t o o b t a i n a s a t i s f a c t o r y s o l u t i o n .
1 . 2 . 1 P r o b l e m S p e c i c a t i o n a n d G e o m e t r y P r e p a r a t i o n
T h e r s t s t e p i n v o l v e s t h e s p e c i c a t i o n o f t h e p r o b l e m , i n c l u d i n g t h e g e o m e t r y , o w
c o n d i t i o n s , a n d t h e r e q u i r e m e n t s o f t h e s i m u l a t i o n . T h e g e o m e t r y m a y r e s u l t f r o m
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1 . 2 . B A C K G R O U N D 3
m e a s u r e m e n t s o f a n e x i s t i n g c o n g u r a t i o n o r m a y b e a s s o c i a t e d w i t h a d e s i g n s t u d y .
A l t e r n a t i v e l y , i n a d e s i g n c o n t e x t , n o g e o m e t r y n e e d b e s u p p l i e d . I n s t e a d , a s e t
o f o b j e c t i v e s a n d c o n s t r a i n t s m u s t b e s p e c i e d . F l o w c o n d i t i o n s m i g h t i n c l u d e , f o r
e x a m p l e , t h e R e y n o l d s n u m b e r a n d M a c h n u m b e r f o r t h e o w o v e r a n a i r f o i l . T h e
r e q u i r e m e n t s o f t h e s i m u l a t i o n i n c l u d e i s s u e s s u c h a s t h e l e v e l o f a c c u r a c y n e e d e d , t h e
t u r n a r o u n d t i m e r e q u i r e d , a n d t h e s o l u t i o n p a r a m e t e r s o f i n t e r e s t . T h e r s t t w o o f
t h e s e r e q u i r e m e n t s a r e o f t e n i n c o n i c t a n d c o m p r o m i s e i s n e c e s s a r y . A s a n e x a m p l e
o f s o l u t i o n p a r a m e t e r s o f i n t e r e s t i n c o m p u t i n g t h e o w e l d a b o u t a n a i r f o i l , o n e m a y
b e i n t e r e s t e d i n i ) t h e l i f t a n d p i t c h i n g m o m e n t o n l y , i i ) t h e d r a g a s w e l l a s t h e l i f t
a n d p i t c h i n g m o m e n t , o r i i i ) t h e d e t a i l s o f t h e o w a t s o m e s p e c i c l o c a t i o n .
1 . 2 . 2 S e l e c t i o n o f G o v e r n i n g E q u a t i o n s a n d B o u n d a r y C o n -
d i t i o n s
O n c e t h e p r o b l e m h a s b e e n s p e c i e d , a n a p p r o p r i a t e s e t o f g o v e r n i n g e q u a t i o n s a n d
b o u n d a r y c o n d i t i o n s m u s t b e s e l e c t e d . I t i s g e n e r a l l y a c c e p t e d t h a t t h e p h e n o m e n a o f
i m p o r t a n c e t o t h e e l d o f c o n t i n u u m u i d d y n a m i c s a r e g o v e r n e d b y t h e c o n s e r v a t i o n
o f m a s s , m o m e n t u m , a n d e n e r g y . T h e p a r t i a l d i e r e n t i a l e q u a t i o n s r e s u l t i n g f r o m
t h e s e c o n s e r v a t i o n l a w s a r e r e f e r r e d t o a s t h e N a v i e r - S t o k e s e q u a t i o n s . H o w e v e r , i n
t h e i n t e r e s t o f e c i e n c y , i t i s a l w a y s p r u d e n t t o c o n s i d e r s o l v i n g s i m p l i e d f o r m s
o f t h e N a v i e r - S t o k e s e q u a t i o n s w h e n t h e s i m p l i c a t i o n s r e t a i n t h e p h y s i c s w h i c h a r e
e s s e n t i a l t o t h e g o a l s o f t h e s i m u l a t i o n . P o s s i b l e s i m p l i e d g o v e r n i n g e q u a t i o n s i n c l u d e
t h e p o t e n t i a l - o w e q u a t i o n s , t h e E u l e r e q u a t i o n s , a n d t h e t h i n - l a y e r N a v i e r - S t o k e s
e q u a t i o n s . T h e s e m a y b e s t e a d y o r u n s t e a d y a n d c o m p r e s s i b l e o r i n c o m p r e s s i b l e .
B o u n d a r y t y p e s w h i c h m a y b e e n c o u n t e r e d i n c l u d e s o l i d w a l l s , i n o w a n d o u t o w
b o u n d a r i e s , p e r i o d i c b o u n d a r i e s , s y m m e t r y b o u n d a r i e s , e t c . T h e b o u n d a r y c o n d i t i o n s
w h i c h m u s t b e s p e c i e d d e p e n d u p o n t h e g o v e r n i n g e q u a t i o n s . F o r e x a m p l e , a t a s o l i d
w a l l , t h e E u l e r e q u a t i o n s r e q u i r e o w t a n g e n c y t o b e e n f o r c e d , w h i l e t h e N a v i e r - S t o k e s
e q u a t i o n s r e q u i r e t h e n o - s l i p c o n d i t i o n . I f n e c e s s a r y , p h y s i c a l m o d e l s m u s t b e c h o s e n
f o r p r o c e s s e s w h i c h c a n n o t b e s i m u l a t e d w i t h i n t h e s p e c i e d c o n s t r a i n t s . T u r b u l e n c e
i s a n e x a m p l e o f a p h y s i c a l p r o c e s s w h i c h i s r a r e l y s i m u l a t e d i n a p r a c t i c a l c o n t e x t ( a t
t h e t i m e o f w r i t i n g ) a n d t h u s i s o f t e n m o d e l l e d . T h e s u c c e s s o f a s i m u l a t i o n d e p e n d s
g r e a t l y o n t h e e n g i n e e r i n g i n s i g h t i n v o l v e d i n s e l e c t i n g t h e g o v e r n i n g e q u a t i o n s a n d
p h y s i c a l m o d e l s b a s e d o n t h e p r o b l e m s p e c i c a t i o n .
1 . 2 . 3 S e l e c t i o n o f G r i d d i n g S t r a t e g y a n d N u m e r i c a l M e t h o d
N e x t a n u m e r i c a l m e t h o d a n d a s t r a t e g y f o r d i v i d i n g t h e o w d o m a i n i n t o c e l l s , o r
e l e m e n t s , m u s t b e s e l e c t e d . W e c o n c e r n o u r s e l v e s h e r e o n l y w i t h n u m e r i c a l m e t h -
o d s r e q u i r i n g s u c h a t e s s e l l a t i o n o f t h e d o m a i n , w h i c h i s k n o w n a s a g r i d , o r m e s h .
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4 C H A P T E R 1 . I N T R O D U C T I O N
M a n y d i e r e n t g r i d d i n g s t r a t e g i e s e x i s t , i n c l u d i n g s t r u c t u r e d , u n s t r u c t u r e d , h y b r i d ,
c o m p o s i t e , a n d o v e r l a p p i n g g r i d s . F u r t h e r m o r e , t h e g r i d c a n b e a l t e r e d b a s e d o n
t h e s o l u t i o n i n a n a p p r o a c h k n o w n a s s o l u t i o n - a d a p t i v e g r i d d i n g . T h e n u m e r i c a l
m e t h o d s g e n e r a l l y u s e d i n C F D c a n b e c l a s s i e d a s n i t e - d i e r e n c e , n i t e - v o l u m e ,
n i t e - e l e m e n t , o r s p e c t r a l m e t h o d s . T h e c h o i c e s o f a n u m e r i c a l m e t h o d a n d a g r i d -
d i n g s t r a t e g y a r e s t r o n g l y i n t e r d e p e n d e n t . F o r e x a m p l e , t h e u s e o f n i t e - d i e r e n c e
m e t h o d s i s t y p i c a l l y r e s t r i c t e d t o s t r u c t u r e d g r i d s . H e r e a g a i n , t h e s u c c e s s o f a s i m -
u l a t i o n c a n d e p e n d o n a p p r o p r i a t e c h o i c e s f o r t h e p r o b l e m o r c l a s s o f p r o b l e m s o f
i n t e r e s t .
1 . 2 . 4 A s s e s s m e n t a n d I n t e r p r e t a t i o n o f R e s u l t s
F i n a l l y , t h e r e s u l t s o f t h e s i m u l a t i o n m u s t b e a s s e s s e d a n d i n t e r p r e t e d . T h i s s t e p c a n
r e q u i r e p o s t - p r o c e s s i n g o f t h e d a t a , f o r e x a m p l e c a l c u l a t i o n o f f o r c e s a n d m o m e n t s ,
a n d c a n b e a i d e d b y s o p h i s t i c a t e d o w v i s u a l i z a t i o n t o o l s a n d e r r o r e s t i m a t i o n t e c h -
n i q u e s . I t i s c r i t i c a l t h a t t h e m a g n i t u d e o f b o t h n u m e r i c a l a n d p h y s i c a l - m o d e l e r r o r s
b e w e l l u n d e r s t o o d .
1 . 3 O v e r v i e w
I t s h o u l d b e c l e a r t h a t s u c c e s s f u l s i m u l a t i o n o f u i d o w s c a n i n v o l v e a w i d e r a n g e o f
i s s u e s f r o m g r i d g e n e r a t i o n t o t u r b u l e n c e m o d e l l i n g t o t h e a p p l i c a b i l i t y o f v a r i o u s s i m -
p l i e d f o r m s o f t h e N a v i e r - S t o k e s e q u a t i o n s . M a n y o f t h e s e i s s u e s a r e n o t a d d r e s s e d
i n t h i s b o o k . S o m e o f t h e m a r e p r e s e n t e d i n t h e b o o k s b y A n d e r s o n , T a n n e h i l l , a n d
P l e t c h e r 1 ] a n d H i r s c h 2 ] . I n s t e a d w e f o c u s o n n u m e r i c a l m e t h o d s , w i t h e m p h a s i s
o n n i t e - d i e r e n c e a n d n i t e - v o l u m e m e t h o d s f o r t h e E u l e r a n d N a v i e r - S t o k e s e q u a -
t i o n s . R a t h e r t h a n p r e s e n t i n g t h e d e t a i l s o f t h e m o s t a d v a n c e d m e t h o d s , w h i c h a r e
s t i l l e v o l v i n g , w e p r e s e n t a f o u n d a t i o n f o r d e v e l o p i n g , a n a l y z i n g , a n d u n d e r s t a n d i n g
s u c h m e t h o d s .
F o r t u n a t e l y , t o d e v e l o p , a n a l y z e , a n d u n d e r s t a n d m o s t n u m e r i c a l m e t h o d s u s e d t o
n d s o l u t i o n s f o r t h e c o m p l e t e c o m p r e s s i b l e N a v i e r - S t o k e s e q u a t i o n s , w e c a n m a k e u s e
o f m u c h s i m p l e r e x p r e s s i o n s , t h e s o - c a l l e d \ m o d e l " e q u a t i o n s . T h e s e m o d e l e q u a t i o n s
i s o l a t e c e r t a i n a s p e c t s o f t h e p h y s i c s c o n t a i n e d i n t h e c o m p l e t e s e t o f e q u a t i o n s . H e n c e
t h e i r n u m e r i c a l s o l u t i o n c a n i l l u s t r a t e t h e p r o p e r t i e s o f a g i v e n n u m e r i c a l m e t h o d
w h e n a p p l i e d t o a m o r e c o m p l i c a t e d s y s t e m o f e q u a t i o n s w h i c h g o v e r n s s i m i l a r p h y s -
i c a l p h e n o m e n a . A l t h o u g h t h e m o d e l e q u a t i o n s a r e e x t r e m e l y s i m p l e a n d e a s y t o
s o l v e , t h e y h a v e b e e n c a r e f u l l y s e l e c t e d t o b e r e p r e s e n t a t i v e , w h e n u s e d i n t e l l i g e n t l y ,
o f d i c u l t i e s a n d c o m p l e x i t i e s t h a t a r i s e i n r e a l i s t i c t w o - a n d t h r e e - d i m e n s i o n a l u i d
o w s i m u l a t i o n s . W e b e l i e v e t h a t a t h o r o u g h u n d e r s t a n d i n g o f w h a t h a p p e n s w h e n
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1 . 4 . N O T A T I O N 5
n u m e r i c a l a p p r o x i m a t i o n s a r e a p p l i e d t o t h e m o d e l e q u a t i o n s i s a m a j o r r s t s t e p i n
m a k i n g c o n d e n t a n d c o m p e t e n t u s e o f n u m e r i c a l a p p r o x i m a t i o n s t o t h e E u l e r a n d
N a v i e r - S t o k e s e q u a t i o n s . A s a w o r d o f c a u t i o n , h o w e v e r , i t s h o u l d b e n o t e d t h a t ,
a l t h o u g h w e c a n l e a r n a g r e a t d e a l b y s t u d y i n g n u m e r i c a l m e t h o d s a s a p p l i e d t o t h e
m o d e l e q u a t i o n s a n d c a n u s e t h a t i n f o r m a t i o n i n t h e d e s i g n a n d a p p l i c a t i o n o f n u -
m e r i c a l m e t h o d s t o p r a c t i c a l p r o b l e m s , t h e r e a r e m a n y a s p e c t s o f p r a c t i c a l p r o b l e m s
w h i c h c a n o n l y b e u n d e r s t o o d i n t h e c o n t e x t o f t h e c o m p l e t e p h y s i c a l s y s t e m s .
1 . 4 N o t a t i o n
T h e n o t a t i o n i s g e n e r a l l y e x p l a i n e d a s i t i s i n t r o d u c e d . B o l d t y p e i s r e s e r v e d f o r r e a l
p h y s i c a l v e c t o r s , s u c h a s v e l o c i t y . T h e v e c t o r s y m b o l ~ i s u s e d f o r t h e v e c t o r s ( o r
c o l u m n m a t r i c e s ) w h i c h c o n t a i n t h e v a l u e s o f t h e d e p e n d e n t v a r i a b l e a t t h e n o d e s
o f a g r i d . O t h e r w i s e , t h e u s e o f a v e c t o r c o n s i s t i n g o f a c o l l e c t i o n o f s c a l a r s s h o u l d
b e a p p a r e n t f r o m t h e c o n t e x t a n d i s n o t i d e n t i e d b y a n y s p e c i a l n o t a t i o n . F o r
e x a m p l e , t h e v a r i a b l e u c a n d e n o t e a s c a l a r C a r t e s i a n v e l o c i t y c o m p o n e n t i n t h e E u l e r
a n d N a v i e r - S t o k e s e q u a t i o n s , a s c a l a r q u a n t i t y i n t h e l i n e a r c o n v e c t i o n a n d d i u s i o n
e q u a t i o n s , a n d a v e c t o r c o n s i s t i n g o f a c o l l e c t i o n o f s c a l a r s i n o u r p r e s e n t a t i o n o f
h y p e r b o l i c s y s t e m s . S o m e o f t h e a b b r e v i a t i o n s u s e d t h r o u g h o u t t h e t e x t a r e l i s t e d
a n d d e n e d b e l o w .
P D E P a r t i a l d i e r e n t i a l e q u a t i o n
O D E O r d i n a r y d i e r e n t i a l e q u a t i o n
O E O r d i n a r y d i e r e n c e e q u a t i o n
R H S R i g h t - h a n d s i d e
P . S . P a r t i c u l a r s o l u t i o n o f a n O D E o r s y s t e m o f O D E ' s
S . S . F i x e d ( t i m e - i n v a r i a n t ) s t e a d y - s t a t e s o l u t i o n
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C h a p t e r 2
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t i o n - l a w f o r m , w h i c h i s u s e f u l i n u n d e r s t a n d i n g t h e c o n c e p t s i n v o l v e d i n n i t e - v o l u m e
s c h e m e s . T h e e q u a t i o n s a r e t h e n r e c a s t i n t o d i v e r g e n c e f o r m , w h i c h i s n a t u r a l f o r
n i t e - d i e r e n c e s c h e m e s . T h e E u l e r a n d N a v i e r - S t o k e s e q u a t i o n s a r e b r i e y d i s c u s s e d
i n t h i s C h a p t e r . T h e m a i n f o c u s , t h o u g h , w i l l b e o n r e p r e s e n t a t i v e m o d e l e q u a t i o n s ,
i n p a r t i c u l a r , t h e c o n v e c t i o n a n d d i u s i o n e q u a t i o n s . T h e s e e q u a t i o n s c o n t a i n m a n y
o f t h e s a l i e n t m a t h e m a t i c a l a n d p h y s i c a l f e a t u r e s o f t h e f u l l N a v i e r - S t o k e s e q u a t i o n s .
T h e c o n c e p t s o f c o n v e c t i o n a n d d i u s i o n a r e p r e v a l e n t i n o u r d e v e l o p m e n t o f n u -
m e r i c a l m e t h o d s f o r c o m p u t a t i o n a l u i d d y n a m i c s , a n d t h e r e c u r r i n g u s e o f t h e s e
m o d e l e q u a t i o n s a l l o w s u s t o d e v e l o p a c o n s i s t e n t f r a m e w o r k o f a n a l y s i s f o r c o n s i s -
t e n c y , a c c u r a c y , s t a b i l i t y , a n d c o n v e r g e n c e . T h e m o d e l e q u a t i o n s w e s t u d y h a v e t w o
p r o p e r t i e s i n c o m m o n . T h e y a r e l i n e a r p a r t i a l d i e r e n t i a l e q u a t i o n s ( P D E ' s ) w i t h
c o e c i e n t s t h a t a r e c o n s t a n t i n b o t h s p a c e a n d t i m e , a n d t h e y r e p r e s e n t p h e n o m e n a
o f i m p o r t a n c e t o t h e a n a l y s i s o f c e r t a i n a s p e c t s o f u i d d y n a m i c p r o b l e m s .
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e q u a t i o n s , c a n b e w r i t t e n i n t h e f o l l o w i n g i n t e g r a l f o r m :
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e . g . , m a s s , m o m e n t u m , a n d e n e r g y , p e r u n i t v o l u m e . T h e e q u a t i o n i s a s t a t e m e n t o f
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t h e c o n s e r v a t i o n o f t h e s e q u a n t i t i e s i n a n i t e r e g i o n o f s p a c e w i t h v o l u m e V ( t ) a n d
s u r f a c e a r e a S ( t ) o v e r a n i t e i n t e r v a l o f t i m e t
2
; t
1
. I n t w o d i m e n s i o n s , t h e r e g i o n
o f s p a c e , o r c e l l , i s a n a r e a A ( t ) b o u n d e d b y a c l o s e d c o n t o u r C ( t ) . T h e v e c t o r n i s
a u n i t v e c t o r n o r m a l t o t h e s u r f a c e p o i n t i n g o u t w a r d , F i s a s e t o f v e c t o r s , o r t e n s o r ,
c o n t a i n i n g t h e u x o f Q p e r u n i t a r e a p e r u n i t t i m e , a n d P i s t h e r a t e o f p r o d u c t i o n
o f Q p e r u n i t v o l u m e p e r u n i t t i m e . I f a l l v a r i a b l e s a r e c o n t i n u o u s i n t i m e , t h e n E q .
2 . 1 c a n b e r e w r i t t e n a s
d
d t
Z
V ( t )
Q d V +
I
S ( t )
n : F d S =
Z
V ( t )
P d V ( 2 . 2 )
T h o s e m e t h o d s w h i c h m a k e v a r i o u s n u m e r i c a l a p p r o x i m a t i o n s o f t h e i n t e g r a l s i n E q s .
2 . 1 a n d 2 . 2 a n d n d a s o l u t i o n f o r Q o n t h a t b a s i s a r e r e f e r r e d t o a s n i t e - v o l u m e
m e t h o d s . M a n y o f t h e a d v a n c e d c o d e s w r i t t e n f o r C F D a p p l i c a t i o n s a r e b a s e d o n t h e
n i t e - v o l u m e c o n c e p t .
O n t h e o t h e r h a n d , a p a r t i a l d e r i v a t i v e f o r m o f a c o n s e r v a t i o n l a w c a n a l s o b e
d e r i v e d . T h e d i v e r g e n c e f o r m o f E q . 2 . 2 i s o b t a i n e d b y a p p l y i n g G a u s s ' s t h e o r e m t o
t h e u x i n t e g r a l , l e a d i n g t o
@ Q
@ t
+ r : F = P ( 2 . 3 )
w h e r e r : i s t h e w e l l - k n o w n d i v e r g e n c e o p e r a t o r g i v e n , i n C a r t e s i a n c o o r d i n a t e s , b y
r :
i
@
@ x
+ j
@
@ y
+ k
@
@ z
!
: ( 2 . 4 )
a n d i j , a n d k a r e u n i t v e c t o r s i n t h e x y , a n d z c o o r d i n a t e d i r e c t i o n s , r e s p e c t i v e l y .
T h o s e m e t h o d s w h i c h m a k e v a r i o u s a p p r o x i m a t i o n s o f t h e d e r i v a t i v e s i n E q . 2 . 3 a n d
n d a s o l u t i o n f o r Q o n t h a t b a s i s a r e r e f e r r e d t o a s n i t e - d i e r e n c e m e t h o d s .
2 . 2 T h e N a v i e r - S t o k e s a n d E u l e r E q u a t i o n s
T h e N a v i e r - S t o k e s e q u a t i o n s f o r m a c o u p l e d s y s t e m o f n o n l i n e a r P D E ' s d e s c r i b i n g
t h e c o n s e r v a t i o n o f m a s s , m o m e n t u m a n d e n e r g y f o r a u i d . F o r a N e w t o n i a n u i d
i n o n e d i m e n s i o n , t h e y c a n b e w r i t t e n a s
@ Q
@ t
+
@ E
@ x
= 0 ( 2 . 5 )
w i t h
Q =
2
6
6
6
6
6
4
u
e
3
7
7
7
7
7
5
E =
2
6
6
6
6
6
4
u
u
2
+ p
u ( e + p )
3
7
7
7
7
7
5
;
2
6
6
6
6
6
4
0
4
3
@ u
@ x
4
3
u
@ u
@ x
+
@ T
@ x
3
7
7
7
7
7
5
( 2 . 6 )
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2 . 2 . T H E N A V I E R - S T O K E S A N D E U L E R E Q U A T I O N S 9
w h e r e i s t h e u i d d e n s i t y , u i s t h e v e l o c i t y , e i s t h e t o t a l e n e r g y p e r u n i t v o l u m e , p i s
t h e p r e s s u r e , T i s t h e t e m p e r a t u r e , i s t h e c o e c i e n t o f v i s c o s i t y , a n d i s t h e t h e r m a l
c o n d u c t i v i t y . T h e t o t a l e n e r g y e i n c l u d e s i n t e r n a l e n e r g y p e r u n i t v o l u m e ( w h e r e
i s t h e i n t e r n a l e n e r g y p e r u n i t m a s s ) a n d k i n e t i c e n e r g y p e r u n i t v o l u m e u
2
= 2 .
T h e s e e q u a t i o n s m u s t b e s u p p l e m e n t e d b y r e l a t i o n s b e t w e e n a n d a n d t h e u i d
s t a t e a s w e l l a s a n e q u a t i o n o f s t a t e , s u c h a s t h e i d e a l g a s l a w . D e t a i l s c a n b e f o u n d
i n A n d e r s o n , T a n n e h i l l , a n d P l e t c h e r 1 ] a n d H i r s c h 2 ] . N o t e t h a t t h e c o n v e c t i v e
u x e s l e a d t o r s t d e r i v a t i v e s i n s p a c e , w h i l e t h e v i s c o u s a n d h e a t c o n d u c t i o n t e r m s
i n v o l v e s e c o n d d e r i v a t i v e s . T h i s f o r m o f t h e e q u a t i o n s i s c a l l e d c o n s e r v a t i o n - l a w o r
c o n s e r v a t i v e f o r m . N o n - c o n s e r v a t i v e f o r m s c a n b e o b t a i n e d b y e x p a n d i n g d e r i v a t i v e s
o f p r o d u c t s u s i n g t h e p r o d u c t r u l e o r b y i n t r o d u c i n g d i e r e n t d e p e n d e n t v a r i a b l e s ,
s u c h a s u a n d p . A l t h o u g h n o n - c o n s e r v a t i v e f o r m s o f t h e e q u a t i o n s a r e a n a l y t i c a l l y
t h e s a m e a s t h e a b o v e f o r m , t h e y c a n l e a d t o q u i t e d i e r e n t n u m e r i c a l s o l u t i o n s i n
t e r m s o f s h o c k s t r e n g t h a n d s h o c k s p e e d , f o r e x a m p l e . T h u s t h e c o n s e r v a t i v e f o r m i s
a p p r o p r i a t e f o r s o l v i n g o w s w i t h f e a t u r e s s u c h a s s h o c k w a v e s .
M a n y o w s o f e n g i n e e r i n g i n t e r e s t a r e s t e a d y ( t i m e - i n v a r i a n t ) , o r a t l e a s t m a y b e
t r e a t e d a s s u c h . F o r s u c h o w s , w e a r e o f t e n i n t e r e s t e d i n t h e s t e a d y - s t a t e s o l u t i o n o f
t h e N a v i e r - S t o k e s e q u a t i o n s , w i t h n o i n t e r e s t i n t h e t r a n s i e n t p o r t i o n o f t h e s o l u t i o n .
T h e s t e a d y s o l u t i o n t o t h e o n e - d i m e n s i o n a l N a v i e r - S t o k e s e q u a t i o n s m u s t s a t i s f y
@ E
@ x
= 0 ( 2 . 7 )
I f w e n e g l e c t v i s c o s i t y a n d h e a t c o n d u c t i o n , t h e E u l e r e q u a t i o n s a r e o b t a i n e d . I n
t w o - d i m e n s i o n a l C a r t e s i a n c o o r d i n a t e s , t h e s e c a n b e w r i t t e n a s
@ Q
@ t
+
@ E
@ x
+
@ F
@ y
= 0 ( 2 . 8 )
w i t h
Q =
2
6
6
6
4
q
1
q
2
q
3
q
4
3
7
7
7
5
=
2
6
6
6
4
u
v
e
3
7
7
7
5
E =
2
6
6
6
4
u
u
2
+ p
u v
u ( e + p )
3
7
7
7
5
F =
2
6
6
6
4
v
u v
v
2
+ p
v ( e + p )
3
7
7
7
5
( 2 . 9 )
w h e r e u a n d v a r e t h e C a r t e s i a n v e l o c i t y c o m p o n e n t s . L a t e r o n w e w i l l m a k e u s e o f
t h e f o l l o w i n g f o r m o f t h e E u l e r e q u a t i o n s a s w e l l :
@ Q
@ t
+ A
@ Q
@ x
+ B
@ Q
@ y
= 0 ( 2 . 1 0 )
T h e m a t r i c e s A =
@ E
@ Q
a n d B =
@ F
@ Q
a r e k n o w n a s t h e u x J a c o b i a n s . T h e u x v e c t o r s
g i v e n a b o v e a r e w r i t t e n i n t e r m s o f t h e p r i m i t i v e v a r i a b l e s , , u , v , a n d p . I n o r d e r
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1 0 C H A P T E R 2 . C O N S E R V A T I O N L A W S A N D T H E M O D E L E Q U A T I O N S
t o d e r i v e t h e u x J a c o b i a n m a t r i c e s , w e m u s t r s t w r i t e t h e u x v e c t o r s E a n d F i n
t e r m s o f t h e c o n s e r v a t i v e v a r i a b l e s , q
1
, q
2
, q
3
, a n d q
4
, a s f o l l o w s :
E =
2
6
6
6
6
6
6
6
6
6
6
6
4
E
1
E
2
E
3
E
4
3
7
7
7
7
7
7
7
7
7
7
7
5
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
q
2
( ; 1 ) q
4
+
3 ;
2
q
2
2
q
1
;
; 1
2
q
2
3
q
1
q
3
q
2
q
1
q
4
q
2
q
1
;
; 1
2
q
3
2
q
2
1
+
q
2
3
q
2
q
2
1
3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
( 2 . 1 1 )
F =
2
6
6
6
6
6
6
6
6
6
6
6
4
F
1
F
2
F
3
F
4
3
7
7
7
7
7
7
7
7
7
7
7
5
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
4
q
3
q
3
q
2
q
1
( ; 1 ) q
4
+
3 ;
2
q
2
3
q
1
;
; 1
2
q
2
2
q
1
q
4
q
3
q
1
;
; 1
2
q
2
2
q
3
q
2
1
+
q
3
3
q
2
1
3
7
7
7
7
7
7
7
7
7
7
7
7
7
5
( 2 . 1 2 )
W e h a v e a s s u m e d t h a t t h e p r e s s u r e s a t i s e s p = ( ; 1 ) e ; ( u
2
+ v
2
) = 2 ] f r o m t h e
i d e a l g a s l a w , w h e r e i s t h e r a t i o o f s p e c i c h e a t s , c
p
= c
v
. F r o m t h i s i t f o l l o w s t h a t
t h e u x J a c o b i a n o f E c a n b e w r i t t e n i n t e r m s o f t h e c o n s e r v a t i v e v a r i a b l e s a s
A =
@ E
i
@ q
j
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
4
0 1 0 0
a
2 1
( 3 ; )
q
2
q
1
( 1 ; )
q
3
q
1
; 1
;
q
2
q
1
q
3
q
1
q
3
q
1
q
2
q
1
0
a
4 1
a
4 2
a
4 3
q
2
q
1
3
7
7
7
7
7
7
7
7
7
7
7
7
7
5
( 2 . 1 3 )
w h e r e
a
2 1
=
; 1
2
q
3
q
1
!
2
;
3 ;
2
q
2
q
1
!
2
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2 . 2 . T H E N A V I E R - S T O K E S A N D E U L E R E Q U A T I O N S 1 1
a
4 1
= ( ; 1 )
2
4
q
2
q
1
!
3
+
q
3
q
1
!
2
q
2
q
1
!
3
5
;
q
4
q
1
!
q
2
q
1
!
a
4 2
=
q
4
q
1
!
;
; 1
2
2
4
3
q
2
q
1
!
2
+
q
3
q
1
!
2
3
5
a
4 3
= ; ( ; 1 )
q
2
q
1
!
q
3
q
1
!
( 2 . 1 4 )
a n d i n t e r m s o f t h e p r i m i t i v e v a r i a b l e s a s
A =
2
6
6
6
6
6
6
6
6
6
6
6
4
0 1 0 0
a
2 1
( 3 ; ) u ( 1 ; ) v ( ; 1 )
; u v v u 0
a
4 1
a
4 2
a
4 3
u
3
7
7
7
7
7
7
7
7
7
7
7
5
( 2 . 1 5 )
w h e r e
a
2 1
=
; 1
2
v
2
;
3 ;
2
u
2
a
4 1
= ( ; 1 ) u ( u
2
+ v
2
) ;
u e
a
4 2
=
e
;
; 1
2
( 3 u
2
+ v
2
)
a
4 3
= ( 1 ; ) u v ( 2 . 1 6 )
D e r i v a t i o n o f t h e t w o f o r m s o f B = @ F = @ Q i s s i m i l a r . T h e e i g e n v a l u e s o f t h e u x
J a c o b i a n m a t r i c e s a r e p u r e l y r e a l . T h i s i s t h e d e n i n g f e a t u r e o f h y p e r b o l i c s y s t e m s
o f P D E ' s , w h i c h a r e f u r t h e r d i s c u s s e d i n S e c t i o n 2 . 5 . T h e h o m o g e n e o u s p r o p e r t y o f
t h e E u l e r e q u a t i o n s i s d i s c u s s e d i n A p p e n d i x C .
T h e N a v i e r - S t o k e s e q u a t i o n s i n c l u d e b o t h c o n v e c t i v e a n d d i u s i v e u x e s . T h i s
m o t i v a t e s t h e c h o i c e o f o u r t w o s c a l a r m o d e l e q u a t i o n s a s s o c i a t e d w i t h t h e p h y s i c s
o f c o n v e c t i o n a n d d i u s i o n . F u r t h e r m o r e , a s p e c t s o f c o n v e c t i v e p h e n o m e n a a s s o c i -
a t e d w i t h c o u p l e d s y s t e m s o f e q u a t i o n s s u c h a s t h e E u l e r e q u a t i o n s a r e i m p o r t a n t i n
d e v e l o p i n g n u m e r i c a l m e t h o d s a n d b o u n d a r y c o n d i t i o n s . T h u s w e a l s o s t u d y l i n e a r
h y p e r b o l i c s y s t e m s o f P D E ' s .
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1 2 C H A P T E R 2 . C O N S E R V A T I O N L A W S A N D T H E M O D E L E Q U A T I O N S
2 . 3 T h e L i n e a r C o n v e c t i o n E q u a t i o n
2 . 3 . 1 D i e r e n t i a l F o r m
T h e s i m p l e s t l i n e a r m o d e l f o r c o n v e c t i o n a n d w a v e p r o p a g a t i o n i s t h e l i n e a r c o n v e c t i o n
e q u a t i o n g i v e n b y t h e f o l l o w i n g P D E :
@ u
@ t
+ a
@ u
@ x
= 0 ( 2 . 1 7 )
H e r e u ( x t ) i s a s c a l a r q u a n t i t y p r o p a g a t i n g w i t h s p e e d a , a r e a l c o n s t a n t w h i c h m a y
b e p o s i t i v e o r n e g a t i v e . T h e m a n n e r i n w h i c h t h e b o u n d a r y c o n d i t i o n s a r e s p e c i e d
s e p a r a t e s t h e f o l l o w i n g t w o p h e n o m e n a f o r w h i c h t h i s e q u a t i o n i s a m o d e l :
( 1 ) I n o n e t y p e , t h e s c a l a r q u a n t i t y u i s g i v e n o n o n e b o u n d a r y , c o r r e s p o n d i n g
t o a w a v e e n t e r i n g t h e d o m a i n t h r o u g h t h i s \ i n o w " b o u n d a r y . N o b o u n d -
a r y c o n d i t i o n i s s p e c i e d a t t h e o p p o s i t e s i d e , t h e \ o u t o w " b o u n d a r y . T h i s
i s c o n s i s t e n t i n t e r m s o f t h e w e l l - p o s e d n e s s o f a 1
s t
- o r d e r P D E . H e n c e t h e
w a v e l e a v e s t h e d o m a i n t h r o u g h t h e o u t o w b o u n d a r y w i t h o u t d i s t o r t i o n o r
r e e c t i o n . T h i s t y p e o f p h e n o m e n o n i s r e f e r r e d t o , s i m p l y , a s t h e c o n v e c t i o n
p r o b l e m . I t r e p r e s e n t s m o s t o f t h e \ u s u a l " s i t u a t i o n s e n c o u n t e r e d i n c o n v e c t -
i n g s y s t e m s . N o t e t h a t t h e l e f t - h a n d b o u n d a r y i s t h e i n o w b o u n d a r y w h e n
a i s p o s i t i v e , w h i l e t h e r i g h t - h a n d b o u n d a r y i s t h e i n o w b o u n d a r y w h e n a i s
n e g a t i v e .
( 2 ) I n t h e o t h e r t y p e , t h e o w b e i n g s i m u l a t e d i s p e r i o d i c . A t a n y g i v e n t i m e ,
w h a t e n t e r s o n o n e s i d e o f t h e d o m a i n m u s t b e t h e s a m e a s t h a t w h i c h i s
l e a v i n g o n t h e o t h e r . T h i s i s r e f e r r e d t o a s t h e b i c o n v e c t i o n p r o b l e m . I t i s
t h e s i m p l e s t t o s t u d y a n d s e r v e s t o i l l u s t r a t e m a n y o f t h e b a s i c p r o p e r t i e s o f
n u m e r i c a l m e t h o d s a p p l i e d t o p r o b l e m s i n v o l v i n g c o n v e c t i o n , w i t h o u t s p e c i a l
c o n s i d e r a t i o n o f b o u n d a r i e s . H e n c e , w e p a y a g r e a t d e a l o f a t t e n t i o n t o i t i n
t h e i n i t i a l c h a p t e r s .
N o w l e t u s c o n s i d e r a s i t u a t i o n i n w h i c h t h e i n i t i a l c o n d i t i o n i s g i v e n b y u ( x 0 ) =
u
0
( x ) , a n d t h e d o m a i n i s i n n i t e . I t i s e a s y t o s h o w b y s u b s t i t u t i o n t h a t t h e e x a c t
s o l u t i o n t o t h e l i n e a r c o n v e c t i o n e q u a t i o n i s t h e n
u ( x t ) = u
0
( x ; a t ) ( 2 . 1 8 )
T h e i n i t i a l w a v e f o r m p r o p a g a t e s u n a l t e r e d w i t h s p e e d j a j t o t h e r i g h t i f a i s p o s i t i v e
a n d t o t h e l e f t i f a i s n e g a t i v e . W i t h p e r i o d i c b o u n d a r y c o n d i t i o n s , t h e w a v e f o r m
t r a v e l s t h r o u g h o n e b o u n d a r y a n d r e a p p e a r s a t t h e o t h e r b o u n d a r y , e v e n t u a l l y r e -
t u r n i n g t o i t s i n i t i a l p o s i t i o n . I n t h i s c a s e , t h e p r o c e s s c o n t i n u e s f o r e v e r w i t h o u t a n y
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2 . 3 . T H E L I N E A R C O N V E C T I O N E Q U A T I O N 1 3
c h a n g e i n t h e s h a p e o f t h e s o l u t i o n . P r e s e r v i n g t h e s h a p e o f t h e i n i t i a l c o n d i t i o n
u
0
( x ) c a n b e a d i c u l t c h a l l e n g e f o r a n u m e r i c a l m e t h o d .
2 . 3 . 2 S o l u t i o n i n W a v e S p a c e
W e n o w e x a m i n e t h e b i c o n v e c t i o n p r o b l e m i n m o r e d e t a i l . L e t t h e d o m a i n b e g i v e n
b y 0 x 2 . W e r e s t r i c t o u r a t t e n t i o n t o i n i t i a l c o n d i t i o n s i n t h e f o r m
u ( x 0 ) = f ( 0 ) e
i x
( 2 . 1 9 )
w h e r e f ( 0 ) i s a c o m p l e x c o n s t a n t , a n d i s t h e w a v e n u m b e r . I n o r d e r t o s a t i s f y t h e
p e r i o d i c b o u n d a r y c o n d i t i o n s , m u s t b e a n i n t e g e r . I t i s a m e a s u r e o f t h e n u m b e r o f
w a v e l e n g t h s w i t h i n t h e d o m a i n . W i t h s u c h a n i n i t i a l c o n d i t i o n , t h e s o l u t i o n c a n b e
w r i t t e n a s
u ( x t ) = f ( t ) e
i x
( 2 . 2 0 )
w h e r e t h e t i m e d e p e n d e n c e i s c o n t a i n e d i n t h e c o m p l e x f u n c t i o n f ( t ) . S u b s t i t u t i n g
t h i s s o l u t i o n i n t o t h e l i n e a r c o n v e c t i o n e q u a t i o n , E q . 2 . 1 7 , w e n d t h a t f ( t ) s a t i s e s
t h e f o l l o w i n g o r d i n a r y d i e r e n t i a l e q u a t i o n ( O D E )
d f
d t
= ; i a f ( 2 . 2 1 )
w h i c h h a s t h e s o l u t i o n
f ( t ) = f ( 0 ) e
; i a t
( 2 . 2 2 )
S u b s t i t u t i n g f ( t ) i n t o E q . 2 . 2 0 g i v e s t h e f o l l o w i n g s o l u t i o n
u ( x t ) = f ( 0 ) e
i ( x ; a t )
= f ( 0 ) e
i ( x ; ! t )
( 2 . 2 3 )
w h e r e t h e f r e q u e n c y , ! , t h e w a v e n u m b e r , , a n d t h e p h a s e s p e e d , a , a r e r e l a t e d b y
! = a ( 2 . 2 4 )
T h e r e l a t i o n b e t w e e n t h e f r e q u e n c y a n d t h e w a v e n u m b e r i s k n o w n a s t h e d i s p e r s i o n
r e l a t i o n . T h e l i n e a r r e l a t i o n g i v e n b y E q . 2 . 2 4 i s c h a r a c t e r i s t i c o f w a v e p r o p a g a t i o n
i n a n o n d i s p e r s i v e m e d i u m . T h i s m e a n s t h a t t h e p h a s e s p e e d i s t h e s a m e f o r a l l
w a v e n u m b e r s . A s w e s h a l l s e e l a t e r , m o s t n u m e r i c a l m e t h o d s i n t r o d u c e s o m e d i s p e r -
s i o n t h a t i s , i n a s i m u l a t i o n , w a v e s w i t h d i e r e n t w a v e n u m b e r s t r a v e l a t d i e r e n t
s p e e d s .
A n a r b i t r a r y i n i t i a l w a v e f o r m c a n b e p r o d u c e d b y s u m m i n g i n i t i a l c o n d i t i o n s o f
t h e f o r m o f E q . 2 . 1 9 . F o r M m o d e s , o n e o b t a i n s
u ( x 0 ) =
M
X
m = 1
f
m
( 0 ) e
i
m
x
( 2 . 2 5 )
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1 4 C H A P T E R 2 . C O N S E R V A T I O N L A W S A N D T H E M O D E L E Q U A T I O N S
w h e r e t h e w a v e n u m b e r s a r e o f t e n o r d e r e d s u c h t h a t
1
2
M
. S i n c e t h e
w a v e e q u a t i o n i s l i n e a r , t h e s o l u t i o n i s o b t a i n e d b y s u m m i n g s o l u t i o n s o f t h e f o r m o f
E q . 2 . 2 3 , g i v i n g
u ( x t ) =
M
X
m = 1
f
m
( 0 ) e
i
m
( x ; a t )
( 2 . 2 6 )
D i s p e r s i o n a n d d i s s i p a t i o n r e s u l t i n g f r o m a n u m e r i c a l a p p r o x i m a t i o n w i l l c a u s e t h e
s h a p e o f t h e s o l u t i o n t o c h a n g e f r o m t h a t o f t h e o r i g i n a l w a v e f o r m .
2 . 4 T h e D i u s i o n E q u a t i o n
2 . 4 . 1 D i e r e n t i a l F o r m
D i u s i v e u x e s a r e a s s o c i a t e d w i t h m o l e c u l a r m o t i o n i n a c o n t i n u u m u i d . A s i m p l e
l i n e a r m o d e l e q u a t i o n f o r a d i u s i v e p r o c e s s i s
@ u
@ t
=
@
2
u
@ x
2
( 2 . 2 7 )
w h e r e i s a p o s i t i v e r e a l c o n s t a n t . F o r e x a m p l e , w i t h u r e p r e s e n t i n g t h e t e m p e r a -
t u r e , t h i s p a r a b o l i c P D E g o v e r n s t h e d i u s i o n o f h e a t i n o n e d i m e n s i o n . B o u n d a r y
c o n d i t i o n s c a n b e p e r i o d i c , D i r i c h l e t ( s p e c i e d u ) , N e u m a n n ( s p e c i e d @ u = @ x ) , o r
m i x e d D i r i c h l e t / N e u m a n n .
I n c o n t r a s t t o t h e l i n e a r c o n v e c t i o n e q u a t i o n , t h e d i u s i o n e q u a t i o n h a s a n o n t r i v i a l
s t e a d y - s t a t e s o l u t i o n , w h i c h i s o n e t h a t s a t i s e s t h e g o v e r n i n g P D E w i t h t h e p a r t i a l
d e r i v a t i v e i n t i m e e q u a l t o z e r o . I n t h e c a s e o f E q . 2 . 2 7 , t h e s t e a d y - s t a t e s o l u t i o n
m u s t s a t i s f y
@
2
u
@ x
2
= 0 ( 2 . 2 8 )
T h e r e f o r e , u m u s t v a r y l i n e a r l y w i t h x a t s t e a d y s t a t e s u c h t h a t t h e b o u n d a r y c o n -
d i t i o n s a r e s a t i s e d . O t h e r s t e a d y - s t a t e s o l u t i o n s a r e o b t a i n e d i f a s o u r c e t e r m g ( x )
i s a d d e d t o E q . 2 . 2 7 , a s f o l l o w s :
@ u
@ t
=
"
@
2
u
@ x
2
; g ( x )
#
( 2 . 2 9 )
g i v i n g a s t e a d y s t a t e - s o l u t i o n w h i c h s a t i s e s
@
2
u
@ x
2
; g ( x ) = 0 ( 2 . 3 0 )
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2 . 4 . T H E D I F F U S I O N E Q U A T I O N 1 5
I n t w o d i m e n s i o n s , t h e d i u s i o n e q u a t i o n b e c o m e s
@ u
@ t
=
"
@
2
u
@ x
2
+
@
2
u
@ y
2
; g ( x y )
#
( 2 . 3 1 )
w h e r e g ( x y ) i s a g a i n a s o u r c e t e r m . T h e c o r r e s p o n d i n g s t e a d y e q u a t i o n i s
@
2
u
@ x
2
+
@
2
u
@ y
2
; g ( x y ) = 0 ( 2 . 3 2 )
W h i l e E q . 2 . 3 1 i s p a r a b o l i c , E q . 2 . 3 2 i s e l l i p t i c . T h e l a t t e r i s k n o w n a s t h e P o i s s o n
e q u a t i o n f o r n o n z e r o g , a n d a s L a p l a c e ' s e q u a t i o n f o r z e r o g .
2 . 4 . 2 S o l u t i o n i n W a v e S p a c e
W e n o w c o n s i d e r a s e r i e s s o l u t i o n t o E q . 2 . 2 7 . L e t t h e d o m a i n b e g i v e n b y 0 x
w i t h b o u n d a r y c o n d i t i o n s u ( 0 ) = u
a
, u ( ) = u
b
. I t i s c l e a r t h a t t h e s t e a d y - s t a t e
s o l u t i o n i s g i v e n b y a l i n e a r f u n c t i o n w h i c h s a t i s e s t h e b o u n d a r y c o n d i t i o n s , i . e . ,
h ( x ) = u
a
+ ( u
b
; u
a
) x = . L e t t h e i n i t i a l c o n d i t i o n b e
u ( x 0 ) =
M
X
m = 1
f
m
( 0 ) s i n
m
x + h ( x ) ( 2 . 3 3 )
w h e r e m u s t b e a n i n t e g e r i n o r d e r t o s a t i s f y t h e b o u n d a r y c o n d i t i o n s . A s o l u t i o n
o f t h e f o r m
u ( x t ) =
M
X
m = 1
f
m
( t ) s i n
m
x + h ( x ) ( 2 . 3 4 )
s a t i s e s t h e i n i t i a l a n d b o u n d a r y c o n d i t i o n s . S u b s t i t u t i n g t h i s f o r m i n t o E q . 2 . 2 7
g i v e s t h e f o l l o w i n g O D E f o r f
m
:
d f
m
d t
= ;
2
m
f
m
( 2 . 3 5 )
a n d w e n d
f
m
( t ) = f
m
( 0 ) e
;
2
m
t
( 2 . 3 6 )
S u b s t i t u t i n g f
m
( t ) i n t o e q u a t i o n 2 . 3 4 , w e o b t a i n
u ( x t ) =
M
X
m = 1
f
m
( 0 ) e
;
2
m
t
s i n
m
x + h ( x ) ( 2 . 3 7 )
T h e s t e a d y - s t a t e s o l u t i o n ( t ! 1 ) i s s i m p l y h ( x ) . E q . 2 . 3 7 s h o w s t h a t h i g h w a v e n u m -
b e r c o m p o n e n t s ( l a r g e
m
) o f t h e s o l u t i o n d e c a y m o r e r a p i d l y t h a n l o w w a v e n u m b e r
c o m p o n e n t s , c o n s i s t e n t w i t h t h e p h y s i c s o f d i u s i o n .
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1 6 C H A P T E R 2 . C O N S E R V A T I O N L A W S A N D T H E M O D E L E Q U A T I O N S
2 . 5 L i n e a r H y p e r b o l i c S y s t e m s
T h e E u l e r e q u a t i o n s , E q . 2 . 8 , f o r m a h y p e r b o l i c s y s t e m o f p a r t i a l d i e r e n t i a l e q u a -
t i o n s . O t h e r s y s t e m s o f e q u a t i o n s g o v e r n i n g c o n v e c t i o n a n d w a v e p r o p a g a t i o n p h e -
n o m e n a , s u c h a s t h e M a x w e l l e q u a t i o n s d e s c r i b i n g t h e p r o p a g a t i o n o f e l e c t r o m a g n e t i c
w a v e s , a r e a l s o o f h y p e r b o l i c t y p e . M a n y a s p e c t s o f n u m e r i c a l m e t h o d s f o r s u c h s y s -
t e m s c a n b e u n d e r s t o o d b y s t u d y i n g a o n e - d i m e n s i o n a l c o n s t a n t - c o e c i e n t l i n e a r
s y s t e m o f t h e f o r m
@ u
@ t
+ A
@ u
@ x
= 0 ( 2 . 3 8 )
w h e r e u = u ( x t ) i s a v e c t o r o f l e n g t h m a n d A i s a r e a l m m m a t r i x . F o r
c o n s e r v a t i o n l a w s , t h i s e q u a t i o n c a n a l s o b e w r i t t e n i n t h e f o r m
@ u
@ t
+
@ f
@ x
= 0 ( 2 . 3 9 )
w h e r e f i s t h e u x v e c t o r a n d A =
@ f
@ u
i s t h e u x J a c o b i a n m a t r i x . T h e e n t r i e s i n t h e
u x J a c o b i a n a r e
a
i j
=
@ f
i
@ u
j
( 2 . 4 0 )
T h e u x J a c o b i a n f o r t h e E u l e r e q u a t i o n s i s d e r i v e d i n S e c t i o n 2 . 2 .
S u c h a s y s t e m i s h y p e r b o l i c i f A i s d i a g o n a l i z a b l e w i t h r e a l e i g e n v a l u e s .
1
T h u s
= X
; 1
A X ( 2 . 4 1 )
w h e r e i s a d i a g o n a l m a t r i x c o n t a i n i n g t h e e i g e n v a l u e s o f A , a n d X i s t h e m a t r i x
o f r i g h t e i g e n v e c t o r s . P r e m u l t i p l y i n g E q . 2 . 3 8 b y X
; 1
, p o s t m u l t i p l y i n g A b y t h e
p r o d u c t X X
; 1
, a n d n o t i n g t h a t X a n d X
; 1
a r e c o n s t a n t s , w e o b t a i n
@ X
; 1
u
@ t
+
@
z } | {
X
; 1
A X X
; 1
u
@ x
= 0 ( 2 . 4 2 )
W i t h w = X
; 1
u , t h i s c a n b e r e w r i t t e n a s
@ w
@ t
+
@ w
@ x
= 0 ( 2 . 4 3 )
W h e n w r i t t e n i n t h i s m a n n e r , t h e e q u a t i o n s h a v e b e e n d e c o u p l e d i n t o m s c a l a r e q u a -
t i o n s o f t h e f o r m
@ w
i
@ t
+
i
@ w
i
@ x
= 0 ( 2 . 4 4 )
1
S e e A p p e n d i x A f o r a b r i e f r e v i e w o f s o m e b a s i c r e l a t i o n s a n d d e n i t i o n s f r o m l i n e a r a l g e b r a .
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2 . 6 . P R O B L E M S 1 7
T h e e l e m e n t s o f w a r e k n o w n a s c h a r a c t e r i s t i c v a r i a b l e s . E a c h c h a r a c t e r i s t i c v a r i a b l e
s a t i s e s t h e l i n e a r c o n v e c t i o n e q u a t i o n w i t h t h e s p e e d g i v e n b y t h e c o r r e s p o n d i n g
e i g e n v a l u e o f A .
B a s e d o n t h e a b o v e , w e s e e t h a t a h y p e r b o l i c s y s t e m i n t h e f o r m o f E q . 2 . 3 8 h a s a
s o l u t i o n g i v e n b y t h e s u p e r p o s i t i o n o f w a v e s w h i c h c a n t r a v e l i n e i t h e r t h e p o s i t i v e o r
n e g a t i v e d i r e c t i o n s a n d a t v a r y i n g s p e e d s . W h i l e t h e s c a l a r l i n e a r c o n v e c t i o n e q u a t i o n
i s c l e a r l y a n e x c e l l e n t m o d e l e q u a t i o n f o r h y p e r b o l i c s y s t e m s , w e m u s t e n s u r e t h a t
o u r n u m e r i c a l m e t h o d s a r e a p p r o p r i a t e f o r w a v e s p e e d s o f a r b i t r a r y s i g n a n d p o s s i b l y
w i d e l y v a r y i n g m a g n i t u d e s .
T h e o n e - d i m e n s i o n a l E u l e r e q u a t i o n s c a n a l s o b e d i a g o n a l i z e d , l e a d i n g t o t h r e e
e q u a t i o n s i n t h e f o r m o f t h e l i n e a r c o n v e c t i o n e q u a t i o n , a l t h o u g h t h e y r e m a i n n o n -
l i n e a r , o f c o u r s e . T h e e i g e n v a l u e s o f t h e u x J a c o b i a n m a t r i x , o r w a v e s p e e d s , a r e
u u + c , a n d u ; c , w h e r e u i s t h e l o c a l u i d v e l o c i t y , a n d c =
q
p = i s t h e l o c a l
s p e e d o f s o u n d . T h e s p e e d u i s a s s o c i a t e d w i t h c o n v e c t i o n o f t h e u i d , w h i l e u + c
a n d u ; c a r e a s s o c i a t e d w i t h s o u n d w a v e s . T h e r e f o r e , i n a s u p e r s o n i c o w , w h e r e
j u j > c , a l l o f t h e w a v e s p e e d s h a v e t h e s a m e s i g n . I n a s u b s o n i c o w , w h e r e j u j < c ,
w a v e s p e e d s o f b o t h p o s i t i v e a n d n e g a t i v e s i g n a r e p r e s e n t , c o r r e s p o n d i n g t o t h e f a c t
t h a t s o u n d w a v e s c a n t r a v e l u p s t r e a m i n a s u b s o n i c o w .
T h e s i g n s o f t h e e i g e n v a l u e s o f t h e m a t r i x A a r e a l s o i m p o r t a n t i n d e t e r m i n i n g
s u i t a b l e b o u n d a r y c o n d i t i o n s . T h e c h a r a c t e r i s t i c v a r i a b l e s e a c h s a t i s f y t h e l i n e a r c o n -
v e c t i o n e q u a t i o n w i t h t h e w a v e s p e e d g i v e n b y t h e c o r r e s p o n d i n g e i g e n v a l u e . T h e r e -
f o r e , t h e b o u n d a r y c o n d i t i o n s c a n b e s p e c i e d a c c o r d i n g l y . T h a t i s , c h a r a c t e r i s t i c
v a r i a b l e s a s s o c i a t e d w i t h p o s i t i v e e i g e n v a l u e s c a n b e s p e c i e d a t t h e l e f t b o u n d a r y ,
w h i c h c o r r e s p o n d s t o i n o w f o r t h e s e v a r i a b l e s . C h a r a c t e r i s t i c v a r i a b l e s a s s o c i a t e d
w i t h n e g a t i v e e i g e n v a l u e s c a n b e s p e c i e d a t t h e r i g h t b o u n d a r y , w h i c h i s t h e i n -
o w b o u n d a r y f o r t h e s e v a r i a b l e s . W h i l e o t h e r b o u n d a r y c o n d i t i o n t r e a t m e n t s a r e
p o s s i b l e , t h e y m u s t b e c o n s i s t e n t w i t h t h i s a p p r o a c h .
2 . 6 P r o b l e m s
1 . S h o w t h a t t h e 1 - D E u l e r e q u a t i o n s c a n b e w r i t t e n i n t e r m s o f t h e p r i m i t i v e
v a r i a b l e s R = u p ]
T
a s f o l l o w s :
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@ t
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@ R
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3
7
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1 8 C H A P T E R 2 . C O N S E R V A T I O N L A W S A N D T H E M O D E L E Q U A T I O N S
A s s u m e a n i d e a l g a s , p = ( ; 1 ) ( e ; u
2
= 2 ) .
2 . F i n d t h e e i g e n v a l u e s a n d e i g e n v e c t o r s o f t h e m a t r i x M d e r i v e d i n q u e s t i o n 1 .
3 . D e r i v e t h e u x J a c o b i a n m a t r i x A = @ E = @ Q f o r t h e 1 - D E u l e r e q u a t i o n s r e s u l t -
i n g f r o m t h e c o n s e r v a t i v e v a r i a b l e f o r m u l a t i o n ( E q . 2 . 5 ) . F i n d i t s e i g e n v a l u e s
a n d c o m p a r e w i t h t h o s e o b t a i n e d i n q u e s t i o n 2 .
4 . S h o w t h a t t h e t w o m a t r i c e s M a n d A d e r i v e d i n q u e s t i o n s 1 a n d 3 , r e s p e c t i v e l y ,
a r e r e l a t e d b y a s i m i l a r i t y t r a n s f o r m . ( H i n t : m a k e u s e o f t h e m a t r i x S =
@ Q = @ R . )
5 . W r i t e t h e 2 - D d i u s i o n e q u a t i o n , E q . 2 . 3 1 , i n t h e f o r m o f E q . 2 . 2 .
6 . G i v e n t h e i n i t i a l c o n d i t i o n u ( x 0 ) = s i n x d e n e d o n 0 x 2 , w r i t e i t i n t h e
f o r m o f E q . 2 . 2 5 , t h a t i s , n d t h e n e c e s s a r y v a l u e s o f f
m
( 0 ) . ( H i n t : u s e M = 2
w i t h
1
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2
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o n l y a t x = 2 j = 4 , j = 0 1 2 3 . F i n d t h e v a l u e s o f f
m
( 0 ) r e q u i r e d t o r e p r o d u c e
t h e i n i t i a l c o n d i t i o n a t t h e s e d i s c r e t e p o i n t s u s i n g M = 4 w i t h
m
= m ; 1 .
7 . P l o t t h e r s t t h r e e b a s i s f u n c t i o n s u s e d i n c o n s t r u c t i n g t h e e x a c t s o l u t i o n t o
t h e d i u s i o n e q u a t i o n i n S e c t i o n 2 . 4 . 2 . N e x t c o n s i d e r a s o l u t i o n w i t h b o u n d a r y
c o n d i t i o n s u
a
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b
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m
( 0 ) = 1
f o r 1 m 3 , f
m
( 0 ) = 0 f o r m > 3 . P l o t t h e i n i t i a l c o n d i t i o n o n t h e d o m a i n
0 x . P l o t t h e s o l u t i o n a t t = 1 w i t h = 1 .
8 . W r i t e t h e c l a s s i c a l w a v e e q u a t i o n @
2
u = @ t
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T
. F i n d t h e e i g e n v a l u e s a n d e i g e n v e c t o r s o f A .
9 . T h e C a u c h y - R i e m a n n e q u a t i o n s a r e f o r m e d f r o m t h e c o u p l i n g o f t h e s t e a d y
c o m p r e s s i b l e c o n t i n u i t y ( c o n s e r v a t i o n o f m a s s ) e q u a t i o n
@ u
@ x
+
@ v
@ y
= 0
a n d t h e v o r t i c i t y d e n i t i o n
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@ v
@ x
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2 . 6 . P R O B L E M S 1 9
w h e r e ! = 0 f o r i r r o t a t i o n a l o w . F o r i s e n t r o p i c a n d h o m e n t h a l p i c o w , t h e
s y s t e m i s c l o s e d b y t h e r e l a t i o n
=
1 ;
; 1
2
u
2
+ v
2
; 1
1
; 1
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P D E ' s , w e h a v e
@ f ( q )
@ x
+
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w h e r e
q =
u
v
f =
; u
v
g =
; v
; u
O n e a p p r o a c h t o s o l v i n g t h e s e e q u a t i o n s i s t o a d d a t i m e - d e p e n d e n t t e r m a n d
n d t h e s t e a d y s o l u t i o n o f t h e f o l l o w i n g e q u a t i o n :
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h y p e r b o l i c , i . e . , h a s r e a l e i g e n v a l u e s .
( d ) A r e t h e a b o v e u x e s h o m o g e n e o u s ? ( S e e A p p e n d i x C . )
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C h a p t e r 4
T H E S E M I - D I S C R E T E
A P P R O A C H
O n e s t r a t e g y f o r o b t a i n i n g n i t e - d i e r e n c e a p p r o x i m a t i o n s t o a P D E i s t o s t a r t b y
d i e r e n c i n g t h e s p a c e d e r i v a t i v e s o n l y , w i t h o u t a p p r o x i m a t i n g t h e t i m e d e r i v a t i v e .
I n t h e f o l l o w i n g c h a p t e r s , w e p r o c e e d w i t h a n a n a l y s i s m a k i n g c o n s i d e r a b l e u s e o f
t h i s c o n c e p t , w h i c h w e r e f e r t o a s t h e s e m i - d i s c r e t e a p p r o a c h . D i e r e n c i n g t h e s p a c e
d e r i v a t i v e s c o n v e r t s t h e b a s i c P D E i n t o a s e t o f c o u p l e d O D E ' s . I n t h e m o s t g e n e r a l
n o t a t i o n , t h e s e O D E ' s w o u l d b e e x p r e s s e d i n t h e f o r m
d ~u
d t
=
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F ( ~ u t ) ( 4 . 1 )
w h i c h i n c l u d e s a l l m a n n e r o f n o n l i n e a r a n d t i m e - d e p e n d e n t p o s s i b i l i t i e s . O n o c c a s i o n ,
w e u s e t h i s f o r m , b u t t h e r e s t o f t h i s c h a p t e r i s d e v o t e d t o a m o r e s p e c i a l i z e d m a t r i x
n o t a t i o n d e s c r i b e d b e l o w .
A n o t h e r s t r a t e g y f o r c o n s t r u c t i n g a n i t e - d i e r e n c e a p p r o x i m a t i o n t o a P D E i s
t o a p p r o x i m a t e a l l t h e p a r t i a l d e r i v a t i v e s a t o n c e . T h i s g e n e r a l l y l e a d s t o a p o i n t
d i e r e n c e o p e r a t o r ( s e e S e c t i o n 3 . 3 . 1 ) w h i c h , i n t u r n , c a n b e u s e d f o r t h e t i m e a d v a n c e
o f t h e s o l u t i o n a t a n y g i v e n p o i n t i n t h e m e s h . A s a n e x a m p l e l e t u s c o n s i d e r t h e
m o d e l e q u a t i o n f o r d i u s i o n
@ u
@ t
=
@
2
u
@ x
2
U s i n g t h r e e - p o i n t c e n t r a l - d i e r e n c i n g s c h e m e s f o r b o t h t h e t i m e a n d s p a c e d e r i v a t i v e s ,
w e n d
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5 2 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
C l e a r l y E q . 4 . 2 i s a d i e r e n c e e q u a t i o n w h i c h c a n b e u s e d a t t h e s p a c e p o i n t j t o
a d v a n c e t h e v a l u e o f u f r o m t h e p r e v i o u s t i m e l e v e l s n a n d n ; 1 t o t h e l e v e l n + 1 .
I t i s a f u l l d i s c r e t i z a t i o n o f t h e P D E . N o t e , h o w e v e r , t h a t t h e s p a t i a l a n d t e m p o r a l
d i s c r e t i z a t i o n s a r e s e p a r a b l e . T h u s , t h i s m e t h o d h a s a n i n t e r m e d i a t e s e m i - d i s c r e t e
f o r m a n d c a n b e a n a l y z e d b y t h e m e t h o d s d i s c u s s e d i n t h e n e x t f e w c h a p t e r s .
A n o t h e r p o s s i b i l i t y i s t o r e p l a c e t h e v a l u e o f u
( n )
j
i n t h e r i g h t h a n d s i d e o f E q . 4 . 2
w i t h t h e t i m e a v e r a g e o f u a t t h a t p o i n t , n a m e l y ( u
( n + 1 )
j
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) = 2 . T h i s r e s u l t s i n
t h e f o r m u l a
u
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j
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j
+
2 h
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2
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1
A
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( n )
j ; 1
3
5
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w h i c h c a n b e s o l v e d f o r u
( n + 1 )
a n d t i m e a d v a n c e d a t t h e p o i n t j . I n t h i s c a s e , t h e
s p a t i a l a n d t e m p o r a l d i s c r e t i z a t i o n s a r e n o t s e p a r a b l e , a n d n o s e m i - d i s c r e t e f o r m
e x i s t s .
E q u a t i o n 4 . 2 i s s o m e t i m e s c a l l e d R i c h a r d s o n ' s m e t h o d o f o v e r l a p p i n g s t e p s a n d
E q . 4 . 3 i s r e f e r r e d t o a s t h e D u F o r t - F r a n k e l m e t h o d . A s w e s h a l l s e e l a t e r o n , t h e r e
a r e s u b t l e p o i n t s t o b e m a d e a b o u t u s i n g t h e s e m e t h o d s t o n d a n u m e r i c a l s o l u t i o n
t o t h e d i u s i o n e q u a t i o n . T h e r e a r e a n u m b e r o f i s s u e s c o n c e r n i n g t h e a c c u r a c y ,
s t a b i l i t y , a n d c o n v e r g e n c e o f E q s . 4 . 2 a n d 4 . 3 w h i c h w e c a n n o t c o m m e n t o n u n t i l w e
d e v e l o p a f r a m e w o r k f o r s u c h i n v e s t i g a t i o n s . W e i n t r o d u c e t h e s e m e t h o d s h e r e o n l y t o
d i s t i n g u i s h b e t w e e n m e t h o d s i n w h i c h t h e t e m p o r a l a n d s p a t i a l t e r m s a r e d i s c r e t i z e d
s e p a r a t e l y a n d t h o s e f o r w h i c h n o s u c h s e p a r a t i o n i s p o s s i b l e . F o r t h e t i m e b e i n g , w e
s h a l l s e p a r a t e t h e s p a c e d i e r e n c e a p p r o x i m a t i o n s f r o m t h e t i m e d i e r e n c i n g . I n t h i s
a p p r o a c h , w e r e d u c e t h e g o v e r n i n g P D E ' s t o O D E ' s b y d i s c r e t i z i n g t h e s p a t i a l t e r m s
a n d u s e t h e w e l l - d e v e l o p e d t h e o r y o f O D E s o l u t i o n s t o a i d u s i n t h e d e v e l o p m e n t o f
a n a n a l y s i s o f a c c u r a c y a n d s t a b i l i t y .
4 . 1 R e d u c t i o n o f P D E ' s t o O D E ' s
4 . 1 . 1 T h e M o d e l O D E ' s
F i r s t l e t u s c o n s i d e r t h e m o d e l P D E ' s f o r d i u s i o n a n d b i c o n v e c t i o n d e s c r i b e d i n
C h a p t e r 2 . I n t h e s e s i m p l e c a s e s , w e c a n a p p r o x i m a t e t h e s p a c e d e r i v a t i v e s w i t h
d i e r e n c e o p e r a t o r s a n d e x p r e s s t h e r e s u l t i n g O D E ' s w i t h a m a t r i x f o r m u l a t i o n . T h i s
i s a s i m p l e a n d n a t u r a l f o r m u l a t i o n w h e n t h e O D E ' s a r e l i n e a r .
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4 . 1 . R E D U C T I O N O F P D E ' S T O O D E ' S 5 3
M o d e l O D E f o r D i u s i o n
F o r e x a m p l e , u s i n g t h e 3 - p o i n t c e n t r a l - d i e r e n c i n g s c h e m e t o r e p r e s e n t t h e s e c o n d
d e r i v a t i v e i n t h e s c a l a r P D E g o v e r n i n g d i u s i o n l e a d s t o t h e f o l l o w i n g O D E d i u s i o n
m o d e l
d
~
u
d t
=
x
2
B ( 1 ; 2 1 )
~
u +
~
( b c ) ( 4 . 4 )
w i t h D i r i c h l e t b o u n d a r y c o n d i t i o n s f o l d e d i n t o t h e
~
( b c ) v e c t o r .
M o d e l O D E f o r B i c o n v e c t i o n
T h e t e r m b i c o n v e c t i o n w a s i n t r o d u c e d i n S e c t i o n 2 . 3 . I t i s u s e d f o r t h e s c a l a r c o n -
v e c t i o n m o d e l w h e n t h e b o u n d a r y c o n d i t i o n s a r e p e r i o d i c . I n t h i s c a s e , t h e 3 - p o i n t
c e n t r a l - d i e r e n c i n g a p p r o x i m a t i o n p r o d u c e s t h e O D E m o d e l g i v e n b y
d
~
u
d t
= ;
a
2 x
B
p
( ; 1 0 1 )
~
u ( 4 . 5 )
w h e r e t h e b o u n d a r y c o n d i t i o n v e c t o r i s a b s e n t b e c a u s e t h e o w i s p e r i o d i c .
E q s . 4 . 4 a n d 4 . 5 a r e t h e m o d e l O D E ' s f o r d i u s i o n a n d b i c o n v e c t i o n o f a s c a l a r i n
o n e d i m e n s i o n . T h e y a r e l i n e a r w i t h c o e c i e n t m a t r i c e s w h i c h a r e i n d e p e n d e n t o f x
a n d t .
4 . 1 . 2 T h e G e n e r i c M a t r i x F o r m
T h e g e n e r i c m a t r i x f o r m o f a s e m i - d i s c r e t e a p p r o x i m a t i o n i s e x p r e s s e d b y t h e e q u a t i o n
d
~
u
d t
= A
~
u ;
~
f ( t ) ( 4 . 6 )
N o t e t h a t t h e e l e m e n t s i n t h e m a t r i x A d e p e n d u p o n b o t h t h e P D E a n d t h e t y p e o f
d i e r e n c i n g s c h e m e c h o s e n f o r t h e s p a c e t e r m s . T h e v e c t o r
~
f ( t ) i s u s u a l l y d e t e r m i n e d
b y t h e b o u n d a r y c o n d i t i o n s a n d p o s s i b l y s o u r c e t e r m s . I n g e n e r a l , e v e n t h e E u l e r
a n d N a v i e r - S t o k e s e q u a t i o n s c a n b e e x p r e s s e d i n t h e f o r m o f E q . 4 . 6 . I n s u c h c a s e s
t h e e q u a t i o n s a r e n o n l i n e a r , t h a t i s , t h e e l e m e n t s o f A d e p e n d o n t h e s o l u t i o n ~u a n d
a r e u s u a l l y d e r i v e d b y n d i n g t h e J a c o b i a n o f a u x v e c t o r . A l t h o u g h t h e e q u a t i o n s
a r e n o n l i n e a r , t h e l i n e a r a n a l y s i s p r e s e n t e d i n t h i s b o o k l e a d s t o d i a g n o s t i c s t h a t a r e
s u r p r i s i n g l y a c c u r a t e w h e n u s e d t o e v a l u a t e m a n y a s p e c t s o f n u m e r i c a l m e t h o d s a s
t h e y a p p l y t o t h e E u l e r a n d N a v i e r - S t o k e s e q u a t i o n s .
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5 4 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
4 . 2 E x a c t S o l u t i o n s o f L i n e a r O D E ' s
I n o r d e r t o a d v a n c e E q . 4 . 1 i n t i m e , t h e s y s t e m o f O D E ' s m u s t b e i n t e g r a t e d u s i n g a
t i m e - m a r c h i n g m e t h o d . I n o r d e r t o a n a l y z e t i m e - m a r c h i n g m e t h o d s , w e w i l l m a k e u s e
o f e x a c t s o l u t i o n s o f c o u p l e d s y s t e m s o f O D E ' s , w h i c h e x i s t u n d e r c e r t a i n c o n d i t i o n s .
T h e O D E ' s r e p r e s e n t e d b y E q . 4 . 1 a r e s a i d t o b e l i n e a r i f F i s l i n e a r l y d e p e n d e n t o n
u ( i . e . , i f @ F = @ u = A w h e r e A i s i n d e p e n d e n t o f u ) . A s w e h a v e a l r e a d y p o i n t e d o u t ,
w h e n t h e O D E ' s a r e l i n e a r t h e y c a n b e e x p r e s s e d i n a m a t r i x n o t a t i o n a s E q . 4 . 6 i n
w h i c h t h e c o e c i e n t m a t r i x , A , i s i n d e p e n d e n t o f u . I f A d o e s d e p e n d e x p l i c i t l y o n
t , t h e g e n e r a l s o l u t i o n c a n n o t b e w r i t t e n w h e r e a s , i f A d o e s n o t d e p e n d e x p l i c i t l y o n
t , t h e g e n e r a l s o l u t i o n t o E q . 4 . 6 c a n b e w r i t t e n . T h i s h o l d s r e g a r d l e s s o f w h e t h e r o r
n o t t h e f o r c i n g f u n c t i o n ,
~
f , d e p e n d s e x p l i c i t l y o n t .
A s w e s h a l l s o o n s e e , t h e e x a c t s o l u t i o n o f E q . 4 . 6 c a n b e w r i t t e n i n t e r m s o f
t h e e i g e n v a l u e s a n d e i g e n v e c t o r s o f A . T h i s w i l l l e a d u s t o a r e p r e s e n t a t i v e s c a l a r
e q u a t i o n f o r u s e i n a n a l y z i n g t i m e - m a r c h i n g m e t h o d s . T h e s e i d e a s a r e d e v e l o p e d i n
t h e f o l l o w i n g s e c t i o n s .
4 . 2 . 1 E i g e n s y s t e m s o f S e m i - D i s c r e t e L i n e a r F o r m s
C o m p l e t e S y s t e m s
A n M M m a t r i x i s r e p r e s e n t e d b y a c o m p l e t e e i g e n s y s t e m i f i t h a s a c o m p l e t e s e t
o f l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s ( s e e A p p e n d i x A ) . A n e i g e n v e c t o r ,
~
x
m
, a n d i t s
c o r r e s p o n d i n g e i g e n v a l u e ,
m
, h a v e t h e p r o p e r t y t h a t
A
~
x
m
=
m
~
x
m
o r
A ;
m
I ]
~
x
m
= 0 ( 4 . 7 )
T h e e i g e n v a l u e s a r e t h e r o o t s o f t h e e q u a t i o n
d e t A ; I ] = 0
W e f o r m t h e r i g h t - h a n d e i g e n v e c t o r m a t r i x o f a c o m p l e t e s y s t e m b y l l i n g i t s c o l u m n s
w i t h t h e e i g e n v e c t o r s
~
x
m
:
X =
h
~
x
1
~
x
2
: : :
~
x
M
i
T h e i n v e r s e i s t h e l e f t - h a n d e i g e n v e c t o r m a t r i x , a n d t o g e t h e r t h e y h a v e t h e p r o p e r t y
t h a t
X
; 1
A X = ( 4 . 8 )
w h e r e i s a d i a g o n a l m a t r i x w h o s e e l e m e n t s a r e t h e e i g e n v a l u e s o f A .
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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4 . 2 . E X A C T S O L U T I O N S O F L I N E A R O D E ' S 5 5
D e f e c t i v e S y s t e m s
I f a n M M m a t r i x d o e s n o t h a v e a c o m p l e t e s e t o f l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s ,
i t c a n n o t b e t r a n s f o r m e d t o a d i a g o n a l m a t r i x o f s c a l a r s , a n d i t i s s a i d t o b e d e f e c t i v e .
I t c a n , h o w e v e r , b e t r a n s f o r m e d t o a d i a g o n a l s e t o f b l o c k s , s o m e o f w h i c h m a y b e
s c a l a r s ( s e e A p p e n d i x A ) . I n g e n e r a l , t h e r e e x i s t s s o m e S w h i c h t r a n s f o r m s a n y
m a t r i x A s u c h t h a t
S
; 1
A S = J
w h e r e
J =
2
6
6
6
6
6
6
4
J
1
J
2
.
.
.
J
m
.
.
.
3
7
7
7
7
7
7
5
a n d
J
( n )
m
=
2
6
6
6
6
4
m
1
m
.
.
.
.
.
.
1
m
3
7
7
7
7
5
1
.
.
.
.
.
.
n
T h e m a t r i x J i s s a i d t o b e i n J o r d a n c a n o n i c a l f o r m , a n d a n e i g e n v a l u e w i t h m u l t i -
p l i c i t y n w i t h i n a J o r d a n b l o c k i s s a i d t o b e a d e f e c t i v e e i g e n v a l u e . D e f e c t i v e s y s t e m s
p l a y a r o l e i n n u m e r i c a l s t a b i l i t y a n a l y s i s .
4 . 2 . 2 S i n g l e O D E ' s o f F i r s t - a n d S e c o n d - O r d e r
F i r s t - O r d e r E q u a t i o n s
T h e s i m p l e s t n o n h o m o g e n e o u s O D E o f i n t e r e s t i s g i v e n b y t h e s i n g l e , r s t - o r d e r
e q u a t i o n
d u
d t
= u + a e
t
( 4 . 9 )
w h e r e , a , a n d a r e s c a l a r s , a l l o f w h i c h c a n b e c o m p l e x n u m b e r s . T h e e q u a t i o n
i s l i n e a r b e c a u s e d o e s n o t d e p e n d o n u , a n d h a s a g e n e r a l s o l u t i o n b e c a u s e d o e s
n o t d e p e n d o n t . I t h a s a s t e a d y - s t a t e s o l u t i o n i f t h e r i g h t - h a n d s i d e i s i n d e p e n d e n t
o f t , i . e . , i f = 0 , a n d i s h o m o g e n e o u s i f t h e f o r c i n g f u n c t i o n i s z e r o , i . e . , i f a = 0 .
A l t h o u g h i t i s q u i t e s i m p l e , t h e n u m e r i c a l a n a l y s i s o f E q . 4 . 9 d i s p l a y s m a n y o f t h e
f u n d a m e n t a l p r o p e r t i e s a n d i s s u e s i n v o l v e d i n t h e c o n s t r u c t i o n a n d s t u d y o f m o s t
p o p u l a r t i m e - m a r c h i n g m e t h o d s . T h i s t h e m e w i l l b e d e v e l o p e d a s w e p r o c e e d .
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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5 6 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
T h e e x a c t s o l u t i o n o f E q . 4 . 9 i s , f o r 6= ,
u ( t ) = c
1
e
t
+
a e
t
;
w h e r e c
1
i s a c o n s t a n t d e t e r m i n e d b y t h e i n i t i a l c o n d i t i o n s . I n t e r m s o f t h e i n i t i a l
v a l u e o f u , i t c a n b e w r i t t e n
u ( t ) = u ( 0 ) e
t
+ a
e
t
; e
t
;
T h e i n t e r e s t i n g q u e s t i o n c a n a r i s e : W h a t h a p p e n s t o t h e s o l u t i o n o f E q . 4 . 9 w h e n
= ? T h i s i s e a s i l y f o u n d b y s e t t i n g = + , s o l v i n g , a n d t h e n t a k i n g t h e l i m i t
a s ! 0 . U s i n g t h i s l i m i t i n g d e v i c e , w e n d t h a t t h e s o l u t i o n t o
d u
d t
= u + a e
t
( 4 . 1 0 )
i s g i v e n b y
u ( t ) = u ( 0 ) + a t ] e
t
A s w e s h a l l s o o n s e e , t h i s s o l u t i o n i s r e q u i r e d f o r t h e a n a l y s i s o f d e f e c t i v e s y s t e m s .
S e c o n d - O r d e r E q u a t i o n s
T h e h o m o g e n e o u s f o r m o f a s e c o n d - o r d e r e q u a t i o n i s g i v e n b y
d
2
u
d t
2
+ a
1
d u
d t
+ a
0
u = 0 ( 4 . 1 1 )
w h e r e a
1
a n d a
0
a r e c o m p l e x c o n s t a n t s . N o w w e i n t r o d u c e t h e d i e r e n t i a l o p e r a t o r
D s u c h t h a t
D
d
d t
a n d f a c t o r u ( t ) o u t o f E q . 4 . 1 1 , g i v i n g
( D
2
+ a
1
D + a
0
) u ( t ) = 0
T h e p o l y n o m i a l i n D i s r e f e r r e d t o a s a c h a r a c t e r i s t i c p o l y n o m i a l a n d d e s i g n a t e d
P ( D ) . C h a r a c t e r i s t i c p o l y n o m i a l s a r e f u n d a m e n t a l t o t h e a n a l y s i s o f b o t h O D E ' s a n d
O E ' s , s i n c e t h e r o o t s o f t h e s e p o l y n o m i a l s d e t e r m i n e t h e s o l u t i o n s o f t h e e q u a t i o n s .
F o r O D E ' s , w e o f t e n l a b e l t h e s e r o o t s i n o r d e r o f i n c r e a s i n g m o d u l u s a s
1
,
2
, ,
m
,
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4 . 2 . E X A C T S O L U T I O N S O F L I N E A R O D E ' S 5 7
,
M
. T h e y a r e f o u n d b y s o l v i n g t h e e q u a t i o n P ( ) = 0 . I n o u r s i m p l e e x a m p l e ,
t h e r e w o u l d b e t w o r o o t s ,
1
a n d
2
, d e t e r m i n e d f r o m
P ( ) =
2
+ a
1
+ a
0
= 0 ( 4 . 1 2 )
a n d t h e s o l u t i o n t o E q . 4 . 1 1 i s g i v e n b y
u ( t ) = c
1
e
1
t
+ c
2
e
2
t
( 4 . 1 3 )
w h e r e c
1
a n d c
2
a r e c o n s t a n t s d e t e r m i n e d f r o m i n i t i a l c o n d i t i o n s . T h e p r o o f o f t h i s
i s s i m p l e a n d i s f o u n d b y s u b s t i t u t i n g E q . 4 . 1 3 i n t o E q . 4 . 1 1 . O n e n d s t h e r e s u l t
c
1
e
1
t
(
2
1
+ a
1
1
+ a
0
) + c
2
e
2
t
(
2
2
+ a
1
2
+ a
0
)
w h i c h i s i d e n t i c a l l y z e r o f o r a l l c
1
, c
2
, a n d t i f a n d o n l y i f t h e ' s s a t i s f y E q . 4 . 1 2 .
4 . 2 . 3 C o u p l e d F i r s t - O r d e r O D E ' s
A C o m p l e t e S y s t e m
A s e t o f c o u p l e d , r s t - o r d e r , h o m o g e n e o u s e q u a t i o n s i s g i v e n b y
u
0
1
= a
1 1
u
1
+ a
1 2
u
2
u
0
2
= a
2 1
u
1
+ a
2 2
u
2
( 4 . 1 4 )
w h i c h c a n b e w r i t t e n
~
u
0
= A
~
u
~
u = u
1
u
2
]
T
A = ( a
i j
) =
"
a
1 1
a
1 2
a
2 1
a
2 2
#
C o n s i d e r t h e p o s s i b i l i t y t h a t a s o l u t i o n i s r e p r e s e n t e d b y
u
1
= c
1
x
1 1
e
1
t
+ c
2
x
1 2
e
2
t
u
2
= c
1
x
2 1
e
1
t
+ c
2
x
2 2
e
2
t
( 4 . 1 5 )
B y s u b s t i t u t i o n , t h e s e a r e i n d e e d s o l u t i o n s t o E q . 4 . 1 4 i f a n d o n l y i f
"
a
1 1
a
1 2
a
2 1
a
2 2
# "
x
1 1
x
2 1
#
=
1
"
x
1 1
x
2 1
#
"
a
1 1
a
1 2
a
2 1
a
2 2
# "
x
1 2
x
2 2
#
=
2
"
x
1 2
x
2 2
#
( 4 . 1 6 )
N o t i c e t h a t a h i g h e r - o r d e r e q u a t i o n c a n b e r e d u c e d t o a c o u p l e d s e t o f r s t - o r d e r
e q u a t i o n s b y i n t r o d u c i n g a n e w s e t o f d e p e n d e n t v a r i a b l e s . T h u s , b y s e t t i n g
u
1
= u
0
u
2
= u
w e n d E q . 4 . 1 1 c a n b e w r i t t e n
u
0
1
= ; a
1
u
1
; a
0
u
2
u
0
2
= u
1
( 4 . 1 7 )
w h i c h i s a s u b s e t o f E q . 4 . 1 4 .
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5 8 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
A D e r o g a t o r y S y s t e m
E q . 4 . 1 5 i s s t i l l a s o l u t i o n t o E q . 4 . 1 4 i f
1
=
2
= , p r o v i d e d t w o l i n e a r l y i n d e p e n d e n t
v e c t o r s e x i s t t o s a t i s f y E q . 4 . 1 6 w i t h A = . I n t h i s c a s e
0
0
1
0
=
1
0
a n d
0
0
0
1
=
0
1
p r o v i d e s u c h a s o l u t i o n . T h i s i s t h e c a s e w h e r e A h a s a c o m p l e t e s e t o f e i g e n v e c t o r s
a n d i s n o t d e f e c t i v e .
A D e f e c t i v e S y s t e m
I f A i s d e f e c t i v e , t h e n i t c a n b e r e p r e s e n t e d b y t h e J o r d a n c a n o n i c a l f o r m
"
u
0
1
u
0
2
#
=
"
0
1
# "
u
1
u
2
#
( 4 . 1 8 )
w h o s e s o l u t i o n i s n o t o b v i o u s . H o w e v e r , i n t h i s c a s e , o n e c a n s o l v e t h e t o p e q u a t i o n
r s t , g i v i n g u
1
( t ) = u
1
( 0 ) e
t
. T h e n , s u b s t i t u t i n g t h i s r e s u l t i n t o t h e s e c o n d e q u a t i o n ,
o n e n d s
d u
2
d t
= u
2
+ u
1
( 0 ) e
t
w h i c h i s i d e n t i c a l i n f o r m t o E q . 4 . 1 0 a n d h a s t h e s o l u t i o n
u
2
( t ) = u
2
( 0 ) + u
1
( 0 ) t ] e
t
F r o m t h i s t h e r e a d e r s h o u l d b e a b l e t o v e r i f y t h a t
u
3
( t ) =
a + b t + c t
2
e
t
i s a s o l u t i o n t o
2
6
4
u
0
1
u
0
2
u
0
3
3
7
5
=
2
6
4
1
1
3
7
5
2
6
4
u
1
u
2
u
3
3
7
5
i f
a = u
3
( 0 ) b = u
2
( 0 ) c =
1
2
u
1
( 0 ) ( 4 . 1 9 )
T h e g e n e r a l s o l u t i o n t o s u c h d e f e c t i v e s y s t e m s i s l e f t a s a n e x e r c i s e .
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4 . 2 . E X A C T S O L U T I O N S O F L I N E A R O D E ' S 5 9
4 . 2 . 4 G e n e r a l S o l u t i o n o f C o u p l e d O D E ' s w i t h C o m p l e t e E i g e n -
s y s t e m s
L e t u s c o n s i d e r a s e t o f c o u p l e d , n o n h o m o g e n e o u s , l i n e a r , r s t - o r d e r O D E ' s w i t h
c o n s t a n t c o e c i e n t s w h i c h m i g h t h a v e b e e n d e r i v e d b y s p a c e d i e r e n c i n g a s e t o f
P D E ' s . R e p r e s e n t t h e m b y t h e e q u a t i o n
d
~
u
d t
= A
~
u ;
~
f ( t ) ( 4 . 2 0 )
O u r a s s u m p t i o n i s t h a t t h e M M m a t r i x A h a s a c o m p l e t e e i g e n s y s t e m
1
a n d c a n
b e t r a n s f o r m e d b y t h e l e f t a n d r i g h t e i g e n v e c t o r m a t r i c e s , X
; 1
a n d X , t o a d i a g o n a l
m a t r i x h a v i n g d i a g o n a l e l e m e n t s w h i c h a r e t h e e i g e n v a l u e s o f A , s e e S e c t i o n 4 . 2 . 1 .
N o w l e t u s m u l t i p l y E q . 4 . 2 0 f r o m t h e l e f t b y X
; 1
a n d i n s e r t t h e i d e n t i t y c o m b i n a t i o n
X X
; 1
= I b e t w e e n A a n d
~
u . T h e r e r e s u l t s
X
; 1
d
~
u
d t
= X
; 1
A X X
; 1
~
u ; X
; 1
~
f ( t ) ( 4 . 2 1 )
S i n c e A i s i n d e p e n d e n t o f b o t h ~u a n d t , t h e e l e m e n t s i n X
; 1
a n d X a r e a l s o i n d e p e n -
d e n t o f b o t h ~u a n d t , a n d E q . 4 . 2 1 c a n b e m o d i e d t o
d
d t
X
; 1
~
u = X
; 1
~
u ; X
; 1
~
f ( t )
F i n a l l y , b y i n t r o d u c i n g t h e n e w v a r i a b l e s
~
w a n d
~
g s u c h t h a t
~
w = X
; 1
~
u
~
g ( t ) = X
; 1
~
f ( t ) ( 4 . 2 2 )
w e r e d u c e E q . 4 . 2 0 t o a n e w a l g e b r a i c f o r m
d
~
w
d t
=
~
w ;
~
g ( t ) ( 4 . 2 3 )
I t i s i m p o r t a n t a t t h i s p o i n t t o r e v i e w t h e r e s u l t s o f t h e p r e v i o u s p a r a g r a p h . N o t i c e
t h a t E q s . 4 . 2 0 a n d 4 . 2 3 a r e e x p r e s s i n g e x a c t l y t h e s a m e e q u a l i t y . T h e o n l y d i e r e n c e
b e t w e e n t h e m w a s b r o u g h t a b o u t b y a l g e b r a i c m a n i p u l a t i o n s w h i c h r e g r o u p e d t h e
v a r i a b l e s . H o w e v e r , t h i s r e g r o u p i n g i s c r u c i a l f o r t h e s o l u t i o n p r o c e s s b e c a u s e E q s .
1
I n t h e f o l l o w i n g , w e e x c l u d e d e f e c t i v e s y s t e m s , n o t b e c a u s e t h e y c a n n o t b e a n a l y z e d ( t h e e x a m p l e
a t t h e c o n c l u s i o n o f t h e p r e v i o u s s e c t i o n p r o v e s o t h e r w i s e ) , b u t b e c a u s e t h e y a r e o n l y o f l i m i t e d
i n t e r e s t i n t h e g e n e r a l d e v e l o p m e n t o f o u r t h e o r y .
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6 0 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
4 . 2 3 a r e n o l o n g e r c o u p l e d . T h e y c a n b e w r i t t e n l i n e b y l i n e a s a s e t o f i n d e p e n d e n t ,
s i n g l e , r s t - o r d e r e q u a t i o n s , t h u s
w
0
1
=
1
w
1
; g
1
( t )
.
.
.
w
0
m
=
m
w
m
; g
m
( t )
.
.
.
w
0
M
=
M
w
M
; g
M
( t ) ( 4 . 2 4 )
F o r a n y g i v e n s e t o f g
m
( t ) e a c h o f t h e s e e q u a t i o n s c a n b e s o l v e d s e p a r a t e l y a n d t h e n
r e c o u p l e d , u s i n g t h e i n v e r s e o f t h e r e l a t i o n s g i v e n i n E q s . 4 . 2 2 :
~
u ( t ) = X
~
w ( t )
=
M
X
m = 1
w
m
( t )
~
x
m
( 4 . 2 5 )
w h e r e ~x
m
i s t h e m ' t h c o l u m n o f X , i . e . , t h e e i g e n v e c t o r c o r r e s p o n d i n g t o
m
.
W e n e x t f o c u s o n t h e v e r y i m p o r t a n t s u b s e t o f E q . 4 . 2 0 w h e n n e i t h e r A n o r
~
f h a s
a n y e x p l i c i t d e p e n d e n c e o n t . I n s u c h a c a s e , t h e g
m
i n E q s . 4 . 2 3 a n d 4 . 2 4 a r e a l s o
t i m e i n v a r i a n t a n d t h e s o l u t i o n t o a n y l i n e i n E q . 4 . 2 4 i s
w
m
( t ) = c
m
e
m
t
+
1
m
g
m
w h e r e t h e c
m
a r e c o n s t a n t s t h a t d e p e n d o n t h e i n i t i a l c o n d i t i o n s . T r a n s f o r m i n g b a c k
t o t h e u - s y s t e m g i v e s
~
u ( t ) = X
~
w ( t )
=
M
X
m = 1
w
m
( t )
~
x
m
=
M
X
m = 1
c
m
e
m
t
~
x
m
+
M
X
m = 1
1
m
g
m
~
x
m
=
M
X
m = 1
c
m
e
m
t
~
x
m
+ X
; 1
X
; 1
~
f
=
M
X
m = 1
c
m
e
m
t
~
x
m
+ A
; 1
~
f
| { z }
T r a n s i e n t
| { z }
S t e a d y - s t a t e
( 4 . 2 6 )
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4 . 3 . R E A L S P A C E A N D E I G E N S P A C E 6 1
N o t e t h a t t h e s t e a d y - s t a t e s o l u t i o n i s A
; 1
~
f , a s m i g h t b e e x p e c t e d .
T h e r s t g r o u p o f t e r m s o n t h e r i g h t s i d e o f t h i s e q u a t i o n i s r e f e r r e d t o c l a s s i c a l l y
a s t h e c o m p l e m e n t a r y s o l u t i o n o r t h e s o l u t i o n o f t h e h o m o g e n e o u s e q u a t i o n s . T h e
s e c o n d g r o u p i s r e f e r r e d t o c l a s s i c a l l y a s t h e p a r t i c u l a r s o l u t i o n o r t h e p a r t i c u l a r
i n t e g r a l . I n o u r a p p l i c a t i o n t o u i d d y n a m i c s , i t i s m o r e i n s t r u c t i v e t o r e f e r t o t h e s e
g r o u p s a s t h e t r a n s i e n t a n d s t e a d y - s t a t e s o l u t i o n s , r e s p e c t i v e l y . A n a l t e r n a t i v e , b u t
e n t i r e l y e q u i v a l e n t , f o r m o f t h e s o l u t i o n i s
~
u ( t ) = c
1
e
1
t
~
x
1
+ + c
m
e
m
t
~
x
m
+ + c
M
e
M
t
~
x
M
+ A
; 1
~
f ( 4 . 2 7 )
4 . 3 R e a l S p a c e a n d E i g e n s p a c e
4 . 3 . 1 D e n i t i o n
F o l l o w i n g t h e s e m i - d i s c r e t e a p p r o a c h d i s c u s s e d i n S e c t i o n 4 . 1 , w e r e d u c e t h e p a r t i a l
d i e r e n t i a l e q u a t i o n s t o a s e t o f o r d i n a r y d i e r e n t i a l e q u a t i o n s r e p r e s e n t e d b y t h e
g e n e r i c f o r m
d
~
u
d t
= A
~
u ;
~
f ( 4 . 2 8 )
T h e d e p e n d e n t v a r i a b l e ~u r e p r e s e n t s s o m e p h y s i c a l q u a n t i t y o r q u a n t i t i e s w h i c h r e l a t e
t o t h e p r o b l e m o f i n t e r e s t . F o r t h e m o d e l p r o b l e m s o n w h i c h w e a r e f o c u s i n g m o s t
o f o u r a t t e n t i o n , t h e e l e m e n t s o f A a r e i n d e p e n d e n t o f b o t h u a n d t . T h i s p e r m i t s
u s t o s a y a g r e a t d e a l a b o u t s u c h p r o b l e m s a n d s e r v e s a s t h e b a s i s f o r t h i s s e c t i o n .
I n p a r t i c u l a r , w e c a n d e v e l o p s o m e v e r y i m p o r t a n t a n d f u n d a m e n t a l c o n c e p t s t h a t
u n d e r l y t h e g l o b a l p r o p e r t i e s o f t h e n u m e r i c a l s o l u t i o n s t o t h e m o d e l p r o b l e m s . H o w
t h e s e r e l a t e t o t h e n u m e r i c a l s o l u t i o n s o f m o r e p r a c t i c a l p r o b l e m s d e p e n d s u p o n t h e
p r o b l e m a n d , t o a m u c h g r e a t e r e x t e n t , o n t h e c l e v e r n e s s o f t h e r e l a t o r .
W e b e g i n b y d e v e l o p i n g t h e c o n c e p t o f \ s p a c e s " . T h a t i s , w e i d e n t i f y d i e r e n t
m a t h e m a t i c a l r e f e r e n c e f r a m e s ( s p a c e s ) a n d v i e w o u r s o l u t i o n s f r o m w i t h i n e a c h .
I n t h i s w a y , w e g e t d i e r e n t p e r s p e c t i v e s o f t h e s a m e s o l u t i o n , a n d t h i s c a n a d d
s i g n i c a n t l y t o o u r u n d e r s t a n d i n g .
T h e m o s t n a t u r a l r e f e r e n c e f r a m e i s t h e p h y s i c a l o n e . W e s a y
I f a s o l u t i o n i s e x p r e s s e d i n t e r m s o f
~
u , i t i s s a i d
t o b e i n r e a l s p a c e .
T h e r e i s , h o w e v e r , a n o t h e r v e r y u s e f u l f r a m e . W e s a w i n S e c t i o n s 4 . 2 . 1 a n d 4 . 2 t h a t
p r e - a n d p o s t - m u l t i p l i c a t i o n o f A b y t h e a p p r o p r i a t e s i m i l a r i t y m a t r i c e s t r a n s f o r m s A
i n t o a d i a g o n a l m a t r i x , c o m p o s e d , i n t h e m o s t g e n e r a l c a s e , o f J o r d a n b l o c k s o r , i n t h e
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4 . 3 . R E A L S P A C E A N D E I G E N S P A C E 6 3
t r i d i a g o n a l s , B ( M : a b c ) a n d B
p
( M : a b c ) , a r e g i v e n i n A p p e n d i x B . F r o m S e c t i o n
B . 1 , w e n d f o r t h e m o d e l d i u s i o n e q u a t i o n w i t h D i r i c h l e t b o u n d a r y c o n d i t i o n s
m
=
x
2
; 2 + 2 c o s
m
M + 1
=
; 4
x
2
s i n
2
m
2 ( M + 1 )
!
m = 1 2 M ( 4 . 2 9 )
a n d , f r o m S e c t i o n B . 4 , f o r t h e m o d e l b i c o n v e c t i o n e q u a t i o n
m
=
; i a
x
s i n
2 m
M
m = 0 1 M ; 1
= ; i
m
a m = 0 1 M ; 1 ( 4 . 3 0 )
w h e r e
m
=
s i n
m
x
x
m = 0 1 M ; 1 ( 4 . 3 1 )
i s t h e m o d i e d w a v e n u m b e r f r o m S e c t i o n 3 . 5 ,
m
= m , a n d x = 2 = M . N o t i c e
t h a t t h e d i u s i o n e i g e n v a l u e s a r e r e a l a n d n e g a t i v e w h i l e t h o s e r e p r e s e n t i n g p e r i o d i c
c o n v e c t i o n a r e a l l p u r e i m a g i n a r y . T h e i n t e r p r e t a t i o n o f t h i s r e s u l t p l a y s a v e r y
i m p o r t a n t r o l e l a t e r i n o u r s t a b i l i t y a n a l y s i s .
4 . 3 . 3 E i g e n v e c t o r s o f t h e M o d e l E q u a t i o n s
N e x t w e c o n s i d e r t h e e i g e n v e c t o r s o f t h e t w o m o d e l e q u a t i o n s . T h e s e f o l l o w a s s p e c i a l
c a s e s f r o m t h e r e s u l t s g i v e n i n A p p e n d i x B .
T h e D i u s i o n M o d e l
C o n s i d e r E q . 4 . 4 , t h e m o d e l O D E ' s f o r d i u s i o n . F i r s t , t o h e l p v i s u a l i z e t h e m a t r i x
s t r u c t u r e , w e p r e s e n t r e s u l t s f o r a s i m p l e 4 - p o i n t m e s h a n d t h e n w e g i v e t h e g e n e r a l
c a s e . T h e r i g h t - h a n d e i g e n v e c t o r m a t r i x X i s g i v e n b y
2
6
6
6
4
s i n ( x
1
) s i n ( 2 x
1
) s i n ( 3 x
1
) s i n ( 4 x
1
)
s i n ( x
2
) s i n ( 2 x
2
) s i n ( 3 x
2
) s i n ( 4 x
2
)
s i n ( x
3
) s i n ( 2 x
3
) s i n ( 3 x
3
) s i n ( 4 x
3
)
s i n ( x
4
) s i n ( 2 x
4
) s i n ( 3 x
4
) s i n ( 4 x
4
)
3
7
7
7
5
T h e c o l u m n s o f t h e m a t r i x a r e p r o p o r t i o n a l t o t h e e i g e n v e c t o r s . R e c a l l t h a t x
j
=
j x = j = ( M + 1 ) , s o i n g e n e r a l t h e r e l a t i o n ~u = X ~ w c a n b e w r i t t e n a s
u
j
=
M
X
m = 1
w
m
s i n m x
j
j = 1 2 M ( 4 . 3 2 )
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6 4 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
F o r t h e i n v e r s e , o r l e f t - h a n d e i g e n v e c t o r m a t r i x X
; 1
, w e n d
2
6
6
6
4
s i n ( x
1
) s i n ( x
2
) s i n ( x
3
) s i n ( x
4
)
s i n ( 2 x
1
) s i n ( 2 x
2
) s i n ( 2 x
3
) s i n ( 2 x
4
)
s i n ( 3 x
1
) s i n ( 3 x
2
) s i n ( 3 x
3
) s i n ( 3 x
4
)
s i n ( 4 x
1
) s i n ( 4 x
2
) s i n ( 4 x
3
) s i n ( 4 x
4
)
3
7
7
7
5
T h e r o w s o f t h e m a t r i x a r e p r o p o r t i o n a l t o t h e e i g e n v e c t o r s . I n g e n e r a l ~ w = X
; 1
~u
g i v e s
w
m
=
M
X
j = 1
u
j
s i n m x
j
m = 1 2 M ( 4 . 3 3 )
I n t h e e l d o f h a r m o n i c a n a l y s i s , E q . 4 . 3 3 r e p r e s e n t s a s i n e t r a n s f o r m o f t h e f u n c -
t i o n u ( x ) f o r a n M - p o i n t s a m p l e b e t w e e n t h e b o u n d a r i e s x = 0 a n d x = w i t h t h e
c o n d i t i o n u ( 0 ) = u ( ) = 0 . S i m i l a r l y , E q . 4 . 3 2 r e p r e s e n t s t h e s i n e s y n t h e s i s t h a t
c o m p a n i o n s t h e s i n e t r a n s f o r m g i v e n b y E q . 4 . 3 3 . I n s u m m a r y ,
F o r t h e m o d e l d i u s i o n e q u a t i o n :
~
w = X
; 1
~
u i s a s i n e t r a n s f o r m f r o m r e a l s p a c e t o ( s i n e ) w a v e
s p a c e .
~
u = X
~
w i s a s i n e s y n t h e s i s f r o m w a v e s p a c e b a c k t o r e a l
s p a c e .
T h e B i c o n v e c t i o n M o d e l
N e x t c o n s i d e r t h e m o d e l O D E ' s f o r p e r i o d i c c o n v e c t i o n , E q . 4 . 5 . T h e c o e c i e n t
m a t r i c e s f o r t h e s e O D E ' s a r e a l w a y s c i r c u l a n t . F o r o u r m o d e l O D E , t h e r i g h t - h a n d
e i g e n v e c t o r s a r e g i v e n b y
~
x
m
= e
i j ( 2 m = M )
j = 0 1 M ; 1
m = 0 1 M ; 1
W i t h x
j
= j x = j 2 = M , w e c a n w r i t e ~u = X ~ w a s
u
j
=
M ; 1
X
m = 0
w
m
e
i m x
j
j = 0 1 M ; 1 ( 4 . 3 4 )
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4 . 3 . R E A L S P A C E A N D E I G E N S P A C E 6 5
F o r a 4 - p o i n t p e r i o d i c m e s h , w e n d t h e f o l l o w i n g l e f t - h a n d e i g e n v e c t o r m a t r i x
f r o m A p p e n d i x B . 4 :
2
6
6
6
4
w
1
w
2
w
3
w
4
3
7
7
7
5
=
1
4
2
6
6
6
4
1 1 1 1
1 e
; 2 i = 4
e
; 4 i = 4
e
; 6 i = 4
1 e
; 4 i = 4
e
; 8 i = 4
e
; 1 2 i = 4
1 e
; 6 i = 4
e
; 1 2 i = 4
e
; 1 8 i = 4
3
7
7
7
5
2
6
6
6
4
u
1
u
2
u
3
u
4
3
7
7
7
5
= X
; 1
~u
I n g e n e r a l
w
m
=
1
M
M ; 1
X
j = 0
u
j
e
; i m x
j
m = 0 1 M ; 1
T h i s e q u a t i o n i s i d e n t i c a l t o a d i s c r e t e F o u r i e r t r a n s f o r m o f t h e p e r i o d i c d e p e n d e n t
v a r i a b l e
~
u u s i n g a n M - p o i n t s a m p l e b e t w e e n a n d i n c l u d i n g x = 0 a n d x = 2 ; x .
F o r c i r c u l a n t m a t r i c e s , i t i s s t r a i g h t f o r w a r d t o e s t a b l i s h t h e f a c t t h a t t h e r e l a t i o n
~
u = X
~
w r e p r e s e n t s t h e F o u r i e r s y n t h e s i s o f t h e v a r i a b l e
~
w b a c k t o
~
u . I n s u m m a r y ,
F o r a n y c i r c u l a n t s y s t e m :
~
w = X
; 1
~
u i s a c o m p l e x F o u r i e r t r a n s f o r m f r o m r e a l s p a c e t o
w a v e s p a c e .
~
u = X
~
w i s a c o m p l e x F o u r i e r s y n t h e s i s f r o m w a v e s p a c e
b a c k t o r e a l s p a c e .
4 . 3 . 4 S o l u t i o n s o f t h e M o d e l O D E ' s
W e c a n n o w c o m b i n e t h e r e s u l t s o f t h e p r e v i o u s s e c t i o n s t o w r i t e t h e s o l u t i o n s o f o u r
m o d e l O D E ' s .
T h e D i u s i o n E q u a t i o n
F o r t h e d i u s i o n e q u a t i o n , E q . 4 . 2 7 b e c o m e s
u
j
( t ) =
M
X
m = 1
c
m
e
m
t
s i n m x
j
+ ( A
; 1
f )
j
j = 1 2 M ( 4 . 3 5 )
w h e r e
m
=
; 4
x
2
s i n
2
m
2 ( M + 1 )
!
( 4 . 3 6 )
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6 6 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
W i t h t h e m o d i e d w a v e n u m b e r d e n e d a s
m
=
2
x
s i n
m
x
2
( 4 . 3 7 )
a n d u s i n g
m
= m , w e c a n w r i t e t h e O D E s o l u t i o n a s
u
j
( t ) =
M
X
m = 1
c
m
e
;
m
2
t
s i n
m
x
j
+ ( A
; 1
f )
j
j = 1 2 M ( 4 . 3 8 )
T h i s c a n b e c o m p a r e d w i t h t h e e x a c t s o l u t i o n t o t h e P D E , E q . 2 . 3 7 , e v a l u a t e d a t t h e
n o d e s o f t h e g r i d :
u
j
( t ) =
M
X
m = 1
c
m
e
;
m
2
t
s i n
m
x
j
+ h ( x
j
) j = 1 2 M ( 4 . 3 9 )
W e s e e t h a t t h e s o l u t i o n s a r e i d e n t i c a l e x c e p t f o r t h e s t e a d y s o l u t i o n a n d t h e
m o d i e d w a v e n u m b e r i n t h e t r a n s i e n t t e r m . T h e m o d i e d w a v e n u m b e r i s a n a p p r o x -
i m a t i o n t o t h e a c t u a l w a v e n u m b e r . T h e d i e r e n c e b e t w e e n t h e m o d i e d w a v e n u m b e r
a n d t h e a c t u a l w a v e n u m b e r d e p e n d s o n t h e d i e r e n c i n g s c h e m e a n d t h e g r i d r e s o l u -
t i o n . T h i s d i e r e n c e c a u s e s t h e v a r i o u s m o d e s ( o r e i g e n v e c t o r c o m p o n e n t s ) t o d e c a y
a t r a t e s w h i c h d i e r f r o m t h e e x a c t s o l u t i o n . W i t h c o n v e n t i o n a l d i e r e n c i n g s c h e m e s ,
l o w w a v e n u m b e r m o d e s a r e a c c u r a t e l y r e p r e s e n t e d , w h i l e h i g h w a v e n u m b e r m o d e s ( i f
t h e y h a v e s i g n i c a n t a m p l i t u d e s ) c a n h a v e l a r g e e r r o r s .
T h e C o n v e c t i o n E q u a t i o n
F o r t h e b i c o n v e c t i o n e q u a t i o n , w e o b t a i n
u
j
( t ) =
M ; 1
X
m = 0
c
m
e
m
t
e
i
m
x
j
j = 0 1 M ; 1 ( 4 . 4 0 )
w h e r e
m
= ; i
m
a ( 4 . 4 1 )
w i t h t h e m o d i e d w a v e n u m b e r d e n e d i n E q . 4 . 3 1 . W e c a n w r i t e t h i s O D E s o l u t i o n
a s
u
j
( t ) =
M ; 1
X
m = 0
c
m
e
; i
m
a t
e
i
m
x
j
j = 0 1 M ; 1 ( 4 . 4 2 )
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4 . 4 . T H E R E P R E S E N T A T I V E E Q U A T I O N 6 7
a n d c o m p a r e i t t o t h e e x a c t s o l u t i o n o f t h e P D E , E q . 2 . 2 6 , e v a l u a t e d a t t h e n o d e s o f
t h e g r i d :
u
j
( t ) =
M ; 1
X
m = 0
f
m
( 0 ) e
; i
m
a t
e
i
m
x
j
j = 0 1 M ; 1 ( 4 . 4 3 )
O n c e a g a i n t h e d i e r e n c e a p p e a r s t h r o u g h t h e m o d i e d w a v e n u m b e r c o n t a i n e d i n
m
. A s d i s c u s s e d i n S e c t i o n 3 . 5 , t h i s l e a d s t o a n e r r o r i n t h e s p e e d w i t h w h i c h v a r i o u s
m o d e s a r e c o n v e c t e d , s i n c e
i s r e a l . S i n c e t h e e r r o r i n t h e p h a s e s p e e d d e p e n d s o n
t h e w a v e n u m b e r , w h i l e t h e a c t u a l p h a s e s p e e d i s i n d e p e n d e n t o f t h e w a v e n u m b e r ,
t h e r e s u l t i s e r r o n e o u s n u m e r i c a l d i s p e r s i o n . I n t h e c a s e o f n o n - c e n t e r e d d i e r e n c i n g ,
d i s c u s s e d i n C h a p t e r 1 1 , t h e m o d i e d w a v e n u m b e r i s c o m p l e x . T h e f o r m o f E q . 4 . 4 2
s h o w s t h a t t h e i m a g i n a r y p o r t i o n o f t h e m o d i e d w a v e n u m b e r p r o d u c e s n o n p h y s i c a l
d e c a y o r g r o w t h i n t h e n u m e r i c a l s o l u t i o n .
4 . 4 T h e R e p r e s e n t a t i v e E q u a t i o n
I n S e c t i o n 4 . 3 , w e p o i n t e d o u t t h a t E q s . 4 . 2 0 a n d 4 . 2 3 e x p r e s s i d e n t i c a l r e s u l t s b u t i n
t e r m s o f d i e r e n t g r o u p i n g s o f t h e d e p e n d e n t v a r i a b l e s , w h i c h a r e r e l a t e d b y a l g e b r a i c
m a n i p u l a t i o n . T h i s l e a d s t o t h e f o l l o w i n g i m p o r t a n t c o n c e p t :
T h e n u m e r i c a l s o l u t i o n t o a s e t o f l i n e a r O D E ' s ( i n w h i c h A i s
n o t a f u n c t i o n o f t ) i s e n t i r e l y e q u i v a l e n t t o t h e s o l u t i o n o b t a i n e d
i f t h e e q u a t i o n s a r e t r a n s f o r m e d t o e i g e n s p a c e , s o l v e d t h e r e i n
t h e i r u n c o u p l e d f o r m , a n d t h e n r e t u r n e d a s a c o u p l e d s e t t o r e a l
s p a c e .
T h e i m p o r t a n c e o f t h i s c o n c e p t r e s i d e s i n i t s m e s s a g e t h a t w e c a n a n a l y z e t i m e -
m a r c h i n g m e t h o d s b y a p p l y i n g t h e m t o a s i n g l e , u n c o u p l e d e q u a t i o n a n d o u r c o n -
c l u s i o n s w i l l a p p l y i n g e n e r a l . T h i s i s h e l p f u l b o t h i n a n a l y z i n g t h e a c c u r a c y o f
t i m e - m a r c h i n g m e t h o d s a n d i n s t u d y i n g t h e i r s t a b i l i t y , t o p i c s w h i c h a r e c o v e r e d i n
C h a p t e r s 6 a n d 7 .
O u r n e x t o b j e c t i v e i s t o n d a \ t y p i c a l " s i n g l e O D E t o a n a l y z e . W e f o u n d t h e
u n c o u p l e d s o l u t i o n t o a s e t o f O D E ' s i n S e c t i o n 4 . 2 . A t y p i c a l m e m b e r o f t h e f a m i l y
i s
d w
m
d t
=
m
w
m
; g
m
( t ) ( 4 . 4 4 )
T h e g o a l i n o u r a n a l y s i s i s t o s t u d y t y p i c a l b e h a v i o r o f g e n e r a l s i t u a t i o n s , n o t p a r t i c -
u l a r p r o b l e m s . F o r s u c h a p u r p o s e E q . 4 . 4 4 i s n o t q u i t e s a t i s f a c t o r y . T h e r o l e o f
m
i s
c l e a r i t s t a n d s f o r s o m e r e p r e s e n t a t i v e e i g e n v a l u e i n t h e o r i g i n a l A m a t r i x . H o w e v e r ,
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4 . 5 . P R O B L E M S 6 9
4 . C o n s i d e r a g r i d w i t h 1 0 i n t e r i o r p o i n t s s p a n n i n g t h e d o m a i n 0 x . F o r
i n i t i a l c o n d i t i o n s u ( x 0 ) = s i n ( m x ) a n d b o u n d a r y c o n d i t i o n s u ( 0 t ) = u ( t ) =
0 , p l o t t h e e x a c t s o l u t i o n o f t h e d i u s i o n e q u a t i o n w i t h = 1 a t t = 1 w i t h
m = 1 a n d m = 3 . ( P l o t t h e s o l u t i o n a t t h e g r i d n o d e s o n l y . ) C a l c u l a t e t h e
c o r r e s p o n d i n g m o d i e d w a v e n u m b e r s f o r t h e s e c o n d - o r d e r c e n t e r e d o p e r a t o r
f r o m E q . 4 . 3 7 . C a l c u l a t e a n d p l o t t h e c o r r e s p o n d i n g O D E s o l u t i o n s .
5 . C o n s i d e r t h e m a t r i x
A = ; B
p
( 1 0 ; 1 0 1 ) = ( 2 x )
c o r r e s p o n d i n g t o t h e O D E f o r m o f t h e b i c o n v e c t i o n e q u a t i o n r e s u l t i n g f r o m t h e
a p p l i c a t i o n o f s e c o n d - o r d e r c e n t r a l d i e r e n c i n g o n a 1 0 - p o i n t g r i d . N o t e t h a t
t h e d o m a i n i s 0 x 2 a n d x = 2 = 1 0 . T h e g r i d n o d e s a r e g i v e n b y
x
j
= j x j = 0 1 : : : 9 . T h e e i g e n v a l u e s o f t h e a b o v e m a t r i x A , a s w e l l a s t h e
m a t r i c e s X a n d X
; 1
, c a n b e f o u n d f r o m A p p e n d i x B . 4 . U s i n g t h e s e , c o m p u t e
a n d p l o t t h e O D E s o l u t i o n a t t = 2 f o r t h e i n i t i a l c o n d i t i o n u ( x 0 ) = s i n x .
C o m p a r e w i t h t h e e x a c t s o l u t i o n o f t h e P D E . C a l c u l a t e t h e n u m e r i c a l p h a s e
s p e e d f r o m t h e m o d i e d w a v e n u m b e r c o r r e s p o n d i n g t o t h i s i n i t i a l c o n d i t i o n
a n d s h o w t h a t i t i s c o n s i s t e n t w i t h t h e O D E s o l u t i o n . R e p e a t f o r t h e i n i t i a l
c o n d i t i o n u ( x 0 ) = s i n 2 x .
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7 0 C H A P T E R 4 . T H E S E M I - D I S C R E T E A P P R O A C H
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7 2 C H A P T E R 5 . F I N I T E - V O L U M E M E T H O D S
5 . 1 B a s i c C o n c e p t s
T h e b a s i c i d e a o f a n i t e - v o l u m e m e t h o d i s t o s a t i s f y t h e i n t e g r a l f o r m o f t h e c o n -
s e r v a t i o n l a w t o s o m e d e g r e e o f a p p r o x i m a t i o n f o r e a c h o f m a n y c o n t i g u o u s c o n t r o l
v o l u m e s w h i c h c o v e r t h e d o m a i n o f i n t e r e s t . T h u s t h e v o l u m e V i n E q . 5 . 1 i s t h a t o f a
c o n t r o l v o l u m e w h o s e s h a p e i s d e p e n d e n t o n t h e n a t u r e o f t h e g r i d . I n o u r e x a m p l e s ,
w e w i l l c o n s i d e r o n l y c o n t r o l v o l u m e s w h i c h d o n o t v a r y w i t h t i m e . E x a m i n i n g E q .
5 . 1 , w e s e e t h a t s e v e r a l a p p r o x i m a t i o n s m u s t b e m a d e . T h e u x i s r e q u i r e d a t t h e
b o u n d a r y o f t h e c o n t r o l v o l u m e , w h i c h i s a c l o s e d s u r f a c e i n t h r e e d i m e n s i o n s a n d a
c l o s e d c o n t o u r i n t w o d i m e n s i o n s . T h i s u x m u s t t h e n b e i n t e g r a t e d t o n d t h e n e t
u x t h r o u g h t h e b o u n d a r y . S i m i l a r l y , t h e s o u r c e t e r m P m u s t b e i n t e g r a t e d o v e r t h e
c o n t r o l v o l u m e . N e x t a t i m e - m a r c h i n g m e t h o d
1
c a n b e a p p l i e d t o n d t h e v a l u e o f
Z
V
Q d V ( 5 . 2 )
a t t h e n e x t t i m e s t e p .
L e t u s c o n s i d e r t h e s e a p p r o x i m a t i o n s i n m o r e d e t a i l . F i r s t , w e n o t e t h a t t h e
a v e r a g e v a l u e o f Q i n a c e l l w i t h v o l u m e V i s
Q
1
V
Z
V
Q d V ( 5 . 3 )
a n d E q . 5 . 1 c a n b e w r i t t e n a s
V
d
d t
Q +
I
S
n : F d S =
Z
V
P d V ( 5 . 4 )
f o r a c o n t r o l v o l u m e w h i c h d o e s n o t v a r y w i t h t i m e . T h u s a f t e r a p p l y i n g a t i m e -
m a r c h i n g m e t h o d , w e h a v e u p d a t e d v a l u e s o f t h e c e l l - a v e r a g e d q u a n t i t i e s
Q . I n o r d e r
t o e v a l u a t e t h e u x e s , w h i c h a r e a f u n c t i o n o f Q , a t t h e c o n t r o l - v o l u m e b o u n d a r y , Q
c a n b e r e p r e s e n t e d w i t h i n t h e c e l l b y s o m e p i e c e w i s e a p p r o x i m a t i o n w h i c h p r o d u c e s
t h e c o r r e c t v a l u e o f
Q . T h i s i s a f o r m o f i n t e r p o l a t i o n o f t e n r e f e r r e d t o a s r e c o n -
s t r u c t i o n . A s w e s h a l l s e e i n o u r e x a m p l e s , e a c h c e l l w i l l h a v e a d i e r e n t p i e c e w i s e
a p p r o x i m a t i o n t o Q . W h e n t h e s e a r e u s e d t o c a l c u l a t e F ( Q ) , t h e y w i l l g e n e r a l l y
p r o d u c e d i e r e n t a p p r o x i m a t i o n s t o t h e u x a t t h e b o u n d a r y b e t w e e n t w o c o n t r o l
v o l u m e s , t h a t i s , t h e u x w i l l b e d i s c o n t i n u o u s . A n o n d i s s i p a t i v e s c h e m e a n a l o g o u s
t o c e n t e r e d d i e r e n c i n g i s o b t a i n e d b y t a k i n g t h e a v e r a g e o f t h e s e t w o u x e s . A n o t h e r
a p p r o a c h k n o w n a s u x - d i e r e n c e s p l i t t i n g i s d e s c r i b e d i n C h a p t e r 1 1 .
T h e b a s i c e l e m e n t s o f a n i t e - v o l u m e m e t h o d a r e t h u s t h e f o l l o w i n g :
1
T i m e - m a r c h i n g m e t h o d s w i l l b e d i s c u s s e d i n t h e n e x t c h a p t e r .
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5 . 2 . M O D E L E Q U A T I O N S I N I N T E G R A L F O R M 7 3
1 . G i v e n t h e v a l u e o f
Q f o r e a c h c o n t r o l v o l u m e , c o n s t r u c t a n a p p r o x i m a t i o n t o
Q ( x y z ) i n e a c h c o n t r o l v o l u m e . U s i n g t h i s a p p r o x i m a t i o n , n d Q a t t h e
c o n t r o l - v o l u m e b o u n d a r y . E v a l u a t e F ( Q ) a t t h e b o u n d a r y . S i n c e t h e r e i s a
d i s t i n c t a p p r o x i m a t i o n t o Q ( x y z ) i n e a c h c o n t r o l v o l u m e , t w o d i s t i n c t v a l u e s
o f t h e u x w i l l g e n e r a l l y b e o b t a i n e d a t a n y p o i n t o n t h e b o u n d a r y b e t w e e n t w o
c o n t r o l v o l u m e s .
2 . A p p l y s o m e s t r a t e g y f o r r e s o l v i n g t h e d i s c o n t i n u i t y i n t h e u x a t t h e c o n t r o l -
v o l u m e b o u n d a r y t o p r o d u c e a s i n g l e v a l u e o f F ( Q ) a t a n y p o i n t o n t h e b o u n d -
a r y . T h i s i s s u e i s d i s c u s s e d i n S e c t i o n 1 1 . 4 . 2 .
3 . I n t e g r a t e t h e u x t o n d t h e n e t u x t h r o u g h t h e c o n t r o l - v o l u m e b o u n d a r y
u s i n g s o m e s o r t o f q u a d r a t u r e .
4 . A d v a n c e t h e s o l u t i o n i n t i m e t o o b t a i n n e w v a l u e s o f
Q .
T h e o r d e r o f a c c u r a c y o f t h e m e t h o d i s d e p e n d e n t o n e a c h o f t h e a p p r o x i m a t i o n s .
T h e s e i d e a s s h o u l d b e c l a r i e d b y t h e e x a m p l e s i n t h e r e m a i n d e r o f t h i s c h a p t e r .
I n o r d e r t o i n c l u d e d i u s i v e u x e s , t h e f o l l o w i n g r e l a t i o n b e t w e e n r Q a n d Q i s
s o m e t i m e s u s e d :
Z
V
r Q d V =
I
S
n Q d S ( 5 . 5 )
o r , i n t w o d i m e n s i o n s ,
Z
A
r Q d A =
I
C
n Q d l ( 5 . 6 )
w h e r e t h e u n i t v e c t o r n p o i n t s o u t w a r d f r o m t h e s u r f a c e o r c o n t o u r .
5 . 2 M o d e l E q u a t i o n s i n I n t e g r a l F o r m
5 . 2 . 1 T h e L i n e a r C o n v e c t i o n E q u a t i o n
A t w o - d i m e n s i o n a l f o r m o f t h e l i n e a r c o n v e c t i o n e q u a t i o n c a n b e w r i t t e n a s
@ u
@ t
+ a c o s
@ u
@ x
+ a s i n
@ u
@ y
= 0 ( 5 . 7 )
T h i s P D E g o v e r n s a s i m p l e p l a n e w a v e c o n v e c t i n g t h e s c a l a r q u a n t i t y , u ( x y t ) w i t h
s p e e d a a l o n g a s t r a i g h t l i n e m a k i n g a n a n g l e w i t h r e s p e c t t o t h e x - a x i s . T h e
o n e - d i m e n s i o n a l f o r m i s r e c o v e r e d w i t h = 0 .
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7 4 C H A P T E R 5 . F I N I T E - V O L U M E M E T H O D S
F o r u n i t s p e e d a , t h e t w o - d i m e n s i o n a l l i n e a r c o n v e c t i o n e q u a t i o n i s o b t a i n e d f r o m
t h e g e n e r a l d i v e r g e n c e f o r m , E q . 2 . 3 , w i t h
Q = u ( 5 . 8 )
F = i u c o s + j u s i n ( 5 . 9 )
P = 0 ( 5 . 1 0 )
S i n c e Q i s a s c a l a r , F i s s i m p l y a v e c t o r . S u b s t i t u t i n g t h e s e e x p r e s s i o n s i n t o a t w o -
d i m e n s i o n a l f o r m o f E q . 2 . 2 g i v e s t h e f o l l o w i n g i n t e g r a l f o r m
d
d t
Z
A
u d A +
I
C
n : ( i u c o s + j u s i n ) d s = 0 ( 5 . 1 1 )
w h e r e A i s t h e a r e a o f t h e c e l l w h i c h i s b o u n d e d b y t h e c l o s e d c o n t o u r C .
5 . 2 . 2 T h e D i u s i o n E q u a t i o n
T h e i n t e g r a l f o r m o f t h e t w o - d i m e n s i o n a l d i u s i o n e q u a t i o n w i t h n o s o u r c e t e r m a n d
u n i t d i u s i o n c o e c i e n t i s o b t a i n e d f r o m t h e g e n e r a l d i v e r g e n c e f o r m , E q . 2 . 3 , w i t h
Q = u ( 5 . 1 2 )
F = ; r u ( 5 . 1 3 )
=
;
i
@ u
@ x
+ j
@ u
@ y
!
( 5 . 1 4 )
P = 0 ( 5 . 1 5 )
U s i n g t h e s e , w e n d
d
d t
Z
A
u d A =
I
C
n :
i
@ u
@ x
+ j
@ u
@ y
!
d s ( 5 . 1 6 )
t o b e t h e i n t e g r a l f o r m o f t h e t w o - d i m e n s i o n a l d i u s i o n e q u a t i o n .
5 . 3 O n e - D i m e n s i o n a l E x a m p l e s
W e r e s t r i c t o u r a t t e n t i o n t o a s c a l a r d e p e n d e n t v a r i a b l e u a n d a s c a l a r u x f , a s i n
t h e m o d e l e q u a t i o n s . W e c o n s i d e r a n e q u i s p a c e d g r i d w i t h s p a c i n g x . T h e n o d e s o f
t h e g r i d a r e l o c a t e d a t x
j
= j x a s u s u a l . C o n t r o l v o l u m e j e x t e n d s f r o m x
j
; x = 2
t o x
j
+ x = 2 , a s s h o w n i n F i g . 5 . 1 . W e w i l l u s e t h e f o l l o w i n g n o t a t i o n :
x
j ; 1 = 2
= x
j
; x = 2 x
j + 1 = 2
= x
j
+ x = 2 ( 5 . 1 7 )
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5 . 3 . O N E - D I M E N S I O N A L E X A M P L E S 7 5
j j+1 j-1 j+2 j-2
j-1/2 j+1/2
∆ x
L RL R
F i g u r e 5 . 1 : C o n t r o l v o l u m e i n o n e d i m e n s i o n .
u
j 1 = 2
= u ( x
j 1 = 2
) f
j 1 = 2
= f ( u
j 1 = 2
) ( 5 . 1 8 )
W i t h t h e s e d e n i t i o n s , t h e c e l l - a v e r a g e v a l u e b e c o m e s
u
j
( t )
1
x
Z
x
j + 1 = 2
x
j ; 1 = 2
u ( x t ) d x ( 5 . 1 9 )
a n d t h e i n t e g r a l f o r m b e c o m e s
d
d t
( x u
j
) + f
j + 1 = 2
; f
j ; 1 = 2
=
Z
x
j + 1 = 2
x
j ; 1 = 2
P d x ( 5 . 2 0 )
N o w w i t h = x ; x
j
, w e c a n e x p a n d u ( x ) i n E q . 5 . 1 9 i n a T a y l o r s e r i e s a b o u t x
j
( w i t h t x e d ) t o g e t
u
j
1
x
Z
x = 2
; x = 2
2
4
u
j
+
@ u
@ x
!
j
+
2
2
@
2
u
@ x
2
!
j
+
3
6
@
3
u
@ x
3
!
j
+ : : :
3
5
d
= u
j
+
x
2
2 4
@
2
u
@ x
2
!
j
+
x
4
1 9 2 0
@
4
u
@ x
4
!
j
+ O ( x
6
) ( 5 . 2 1 )
o r
u
j
= u
j
+ O ( x
2
) ( 5 . 2 2 )
w h e r e u
j
i s t h e v a l u e a t t h e c e n t e r o f t h e c e l l . H e n c e t h e c e l l - a v e r a g e v a l u e a n d t h e
v a l u e a t t h e c e n t e r o f t h e c e l l d i e r b y a t e r m o f s e c o n d o r d e r .
5 . 3 . 1 A S e c o n d - O r d e r A p p r o x i m a t i o n t o t h e C o n v e c t i o n E q u a -
t i o n
I n o n e d i m e n s i o n , t h e i n t e g r a l f o r m o f t h e l i n e a r c o n v e c t i o n e q u a t i o n , E q . 5 . 1 1 , b e -
c o m e s
x
d u
j
d t
+ f
j + 1 = 2
; f
j ; 1 = 2
= 0 ( 5 . 2 3 )
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5 . 3 . O N E - D I M E N S I O N A L E X A M P L E S 7 7
T h i s i s i d e n t i c a l t o t h e e x p r e s s i o n o b t a i n e d u s i n g s e c o n d - o r d e r c e n t e r e d d i e r e n c e s ,
e x c e p t i t i s w r i t t e n i n t e r m s o f t h e c e l l a v e r a g e
~
u , r a t h e r t h a n t h e n o d a l v a l u e s , ~u .
H e n c e o u r a n a l y s i s a n d u n d e r s t a n d i n g o f t h e e i g e n s y s t e m o f t h e m a t r i x B
p
( ; 1 0 1 )
i s r e l e v a n t t o n i t e - v o l u m e m e t h o d s a s w e l l a s n i t e - d i e r e n c e m e t h o d s . S i n c e t h e
e i g e n v a l u e s o f B
p
(
; 1 0 1 ) a r e p u r e i m a g i n a r y , w e c a n c o n c l u d e t h a t t h e u s e o f t h e
a v e r a g e o f t h e u x e s o n e i t h e r s i d e o f t h e c e l l b o u n d a r y , a s i n E q s . 5 . 2 9 a n d 5 . 3 0 , c a n
l e a d t o a n o n d i s s i p a t i v e n i t e - v o l u m e m e t h o d .
5 . 3 . 2 A F o u r t h - O r d e r A p p r o x i m a t i o n t o t h e C o n v e c t i o n E q u a -
t i o n
L e t u s r e p l a c e t h e p i e c e w i s e c o n s t a n t a p p r o x i m a t i o n i n S e c t i o n 5 . 3 . 1 w i t h a p i e c e w i s e
q u a d r a t i c a p p r o x i m a t i o n a s f o l l o w s
u ( ) = a
2
+ b + c ( 5 . 3 3 )
w h e r e i s a g a i n e q u a l t o x ; x
j
. T h e t h r e e p a r a m e t e r s a , b , a n d c a r e c h o s e n t o
s a t i s f y t h e f o l l o w i n g c o n s t r a i n t s :
1
x
Z
; x = 2
; 3 x = 2
u ( ) d = u
j ; 1
1
x
Z
x = 2
; x = 2
u ( ) d = u
j
( 5 . 3 4 )
1
x
Z
3 x = 2
x = 2
u ( ) d = u
j + 1
T h e s e c o n s t r a i n t s l e a d t o
a =
u
j + 1
; 2 u
j
+ u
j ; 1
2 x
2
b =
u
j + 1
; u
j ; 1
2 x
( 5 . 3 5 )
c =
; u
j ; 1
+ 2 6 u
j
; u
j + 1
2 4
W i t h t h e s e v a l u e s o f a , b , a n d c , t h e p i e c e w i s e q u a d r a t i c a p p r o x i m a t i o n p r o d u c e s
t h e f o l l o w i n g v a l u e s a t t h e c e l l b o u n d a r i e s :
u
L
j + 1 = 2
=
1
6
( 2 u
j + 1
+ 5 u
j
; u
j ; 1
) ( 5 . 3 6 )
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7 8 C H A P T E R 5 . F I N I T E - V O L U M E M E T H O D S
u
R
j ; 1 = 2
=
1
6
( ; u
j + 1
+ 5 u
j
+ 2 u
j ; 1
) ( 5 . 3 7 )
u
R
j + 1 = 2
=
1
6
( ; u
j + 2
+ 5 u
j + 1
+ 2 u
j
) ( 5 . 3 8 )
u
L
j ; 1 = 2
=
1
6
( 2 u
j
+ 5 u
j ; 1
; u
j ; 2
) ( 5 . 3 9 )
u s i n g t h e n o t a t i o n d e n e d i n S e c t i o n 5 . 3 . 1 . R e c a l l i n g t h a t f = u , w e a g a i n u s e t h e
a v e r a g e o f t h e u x e s o n e i t h e r s i d e o f t h e b o u n d a r y t o o b t a i n
f
j + 1 = 2
=
1
2
f ( u
L
j + 1 = 2
) + f ( u
R
j + 1 = 2
) ]
=
1
1 2
( ; u
j + 2
+ 7 u
j + 1
+ 7 u
j
; u
j ; 1
) ( 5 . 4 0 )
a n d
f
j ; 1 = 2
=
1
2
f ( u
L
j ; 1 = 2
) + f ( u
R
j ; 1 = 2
) ]
=
1
1 2
( ; u
j + 1
+ 7 u
j
+ 7 u
j ; 1
; u
j ; 2
) ( 5 . 4 1 )
S u b s t i t u t i n g t h e s e e x p r e s s i o n s i n t o t h e i n t e g r a l f o r m , E q . 5 . 2 3 , g i v e s
x
d u
j
d t
+
1
1 2
( ; u
j + 2
+ 8 u
j + 1
; 8 u
j ; 1
+ u
j ; 2
) = 0 ( 5 . 4 2 )
T h i s i s a f o u r t h - o r d e r a p p r o x i m a t i o n t o t h e i n t e g r a l f o r m o f t h e e q u a t i o n , a s c a n b e
v e r i e d u s i n g T a y l o r s e r i e s e x p a n s i o n s ( s e e q u e s t i o n 1 a t t h e e n d o f t h i s c h a p t e r ) .
W i t h p e r i o d i c b o u n d a r y c o n d i t i o n s , t h e f o l l o w i n g s e m i - d i s c r e t e f o r m i s o b t a i n e d :
d
~
u
d t
=
;
1
1 2 x
B
p
( 1
; 8 0 8
; 1 )
~
u ( 5 . 4 3 )
T h i s i s a s y s t e m o f O D E ' s g o v e r n i n g t h e e v o l u t i o n o f t h e c e l l - a v e r a g e d a t a .
5 . 3 . 3 A S e c o n d - O r d e r A p p r o x i m a t i o n t o t h e D i u s i o n E q u a -
t i o n
I n t h i s s e c t i o n , w e d e s c r i b e t w o a p p r o a c h e s t o d e r i v i n g a n i t e - v o l u m e a p p r o x i m a t i o n
t o t h e d i u s i o n e q u a t i o n . T h e r s t a p p r o a c h i s s i m p l e r t o e x t e n d t o m u l t i d i m e n s i o n s ,
w h i l e t h e s e c o n d a p p r o a c h i s m o r e s u i t e d t o e x t e n s i o n t o h i g h e r o r d e r a c c u r a c y .
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5 . 3 . O N E - D I M E N S I O N A L E X A M P L E S 7 9
I n o n e d i m e n s i o n , t h e i n t e g r a l f o r m o f t h e d i u s i o n e q u a t i o n , E q . 5 . 1 6 , b e c o m e s
x
d u
j
d t
+ f
j + 1 = 2
; f
j ; 1 = 2
= 0 ( 5 . 4 4 )
w i t h f = ; r u = ; @ u = @ x . A l s o , E q . 5 . 6 b e c o m e s
Z
b
a
@ u
@ x
d x = u ( b ) ; u ( a ) ( 5 . 4 5 )
W e c a n t h u s w r i t e t h e f o l l o w i n g e x p r e s s i o n f o r t h e a v e r a g e v a l u e o f t h e g r a d i e n t o f u
o v e r t h e i n t e r v a l x
j
x x
j + 1
:
1
x
Z
x
j + 1
x
j
@ u
@ x
d x =
1
x
( u
j + 1
; u
j
) ( 5 . 4 6 )
F r o m E q . 5 . 2 2 , w e k n o w t h a t t h e v a l u e o f a c o n t i n u o u s f u n c t i o n a t t h e c e n t e r o f a g i v e n
i n t e r v a l i s e q u a l t o t h e a v e r a g e v a l u e o f t h e f u n c t i o n o v e r t h e i n t e r v a l t o s e c o n d - o r d e r
a c c u r a c y . H e n c e , t o s e c o n d - o r d e r , w e c a n w r i t e
f
j + 1 = 2
= ;
@ u
@ x
!
j + 1 = 2
= ;
1
x
( u
j + 1
; u
j
) ( 5 . 4 7 )
S i m i l a r l y ,
f
j ; 1 = 2
= ;
1
x
( u
j
; u
j ; 1
) ( 5 . 4 8 )
S u b s t i t u t i n g t h e s e i n t o t h e i n t e g r a l f o r m , E q . 5 . 4 4 , w e o b t a i n
x
d u
j
d t
=
1
x
( u
j ; 1
; 2 u
j
+ u
j + 1
) ( 5 . 4 9 )
o r , w i t h D i r i c h l e t b o u n d a r y c o n d i t i o n s ,
d
~
u
d t
=
1
x
2
B ( 1
; 2 1 )
~
u +
~
b c
( 5 . 5 0 )
T h i s p r o v i d e s a s e m i - d i s c r e t e n i t e - v o l u m e a p p r o x i m a t i o n t o t h e d i u s i o n e q u a t i o n ,
a n d w e s e e t h a t t h e p r o p e r t i e s o f t h e m a t r i x B ( 1 ; 2 1 ) a r e r e l e v a n t t o t h e s t u d y o f
n i t e - v o l u m e m e t h o d s a s w e l l a s n i t e - d i e r e n c e m e t h o d s .
F o r o u r s e c o n d a p p r o a c h , w e u s e a p i e c e w i s e q u a d r a t i c a p p r o x i m a t i o n a s i n S e c t i o n
5 . 3 . 2 . F r o m E q . 5 . 3 3 w e h a v e
@ u
@ x
=
@ u
@
= 2 a + b ( 5 . 5 1 )
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8 2 C H A P T E R 5 . F I N I T E - V O L U M E M E T H O D S
F o r E q . 5 . 1 1 , t h e t w o - d i m e n s i o n a l l i n e a r c o n v e c t i o n e q u a t i o n , w e h a v e f o r s i d e
n
Z
F d l = n
( i c o s + j s i n )
Z
` = 2
; ` = 2
u
( ) d ( 5 . 5 8 )
w h e r e i s a l e n g t h m e a s u r e d f r o m t h e m i d d l e o f a s i d e . M a k i n g t h e c h a n g e o f
v a r i a b l e z = = , o n e h a s t h e e x p r e s s i o n
Z
` = 2
; ` = 2
u ( ) d =
Z
1 = 2
; 1 = 2
u ( z ) d z ( 5 . 5 9 )
T h e n , i n t e r m s o f u a n d t h e h e x a g o n a r e a A , w e h a v e
d
d t
Z
A
u d A +
5
X
= 0
n
( i c o s + j s i n )
"
Z
1 = 2
; 1 = 2
u ( z ) d z
#
= 0 ( 5 . 6 0 )
T h e v a l u e s o f n
( i c o s + j s i n ) a r e g i v e n b y t h e e x p r e s s i o n s i n T a b l e 5 . 2 . T h e r e
a r e n o n u m e r i c a l a p p r o x i m a t i o n s i n E q . 5 . 6 0 . T h a t i s , i f t h e i n t e g r a l s i n t h e e q u a t i o n
a r e e v a l u a t e d e x a c t l y , t h e i n t e g r a t e d t i m e r a t e o f c h a n g e o f t h e i n t e g r a l o f u o v e r t h e
a r e a o f t h e h e x a g o n i s k n o w n e x a c t l y .
S i d e n
( i c o s + j s i n )
0 ( c o s ;
p
3 s i n ) = 2
1 c o s
2 ( c o s +
p
3 s i n ) = 2
3 ( ; c o s +
p
3 s i n ) = 2
4 ; c o s
5 ( ; c o s ;
p
3 s i n ) = 2
T a b l e 5 . 2 . W e i g h t s o f u x i n t e g r a l s , s e e E q . 5 . 6 0 .
I n t r o d u c i n g t h e c e l l a v e r a g e ,
Z
A
u d A = A u
p
( 5 . 6 1 )
a n d t h e p i e c e w i s e - c o n s t a n t a p p r o x i m a t i o n u = u
p
o v e r t h e e n t i r e h e x a g o n , t h e a p -
p r o x i m a t i o n t o t h e u x i n t e g r a l b e c o m e s t r i v i a l . T a k i n g t h e a v e r a g e o f t h e u x o n
e i t h e r s i d e o f e a c h e d g e o f t h e h e x a g o n g i v e s f o r e d g e 1 :
Z
1
u ( z ) d z =
u
p
+ u
a
2
Z
1 = 2
; 1 = 2
d z =
u
p
+ u
a
2
( 5 . 6 2 )
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5 . 5 . P R O B L E M S 8 3
S i m i l a r l y , w e h a v e f o r t h e o t h e r v e e d g e s :
Z
2
u ( z ) d z =
u
p
+ u
b
2
( 5 . 6 3 )
Z
3
u ( z ) d z =
u
p
+ u
c
2
( 5 . 6 4 )
Z
4
u ( z ) d z =
u
p
+ u
d
2
( 5 . 6 5 )
Z
5
u ( z ) d z =
u
p
+ u
e
2
( 5 . 6 6 )
Z
0
u ( z ) d z =
u
p
+ u
f
2
( 5 . 6 7 )
S u b s t i t u t i n g t h e s e i n t o E q . 5 . 6 0 , a l o n g w i t h t h e e x p r e s s i o n s i n T a b l e 5 . 2 , w e o b t a i n
A
d u
p
d t
+
2
( 2 c o s ) ( u
a
; u
d
) + ( c o s +
p
3 s i n ) ( u
b
; u
e
)
+ ( ; c o s +
p
3 s i n ) ( u
c
; u
f
) ] = 0 ( 5 . 6 8 )
o r
d u
p
d t
+
1
3
( 2 c o s ) ( u
a
; u
d
) + ( c o s +
p
3 s i n ) ( u
b
; u
e
)
+ ( ; c o s +
p
3 s i n ) ( u
c
; u
f
) ] = 0 ( 5 . 6 9 )
T h e r e a d e r c a n v e r i f y , u s i n g T a y l o r s e r i e s e x p a n s i o n s , t h a t t h i s i s a s e c o n d - o r d e r
a p p r o x i m a t i o n t o t h e i n t e g r a l f o r m o f t h e t w o - d i m e n s i o n a l l i n e a r c o n v e c t i o n e q u a t i o n .
5 . 5 P r o b l e m s
1 . U s e T a y l o r s e r i e s t o v e r i f y t h a t E q . 5 . 4 2 i s a f o u r t h - o r d e r a p p r o x i m a t i o n t o E q .
5 . 2 3 .
2 . F i n d t h e s e m i - d i s c r e t e O D E f o r m g o v e r n i n g t h e c e l l - a v e r a g e d a t a r e s u l t i n g f r o m
t h e u s e o f a l i n e a r a p p r o x i m a t i o n i n d e v e l o p i n g a n i t e - v o l u m e m e t h o d f o r t h e
l i n e a r c o n v e c t i o n e q u a t i o n . U s e t h e f o l l o w i n g l i n e a r a p p r o x i m a t i o n :
u ( ) = a + b
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8 4 C H A P T E R 5 . F I N I T E - V O L U M E M E T H O D S
w h e r e b = u
j
a n d
a =
u
j + 1
; u
j ; 1
2 x
a n d u s e t h e a v e r a g e u x a t t h e c e l l i n t e r f a c e .
3 . U s i n g t h e r s t a p p r o a c h g i v e n i n S e c t i o n 5 . 3 . 3 , d e r i v e a n i t e - v o l u m e a p p r o x -
i m a t i o n t o t h e s p a t i a l t e r m s i n t h e t w o - d i m e n s i o n a l d i u s i o n e q u a t i o n o n a
s q u a r e g r i d .
4 . R e p e a t q u e s t i o n 3 f o r a g r i d c o n s i s t i n g o f e q u i l a t e r a l t r i a n g l e s .
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C h a p t e r 6
T I M E - M A R C H I N G M E T H O D S
F O R O D E ' S
A f t e r d i s c r e t i z i n g t h e s p a t i a l d e r i v a t i v e s i n t h e g o v e r n i n g P D E ' s ( s u c h a s t h e N a v i e r -
S t o k e s e q u a t i o n s ) , w e o b t a i n a c o u p l e d s y s t e m o f n o n l i n e a r O D E ' s i n t h e f o r m
d ~u
d t
=
~
F ( ~ u t ) ( 6 . 1 )
T h e s e c a n b e i n t e g r a t e d i n t i m e u s i n g a t i m e - m a r c h i n g m e t h o d t o o b t a i n a t i m e -
a c c u r a t e s o l u t i o n t o a n u n s t e a d y o w p r o b l e m . F o r a s t e a d y o w p r o b l e m , s p a t i a l
d i s c r e t i z a t i o n l e a d s t o a c o u p l e d s y s t e m o f n o n l i n e a r a l g e b r a i c e q u a t i o n s i n t h e f o r m
~
F ( ~u ) = 0 ( 6 . 2 )
A s a r e s u l t o f t h e n o n l i n e a r i t y o f t h e s e e q u a t i o n s , s o m e s o r t o f i t e r a t i v e m e t h o d i s
r e q u i r e d t o o b t a i n a s o l u t i o n . F o r e x a m p l e , o n e c a n c o n s i d e r t h e u s e o f N e w t o n ' s
m e t h o d , w h i c h i s w i d e l y u s e d f o r n o n l i n e a r a l g e b r a i c e q u a t i o n s ( S e e S e c t i o n 6 . 1 0 . 3 . ) .
T h i s p r o d u c e s a n i t e r a t i v e m e t h o d i n w h i c h a c o u p l e d s y s t e m o f l i n e a r a l g e b r a i c
e q u a t i o n s m u s t b e s o l v e d a t e a c h i t e r a t i o n . T h e s e c a n b e s o l v e d i t e r a t i v e l y u s i n g
r e l a x a t i o n m e t h o d s , w h i c h w i l l b e d i s c u s s e d i n C h a p t e r 9 , o r d i r e c t l y u s i n g G a u s s i a n
e l i m i n a t i o n o r s o m e v a r i a t i o n t h e r e o f .
A l t e r n a t i v e l y , o n e c a n c o n s i d e r a t i m e - d e p e n d e n t p a t h t o t h e s t e a d y s t a t e a n d u s e
a t i m e - m a r c h i n g m e t h o d t o i n t e g r a t e t h e u n s t e a d y f o r m o f t h e e q u a t i o n s u n t i l t h e
s o l u t i o n i s s u c i e n t l y c l o s e t o t h e s t e a d y s o l u t i o n . T h e s u b j e c t o f t h e p r e s e n t c h a p t e r ,
t i m e - m a r c h i n g m e t h o d s f o r O D E ' s , i s t h u s r e l e v a n t t o b o t h s t e a d y a n d u n s t e a d y o w
p r o b l e m s . W h e n u s i n g a t i m e - m a r c h i n g m e t h o d t o c o m p u t e s t e a d y o w s , t h e g o a l i s
s i m p l y t o r e m o v e t h e t r a n s i e n t p o r t i o n o f t h e s o l u t i o n a s q u i c k l y a s p o s s i b l e t i m e -
a c c u r a c y i s n o t r e q u i r e d . T h i s m o t i v a t e s t h e s t u d y o f s t a b i l i t y a n d s t i n e s s , t o p i c s
w h i c h a r e c o v e r e d i n t h e n e x t t w o c h a p t e r s .
8 5
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8 6 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
A p p l i c a t i o n o f a s p a t i a l d i s c r e t i z a t i o n t o a P D E p r o d u c e s a c o u p l e d s y s t e m o f
O D E ' s . A p p l i c a t i o n o f a t i m e - m a r c h i n g m e t h o d t o a n O D E p r o d u c e s a n o r d i n a r y
d i e r e n c e e q u a t i o n ( O E ) . I n e a r l i e r c h a p t e r s , w e d e v e l o p e d e x a c t s o l u t i o n s t o o u r
m o d e l P D E ' s a n d O D E ' s . I n t h i s c h a p t e r w e w i l l p r e s e n t s o m e b a s i c t h e o r y o f l i n e a r
O E ' s w h i c h c l o s e l y p a r a l l e l s t h a t f o r l i n e a r O D E ' s , a n d , u s i n g t h i s t h e o r y , w e w i l l
d e v e l o p e x a c t s o l u t i o n s f o r t h e m o d e l O E ' s a r i s i n g f r o m t h e a p p l i c a t i o n o f t i m e -
m a r c h i n g m e t h o d s t o t h e m o d e l O D E ' s .
6 . 1 N o t a t i o n
U s i n g t h e s e m i - d i s c r e t e a p p r o a c h , w e r e d u c e o u r P D E t o a s e t o f c o u p l e d O D E ' s
r e p r e s e n t e d i n g e n e r a l b y E q . 4 . 1 . H o w e v e r , f o r t h e p u r p o s e o f t h i s c h a p t e r , w e n e e d
o n l y c o n s i d e r t h e s c a l a r c a s e
d u
d t
= u
0
= F ( u t ) ( 6 . 3 )
A l t h o u g h w e u s e u t o r e p r e s e n t t h e d e p e n d e n t v a r i a b l e , r a t h e r t h a n w , t h e r e a d e r
s h o u l d r e c a l l t h e a r g u m e n t s m a d e i n C h a p t e r 4 t o j u s t i f y t h e s t u d y o f a s c a l a r O D E .
O u r r s t t a s k i s t o n d n u m e r i c a l a p p r o x i m a t i o n s t h a t c a n b e u s e d t o c a r r y o u t t h e
t i m e i n t e g r a t i o n o f E q . 6 . 3 t o s o m e g i v e n a c c u r a c y , w h e r e a c c u r a c y c a n b e m e a s u r e d
e i t h e r i n a l o c a l o r a g l o b a l s e n s e . W e t h e n f a c e a f u r t h e r t a s k c o n c e r n i n g t h e n u m e r i c a l
s t a b i l i t y o f t h e r e s u l t i n g m e t h o d s , b u t w e p o s t p o n e s u c h c o n s i d e r a t i o n s t o t h e n e x t
c h a p t e r .
I n C h a p t e r 2 , w e i n t r o d u c e d t h e c o n v e n t i o n t h a t t h e n s u b s c r i p t , o r t h e ( n ) s u -
p e r s c r i p t , a l w a y s p o i n t s t o a d i s c r e t e t i m e v a l u e , a n d h r e p r e s e n t s t h e t i m e i n t e r v a l
t . C o m b i n i n g t h i s n o t a t i o n w i t h E q . 6 . 3 g i v e s
u
0
n
= F
n
= F ( u
n
t
n
) t
n
= n h
O f t e n w e n e e d a m o r e s o p h i s t i c a t e d n o t a t i o n f o r i n t e r m e d i a t e t i m e s t e p s i n v o l v i n g
t e m p o r a r y c a l c u l a t i o n s d e n o t e d b y ~ u , u , e t c . F o r t h e s e w e u s e t h e n o t a t i o n
~ u
0
n +
=
~
F
n +
= F ( ~ u
n +
t
n
+ h )
T h e c h o i c e o f u
0
o r F t o e x p r e s s t h e d e r i v a t i v e i n a s c h e m e i s a r b i t r a r y . T h e y a r e
b o t h c o m m o n l y u s e d i n t h e l i t e r a t u r e o n O D E ' s .
T h e m e t h o d s w e s t u d y a r e t o b e a p p l i e d t o l i n e a r o r n o n l i n e a r O D E ' s , b u t t h e
m e t h o d s t h e m s e l v e s a r e f o r m e d b y l i n e a r c o m b i n a t i o n s o f t h e d e p e n d e n t v a r i a b l e a n d
i t s d e r i v a t i v e a t v a r i o u s t i m e i n t e r v a l s . T h e y a r e r e p r e s e n t e d c o n c e p t u a l l y b y
u
n + 1
= f
1
u
0
n + 1
0
u
0
n
; 1
u
0
n ; 1
0
u
n
; 1
u
n ; 1
( 6 . 4 )
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8 8 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
s u c h a l i n e a r O D E i n t o a l i n e a r O E . T h e l a t t e r a r e s u b j e c t t o a w h o l e b o d y o f
a n a l y s i s t h a t i s s i m i l a r i n m a n y r e s p e c t s t o , a n d j u s t a s p o w e r f u l a s , t h e t h e o r y o f
O D E ' s . W e n e x t c o n s i d e r e x a m p l e s o f t h i s c o n v e r s i o n p r o c e s s a n d t h e n g o i n t o t h e
g e n e r a l t h e o r y o n s o l v i n g O E ' s .
A p p l y t h e s i m p l e e x p l i c i t E u l e r s c h e m e , E q . 6 . 5 , t o E q . 6 . 8 . T h e r e r e s u l t s
u
n + 1
= u
n
+ h ( u
n
+ a e
h n
)
o r
u
n + 1
; ( 1 + h ) u
n
= h a e
h n
( 6 . 9 )
E q . 6 . 9 i s a l i n e a r O E , w i t h c o n s t a n t c o e c i e n t s , e x p r e s s e d i n t e r m s o f t h e d e p e n -
d e n t v a r i a b l e u
n
a n d t h e i n d e p e n d e n t v a r i a b l e n . A s a n o t h e r e x a m p l e , a p p l y i n g t h e
i m p l i c i t E u l e r m e t h o d , E q . 6 . 6 , t o E q . 6 . 8 , w e n d
u
n + 1
= u
n
+ h
u
n + 1
+ a e
h ( n + 1 )
o r
( 1 ; h ) u
n + 1
; u
n
= h e
h
a e
h n
( 6 . 1 0 )
A s a n a l e x a m p l e , t h e p r e d i c t o r - c o r r e c t o r s e q u e n c e , E q . 6 . 7 , g i v e s
~ u
n + 1
; ( 1 + h ) u
n
= a h e
h n
;
1
2
( 1 + h ) ~ u
n + 1
+ u
n + 1
;
1
2
u
n
=
1
2
a h e
h ( n + 1 )
( 6 . 1 1 )
w h i c h i s a c o u p l e d s e t o f l i n e a r O E ' s w i t h c o n s t a n t c o e c i e n t s . N o t e t h a t t h e r s t
l i n e i n E q . 6 . 1 1 i s i d e n t i c a l t o E q . 6 . 9 , s i n c e t h e p r e d i c t o r s t e p i n E q . 6 . 7 i s s i m p l y
t h e e x p l i c i t E u l e r m e t h o d . T h e s e c o n d l i n e i n E q . 6 . 1 1 i s o b t a i n e d b y n o t i n g t h a t
~ u
0
n + 1
= F ( ~ u
n + 1
t
n
+ h )
= ~ u
n + 1
+ a e
h ( n + 1 )
( 6 . 1 2 )
N o w w e n e e d t o d e v e l o p t e c h n i q u e s f o r a n a l y z i n g t h e s e d i e r e n c e e q u a t i o n s s o t h a t
w e c a n c o m p a r e t h e m e r i t s o f t h e t i m e - m a r c h i n g m e t h o d s t h a t g e n e r a t e d t h e m .
6 . 3 S o l u t i o n o f L i n e a r O E ' s W i t h C o n s t a n t C o -
e c i e n t s
T h e t e c h n i q u e s f o r s o l v i n g l i n e a r d i e r e n c e e q u a t i o n s w i t h c o n s t a n t c o e c i e n t s i s a s
w e l l d e v e l o p e d a s t h a t f o r O D E ' s a n d t h e t h e o r y f o l l o w s a r e m a r k a b l y p a r a l l e l p a t h .
T h i s i s d e m o n s t r a t e d b y r e p e a t i n g s o m e o f t h e d e v e l o p m e n t s i n S e c t i o n 4 . 2 , b u t f o r
d i e r e n c e r a t h e r t h a n d i e r e n t i a l e q u a t i o n s .
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6 . 3 . S O L U T I O N O F L I N E A R O E ' S W I T H C O N S T A N T C O E F F I C I E N T S 8 9
6 . 3 . 1 F i r s t - a n d S e c o n d - O r d e r D i e r e n c e E q u a t i o n s
F i r s t - O r d e r E q u a t i o n s
T h e s i m p l e s t n o n h o m o g e n e o u s O E o f i n t e r e s t i s g i v e n b y t h e s i n g l e r s t - o r d e r e q u a -
t i o n
u
n + 1
= u
n
+ a b
n
( 6 . 1 3 )
w h e r e , a , a n d b a r e , i n g e n e r a l , c o m p l e x p a r a m e t e r s . T h e i n d e p e n d e n t v a r i a b l e i s
n o w n r a t h e r t h a n t , a n d s i n c e t h e e q u a t i o n s a r e l i n e a r a n d h a v e c o n s t a n t c o e c i e n t s ,
i s n o t a f u n c t i o n o f e i t h e r n o r u . T h e e x a c t s o l u t i o n o f E q . 6 . 1 3 i s
u
n
= c
1
n
+
a b
n
b ;
w h e r e c
1
i s a c o n s t a n t d e t e r m i n e d b y t h e i n i t i a l c o n d i t i o n s . I n t e r m s o f t h e i n i t i a l
v a l u e o f u i t c a n b e w r i t t e n
u
n
= u
0
n
+ a
b
n
;
n
b ;
J u s t a s i n t h e d e v e l o p m e n t o f E q . 4 . 1 0 , o n e c a n r e a d i l y s h o w t h a t t h e s o l u t i o n o f t h e
d e f e c t i v e c a s e , ( b = ) ,
u
n + 1
= u
n
+ a
n
i s
u
n
=
h
u
0
+ a n
; 1
i
n
T h i s c a n a l l b e e a s i l y v e r i e d b y s u b s t i t u t i o n .
S e c o n d - O r d e r E q u a t i o n s
T h e h o m o g e n e o u s f o r m o f a s e c o n d - o r d e r d i e r e n c e e q u a t i o n i s g i v e n b y
u
n + 2
+ a
1
u
n + 1
+ a
0
u
n
= 0 ( 6 . 1 4 )
I n s t e a d o f t h e d i e r e n t i a l o p e r a t o r D
d
d t
u s e d f o r O D E ' s , w e u s e f o r O E ' s t h e
d i e r e n c e o p e r a t o r E ( c o m m o n l y r e f e r r e d t o a s t h e d i s p l a c e m e n t o r s h i f t o p e r a t o r )
a n d d e n e d f o r m a l l y b y t h e r e l a t i o n s
u
n + 1
= E u
n
u
n + k
= E
k
u
n
F u r t h e r n o t i c e t h a t t h e d i s p l a c e m e n t o p e r a t o r a l s o a p p l i e s t o e x p o n e n t s , t h u s
b
b
n
= b
n +
= E
b
n
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9 0 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
w h e r e c a n b e a n y f r a c t i o n o r i r r a t i o n a l n u m b e r .
T h e r o l e s o f D a n d E a r e t h e s a m e i n s o f a r a s o n c e t h e y h a v e b e e n i n t r o d u c e d t o
t h e b a s i c e q u a t i o n s t h e v a l u e o f u ( t ) o r u
n
c a n b e f a c t o r e d o u t . T h u s E q . 6 . 1 4 c a n
n o w b e r e - e x p r e s s e d i n a n o p e r a t i o n a l n o t i o n a s
( E
2
+ a
1
E + a
0
) u
n
= 0 ( 6 . 1 5 )
w h i c h m u s t b e z e r o f o r a l l u
n
. E q . 6 . 1 5 i s k n o w n a s t h e o p e r a t i o n a l f o r m o f E q . 6 . 1 4 .
T h e o p e r a t i o n a l f o r m c o n t a i n s a c h a r a c t e r i s t i c p o l y n o m i a l P ( E ) w h i c h p l a y s t h e s a m e
r o l e f o r d i e r e n c e e q u a t i o n s t h a t P ( D ) p l a y e d f o r d i e r e n t i a l e q u a t i o n s t h a t i s , i t s
r o o t s d e t e r m i n e t h e s o l u t i o n t o t h e O E . I n t h e a n a l y s i s o f O E ' s , w e l a b e l t h e s e
r o o t s
1
,
2
, , e t c , a n d r e f e r t o t h e m a s t h e - r o o t s . T h e y a r e f o u n d b y s o l v i n g t h e
e q u a t i o n P ( ) = 0 . I n t h e s i m p l e e x a m p l e g i v e n a b o v e , t h e r e a r e j u s t t w o r o o t s
a n d i n t e r m s o f t h e m t h e s o l u t i o n c a n b e w r i t t e n
u
n
= c
1
(
1
)
n
+ c
2
(
2
)
n
( 6 . 1 6 )
w h e r e c
1
a n d c
2
d e p e n d u p o n t h e i n i t i a l c o n d i t i o n s . T h e f a c t t h a t E q . 6 . 1 6 i s a
s o l u t i o n t o E q . 6 . 1 4 f o r a l l c
1
c
2
a n d n s h o u l d b e v e r i e d b y s u b s t i t u t i o n .
6 . 3 . 2 S p e c i a l C a s e s o f C o u p l e d F i r s t - O r d e r E q u a t i o n s
A C o m p l e t e S y s t e m
C o u p l e d , r s t - o r d e r , l i n e a r h o m o g e n e o u s d i e r e n c e e q u a t i o n s h a v e t h e f o r m
u
( n + 1 )
1
= c
1 1
u
( n )
1
+ c
1 2
u
( n )
2
u
( n + 1 )
2
= c
2 1
u
( n )
1
+ c
2 2
u
( n )
2
( 6 . 1 7 )
w h i c h c a n a l s o b e w r i t t e n
~
u
n + 1
= C
~
u
n
~
u
n
=
h
u
( n )
1
u
( n )
2
i
T
C =
c
1 1
c
1 2
c
2 1
c
2 2
T h e o p e r a t i o n a l f o r m o f E q . 6 . 1 7 c a n b e w r i t t e n
( c
1 1
; E ) c
1 2
c
2 1
( c
2 2
; E )
u
1
u
2
( n )
= C ; E I ]
~
u
n
= 0
w h i c h m u s t b e z e r o f o r a l l u
1
a n d u
2
. A g a i n w e a r e l e d t o a c h a r a c t e r i s t i c p o l y n o m i a l ,
t h i s t i m e h a v i n g t h e f o r m P ( E ) = d e t C ; E I ] . T h e - r o o t s a r e f o u n d f r o m
P ( ) = d e t
( c
1 1
; ) c
1 2
c
2 1
( c
2 2
; )
= 0
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6 . 4 . S O L U T I O N O F T H E R E P R E S E N T A T I V E O E ' S 9 1
O b v i o u s l y t h e
k
a r e t h e e i g e n v a l u e s o f C a n d , f o l l o w i n g t h e l o g i c o f S e c t i o n 4 . 2 ,
i f
~
x a r e i t s e i g e n v e c t o r s , t h e s o l u t i o n o f E q . 6 . 1 7 i s
~
u
n
=
2
X
k = 1
c
k
(
k
)
n
~
x
k
w h e r e c
k
a r e c o n s t a n t s d e t e r m i n e d b y t h e i n i t i a l c o n d i t i o n s .
A D e f e c t i v e S y s t e m
T h e s o l u t i o n o f O E ' s w i t h d e f e c t i v e e i g e n s y s t e m s f o l l o w s c l o s e l y t h e l o g i c i n S e c t i o n
4 . 2 . 2 f o r d e f e c t i v e O D E ' s . F o r e x a m p l e , o n e c a n s h o w t h a t t h e s o l u t i o n t o
2
6
4
u
n + 1
u
n + 1
u
n + 1
3
7
5
=
2
6
4
1
1
3
7
5
2
6
4
u
n
u
n
u
n
3
7
5
i s
u
n
= u
0
n
u
n
=
h
u
0
+ u
0
n
; 1
i
n
u
n
=
"
u
0
+ u
0
n
; 1
+ u
0
n ( n ; 1 )
2
; 2
#
n
( 6 . 1 8 )
6 . 4 S o l u t i o n o f t h e R e p r e s e n t a t i v e O E ' s
6 . 4 . 1 T h e O p e r a t i o n a l F o r m a n d i t s S o l u t i o n
E x a m p l e s o f t h e n o n h o m o g e n e o u s , l i n e a r , r s t - o r d e r o r d i n a r y d i e r e n c e e q u a t i o n s ,
p r o d u c e d b y a p p l y i n g a t i m e - m a r c h i n g m e t h o d t o t h e r e p r e s e n t a t i v e e q u a t i o n , a r e
g i v e n b y E q s . 6 . 9 t o 6 . 1 1 . U s i n g t h e d i s p l a c e m e n t o p e r a t o r , E , t h e s e e q u a t i o n s c a n
b e w r i t t e n
E ; ( 1 + h ) ] u
n
= h a e
h n
( 6 . 1 9 )
( 1 ; h ) E ; 1 ] u
n
= h E a e
h n
( 6 . 2 0 )
"
E ; ( 1 + h )
;
1
2
( 1 + h ) E E ;
1
2
# "
~ u
u
#
n
= h
"
1
1
2
E
#
a e
h n
( 6 . 2 1 )
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9 2 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
A l l t h r e e o f t h e s e e q u a t i o n s a r e s u b s e t s o f t h e o p e r a t i o n a l f o r m o f t h e r e p r e s e n t a t i v e
O E
P ( E ) u
n
= Q ( E ) a e
h n
( 6 . 2 2 )
w h i c h i s p r o d u c e d b y a p p l y i n g t i m e - m a r c h i n g m e t h o d s t o t h e r e p r e s e n t a t i v e O D E , E q .
4 . 4 5 . W e c a n e x p r e s s i n t e r m s o f E q . 6 . 2 2 a l l m a n n e r o f s t a n d a r d t i m e - m a r c h i n g m e t h -
o d s h a v i n g m u l t i p l e t i m e s t e p s a n d v a r i o u s t y p e s o f i n t e r m e d i a t e p r e d i c t o r - c o r r e c t o r
f a m i l i e s . T h e t e r m s P ( E ) a n d Q ( E ) a r e p o l y n o m i a l s i n E r e f e r r e d t o a s t h e c h a r a c -
t e r i s t i c p o l y n o m i a l a n d t h e p a r t i c u l a r p o l y n o m i a l , r e s p e c t i v e l y .
T h e g e n e r a l s o l u t i o n o f E q . 6 . 2 2 c a n b e e x p r e s s e d a s
u
n
=
K
X
k = 1
c
k
(
k
)
n
+ a e
h n
Q ( e
h
)
P ( e
h
)
( 6 . 2 3 )
w h e r e
k
a r e t h e K r o o t s o f t h e c h a r a c t e r i s t i c p o l y n o m i a l , P ( ) = 0 . W h e n d e t e r m i -
n a n t s a r e i n v o l v e d i n t h e c o n s t r u c t i o n o f P ( E ) a n d Q ( E ) , a s w o u l d b e t h e c a s e f o r
E q . 6 . 2 1 , t h e r a t i o Q ( E ) = P ( E ) c a n b e f o u n d b y K r a m e r ' s r u l e . K e e p i n m i n d t h a t
f o r m e t h o d s s u c h a s i n E q . 6 . 2 1 t h e r e a r e m u l t i p l e ( t w o i n t h i s c a s e ) s o l u t i o n s , o n e
f o r u
n
a n d ~ u
n
a n d w e a r e u s u a l l y o n l y i n t e r e s t e d i n t h e n a l s o l u t i o n u
n
. N o t i c e a l s o ,
t h e i m p o r t a n t s u b s e t o f t h i s s o l u t i o n w h i c h o c c u r s w h e n = 0 , r e p r e s e n t i n g a t i m e
i n v a r i a n t p a r t i c u l a r s o l u t i o n , o r a s t e a d y s t a t e . I n s u c h a c a s e
u
n
=
K
X
k = 1
c
k
(
k
)
n
+ a
Q ( 1 )
P ( 1 )
6 . 4 . 2 E x a m p l e s o f S o l u t i o n s t o T i m e - M a r c h i n g O E ' s
A s e x a m p l e s o f t h e u s e o f E q s . 6 . 2 2 a n d 6 . 2 3 , w e d e r i v e t h e s o l u t i o n s o f E q s . 6 . 1 9 t o
6 . 2 1 . F o r t h e e x p l i c i t E u l e r m e t h o d , E q . 6 . 1 9 , w e h a v e
P ( E ) = E ; 1 ; h
Q ( E ) = h ( 6 . 2 4 )
a n d t h e s o l u t i o n o f i t s r e p r e s e n t a t i v e O E f o l l o w s i m m e d i a t e l y f r o m E q . 6 . 2 3 :
u
n
= c
1
( 1 + h )
n
+ a e
h n
h
e
h
; 1 ; h
F o r t h e i m p l i c i t E u l e r m e t h o d , E q . 6 . 2 0 , w e h a v e
P ( E ) = ( 1 ; h ) E ; 1
Q ( E ) = h E ( 6 . 2 5 )
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6 . 5 . T H E ; R E L A T I O N 9 5
6 . 5 . 2 T h e P r i n c i p a l - R o o t
B a s e d o n t h e a b o v e w e m a k e t h e f o l l o w i n g o b s e r v a t i o n :
A p p l i c a t i o n o f t h e s a m e t i m e - m a r c h i n g m e t h o d t o a l l o f t h e e q u a t i o n s i n
a c o u p l e d s y s t e m l i n e a r O D E ' s i n t h e f o r m o f E q . 4 . 6 , a l w a y s p r o d u c e s
o n e - r o o t f o r e v e r y - r o o t t h a t s a t i s e s t h e r e l a t i o n
= 1 + h +
1
2
2
h
2
+ +
1
k !
k
h
k
+ O
h
k + 1
w h e r e k i s t h e o r d e r o f t h e t i m e - m a r c h i n g m e t h o d .
( 6 . 3 3 )
W e r e f e r t o t h e r o o t t h a t h a s t h e a b o v e p r o p e r t y a s t h e p r i n c i p a l - r o o t , a n d d e s i g n a t e
i t (
m
)
1
. T h e a b o v e p r o p e r t y c a n b e s t a t e d r e g a r d l e s s o f t h e d e t a i l s o f t h e t i m e -
m a r c h i n g m e t h o d , k n o w i n g o n l y t h a t i t s l e a d i n g e r r o r i s O
h
k + 1
. T h u s t h e p r i n c i p a l
r o o t i s a n a p p r o x i m a t i o n t o e
h
u p t o O
h
k
.
N o t e t h a t a s e c o n d - o r d e r a p p r o x i m a t i o n t o a d e r i v a t i v e w r i t t e n i n t h e f o r m
(
t
u )
n
=
1
2 h
( u
n + 1
; u
n ; 1
) ( 6 . 3 4 )
h a s a l e a d i n g t r u n c a t i o n e r r o r w h i c h i s O ( h
2
) , w h i l e t h e s e c o n d - o r d e r t i m e - m a r c h i n g
m e t h o d w h i c h r e s u l t s f r o m t h i s a p p r o x i m a t i o n , w h i c h i s t h e l e a p f r o g m e t h o d :
u
n + 1
= u
n ; 1
+ 2 h u
0
n
( 6 . 3 5 )
h a s a l e a d i n g t r u n c a t i o n e r r o r O ( h
3
) . T h i s a r i s e s s i m p l y b e c a u s e o f o u r n o t a t i o n
f o r t h e t i m e - m a r c h i n g m e t h o d i n w h i c h w e h a v e m u l t i p l i e d t h r o u g h b y h t o g e t a n
a p p r o x i m a t i o n f o r t h e f u n c t i o n u
n + 1
r a t h e r t h a n t h e d e r i v a t i v e a s i n E q . 6 . 3 4 . T h e
f o l l o w i n g e x a m p l e m a k e s t h i s c l e a r . C o n s i d e r a s o l u t i o n o b t a i n e d a t a g i v e n t i m e T
u s i n g a s e c o n d - o r d e r t i m e - m a r c h i n g m e t h o d w i t h a t i m e s t e p h . N o w c o n s i d e r t h e
s o l u t i o n o b t a i n e d u s i n g t h e s a m e m e t h o d w i t h a t i m e s t e p h = 2 . S i n c e t h e e r r o r p e r
t i m e s t e p i s O ( h
3
) , t h i s i s r e d u c e d b y a f a c t o r o f e i g h t ( c o n s i d e r i n g t h e l e a d i n g t e r m
o n l y ) . H o w e v e r , t w i c e a s m a n y t i m e s t e p s a r e r e q u i r e d t o r e a c h t h e t i m e T . T h e r e f o r e
t h e e r r o r a t t h e e n d o f t h e s i m u l a t i o n i s r e d u c e d b y a f a c t o r o f f o u r , c o n s i s t e n t w i t h
a s e c o n d - o r d e r a p p r o x i m a t i o n .
6 . 5 . 3 S p u r i o u s - R o o t s
W e s a w f r o m E q . 6 . 3 0 t h a t t h e ; r e l a t i o n f o r t h e l e a p f r o g m e t h o d p r o d u c e s t w o
- r o o t s f o r e a c h . O n e o f t h e s e w e i d e n t i e d a s t h e p r i n c i p a l r o o t w h i c h a l w a y s
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9 6 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
h a s t h e p r o p e r t y g i v e n i n 6 . 3 3 . T h e o t h e r i s r e f e r r e d t o a s a s p u r i o u s - r o o t a n d
d e s i g n a t e d (
m
)
2
. I n g e n e r a l , t h e ; r e l a t i o n p r o d u c e d b y a t i m e - m a r c h i n g s c h e m e
c a n r e s u l t i n m u l t i p l e - r o o t s a l l o f w h i c h , e x c e p t f o r t h e p r i n c i p a l o n e , a r e s p u r i o u s .
A l l s p u r i o u s r o o t s a r e d e s i g n a t e d (
m
)
k
w h e r e k = 2 3 . N o m a t t e r w h e t h e r a
- r o o t i s p r i n c i p a l o r s p u r i o u s , i t i s a l w a y s s o m e a l g e b r a i c f u n c t i o n o f t h e p r o d u c t h .
T o e x p r e s s t h i s f a c t w e u s e t h e n o t a t i o n = ( h ) .
I f a t i m e - m a r c h i n g m e t h o d p r o d u c e s s p u r i o u s - r o o t s , t h e s o l u t i o n f o r t h e O E i n
t h e f o r m s h o w n i n E q . 6 . 2 8 m u s t b e m o d i e d . F o l l o w i n g a g a i n t h e m e s s a g e o f S e c t i o n
4 . 4 , w e h a v e
u
n
= c
1 1
(
1
)
n
1
~
x
1
+ + c
m 1
(
m
)
n
1
~
x
m
+ + c
M 1
(
M
)
n
1
~
x
M
+ P : S :
+ c
1 2
(
1
)
n
2
~
x
1
+ + c
m 2
(
m
)
n
2
~
x
m
+ + c
M 2
(
M
)
n
2
~
x
M
+ c
1 3
(
1
)
n
3
~
x
1
+ + c
m 3
(
m
)
n
3
~
x
m
+ + c
M 3
(
M
)
n
3
~
x
M
+ e t c . , i f t h e r e a r e m o r e s p u r i o u s r o o t s ( 6 . 3 6 )
S p u r i o u s r o o t s a r i s e i f a m e t h o d u s e s d a t a f r o m t i m e l e v e l n ; 1 o r e a r l i e r t o
a d v a n c e t h e s o l u t i o n f r o m t i m e l e v e l n t o n + 1 . S u c h r o o t s o r i g i n a t e e n t i r e l y f r o m
t h e n u m e r i c a l a p p r o x i m a t i o n o f t h e t i m e - m a r c h i n g m e t h o d a n d h a v e n o t h i n g t o d o
w i t h t h e O D E b e i n g s o l v e d . H o w e v e r , g e n e r a t i o n o f s p u r i o u s r o o t s d o e s n o t , i n i t s e l f ,
m a k e a m e t h o d i n f e r i o r . I n f a c t , m a n y v e r y a c c u r a t e m e t h o d s i n p r a c t i c a l u s e f o r
i n t e g r a t i n g s o m e f o r m s o f O D E ' s h a v e s p u r i o u s r o o t s .
I t s h o u l d b e m e n t i o n e d t h a t m e t h o d s w i t h s p u r i o u s r o o t s a r e n o t s e l f s t a r t i n g .
F o r e x a m p l e , i f t h e r e i s o n e s p u r i o u s r o o t t o a m e t h o d , a l l o f t h e c o e c i e n t s ( c
m
)
2
i n E q . 6 . 3 6 m u s t b e i n i t i a l i z e d b y s o m e s t a r t i n g p r o c e d u r e . T h e i n i t i a l v e c t o r ~u
0
d o e s n o t p r o v i d e e n o u g h d a t a t o i n i t i a l i z e a l l o f t h e c o e c i e n t s . T h i s r e s u l t s b e c a u s e
m e t h o d s w h i c h p r o d u c e s p u r i o u s r o o t s r e q u i r e d a t a f r o m t i m e l e v e l n ; 1 o r e a r l i e r .
F o r e x a m p l e , t h e l e a p f r o g m e t h o d r e q u i r e s ~u
n ; 1
a n d t h u s c a n n o t b e s t a r t e d u s i n g
o n l y ~u
n
.
P r e s u m a b l y ( i . e . , i f o n e s t a r t s t h e m e t h o d p r o p e r l y ) t h e s p u r i o u s c o e c i e n t s a r e
a l l i n i t i a l i z e d w i t h v e r y s m a l l m a g n i t u d e s , a n d p r e s u m a b l y t h e m a g n i t u d e s o f t h e
s p u r i o u s r o o t s t h e m s e l v e s a r e a l l l e s s t h a n o n e ( s e e C h a p t e r 7 ) . T h e n t h e p r e s e n c e o f
s p u r i o u s r o o t s d o e s n o t c o n t a m i n a t e t h e a n s w e r . T h a t i s , a f t e r s o m e n i t e t i m e t h e
a m p l i t u d e o f t h e e r r o r a s s o c i a t e d w i t h t h e s p u r i o u s r o o t s i s e v e n s m a l l e r t h e n w h e n
i t w a s i n i t i a l i z e d . T h u s w h i l e s p u r i o u s r o o t s m u s t b e c o n s i d e r e d i n s t a b i l i t y a n a l y s i s ,
t h e y p l a y v i r t u a l l y n o r o l e i n a c c u r a c y a n a l y s i s .
6 . 5 . 4 O n e - R o o t T i m e - M a r c h i n g M e t h o d s
T h e r e a r e a n u m b e r o f t i m e - m a r c h i n g m e t h o d s t h a t p r o d u c e o n l y o n e - r o o t f o r e a c h
- r o o t . W e r e f e r t o t h e m a s o n e - r o o t m e t h o d s . T h e y a r e a l s o c a l l e d o n e - s t e p m e t h o d s .
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6 . 6 . A C C U R A C Y M E A S U R E S O F T I M E - M A R C H I N G M E T H O D S 9 7
T h e y h a v e t h e s i g n i c a n t a d v a n t a g e o f b e i n g s e l f - s t a r t i n g w h i c h c a r r i e s w i t h i t t h e
v e r y u s e f u l p r o p e r t y t h a t t h e t i m e - s t e p i n t e r v a l c a n b e c h a n g e d a t w i l l t h r o u g h o u t
t h e m a r c h i n g p r o c e s s . T h r e e o n e - r o o t m e t h o d s w e r e a n a l y z e d i n S e c t i o n 6 . 4 . 2 . A
p o p u l a r m e t h o d h a v i n g t h i s p r o p e r t y , t h e s o - c a l l e d - m e t h o d , i s g i v e n b y t h e f o r m u l a
u
n + 1
= u
n
+ h
h
( 1 ; ) u
0
n
+ u
0
n + 1
i
T h e - m e t h o d r e p r e s e n t s t h e e x p l i c i t E u l e r ( = 0 ) , t h e t r a p e z o i d a l ( =
1
2
) , a n d t h e
i m p l i c i t E u l e r m e t h o d s ( = 1 ) , r e s p e c t i v e l y . I t s ; r e l a t i o n i s
=
1 + ( 1 ; ) h
1 ; h
I t i s i n s t r u c t i v e t o c o m p a r e t h e e x a c t s o l u t i o n t o a s e t o f O D E ' s ( w i t h a c o m p l e t e
e i g e n s y s t e m ) h a v i n g t i m e - i n v a r i a n t f o r c i n g t e r m s w i t h t h e e x a c t s o l u t i o n t o t h e O E ' s
f o r o n e - r o o t m e t h o d s . T h e s e a r e
~
u ( t ) = c
1
e
1
h
n
~
x
1
+ + c
m
e
m
h
n
~
x
m
+ + c
M
e
M
h
n
~
x
M
+ A
; 1
~
f
~
u
n
= c
1
(
1
)
n
~
x
1
+ + c
m
(
m
)
n
~
x
m
+ + c
M
(
M
)
n
~
x
M
+ A
; 1
~
f ( 6 . 3 7 )
r e s p e c t i v e l y . N o t i c e t h a t w h e n t a n d n = 0 , t h e s e e q u a t i o n s a r e i d e n t i c a l , s o t h a t a l l
t h e c o n s t a n t s , v e c t o r s , a n d m a t r i c e s a r e i d e n t i c a l e x c e p t t h e
~
u a n d t h e t e r m s i n s i d e
t h e p a r e n t h e s e s o n t h e r i g h t h a n d s i d e s . T h e o n l y e r r o r m a d e b y i n t r o d u c i n g t h e t i m e
m a r c h i n g i s t h e e r r o r t h a t m a k e s i n a p p r o x i m a t i n g e
h
.
6 . 6 A c c u r a c y M e a s u r e s o f T i m e - M a r c h i n g M e t h -
o d s
6 . 6 . 1 L o c a l a n d G l o b a l E r r o r M e a s u r e s
T h e r e a r e t w o b r o a d c a t e g o r i e s o f e r r o r s t h a t c a n b e u s e d t o d e r i v e a n d e v a l u a t e t i m e -
m a r c h i n g m e t h o d s . O n e i s t h e e r r o r m a d e i n e a c h t i m e s t e p . T h i s i s a l o c a l e r r o r s u c h
a s t h a t f o u n d f r o m a T a y l o r t a b l e a n a l y s i s , s e e S e c t i o n 3 . 4 . I t i s u s u a l l y u s e d a s t h e
b a s i s f o r e s t a b l i s h i n g t h e o r d e r o f a m e t h o d . T h e o t h e r i s t h e e r r o r d e t e r m i n e d a t t h e
e n d o f a g i v e n e v e n t w h i c h h a s c o v e r e d a s p e c i c i n t e r v a l o f t i m e c o m p o s e d o f m a n y
t i m e s t e p s . T h i s i s a g l o b a l e r r o r . I t i s u s e f u l f o r c o m p a r i n g m e t h o d s , a s w e s h a l l s e e
i n C h a p t e r 8 .
I t i s q u i t e c o m m o n t o j u d g e a t i m e - m a r c h i n g m e t h o d o n t h e b a s i s o f r e s u l t s f o u n d
f r o m a T a y l o r t a b l e . H o w e v e r , a T a y l o r s e r i e s a n a l y s i s i s a v e r y l i m i t e d t o o l f o r n d i n g
t h e m o r e s u b t l e p r o p e r t i e s o f a n u m e r i c a l t i m e - m a r c h i n g m e t h o d . F o r e x a m p l e , i t i s
o f n o u s e i n :
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9 8 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
n d i n g s p u r i o u s r o o t s .
e v a l u a t i n g n u m e r i c a l s t a b i l i t y a n d s e p a r a t i n g t h e e r r o r s i n p h a s e a n d a m p l i t u d e .
a n a l y z i n g t h e p a r t i c u l a r s o l u t i o n o f p r e d i c t o r - c o r r e c t o r c o m b i n a t i o n s .
n d i n g t h e g l o b a l e r r o r .
T h e l a t t e r t h r e e o f t h e s e a r e o f c o n c e r n t o u s h e r e , a n d t o s t u d y t h e m w e m a k e u s e o f
t h e m a t e r i a l d e v e l o p e d i n t h e p r e v i o u s s e c t i o n s o f t h i s c h a p t e r . O u r e r r o r m e a s u r e s
a r e b a s e d o n t h e d i e r e n c e b e t w e e n t h e e x a c t s o l u t i o n t o t h e r e p r e s e n t a t i v e O D E ,
g i v e n b y
u ( t ) = c e
t
+
a e
t
;
( 6 . 3 8 )
a n d t h e s o l u t i o n t o t h e r e p r e s e n t a t i v e O E ' s , i n c l u d i n g o n l y t h e c o n t r i b u t i o n f r o m
t h e p r i n c i p a l r o o t , w h i c h c a n b e w r i t t e n a s
u
n
= c
1
(
1
)
n
+ a e
h n
Q ( e
h
)
P ( e
h
)
( 6 . 3 9 )
6 . 6 . 2 L o c a l A c c u r a c y o f t h e T r a n s i e n t S o l u t i o n ( e r
j j e r
!
)
T r a n s i e n t e r r o r
T h e p a r t i c u l a r c h o i c e o f a n e r r o r m e a s u r e , e i t h e r l o c a l o r g l o b a l , i s t o s o m e e x t e n t
a r b i t r a r y . H o w e v e r , a n e c e s s a r y c o n d i t i o n f o r t h e c h o i c e s h o u l d b e t h a t t h e m e a s u r e
c a n b e u s e d c o n s i s t e n t l y f o r a l l m e t h o d s . I n t h e d i s c u s s i o n o f t h e - r e l a t i o n w e
s a w t h a t a l l t i m e - m a r c h i n g m e t h o d s p r o d u c e a p r i n c i p a l - r o o t f o r e v e r y - r o o t t h a t
e x i s t s i n a s e t o f l i n e a r O D E ' s . T h e r e f o r e , a v e r y n a t u r a l l o c a l e r r o r m e a s u r e f o r t h e
t r a n s i e n t s o l u t i o n i s t h e v a l u e o f t h e d i e r e n c e b e t w e e n s o l u t i o n s b a s e d o n t h e s e t w o
r o o t s . W e d e s i g n a t e t h i s b y e r
a n d m a k e t h e f o l l o w i n g d e n i t i o n
e r
e
h
;
1
T h e l e a d i n g e r r o r t e r m c a n b e f o u n d b y e x p a n d i n g i n a T a y l o r s e r i e s a n d c h o o s i n g
t h e r s t n o n v a n i s h i n g t e r m . T h i s i s s i m i l a r t o t h e e r r o r f o u n d f r o m a T a y l o r t a b l e .
T h e o r d e r o f t h e m e t h o d i s t h e l a s t p o w e r o f h m a t c h e d e x a c t l y .
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6 . 6 . A C C U R A C Y M E A S U R E S O F T I M E - M A R C H I N G M E T H O D S 9 9
A m p l i t u d e a n d P h a s e E r r o r
S u p p o s e a e i g e n v a l u e i s i m a g i n a r y . S u c h c a n i n d e e d b e t h e c a s e w h e n w e s t u d y t h e
e q u a t i o n s g o v e r n i n g p e r i o d i c c o n v e c t i o n w h i c h p r o d u c e s h a r m o n i c m o t i o n . F o r s u c h
c a s e s i t i s m o r e m e a n i n g f u l t o e x p r e s s t h e e r r o r i n t e r m s o f a m p l i t u d e a n d p h a s e .
L e t = i ! w h e r e ! i s a r e a l n u m b e r r e p r e s e n t i n g a f r e q u e n c y . T h e n t h e n u m e r i c a l
m e t h o d m u s t p r o d u c e a p r i n c i p a l - r o o t t h a t i s c o m p l e x a n d e x p r e s s i b l e i n t h e f o r m
1
=
r
+ i
i
e
i ! h
( 6 . 4 0 )
F r o m t h i s i t f o l l o w s t h a t t h e l o c a l e r r o r i n a m p l i t u d e i s m e a s u r e d b y t h e d e v i a t i o n o f
j
1
j f r o m u n i t y , t h a t i s
e r
a
= 1 ; j
1
j = 1 ;
q
(
1
)
2
r
+ (
1
)
2
i
a n d t h e l o c a l e r r o r i n p h a s e c a n b e d e n e d a s
e r
!
! h ; t a n
; 1
(
1
)
i
= (
1
)
r
) ] ( 6 . 4 1 )
A m p l i t u d e a n d p h a s e e r r o r s a r e i m p o r t a n t m e a s u r e s o f t h e s u i t a b i l i t y o f t i m e - m a r c h i n g
m e t h o d s f o r c o n v e c t i o n a n d w a v e p r o p a g a t i o n p h e n o m e n a .
T h e a p p r o a c h t o e r r o r a n a l y s i s d e s c r i b e d i n S e c t i o n 3 . 5 c a n b e e x t e n d e d t o t h e
c o m b i n a t i o n o f a s p a t i a l d i s c r e t i z a t i o n a n d a t i m e - m a r c h i n g m e t h o d a p p l i e d t o t h e
l i n e a r c o n v e c t i o n e q u a t i o n . T h e p r i n c i p a l r o o t ,
1
( h ) , i s f o u n d u s i n g = ; i a
,
w h e r e
i s t h e m o d i e d w a v e n u m b e r o f t h e s p a t i a l d i s c r e t i z a t i o n . I n t r o d u c i n g t h e
C o u r a n t n u m b e r , C
n
= a h = x , w e h a v e h = ; i C
n
x . T h u s o n e c a n o b t a i n
v a l u e s o f t h e p r i n c i p a l r o o t o v e r t h e r a n g e 0 x f o r a g i v e n v a l u e o f t h e
C o u r a n t n u m b e r . T h e a b o v e e x p r e s s i o n f o r e r
!
c a n b e n o r m a l i z e d t o g i v e t h e e r r o r
i n t h e p h a s e s p e e d , a s f o l l o w s
e r
p
=
e r
!
! h
= 1 +
t a n
; 1
(
1
)
i
= (
1
)
r
) ]
C
n
x
( 6 . 4 2 )
w h e r e ! = ; a . A p o s i t i v e v a l u e o f e r
p
c o r r e s p o n d s t o p h a s e l a g ( t h e n u m e r i c a l p h a s e
s p e e d i s t o o s m a l l ) , w h i l e a n e g a t i v e v a l u e c o r r e s p o n d s t o p h a s e l e a d ( t h e n u m e r i c a l
p h a s e s p e e d i s t o o l a r g e ) .
6 . 6 . 3 L o c a l A c c u r a c y o f t h e P a r t i c u l a r S o l u t i o n ( e r
)
T h e n u m e r i c a l e r r o r i n t h e p a r t i c u l a r s o l u t i o n i s f o u n d b y c o m p a r i n g t h e p a r t i c u l a r
s o l u t i o n o f t h e O D E w i t h t h a t f o r t h e O E . W e h a v e f o u n d t h e s e t o b e g i v e n b y
P : S :
( O D E )
= a e
t
1
( ; )
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1 0 0 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
a n d
P : S :
( O E )
= a e
t
Q ( e
h
)
P ( e
h
)
r e s p e c t i v e l y . F o r a m e a s u r e o f t h e l o c a l e r r o r i n t h e p a r t i c u l a r s o l u t i o n w e i n t r o d u c e
t h e d e n i t i o n
e r
h
(
P : S :
( O E )
P : S :
( O D E )
; 1
)
( 6 . 4 3 )
T h e m u l t i p l i c a t i o n b y h c o n v e r t s t h e e r r o r f r o m a g l o b a l m e a s u r e t o a l o c a l o n e , s o
t h a t t h e o r d e r o f e r
a n d e r
a r e c o n s i s t e n t . I n o r d e r t o d e t e r m i n e t h e l e a d i n g e r r o r
t e r m , E q . 6 . 4 3 c a n b e w r i t t e n i n t e r m s o f t h e c h a r a c t e r i s t i c a n d p a r t i c u l a r p o l y n o m i a l s
a s
e r
=
c
o
;
n
( ; ) Q
e
h
; P
e
h
o
( 6 . 4 4 )
w h e r e
c
o
= l i m
h ! 0
h ( ; )
P
e
h
T h e v a l u e o f c
o
i s a m e t h o d - d e p e n d e n t c o n s t a n t t h a t i s o f t e n e q u a l t o o n e . I f t h e
f o r c i n g f u n c t i o n i s i n d e p e n d e n t o f t i m e , i s e q u a l t o z e r o , a n d f o r t h i s c a s e , m a n y
n u m e r i c a l m e t h o d s g e n e r a t e a n e r
t h a t i s a l s o z e r o .
T h e a l g e b r a i n v o l v e d i n n d i n g t h e o r d e r o f e r
c a n b e q u i t e t e d i o u s . H o w e v e r ,
t h i s o r d e r i s q u i t e i m p o r t a n t i n d e t e r m i n i n g t h e t r u e o r d e r o f a t i m e - m a r c h i n g m e t h o d
b y t h e p r o c e s s t h a t h a s b e e n o u t l i n e d . A n i l l u s t r a t i o n o f t h i s i s g i v e n i n t h e s e c t i o n
o n R u n g e - K u t t a m e t h o d s .
6 . 6 . 4 T i m e A c c u r a c y F o r N o n l i n e a r A p p l i c a t i o n s
I n p r a c t i c e , t i m e - m a r c h i n g m e t h o d s a r e u s u a l l y a p p l i e d t o n o n l i n e a r O D E ' s , a n d i t
i s n e c e s s a r y t h a t t h e a d v e r t i s e d o r d e r o f a c c u r a c y b e v a l i d f o r t h e n o n l i n e a r c a s e s a s
w e l l a s f o r t h e l i n e a r o n e s . A n e c e s s a r y c o n d i t i o n f o r t h i s t o o c c u r i s t h a t t h e l o c a l
a c c u r a c i e s o f b o t h t h e t r a n s i e n t a n d t h e p a r t i c u l a r s o l u t i o n s b e o f t h e s a m e o r d e r .
M o r e p r e c i s e l y , a t i m e - m a r c h i n g m e t h o d i s s a i d t o b e o f o r d e r k i f
e r
= c
1
( h )
k
1
+ 1
( 6 . 4 5 )
e r
= c
2
( h )
k
2
+ 1
( 6 . 4 6 )
w h e r e k = s m a l l e s t o f ( k
1
k
2
) ( 6 . 4 7 )
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6 . 6 . A C C U R A C Y M E A S U R E S O F T I M E - M A R C H I N G M E T H O D S 1 0 1
T h e r e a d e r s h o u l d b e a w a r e t h a t t h i s i s n o t s u c i e n t . F o r e x a m p l e , t o d e r i v e a l l o f
t h e n e c e s s a r y c o n d i t i o n s f o r t h e f o u r t h - o r d e r R u n g e - K u t t a m e t h o d p r e s e n t e d l a t e r
i n t h i s c h a p t e r t h e d e r i v a t i o n m u s t b e p e r f o r m e d f o r a n o n l i n e a r O D E . H o w e v e r , t h e
a n a l y s i s b a s e d o n a l i n e a r n o n h o m o g e n e o u s O D E p r o d u c e s t h e a p p r o p r i a t e c o n d i t i o n s
f o r t h e m a j o r i t y o f t i m e - m a r c h i n g m e t h o d s u s e d i n C F D .
6 . 6 . 5 G l o b a l A c c u r a c y
I n c o n t r a s t t o t h e l o c a l e r r o r m e a s u r e s w h i c h h a v e j u s t b e e n d i s c u s s e d , w e c a n a l s o
d e n e g l o b a l e r r o r m e a s u r e s . T h e s e a r e u s e f u l w h e n w e c o m e t o t h e e v a l u a t i o n o f
t i m e - m a r c h i n g m e t h o d s f o r s p e c i c p u r p o s e s . T h i s s u b j e c t i s c o v e r e d i n C h a p t e r 8
a f t e r o u r i n t r o d u c t i o n t o s t a b i l i t y i n C h a p t e r 7 .
S u p p o s e w e w i s h t o c o m p u t e s o m e t i m e - a c c u r a t e p h e n o m e n o n o v e r a x e d i n t e r v a l
o f t i m e u s i n g a c o n s t a n t t i m e s t e p . W e r e f e r t o s u c h a c o m p u t a t i o n a s a n \ e v e n t " .
L e t T b e t h e x e d t i m e o f t h e e v e n t a n d h b e t h e c h o s e n s t e p s i z e . T h e n t h e r e q u i r e d
n u m b e r o f t i m e s t e p s , i s N , g i v e n b y t h e r e l a t i o n
T = N h
G l o b a l e r r o r i n t h e t r a n s i e n t
A n a t u r a l e x t e n s i o n o f e r
t o c o v e r t h e e r r o r i n a n e n t i r e e v e n t i s g i v e n b y
E r
e
T
; (
1
( h ) )
N
( 6 . 4 8 )
G l o b a l e r r o r i n a m p l i t u d e a n d p h a s e
I f t h e e v e n t i s p e r i o d i c , w e a r e m o r e c o n c e r n e d w i t h t h e g l o b a l e r r o r i n a m p l i t u d e a n d
p h a s e . T h e s e a r e g i v e n b y
E r
a
= 1 ;
q
(
1
)
2
r
+ (
1
)
2
i
N
( 6 . 4 9 )
a n d
E r
!
N
"
! h ; t a n
; 1
(
1
)
i
(
1
)
r
! #
= ! T ; N t a n
; 1
(
1
)
i
= (
1
)
r
] ( 6 . 5 0 )
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1 0 2 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
G l o b a l e r r o r i n t h e p a r t i c u l a r s o l u t i o n
F i n a l l y , t h e g l o b a l e r r o r i n t h e p a r t i c u l a r s o l u t i o n f o l l o w s n a t u r a l l y b y c o m p a r i n g t h e
s o l u t i o n s t o t h e O D E a n d t h e O E . I t c a n b e m e a s u r e d b y
E r
( ; )
Q
e
h
P
e
h
; 1
6 . 7 L i n e a r M u l t i s t e p M e t h o d s
I n t h e p r e v i o u s s e c t i o n s , w e h a v e d e v e l o p e d t h e f r a m e w o r k o f e r r o r a n a l y s i s f o r t i m e
a d v a n c e m e t h o d s a n d h a v e r a n d o m l y i n t r o d u c e d a f e w m e t h o d s w i t h o u t a d d r e s s i n g
m o t i v a t i o n a l , d e v e l o p m e n t a l o r d e s i g n i s s u e s . I n t h e s u b s e q u e n t s e c t i o n s , w e i n t r o d u c e
c l a s s e s o f m e t h o d s a l o n g w i t h t h e i r a s s o c i a t e d e r r o r a n a l y s i s . W e s h a l l n o t s p e n d m u c h
t i m e o n d e v e l o p m e n t o r d e s i g n o f t h e s e m e t h o d s , s i n c e m o s t o f t h e m h a v e h i s t o r i c
o r i g i n s f r o m a w i d e v a r i e t y o f d i s c i p l i n e s a n d a p p l i c a t i o n s . T h e L i n e a r M u l t i s t e p
M e t h o d s ( L M M ' s ) a r e p r o b a b l y t h e m o s t n a t u r a l e x t e n s i o n t o t i m e m a r c h i n g o f t h e
s p a c e d i e r e n c i n g s c h e m e s i n t r o d u c e d i n C h a p t e r 3 a n d c a n b e a n a l y z e d f o r a c c u r a c y
o r d e s i g n e d u s i n g t h e T a y l o r t a b l e a p p r o a c h o f S e c t i o n 3 . 4 .
6 . 7 . 1 T h e G e n e r a l F o r m u l a t i o n
W h e n a p p l i e d t o t h e n o n l i n e a r O D E
d u
d t
= u
0
= F ( u t )
a l l l i n e a r m u l t i s t e p m e t h o d s c a n b e e x p r e s s e d i n t h e g e n e r a l f o r m
1
X
k = 1 ; K
k
u
n + k
= h
1
X
k = 1 ; K
k
F
n + k
( 6 . 5 1 )
w h e r e t h e n o t a t i o n f o r F i s d e n e d i n S e c t i o n 6 . 1 . T h e m e t h o d s a r e s a i d t o b e l i n e a r
b e c a u s e t h e ' s a n d ' s a r e i n d e p e n d e n t o f u a n d n , a n d t h e y a r e s a i d t o b e K - s t e p
b e c a u s e K t i m e - l e v e l s o f d a t a a r e r e q u i r e d t o m a r c h i n g t h e s o l u t i o n o n e t i m e - s t e p , h .
T h e y a r e e x p l i c i t i f
1
= 0 a n d i m p l i c i t o t h e r w i s e .
W h e n E q . 6 . 5 1 i s a p p l i e d t o t h e r e p r e s e n t a t i v e e q u a t i o n , E q . 6 . 8 , a n d t h e r e s u l t i s
e x p r e s s e d i n o p e r a t i o n a l f o r m , o n e n d s
0
@
1
X
k = 1 ; K
k
E
k
1
A
u
n
= h
0
@
1
X
k = 1 ; K
k
E
k
1
A
( u
n
+ a e
h n
) ( 6 . 5 2 )
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1 0 4 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
T h i s l e a d s t o t h e l i n e a r s y s t e m
2
6
6
6
4
1 1 1 1
2 0 ; 2 ; 4
3 0 3 1 2
4 0 ; 4 ; 3 2
3
7
7
7
5
2
6
6
6
4
1
0
; 1
; 2
3
7
7
7
5
=
2
6
6
6
4
1
1
1
1
3
7
7
7
5
( 6 . 5 5 )
t o s o l v e f o r t h e ' s , r e s u l t i n g i n
1
= 9 = 2 4
0
= 1 9 = 2 4
; 1
= ; 5 = 2 4
; 2
= 1 = 2 4 ( 6 . 5 6 )
w h i c h p r o d u c e s a m e t h o d w h i c h i s f o u r t h - o r d e r a c c u r a t e .
4
W i t h
1
= 0 o n e o b t a i n s
2
6
4
1 1 1
0 ; 2 ; 4
0 3 1 2
3
7
5
2
6
4
0
; 1
; 2
3
7
5
=
2
6
4
1
1
1
3
7
5
( 6 . 5 7 )
g i v i n g
0
= 2 3 = 1 2
; 1
= ; 1 6 = 1 2
; 2
= 5 = 1 2 ( 6 . 5 8 )
T h i s i s t h e t h i r d - o r d e r A d a m s - B a s h f o r t h m e t h o d .
A l i s t o f s i m p l e m e t h o d s , s o m e o f w h i c h a r e v e r y c o m m o n i n C F D a p p l i c a t i o n s ,
i s g i v e n b e l o w t o g e t h e r w i t h i d e n t i f y i n g n a m e s t h a t a r e s o m e t i m e s a s s o c i a t e d w i t h
t h e m . I n t h e f o l l o w i n g m a t e r i a l A B ( n ) a n d A M ( n ) a r e u s e d a s a b b r e v i a t i o n s f o r t h e
( n ) t h o r d e r A d a m s - B a s h f o r t h a n d ( n ) t h o r d e r A d a m s - M o u l t o n m e t h o d s . O n e c a n
v e r i f y t h a t t h e A d a m s t y p e s c h e m e s g i v e n b e l o w s a t i s f y E q s . 6 . 5 5 a n d 6 . 5 7 u p t o t h e
o r d e r o f t h e m e t h o d .
E x p l i c i t M e t h o d s
u
n + 1
= u
n
+ h u
0
n
E u l e r
u
n + 1
= u
n ; 1
+ 2 h u
0
n
L e a p f r o g
u
n + 1
= u
n
+
1
2
h
h
3 u
0
n
; u
0
n ; 1
i
A B 2
u
n + 1
= u
n
+
h
1 2
h
2 3 u
0
n
; 1 6 u
0
n ; 1
+ 5 u
0
n ; 2
i
A B 3
I m p l i c i t M e t h o d s
u
n + 1
= u
n
+ h u
0
n + 1
I m p l i c i t E u l e r
u
n + 1
= u
n
+
1
2
h
h
u
0
n
+ u
0
n + 1
i
T r a p e z o i d a l ( A M 2 )
u
n + 1
=
1
3
h
4 u
n
; u
n ; 1
+ 2 h u
0
n + 1
i
2 n d - o r d e r B a c k w a r d
u
n + 1
= u
n
+
h
1 2
h
5 u
0
n + 1
+ 8 u
0
n
; u
0
n ; 1
i
A M 3
4
R e c a l l f r o m S e c t i o n 6 . 5 . 2 t h a t a k t h - o r d e r t i m e - m a r c h i n g m e t h o d h a s a l e a d i n g t r u n c a t i o n e r r o r
t e r m w h i c h i s O ( h
k + 1
) .
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6 . 7 . L I N E A R M U L T I S T E P M E T H O D S 1 0 5
6 . 7 . 3 T w o - S t e p L i n e a r M u l t i s t e p M e t h o d s
H i g h r e s o l u t i o n C F D p r o b l e m s u s u a l l y r e q u i r e v e r y l a r g e d a t a s e t s t o s t o r e t h e s p a t i a l
i n f o r m a t i o n f r o m w h i c h t h e t i m e d e r i v a t i v e i s c a l c u l a t e d . T h i s l i m i t s t h e i n t e r e s t
i n m u l t i s t e p m e t h o d s t o a b o u t t w o t i m e l e v e l s . T h e m o s t g e n e r a l t w o - s t e p l i n e a r
m u l t i s t e p m e t h o d ( i . e . , K = 2 i n E q . 6 . 5 1 ) , t h a t i s a t l e a s t r s t - o r d e r a c c u r a t e , c a n b e
w r i t t e n a s
( 1 + ) u
n + 1
= ( 1 + 2 ) u
n
; u
n ; 1
] + h
h
u
0
n + 1
+ ( 1 ; + ' ) u
0
n
; ' u
0
n ; 1
i
( 6 . 5 9 )
C l e a r l y t h e m e t h o d s a r e e x p l i c i t i f = 0 a n d i m p l i c i t o t h e r w i s e . A l i s t o f m e t h o d s
c o n t a i n e d i n E q . 6 . 5 9 i s g i v e n i n T a b l e 6 . 2 . N o t i c e t h a t t h e A d a m s m e t h o d s h a v e
= 0 , w h i c h c o r r e s p o n d s t o
; 1
= 0 i n E q . 6 . 5 1 . M e t h o d s w i t h = ; 1 = 2 , w h i c h
c o r r e s p o n d s t o
0
= 0 i n E q . 6 . 5 1 , a r e k n o w n a s M i l n e m e t h o d s .
' M e t h o d O r d e r
0 0 0 E u l e r 1
1 0 0 I m p l i c i t E u l e r 1
1 = 2 0 0 T r a p e z o i d a l o r A M 2 2
1 1 = 2 0 2 n d O r d e r B a c k w a r d 2
3 = 4 0 ; 1 = 4 A d a m s t y p e 2
1 = 3 ; 1 = 2 ; 1 = 3 L e e s T y p e 2
1 = 2 ; 1 = 2 ; 1 = 2 T w o { s t e p t r a p e z o i d a l 2
5 = 9 ; 1 = 6 ; 2 = 9 A { c o n t r a c t i v e 2
0 ; 1 = 2 0 L e a p f r o g 2
0 0 1 = 2 A B 2 2
0 ; 5 = 6 ; 1 = 3 M o s t a c c u r a t e e x p l i c i t 3
1 = 3 ; 1 = 6 0 T h i r d { o r d e r i m p l i c i t 3
5 = 1 2 0 1 = 1 2 A M 3 3
1 = 6 ; 1 = 2 ; 1 = 6 M i l n e 4
T a b l e 6 . 2 . S o m e l i n e a r o n e - a n d t w o - s t e p m e t h o d s , s e e E q . 6 . 5 9 .
O n e c a n s h o w a f t e r a l i t t l e a l g e b r a t h a t b o t h e r
a n d e r
a r e r e d u c e d t o 0 ( h
3
)
( i . e . , t h e m e t h o d s a r e 2 n d - o r d e r a c c u r a t e ) i f
' = ; +
1
2
T h e c l a s s o f a l l 3 r d - o r d e r m e t h o d s i s d e t e r m i n e d b y i m p o s i n g t h e a d d i t i o n a l c o n s t r a i n t
= 2 ;
5
6
F i n a l l y a u n i q u e f o u r t h - o r d e r m e t h o d i s f o u n d b y s e t t i n g = ; ' = ; = 3 =
1
6
.
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1 0 6 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
6 . 8 P r e d i c t o r - C o r r e c t o r M e t h o d s
T h e r e a r e a w i d e v a r i e t y o f p r e d i c t o r - c o r r e c t o r s c h e m e s c r e a t e d a n d u s e d f o r a v a r i e t y
o f p u r p o s e s . T h e i r u s e i n s o l v i n g O D E ' s i s r e l a t i v e l y e a s y t o i l l u s t r a t e a n d u n d e r s t a n d .
T h e i r u s e i n s o l v i n g P D E ' s c a n b e m u c h m o r e s u b t l e a n d d e m a n d s c o n c e p t s
5
w h i c h
h a v e n o c o u n t e r p a r t i n t h e a n a l y s i s o f O D E ' s .
P r e d i c t o r - c o r r e c t o r m e t h o d s c o n s t r u c t e d t o t i m e - m a r c h l i n e a r o r n o n l i n e a r O D E ' s
a r e c o m p o s e d o f s e q u e n c e s o f l i n e a r m u l t i s t e p m e t h o d s , e a c h o f w h i c h i s r e f e r r e d t o
a s a f a m i l y i n t h e s o l u t i o n p r o c e s s . T h e r e m a y b e m a n y f a m i l i e s i n t h e s e q u e n c e , a n d
u s u a l l y t h e n a l f a m i l y h a s a h i g h e r T a y l o r - s e r i e s o r d e r o f a c c u r a c y t h a n t h e i n t e r -
m e d i a t e o n e s . T h e i r u s e i s m o t i v a t e d b y e a s e o f a p p l i c a t i o n a n d i n c r e a s e d e c i e n c y ,
w h e r e m e a s u r e s o f e c i e n c y a r e d i s c u s s e d i n t h e n e x t t w o c h a p t e r s .
A s i m p l e o n e - p r e d i c t o r , o n e - c o r r e c t o r e x a m p l e i s g i v e n b y
~ u
n +
= u
n
+ h u
0
n
u
n + 1
= u
n
+ h
h
~ u
0
n +
+ u
0
n
i
( 6 . 6 0 )
w h e r e t h e p a r a m e t e r s a n d a r e a r b i t r a r y p a r a m e t e r s t o b e d e t e r m i n e d . O n e
c a n a n a l y z e t h i s s e q u e n c e b y a p p l y i n g i t t o t h e r e p r e s e n t a t i v e e q u a t i o n a n d u s i n g
t h e o p e r a t i o n a l t e c h n i q u e s o u t l i n e d i n S e c t i o n 6 . 4 . I t i s e a s y t o s h o w , f o l l o w i n g t h e
e x a m p l e l e a d i n g t o E q . 6 . 2 6 , t h a t
P ( E ) = E
h
E ; 1 ; ( + ) h ;
2
h
2
i
( 6 . 6 1 )
Q ( E ) = E
h E
+ + h ] ( 6 . 6 2 )
C o n s i d e r i n g o n l y l o c a l a c c u r a c y , o n e i s l e d , b y f o l l o w i n g t h e d i s c u s s i o n i n S e c t i o n 6 . 6 ,
t o t h e f o l l o w i n g o b s e r v a t i o n s . F o r t h e m e t h o d t o b e s e c o n d - o r d e r a c c u r a t e b o t h e r
a n d e r
m u s t b e O ( h
3
) . F o r t h i s t o h o l d f o r e r
, i t i s o b v i o u s f r o m E q . 6 . 6 1 t h a t
+ = 1 =
1
2
w h i c h p r o v i d e s t w o e q u a t i o n s f o r t h r e e u n k n o w n s . T h e s i t u a t i o n f o r e r
r e q u i r e s s o m e
a l g e b r a , b u t i t i s n o t d i c u l t t o s h o w u s i n g E q . 6 . 4 4 t h a t t h e s a m e c o n d i t i o n s a l s o
m a k e i t O ( h
3
) . O n e c o n c l u d e s , t h e r e f o r e , t h a t t h e p r e d i c t o r - c o r r e c t o r s e q u e n c e
~ u
n +
= u
n
+ h u
0
n
u
n + 1
= u
n
+
1
2
h
1
~ u
0
n +
+
2 ; 1
u
0
n
( 6 . 6 3 )
i s a s e c o n d - o r d e r a c c u r a t e m e t h o d f o r a n y .
5
S u c h a s a l t e r n a t i n g d i r e c t i o n , f r a c t i o n a l - s t e p , a n d h y b r i d m e t h o d s .
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6 . 9 . R U N G E - K U T T A M E T H O D S 1 0 7
A c l a s s i c a l p r e d i c t o r - c o r r e c t o r s e q u e n c e i s f o r m e d b y f o l l o w i n g a n A d a m s - B a s h f o r t h
p r e d i c t o r o f a n y o r d e r w i t h a n A d a m s - M o u l t o n c o r r e c t o r h a v i n g a n o r d e r o n e h i g h e r .
T h e o r d e r o f t h e c o m b i n a t i o n i s t h e n e q u a l t o t h e o r d e r o f t h e c o r r e c t o r . I f t h e o r d e r
o f t h e c o r r e c t o r i s ( k ) , w e r e f e r t o t h e s e a s A B M ( k ) m e t h o d s . T h e A d a m s - B a s h f o r t h -
M o u l t o n s e q u e n c e f o r k = 3 i s
~ u
n + 1
= u
n
+
1
2
h
h
3 u
0
n
; u
0
n ; 1
i
u
n + 1
= u
n
+
h
1 2
h
5 ~ u
0
n + 1
+ 8 u
0
n
; u
0
n ; 1
i
( 6 . 6 4 )
S o m e s i m p l e , s p e c i c , s e c o n d - o r d e r a c c u r a t e m e t h o d s a r e g i v e n b e l o w . T h e G a z d a g
m e t h o d , w h i c h w e d i s c u s s i n C h a p t e r 8 , i s
~ u
n + 1
= u
n
+
1
2
h
h
3 ~ u
0
n
; ~ u
0
n ; 1
i
u
n + 1
= u
n
+
1
2
h
h
~ u
0
n
+ ~ u
0
n + 1
i
( 6 . 6 5 )
T h e B u r s t e i n m e t h o d , o b t a i n e d f r o m E q . 6 . 6 3 w i t h = 1 = 2 i s
~ u
n + 1 = 2
= u
n
+
1
2
h u
0
n
u
n + 1
= u
n
+ h ~ u
0
n + 1 = 2
( 6 . 6 6 )
a n d , n a l l y , M a c C o r m a c k ' s m e t h o d , p r e s e n t e d e a r l i e r i n t h i s c h a p t e r , i s
~ u
n + 1
= u
n
+ h u
0
n
u
n + 1
=
1
2
u
n
+ ~ u
n + 1
+ h ~ u
0
n + 1
] ( 6 . 6 7 )
N o t e t h a t M a c C o r m a c k ' s m e t h o d c a n a l s o b e w r i t t e n a s
~ u
n + 1
= u
n
+ h u
0
n
u
n + 1
= u
n
+
1
2
h u
0
n
+ ~ u
0
n + 1
] ( 6 . 6 8 )
f r o m w h i c h i t i s c l e a r t h a t i t i s o b t a i n e d f r o m E q . 6 . 6 3 w i t h = 1 .
6 . 9 R u n g e - K u t t a M e t h o d s
T h e r e i s a s p e c i a l s u b s e t o f p r e d i c t o r - c o r r e c t o r m e t h o d s , r e f e r r e d t o a s R u n g e - K u t t a
m e t h o d s ,
6
t h a t p r o d u c e j u s t o n e - r o o t f o r e a c h - r o o t s u c h t h a t ( h ) c o r r e s p o n d s
6
A l t h o u g h i m p l i c i t a n d m u l t i - s t e p R u n g e - K u t t a m e t h o d s e x i s t , w e w i l l c o n s i d e r o n l y s i n g l e - s t e p ,
e x p l i c i t R u n g e - K u t t a m e t h o d s h e r e .
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1 0 8 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
t o t h e T a y l o r s e r i e s e x p a n s i o n o f e
h
o u t t h r o u g h t h e o r d e r o f t h e m e t h o d a n d t h e n
t r u n c a t e s . T h u s f o r a R u n g e - K u t t a m e t h o d o f o r d e r k ( u p t o 4 t h o r d e r ) , t h e p r i n c i p a l
( a n d o n l y ) - r o o t i s g i v e n b y
= 1 + h +
1
2
2
h
2
+ +
1
k !
k
h
k
( 6 . 6 9 )
I t i s n o t p a r t i c u l a r l y d i c u l t t o b u i l d t h i s p r o p e r t y i n t o a m e t h o d , b u t , a s w e p o i n t e d
o u t i n S e c t i o n 6 . 6 . 4 , i t i s n o t s u c i e n t t o g u a r a n t e e k ' t h o r d e r a c c u r a c y f o r t h e s o l u t i o n
o f u
0
= F ( u t ) o r f o r t h e r e p r e s e n t a t i v e e q u a t i o n . T o e n s u r e k ' t h o r d e r a c c u r a c y , t h e
m e t h o d m u s t f u r t h e r s a t i s f y t h e c o n s t r a i n t t h a t
e r
= O ( h
k + 1
) ( 6 . 7 0 )
a n d t h i s i s m u c h m o r e d i c u l t .
T h e m o s t w i d e l y p u b l i c i z e d R u n g e - K u t t a p r o c e s s i s t h e o n e t h a t l e a d s t o t h e
f o u r t h - o r d e r m e t h o d . W e p r e s e n t i t b e l o w i n s o m e d e t a i l . I t i s u s u a l l y i n t r o d u c e d i n
t h e f o r m
k
1
= h F ( u
n
t
n
)
k
2
= h F ( u
n
+ k
1
t
n
+ h )
k
3
= h F ( u
n
+
1
k
1
+
1
k
2
t
n
+
1
h )
k
4
= h F ( u
n
+
2
k
1
+
2
k
2
+
2
k
3
t
n
+
2
h )
f o l l o w e d b y
u ( t
n
+ h ) ; u ( t
n
) =
1
k
1
+
2
k
2
+
3
k
3
+
4
k
4
( 6 . 7 1 )
H o w e v e r , w e p r e f e r t o p r e s e n t i t u s i n g p r e d i c t o r - c o r r e c t o r n o t a t i o n . T h u s , a s c h e m e
e n t i r e l y e q u i v a l e n t t o 6 . 7 1 i s
b
u
n +
= u
n
+ h u
0
n
~ u
n +
1
= u
n
+
1
h u
0
n
+
1
h
b
u
0
n +
u
n +
2
= u
n
+
2
h u
0
n
+
2
h
b
u
0
n +
+
2
h ~ u
0
n +
1
u
n + 1
= u
n
+
1
h u
0
n
+
2
h
b
u
0
n +
+
3
h ~ u
0
n +
1
+
4
h u
0
n +
2
( 6 . 7 2 )
A p p e a r i n g i n E q s . 6 . 7 1 a n d 6 . 7 2 a r e a t o t a l o f 1 3 p a r a m e t e r s w h i c h a r e t o b e
d e t e r m i n e d s u c h t h a t t h e m e t h o d i s f o u r t h - o r d e r a c c o r d i n g t o t h e r e q u i r e m e n t s i n
E q s . 6 . 6 9 a n d 6 . 7 0 . F i r s t o f a l l , t h e c h o i c e s f o r t h e t i m e s a m p l i n g s , ,
1
, a n d
2
, a r e
n o t a r b i t r a r y . T h e y m u s t s a t i s f y t h e r e l a t i o n s
=
1
=
1
+
1
2
=
2
+
2
+
2
( 6 . 7 3 )
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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6 . 9 . R U N G E - K U T T A M E T H O D S 1 0 9
T h e a l g e b r a i n v o l v e d i n n d i n g a l g e b r a i c e q u a t i o n s f o r t h e r e m a i n i n g 1 0 p a r a m e t e r s
i s n o t t r i v i a l , b u t t h e e q u a t i o n s f o l l o w d i r e c t l y f r o m n d i n g P ( E ) a n d Q ( E ) a n d t h e n
s a t i s f y i n g t h e c o n d i t i o n s i n E q s . 6 . 6 9 a n d 6 . 7 0 . U s i n g E q . 6 . 7 3 t o e l i m i n a t e t h e ' s
w e n d f r o m E q . 6 . 6 9 t h e f o u r c o n d i t i o n s
1
+
2
+
3
+
4
= 1 ( 1 )
2
+
3
1
+
4
2
= 1 = 2 ( 2 )
3
1
+
4
(
2
+
1
2
) = 1 = 6 ( 3 )
4
1
2
= 1 = 2 4 ( 4 )
( 6 . 7 4 )
T h e s e f o u r r e l a t i o n s g u a r a n t e e t h a t t h e v e t e r m s i n e x a c t l y m a t c h t h e r s t 5 t e r m s
i n t h e e x p a n s i o n o f e
h
. T o s a t i s f y t h e c o n d i t i o n t h a t e r
= O ( k
5
) , w e h a v e t o f u l l l
f o u r m o r e c o n d i t i o n s
2
2
+
3
2
1
+
4
2
2
= 1 = 3 ( 3 )
2
3
+
3
3
1
+
4
3
2
= 1 = 4 ( 4 )
3
2
1
+
4
(
2
2
+
2
1
2
) = 1 = 1 2 ( 4 )
3
1
1
+
4
2
(
2
+
1
2
) = 1 = 8 ( 4 )
( 6 . 7 5 )
T h e n u m b e r i n p a r e n t h e s e s a t t h e e n d o f e a c h e q u a t i o n i n d i c a t e s t h e o r d e r t h a t
i s t h e b a s i s f o r t h e e q u a t i o n . T h u s i f t h e r s t 3 e q u a t i o n s i n 6 . 7 4 a n d t h e r s t
e q u a t i o n i n 6 . 7 5 a r e a l l s a t i s e d , t h e r e s u l t i n g m e t h o d w o u l d b e t h i r d - o r d e r a c c u r a t e .
A s d i s c u s s e d i n S e c t i o n 6 . 6 . 4 , t h e f o u r t h c o n d i t i o n i n E q . 6 . 7 5 c a n n o t b e d e r i v e d
u s i n g t h e m e t h o d o l o g y p r e s e n t e d h e r e , w h i c h i s b a s e d o n a l i n e a r n o n h o m o g e n o u s
r e p r e s e n t a t i v e O D E . A m o r e g e n e r a l d e r i v a t i o n b a s e d o n a n o n l i n e a r O D E c a n b e
f o u n d i n s e v e r a l b o o k s .
7
T h e r e a r e e i g h t e q u a t i o n s i n 6 . 7 4 a n d 6 . 7 5 w h i c h m u s t b e s a t i s e d b y t h e 1 0
u n k n o w n s . S i n c e t h e e q u a t i o n s a r e o v e r d e t e r m i n e d , t w o p a r a m e t e r s c a n b e s e t a r b i -
t r a r i l y . S e v e r a l c h o i c e s f o r t h e p a r a m e t e r s h a v e b e e n p r o p o s e d , b u t t h e m o s t p o p u l a r
o n e i s d u e t o R u n g e . I t r e s u l t s i n t h e \ s t a n d a r d " f o u r t h - o r d e r R u n g e - K u t t a m e t h o d
e x p r e s s e d i n p r e d i c t o r - c o r r e c t o r f o r m a s
b
u
n + 1 = 2
= u
n
+
1
2
h u
0
n
~ u
n + 1 = 2
= u
n
+
1
2
h
b
u
0
n + 1 = 2
u
n + 1
= u
n
+ h ~ u
0
n + 1 = 2
u
n + 1
= u
n
+
1
6
h
h
u
0
n
+ 2
b
u
0
n + 1 = 2
+ ~ u
0
n + 1 = 2
+ u
0
n + 1
i
( 6 . 7 6 )
7
T h e p r e s e n t a p p r o a c h b a s e d o n a l i n e a r i n h o m o g e n e o u s e q u a t i o n p r o v i d e s a l l o f t h e n e c e s s a r y
c o n d i t i o n s f o r R u n g e - K u t t a m e t h o d s o f u p t o t h i r d o r d e r .
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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1 1 0 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
N o t i c e t h a t t h i s r e p r e s e n t s t h e s i m p l e s e q u e n c e o f c o n v e n t i o n a l l i n e a r m u l t i s t e p m e t h -
o d s r e f e r r e d t o , r e s p e c t i v e l y , a s
E u l e r P r e d i c t o r
E u l e r C o r r e c t o r
L e a p f r o g P r e d i c t o r
M i l n e C o r r e c t o r
9
>
>
>
=
>
>
>
R K 4
O n e c a n e a s i l y s h o w t h a t b o t h t h e B u r s t e i n a n d t h e M a c C o r m a c k m e t h o d s g i v e n b y
E q s . 6 . 6 6 a n d 6 . 6 7 a r e s e c o n d - o r d e r R u n g e - K u t t a m e t h o d s , a n d t h i r d - o r d e r m e t h o d s
c a n b e d e r i v e d f r o m E q s . 6 . 7 2 b y s e t t i n g
4
= 0 a n d s a t i s f y i n g o n l y E q s . 6 . 7 4 a n d t h e
r s t e q u a t i o n i n 6 . 7 5 . I t i s c l e a r t h a t f o r o r d e r s o n e t h r o u g h f o u r , R K m e t h o d s o f o r d e r
k r e q u i r e k e v a l u a t i o n s o f t h e d e r i v a t i v e f u n c t i o n t o a d v a n c e t h e s o l u t i o n o n e t i m e
s t e p . W e s h a l l d i s c u s s t h e c o n s e q u e n c e s o f t h i s i n C h a p t e r 8 . H i g h e r - o r d e r R u n g e -
K u t t a m e t h o d s c a n b e d e v e l o p e d , b u t t h e y r e q u i r e m o r e d e r i v a t i v e e v a l u a t i o n s t h a n
t h e i r o r d e r . F o r e x a m p l e , a f t h - o r d e r m e t h o d r e q u i r e s s i x e v a l u a t i o n s t o a d v a n c e
t h e s o l u t i o n o n e s t e p . I n a n y e v e n t , s t o r a g e r e q u i r e m e n t s r e d u c e t h e u s e f u l n e s s o f
R u n g e - K u t t a m e t h o d s o f o r d e r h i g h e r t h a n f o u r f o r C F D a p p l i c a t i o n s .
6 . 1 0 I m p l e m e n t a t i o n o f I m p l i c i t M e t h o d s
W e h a v e p r e s e n t e d a w i d e v a r i e t y o f t i m e - m a r c h i n g m e t h o d s a n d s h o w n h o w t o d e r i v e
t h e i r ; r e l a t i o n s . I n t h e n e x t c h a p t e r , w e w i l l s e e t h a t t h e s e m e t h o d s c a n h a v e
w i d e l y d i e r e n t p r o p e r t i e s w i t h r e s p e c t t o s t a b i l i t y . T h i s l e a d s t o v a r i o u s t r a d e -
o s w h i c h m u s t b e c o n s i d e r e d i n s e l e c t i n g a m e t h o d f o r a s p e c i c a p p l i c a t i o n . O u r
p r e s e n t a t i o n o f t h e t i m e - m a r c h i n g m e t h o d s i n t h e c o n t e x t o f a l i n e a r s c a l a r e q u a t i o n
o b s c u r e s s o m e o f t h e i s s u e s i n v o l v e d i n i m p l e m e n t i n g a n i m p l i c i t m e t h o d f o r s y s t e m s
o f e q u a t i o n s a n d n o n l i n e a r e q u a t i o n s . T h e s e a r e c o v e r e d i n t h i s S e c t i o n .
6 . 1 0 . 1 A p p l i c a t i o n t o S y s t e m s o f E q u a t i o n s
C o n s i d e r r s t t h e n u m e r i c a l s o l u t i o n o f o u r r e p r e s e n t a t i v e O D E
u
0
= u + a e
t
( 6 . 7 7 )
u s i n g t h e i m p l i c i t E u l e r m e t h o d . F o l l o w i n g t h e s t e p s o u t l i n e d i n S e c t i o n 6 . 2 , w e
o b t a i n e d
( 1 ; h ) u
n + 1
; u
n
= h e
h
a e
h n
( 6 . 7 8 )
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6 . 1 0 . I M P L E M E N T A T I O N O F I M P L I C I T M E T H O D S 1 1 1
S o l v i n g f o r u
n + 1
g i v e s
u
n + 1
=
1
1 ; h
( u
n
+ h e
h
a e
h n
) ( 6 . 7 9 )
T h i s c a l c u l a t i o n d o e s n o t s e e m p a r t i c u l a r l y o n e r o u s i n c o m p a r i s o n w i t h t h e a p p l i c a -
t i o n o f a n e x p l i c i t m e t h o d t o t h i s O D E , r e q u i r i n g o n l y a n a d d i t i o n a l d i v i s i o n .
N o w l e t u s a p p l y t h e i m p l i c i t E u l e r m e t h o d t o o u r g e n e r i c s y s t e m o f e q u a t i o n s
g i v e n b y
~u
0
= A ~u ;
~
f ( t ) ( 6 . 8 0 )
w h e r e ~ u a n d
~
f a r e v e c t o r s a n d w e s t i l l a s s u m e t h a t A i s n o t a f u n c t i o n o f ~u o r t . N o w
t h e e q u i v a l e n t t o E q . 6 . 7 8 i s
( I ; h A ) ~u
n + 1
; ~u
n
= ; h
~
f ( t + h ) ( 6 . 8 1 )
o r
~u
n + 1
= ( I ; h A )
; 1
~u
n
; h
~
f ( t + h ) ] ( 6 . 8 2 )
T h e i n v e r s e i s n o t a c t u a l l y p e r f o r m e d , b u t r a t h e r w e s o l v e E q . 6 . 8 1 a s a l i n e a r s y s t e m
o f e q u a t i o n s . F o r o u r o n e - d i m e n s i o n a l e x a m p l e s , t h e s y s t e m o f e q u a t i o n s w h i c h m u s t
b e s o l v e d i s t r i d i a g o n a l ( e . g . , f o r b i c o n v e c t i o n , A = ; a B
p
( ; 1 0 1 ) = 2 x ) , a n d h e n c e
i t s s o l u t i o n i s i n e x p e n s i v e , b u t i n m u l t i d i m e n s i o n s t h e b a n d w i d t h c a n b e v e r y l a r g e . I n
g e n e r a l , t h e c o s t p e r t i m e s t e p o f a n i m p l i c i t m e t h o d i s l a r g e r t h a n t h a t o f a n e x p l i c i t
m e t h o d . T h e p r i m a r y a r e a o f a p p l i c a t i o n o f i m p l i c i t m e t h o d s i s i n t h e s o l u t i o n o f
s t i O D E ' s , a s w e s h a l l s e e i n C h a p t e r 8 .
6 . 1 0 . 2 A p p l i c a t i o n t o N o n l i n e a r E q u a t i o n s
N o w c o n s i d e r t h e g e n e r a l n o n l i n e a r s c a l a r O D E g i v e n b y
d u
d t
= F ( u t ) ( 6 . 8 3 )
A p p l i c a t i o n o f t h e i m p l i c i t E u l e r m e t h o d g i v e s
u
n + 1
= u
n
+ h F ( u
n + 1
t
n + 1
) ( 6 . 8 4 )
T h i s i s a n o n l i n e a r d i e r e n c e e q u a t i o n . A s a n e x a m p l e , c o n s i d e r t h e n o n l i n e a r O D E
d u
d t
+
1
2
u
2
= 0 ( 6 . 8 5 )
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1 1 2 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
s o l v e d u s i n g i m p l i c i t E u l e r t i m e m a r c h i n g , w h i c h g i v e s
u
n + 1
+ h
1
2
u
2
n + 1
= u
n
( 6 . 8 6 )
w h i c h r e q u i r e s a n o n t r i v i a l m e t h o d t o s o l v e f o r u
n + 1
. T h e r e a r e s e v e r a l d i e r e n t
a p p r o a c h e s o n e c a n t a k e t o s o l v i n g t h i s n o n l i n e a r d i e r e n c e e q u a t i o n . A n i t e r a t i v e
m e t h o d , s u c h a s N e w t o n ' s m e t h o d ( s e e b e l o w ) , c a n b e u s e d . I n p r a c t i c e , t h e \ i n i t i a l
g u e s s " f o r t h i s n o n l i n e a r p r o b l e m c a n b e q u i t e c l o s e t o t h e s o l u t i o n , s i n c e t h e \ i n i t i a l
g u e s s " i s s i m p l y t h e s o l u t i o n a t t h e p r e v i o u s t i m e s t e p , w h i c h i m p l i e s t h a t a l i n e a r i z a -
t i o n a p p r o a c h m a y b e q u i t e s u c c e s s f u l . S u c h a n a p p r o a c h i s d e s c r i b e d i n t h e n e x t
S e c t i o n .
6 . 1 0 . 3 L o c a l L i n e a r i z a t i o n f o r S c a l a r E q u a t i o n s
G e n e r a l D e v e l o p m e n t
L e t u s s t a r t t h e p r o c e s s o f l o c a l l i n e a r i z a t i o n b y c o n s i d e r i n g E q . 6 . 8 3 . I n o r d e r t o
i m p l e m e n t t h e l i n e a r i z a t i o n , w e e x p a n d F ( u t ) a b o u t s o m e r e f e r e n c e p o i n t i n t i m e .
D e s i g n a t e t h e r e f e r e n c e v a l u e b y t
n
a n d t h e c o r r e s p o n d i n g v a l u e o f t h e d e p e n d e n t
v a r i a b l e b y u
n
. A T a y l o r s e r i e s e x p a n s i o n a b o u t t h e s e r e f e r e n c e q u a n t i t i e s g i v e s
F ( u t ) = F ( u
n
t
n
) +
@ F
@ u
!
n
( u ; u
n
) +
@ F
@ t
!
n
( t ; t
n
)
+
1
2
@
2
F
@ u
2
!
n
( u ; u
n
)
2
+
@
2
F
@ u @ t
!
n
( u ; u
n
) ( t ; t
n
)
+
1
2
@
2
F
@ t
2
!
n
( t ; t
n
)
2
+ ( 6 . 8 7 )
O n t h e o t h e r h a n d , t h e e x p a n s i o n o f u ( t ) i n t e r m s o f t h e i n d e p e n d e n t v a r i a b l e t i s
u ( t ) = u
n
+ ( t ; t
n
)
@ u
@ t
!
n
+
1
2
( t ; t
n
)
2
@
2
u
@ t
2
!
n
+ ( 6 . 8 8 )
I f t i s w i t h i n h o f t
n
, b o t h ( t ; t
n
)
k
a n d ( u ; u
n
)
k
a r e O ( h
k
) , a n d E q . 6 . 8 7 c a n b e
w r i t t e n
F ( u t ) = F
n
+
@ F
@ u
!
n
( u ; u
n
) +
@ F
@ t
!
n
( t ; t
n
) + O ( h
2
) ( 6 . 8 9 )
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6 . 1 0 . I M P L E M E N T A T I O N O F I M P L I C I T M E T H O D S 1 1 3
N o t i c e t h a t t h i s i s a n e x p a n s i o n o f t h e d e r i v a t i v e o f t h e f u n c t i o n . T h u s , r e l a t i v e t o t h e
o r d e r o f e x p a n s i o n o f t h e f u n c t i o n , i t r e p r e s e n t s a s e c o n d - o r d e r - a c c u r a t e , l o c a l l y - l i n e a r
a p p r o x i m a t i o n t o F ( u t ) t h a t i s v a l i d i n t h e v i c i n i t y o f t h e r e f e r e n c e s t a t i o n t
n
a n d
t h e c o r r e s p o n d i n g u
n
= u ( t
n
) . W i t h t h i s w e o b t a i n t h e l o c a l l y ( i n t h e n e i g h b o r h o o d
o f t
n
) t i m e - l i n e a r r e p r e s e n t a t i o n o f E q . 6 . 8 3 , n a m e l y
d u
d t
=
@ F
@ u
!
n
u +
F
n
;
@ F
@ u
!
n
u
n
!
+
@ F
@ t
!
n
( t ; t
n
) + O ( h
2
) ( 6 . 9 0 )
I m p l e m e n t a t i o n o f t h e T r a p e z o i d a l M e t h o d
A s a n e x a m p l e o f h o w s u c h a n e x p a n s i o n c a n b e u s e d , c o n s i d e r t h e m e c h a n i c s o f
a p p l y i n g t h e t r a p e z o i d a l m e t h o d f o r t h e t i m e i n t e g r a t i o n o f E q . 6 . 8 3 . T h e t r a p e z o i d a l
m e t h o d i s g i v e n b y
u
n + 1
= u
n
+
1
2
h F
n + 1
+ F
n
] + h O ( h
2
) ( 6 . 9 1 )
w h e r e w e w r i t e h O ( h
2
) t o e m p h a s i z e t h a t t h e m e t h o d i s s e c o n d o r d e r a c c u r a t e . U s i n g
E q . 6 . 8 9 t o e v a l u a t e F
n + 1
= F ( u
n + 1
t
n + 1
) , o n e n d s
u
n + 1
= u
n
+
1
2
h
"
F
n
+
@ F
@ u
!
n
( u
n + 1
; u
n
) + h
@ F
@ t
!
n
+ O ( h
2
) + F
n
#
+ h O ( h
2
) ( 6 . 9 2 )
N o t e t h a t t h e O ( h
2
) t e r m w i t h i n t h e b r a c k e t s ( w h i c h i s d u e t o t h e l o c a l l i n e a r i z a t i o n )
i s m u l t i p l i e d b y h a n d t h e r e f o r e i s t h e s a m e o r d e r a s t h e h O ( h
2
) e r r o r f r o m t h e
T r a p e z o i d a l M e t h o d . T h e u s e o f l o c a l t i m e l i n e a r i z a t i o n u p d a t e d a t t h e e n d o f e a c h
t i m e s t e p , a n d t h e t r a p e z o i d a l t i m e m a r c h , c o m b i n e t o m a k e a s e c o n d - o r d e r - a c c u r a t e
n u m e r i c a l i n t e g r a t i o n p r o c e s s . T h e r e a r e , o f c o u r s e , o t h e r s e c o n d - o r d e r i m p l i c i t t i m e -
m a r c h i n g m e t h o d s t h a t c a n b e u s e d . T h e i m p o r t a n t p o i n t t o b e m a d e h e r e i s t h a t
l o c a l l i n e a r i z a t i o n u p d a t e d a t e a c h t i m e s t e p h a s n o t r e d u c e d t h e o r d e r o f a c c u r a c y o f
a s e c o n d - o r d e r t i m e - m a r c h i n g p r o c e s s .
A v e r y u s e f u l r e o r d e r i n g o f t h e t e r m s i n E q . 6 . 9 2 r e s u l t s i n t h e e x p r e s s i o n
"
1 ;
1
2
h
@ F
@ u
!
n
#
u
n
= h F
n
+
1
2
h
2
@ F
@ t
!
n
( 6 . 9 3 )
w h i c h i s n o w i n t h e d e l t a f o r m w h i c h w i l l b e f o r m a l l y i n t r o d u c e d i n S e c t i o n 1 2 . 6 . I n
m a n y u i d m e c h a n i c a p p l i c a t i o n s t h e n o n l i n e a r f u n c t i o n F i s n o t a n e x p l i c i t f u n c t i o n
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1 1 4 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
o f t . I n s u c h c a s e s t h e p a r t i a l d e r i v a t i v e o f F ( u ) w i t h r e s p e c t t o t i s z e r o a n d E q . 6 . 9 3
s i m p l i e s t o t h e s e c o n d - o r d e r a c c u r a t e e x p r e s s i o n
"
1 ;
1
2
h
@ F
@ u
!
n
#
u
n
= h F
n
( 6 . 9 4 )
N o t i c e t h a t t h e R H S i s e x t r e m e l y s i m p l e . I t i s t h e p r o d u c t o f h a n d t h e R H S o f
t h e b a s i c e q u a t i o n e v a l u a t e d a t t h e p r e v i o u s t i m e s t e p . I n t h i s e x a m p l e , t h e b a s i c
e q u a t i o n w a s t h e s i m p l e s c a l a r e q u a t i o n 6 . 8 3 , b u t f o r o u r a p p l i c a t i o n s , i t i s g e n e r a l l y
t h e s p a c e - d i e r e n c e d f o r m o f t h e s t e a d y - s t a t e e q u a t i o n o f s o m e u i d o w p r o b l e m .
A n u m e r i c a l t i m e - m a r c h i n g p r o c e d u r e u s i n g E q . 6 . 9 4 i s u s u a l l y i m p l e m e n t e d a s
f o l l o w s :
1 . S o l v e f o r t h e e l e m e n t s o f h
~
F
n
, s t o r e t h e m i n a n a r r a y s a y
~
R , a n d s a v e ~u
n
.
2 . S o l v e f o r t h e e l e m e n t s o f t h e m a t r i x m u l t i p l y i n g ~u
n
a n d s t o r e i n s o m e a p p r o -
p r i a t e m a n n e r m a k i n g u s e o f s p a r s e n e s s o r b a n d e d n e s s o f t h e m a t r i x i f p o s s i b l e .
L e t t h i s s t o r a g e a r e a b e r e f e r r e d t o a s B .
3 . S o l v e t h e c o u p l e d s e t o f l i n e a r e q u a t i o n s
B ~u
n
=
~
R
f o r ~u
n
. ( V e r y s e l d o m d o e s o n e n d B
; 1
i n c a r r y i n g o u t t h i s s t e p ) .
4 . F i n d ~u
n + 1
b y a d d i n g ~u
n
t o ~u
n
, t h u s
~u
n + 1
= ~u
n
+ ~u
n
T h e s o l u t i o n f o r ~u
n + 1
i s g e n e r a l l y s t o r e d s u c h t h a t i t o v e r w r i t e s t h e v a l u e o f ~u
n
a n d t h e p r o c e s s i s r e p e a t e d .
I m p l e m e n t a t i o n o f t h e I m p l i c i t E u l e r M e t h o d
W e h a v e s e e n t h a t t h e r s t - o r d e r i m p l i c i t E u l e r m e t h o d c a n b e w r i t t e n
u
n + 1
= u
n
+ h F
n + 1
( 6 . 9 5 )
i f w e i n t r o d u c e E q . 6 . 9 0 i n t o t h i s m e t h o d , r e a r r a n g e t e r m s , a n d r e m o v e t h e e x p l i c i t
d e p e n d e n c e o n t i m e , w e a r r i v e a t t h e f o r m
"
1 ; h
@ F
@ u
!
n
#
u
n
= h F
n
( 6 . 9 6 )
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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6 . 1 0 . I M P L E M E N T A T I O N O F I M P L I C I T M E T H O D S 1 1 5
W e s e e t h a t t h e o n l y d i e r e n c e b e t w e e n t h e i m p l e m e n t a t i o n o f t h e t r a p e z o i d a l m e t h o d
a n d t h e i m p l i c i t E u l e r m e t h o d i s t h e f a c t o r o f
1
2
i n t h e b r a c k e t s o f t h e l e f t s i d e o f
E q s . 6 . 9 4 a n d 6 . 9 6 . O m i s s i o n o f t h i s f a c t o r d e g r a d e s t h e m e t h o d i n t i m e a c c u r a c y b y
o n e o r d e r o f h . W e s h a l l s e e l a t e r t h a t t h i s m e t h o d i s a n e x c e l l e n t c h o i c e f o r s t e a d y
p r o b l e m s .
N e w t o n ' s M e t h o d
C o n s i d e r t h e l i m i t h ! 1 o f E q . 6 . 9 6 o b t a i n e d b y d i v i d i n g b o t h s i d e s b y h a n d
s e t t i n g 1 = h = 0 . T h e r e r e s u l t s
;
@ F
@ u
!
n
u
n
= F
n
( 6 . 9 7 )
o r
u
n + 1
= u
n
;
"
@ F
@ u
!
n
#
; 1
F
n
( 6 . 9 8 )
T h i s i s t h e w e l l - k n o w n N e w t o n m e t h o d f o r n d i n g t h e r o o t s o f a n o n l i n e a r e q u a t i o n
F ( u ) = 0 . T h e f a c t t h a t i t h a s q u a d r a t i c c o n v e r g e n c e i s v e r i e d b y a g l a n c e a t E q s .
6 . 8 7 a n d 6 . 8 8 ( r e m e m b e r t h e d e p e n d e n c e o n t h a s b e e n e l i m i n a t e d f o r t h i s c a s e ) . B y
q u a d r a t i c c o n v e r g e n c e , w e m e a n t h a t t h e e r r o r a f t e r a g i v e n i t e r a t i o n i s p r o p o r t i o n a l
t o t h e s q u a r e o f t h e e r r o r a t t h e p r e v i o u s i t e r a t i o n , w h e r e t h e e r r o r i s t h e d i e r e n c e
b e t w e e n t h e c u r r e n t s o l u t i o n a n d t h e c o n v e r g e d s o l u t i o n . Q u a d r a t i c c o n v e r g e n c e i s
t h u s a v e r y p o w e r f u l p r o p e r t y . U s e o f a n i t e v a l u e o f h i n E q . 6 . 9 6 l e a d s t o l i n e a r
c o n v e r g e n c e , i . e . , t h e e r r o r a t a g i v e n i t e r a t i o n i s s o m e m u l t i p l e o f t h e e r r o r a t t h e
p r e v i o u s i t e r a t i o n . T h e r e a d e r s h o u l d p o n d e r t h e m e a n i n g o f l e t t i n g h ! 1 f o r t h e
t r a p e z o i d a l m e t h o d , g i v e n b y E q . 6 . 9 4 .
6 . 1 0 . 4 L o c a l L i n e a r i z a t i o n f o r C o u p l e d S e t s o f N o n l i n e a r E q u a -
t i o n s
I n o r d e r t o p r e s e n t t h i s c o n c e p t , l e t u s c o n s i d e r a n e x a m p l e i n v o l v i n g s o m e s i m -
p l e b o u n d a r y - l a y e r e q u a t i o n s . W e c h o o s e t h e F a l k n e r - S k a n e q u a t i o n s f r o m c l a s s i c a l
b o u n d a r y - l a y e r t h e o r y . O u r t a s k i s t o a p p l y t h e i m p l i c i t t r a p e z o i d a l m e t h o d t o t h e
e q u a t i o n s
d
3
f
d t
3
+ f
d
2
f
d t
2
+
0
@
1 ;
d f
d t
!
2
1
A
= 0 ( 6 . 9 9 )
H e r e f r e p r e s e n t s a d i m e n s i o n l e s s s t r e a m f u n c t i o n , a n d i s a s c a l i n g f a c t o r .
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1 1 6 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
F i r s t o f a l l w e r e d u c e E q . 6 . 9 9 t o a s e t o f r s t - o r d e r n o n l i n e a r e q u a t i o n s b y t h e
t r a n s f o r m a t i o n s
u
1
=
d
2
f
d t
2
u
2
=
d f
d t
u
3
= f ( 6 . 1 0 0 )
T h i s g i v e s t h e c o u p l e d s e t o f t h r e e n o n l i n e a r e q u a t i o n s
u
0
1
= F
1
= ; u
1
u
3
;
1 ; u
2
2
u
0
2
= F
2
= u
1
u
0
3
= F
3
= u
2
( 6 . 1 0 1 )
a n d t h e s e c a n b e r e p r e s e n t e d i n v e c t o r n o t a t i o n a s
d
~
u
d t
=
~
F (
~
u ) ( 6 . 1 0 2 )
N o w w e s e e k t o m a k e t h e s a m e l o c a l e x p a n s i o n t h a t d e r i v e d E q . 6 . 9 0 , e x c e p t t h a t
t h i s t i m e w e a r e f a c e d w i t h a n o n l i n e a r v e c t o r f u n c t i o n , r a t h e r t h a n a s i m p l e n o n l i n e a r
s c a l a r f u n c t i o n . T h e r e q u i r e d e x t e n s i o n r e q u i r e s t h e e v a l u a t i o n o f a m a t r i x , c a l l e d
t h e J a c o b i a n m a t r i x .
8
L e t u s r e f e r t o t h i s m a t r i x a s A . I t i s d e r i v e d f r o m E q . 6 . 1 0 2
b y t h e f o l l o w i n g p r o c e s s
A = ( a
i j
) = @ F
i
= @ u
j
( 6 . 1 0 3 )
F o r t h e g e n e r a l c a s e i n v o l v i n g a t h i r d o r d e r m a t r i x t h i s i s
A =
2
6
6
6
6
6
6
6
6
6
4
@ F
1
@ u
1
@ F
1
@ u
2
@ F
1
@ u
3
@ F
2
@ u
1
@ F
2
@ u
2
@ F
2
@ u
3
@ F
3
@ u
1
@ F
3
@ u
2
@ F
3
@ u
3
3
7
7
7
7
7
7
7
7
7
5
( 6 . 1 0 4 )
T h e e x p a n s i o n o f
~
F (
~
u ) a b o u t s o m e r e f e r e n c e s t a t e
~
u
n
c a n b e e x p r e s s e d i n a w a y
s i m i l a r t o t h e s c a l a r e x p a n s i o n g i v e n b y e q 6 . 8 7 . O m i t t i n g t h e e x p l i c i t d e p e n d e n c y
o n t h e i n d e p e n d e n t v a r i a b l e t , a n d d e n i n g
~
F
n
a s
~
F (
~
u
n
) , o n e h a s
9
8
R e c a l l t h a t w e d e r i v e d t h e J a c o b i a n m a t r i c e s f o r t h e t w o - d i m e n s i o n a l E u l e r e q u a t i o n s i n S e c t i o n
2 . 2
9
T h e T a y l o r s e r i e s e x p a n s i o n o f a v e c t o r c o n t a i n s a v e c t o r f o r t h e r s t t e r m , a m a t r i x t i m e s a
v e c t o r f o r t h e s e c o n d t e r m , a n d t e n s o r p r o d u c t s f o r t h e t e r m s o f h i g h e r o r d e r .
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6 . 1 1 . P R O B L E M S 1 1 7
~
F (
~
u ) =
~
F
n
+ A
n
~
u ;
~
u
n
+ O ( h
2
) ( 6 . 1 0 5 )
w h e r e t ; t
n
a n d t h e a r g u m e n t f o r O ( h
2
) i s t h e s a m e a s i n t h e d e r i v a t i o n o f E q . 6 . 8 8 .
U s i n g t h i s w e c a n w r i t e t h e l o c a l l i n e a r i z a t i o n o f E q . 6 . 1 0 2 a s
d
~
u
d t
= A
n
~
u +
~
F
n
; A
n
~
u
n
| { z }
\ c o n s t a n t "
+ O ( h
2
) ( 6 . 1 0 6 )
w h i c h i s a l o c a l l y - l i n e a r , s e c o n d - o r d e r - a c c u r a t e a p p r o x i m a t i o n t o a s e t o f c o u p l e d
n o n l i n e a r o r d i n a r y d i e r e n t i a l e q u a t i o n s t h a t i s v a l i d f o r t t
n
+ h . A n y r s t - o r
s e c o n d - o r d e r t i m e - m a r c h i n g m e t h o d , e x p l i c i t o r i m p l i c i t , c o u l d b e u s e d t o i n t e g r a t e
t h e e q u a t i o n s w i t h o u t l o s s i n a c c u r a c y w i t h r e s p e c t t o o r d e r . T h e n u m b e r o f t i m e s ,
a n d t h e m a n n e r i n w h i c h , t h e t e r m s i n t h e J a c o b i a n m a t r i x a r e u p d a t e d a s t h e s o l u t i o n
p r o c e e d s d e p e n d s , o f c o u r s e , o n t h e n a t u r e o f t h e p r o b l e m .
R e t u r n i n g t o o u r s i m p l e b o u n d a r y - l a y e r e x a m p l e , w h i c h i s g i v e n b y E q . 6 . 1 0 1 , w e
n d t h e J a c o b i a n m a t r i x t o b e
A =
2
6
4
; u
3
2 u
2
; u
1
1 0 0
0 1 0
3
7
5
( 6 . 1 0 7 )
T h e s t u d e n t s h o u l d b e a b l e t o d e r i v e r e s u l t s f o r t h i s e x a m p l e t h a t a r e e q u i v a l e n t t o
t h o s e g i v e n f o r t h e s c a l a r c a s e i n E q . 6 . 9 3 . T h u s f o r t h e F a l k n e r - S k a n e q u a t i o n s t h e
t r a p e z o i d a l m e t h o d r e s u l t s i n
2
6
6
6
4
1 +
h
2
( u
3
)
n
; h ( u
2
)
n
h
2
( u
1
)
n
;
h
2
1 0
0 ;
h
2
1
3
7
7
7
5
2
6
4
( u
1
)
n
( u
2
)
n
( u
3
)
n
3
7
5
= h
2
6
4
; ( u
1
u
3
)
n
; ( 1 ; u
2
2
)
n
( u
1
)
n
( u
2
)
n
3
7
5
W e n d
~
u
n + 1
f r o m
~
u
n
+
~
u
n
, a n d t h e s o l u t i o n i s n o w a d v a n c e d o n e s t e p . R e - e v a l u a t e
t h e e l e m e n t s u s i n g
~
u
n + 1
a n d c o n t i n u e . W i t h o u t a n y i t e r a t i n g w i t h i n a s t e p a d v a n c e ,
t h e s o l u t i o n w i l l b e s e c o n d - o r d e r - a c c u r a t e i n t i m e .
6 . 1 1 P r o b l e m s
1 . F i n d a n e x p r e s s i o n f o r t h e n t h t e r m i n t h e F i b o n a c c i s e r i e s , w h i c h i s g i v e n
b y 1 1 2 3 5 8 : : : N o t e t h a t t h e s e r i e s c a n b e e x p r e s s e d a s t h e s o l u t i o n t o a
d i e r e n c e e q u a t i o n o f t h e f o r m u
n + 1
= u
n
+ u
n ; 1
. W h a t i s u
2 5
? ( L e t t h e r s t
t e r m g i v e n a b o v e b e u
0
. )
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1 1 8 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
2 . T h e t r a p e z o i d a l m e t h o d u
n + 1
= u
n
+
1
2
h ( u
0
n + 1
+ u
0
n
) i s u s e d t o s o l v e t h e r e p r e -
s e n t a t i v e O D E .
( a ) W h a t i s t h e r e s u l t i n g O E ?
( b ) W h a t i s i t s e x a c t s o l u t i o n ?
( c ) H o w d o e s t h e e x a c t s t e a d y - s t a t e s o l u t i o n o f t h e O E c o m p a r e w i t h t h e
e x a c t s t e a d y - s t a t e s o l u t i o n o f t h e O D E i f = 0 ?
3 . T h e 2 n d - o r d e r b a c k w a r d m e t h o d i s g i v e n b y
u
n + 1
=
1
3
h
4 u
n
; u
n ; 1
+ 2 h u
0
n + 1
i
( a ) W r i t e t h e O E f o r t h e r e p r e s e n t a t i v e e q u a t i o n . I d e n t i f y t h e p o l y n o m i a l s
P ( E ) a n d Q ( E ) .
( b ) D e r i v e t h e - r e l a t i o n . S o l v e f o r t h e - r o o t s a n d i d e n t i f y t h e m a s p r i n c i p a l
o r s p u r i o u s .
( c ) F i n d e r
a n d t h e r s t t w o n o n v a n i s h i n g t e r m s i n a T a y l o r s e r i e s e x p a n s i o n
o f t h e s p u r i o u s r o o t .
( d ) P e r f o r m a - r o o t t r a c e r e l a t i v e t o t h e u n i t c i r c l e f o r b o t h d i u s i o n a n d
c o n v e c t i o n .
4 . C o n s i d e r t h e t i m e - m a r c h i n g s c h e m e g i v e n b y
u
n + 1
= u
n ; 1
+
2 h
3
( u
0
n + 1
+ u
0
n
+ u
0
n ; 1
)
( a ) W r i t e t h e O E f o r t h e r e p r e s e n t a t i v e e q u a t i o n . I d e n t i f y t h e p o l y n o m i a l s
P ( E ) a n d Q ( E ) .
( b ) D e r i v e t h e ; r e l a t i o n .
( c ) F i n d e r
.
5 . F i n d t h e d i e r e n c e e q u a t i o n w h i c h r e s u l t s f r o m a p p l y i n g t h e G a z d a g p r e d i c t o r -
c o r r e c t o r m e t h o d ( E q . 6 . 6 5 ) t o t h e r e p r e s e n t a t i v e e q u a t i o n . F i n d t h e - r e l a -
t i o n .
6 . C o n s i d e r t h e f o l l o w i n g t i m e - m a r c h i n g m e t h o d :
~ u
n + 1 = 3
= u
n
+ h u
0
n
= 3
u
n + 1 = 2
= u
n
+ h ~ u
0
n + 1 = 3
= 2
u
n + 1
= u
n
+ h u
0
n + 1 = 2
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6 . 1 1 . P R O B L E M S 1 1 9
F i n d t h e d i e r e n c e e q u a t i o n w h i c h r e s u l t s f r o m a p p l y i n g t h i s m e t h o d t o t h e
r e p r e s e n t a t i v e e q u a t i o n . F i n d t h e - r e l a t i o n . F i n d t h e s o l u t i o n t o t h e d i e r -
e n c e e q u a t i o n , i n c l u d i n g t h e h o m o g e n e o u s a n d p a r t i c u l a r s o l u t i o n s . F i n d e r
a n d e r
. W h a t o r d e r i s t h e h o m o g e n e o u s s o l u t i o n ? W h a t o r d e r i s t h e p a r t i c u l a r
s o l u t i o n ? F i n d t h e p a r t i c u l a r s o l u t i o n i f t h e f o r c i n g t e r m i s x e d .
7 . W r i t e a c o m p u t e r p r o g r a m t o s o l v e t h e o n e - d i m e n s i o n a l l i n e a r c o n v e c t i o n e q u a -
t i o n w i t h p e r i o d i c b o u n d a r y c o n d i t i o n s a n d a = 1 o n t h e d o m a i n 0 x 1 .
U s e 2 n d - o r d e r c e n t e r e d d i e r e n c e s i n s p a c e a n d a g r i d o f 5 0 p o i n t s . F o r t h e
i n i t i a l c o n d i t i o n , u s e
u ( x 0 ) = e
; 0 5 ( x ; 0 5 ) = ]
2
w i t h = 0 : 0 8 . U s e t h e e x p l i c i t E u l e r , 2 n d - o r d e r A d a m s - B a s h f o r t h ( A B 2 ) , i m -
p l i c i t E u l e r , t r a p e z o i d a l , a n d 4 t h - o r d e r R u n g e - K u t t a m e t h o d s . F o r t h e e x p l i c i t
E u l e r a n d A B 2 m e t h o d s , u s e a C o u r a n t n u m b e r , a h = x , o f 0 . 1 f o r t h e o t h e r
m e t h o d s , u s e a C o u r a n t n u m b e r o f u n i t y . P l o t t h e s o l u t i o n s o b t a i n e d a t t = 1
c o m p a r e d t o t h e e x a c t s o l u t i o n ( w h i c h i s i d e n t i c a l t o t h e i n i t i a l c o n d i t i o n ) .
8 . R e p e a t p r o b l e m 7 u s i n g 4 t h - o r d e r ( n o n c o m p a c t ) d i e r e n c e s i n s p a c e . U s e o n l y
4 t h - o r d e r R u n g e - K u t t a t i m e m a r c h i n g a t a C o u r a n t n u m b e r o f u n i t y . S h o w
s o l u t i o n s a t t = 1 a n d t = 1 0 c o m p a r e d t o t h e e x a c t s o l u t i o n .
9 . U s i n g t h e c o m p u t e r p r o g r a m w r i t t e n f o r p r o b l e m 7 , c o m p u t e t h e s o l u t i o n a t
t = 1 u s i n g 2 n d - o r d e r c e n t e r e d d i e r e n c e s i n s p a c e c o u p l e d w i t h t h e 4 t h - o r d e r
R u n g e - K u t t a m e t h o d f o r g r i d s o f 1 0 0 , 2 0 0 , a n d 4 0 0 n o d e s . O n a l o g - l o g s c a l e ,
p l o t t h e e r r o r g i v e n b y
v
u
u
u
t
M
X
j = 1
( u
j
; u
e x a c t
j
)
2
M
w h e r e M i s t h e n u m b e r o f g r i d n o d e s a n d u
e x a c t
i s t h e e x a c t s o l u t i o n . F i n d t h e
g l o b a l o r d e r o f a c c u r a c y f r o m t h e p l o t .
1 0 . U s i n g t h e c o m p u t e r p r o g r a m w r i t t e n f o r p r o b l e m 8 , r e p e a t p r o b l e m 9 u s i n g
4 t h - o r d e r ( n o n c o m p a c t ) d i e r e n c e s i n s p a c e .
1 1 . W r i t e a c o m p u t e r p r o g r a m t o s o l v e t h e o n e - d i m e n s i o n a l l i n e a r c o n v e c t i o n e q u a -
t i o n w i t h i n o w - o u t o w b o u n d a r y c o n d i t i o n s a n d a = 1 o n t h e d o m a i n 0
x 1 . L e t u ( 0 t ) = s i n ! t w i t h ! = 1 0 . R u n u n t i l a p e r i o d i c s t e a d y s t a t e
i s r e a c h e d w h i c h i s i n d e p e n d e n t o f t h e i n i t i a l c o n d i t i o n a n d p l o t y o u r s o l u t i o n
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1 2 0 C H A P T E R 6 . T I M E - M A R C H I N G M E T H O D S F O R O D E ' S
c o m p a r e d w i t h t h e e x a c t s o l u t i o n . U s e 2 n d - o r d e r c e n t e r e d d i e r e n c e s i n s p a c e
w i t h a 1 s t - o r d e r b a c k w a r d d i e r e n c e a t t h e o u t o w b o u n d a r y ( a s i n E q . 3 . 6 9 )
t o g e t h e r w i t h 4 t h - o r d e r R u n g e - K u t t a t i m e m a r c h i n g . U s e g r i d s w i t h 1 0 0 , 2 0 0 ,
a n d 4 0 0 n o d e s a n d p l o t t h e e r r o r v s . t h e n u m b e r o f g r i d n o d e s , a s d e s c r i b e d i n
p r o b l e m 9 . F i n d t h e g l o b a l o r d e r o f a c c u r a c y .
1 2 . R e p e a t p r o b l e m 1 1 u s i n g 4 t h - o r d e r ( n o n c o m p a c t ) c e n t e r e d d i e r e n c e s . U s e a
t h i r d - o r d e r f o r w a r d - b i a s e d o p e r a t o r a t t h e i n o w b o u n d a r y ( a s i n E q . 3 . 6 7 ) . A t
t h e l a s t g r i d n o d e , d e r i v e a n d u s e a 3 r d - o r d e r b a c k w a r d o p e r a t o r ( u s i n g n o d e s
j ; 3 , j ; 2 , j ; 1 , a n d j ) a n d a t t h e s e c o n d l a s t n o d e , u s e a 3 r d - o r d e r b a c k w a r d -
b i a s e d o p e r a t o r ( u s i n g n o d e s j ; 2 , j ; 1 , j , a n d j + 1 s e e p r o b l e m 1 i n C h a p t e r
3 ) .
1 3 . U s i n g t h e a p p r o a c h d e s c r i b e d i n S e c t i o n 6 . 6 . 2 , n d t h e p h a s e s p e e d e r r o r , e r
p
,
a n d t h e a m p l i t u d e e r r o r , e r
a
, f o r t h e c o m b i n a t i o n o f s e c o n d - o r d e r c e n t e r e d d i f -
f e r e n c e s a n d 1 s t , 2 n d , 3 r d , a n d 4 t h - o r d e r R u n g e - K u t t a t i m e - m a r c h i n g a t a
C o u r a n t n u m b e r o f u n i t y . A l s o p l o t t h e p h a s e s p e e d e r r o r o b t a i n e d u s i n g e x a c t
i n t e g r a t i o n i n t i m e , i . e . , t h a t o b t a i n e d u s i n g t h e s p a t i a l d i s c r e t i z a t i o n a l o n e .
N o t e t h a t t h e r e q u i r e d - r o o t s f o r t h e v a r i o u s R u n g e - K u t t a m e t h o d s c a n b e
d e d u c e d f r o m E q . 6 . 6 9 , w i t h o u t a c t u a l l y d e r i v i n g t h e m e t h o d s . E x p l a i n y o u r
r e s u l t s .
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b ø ô ù × ô ¤ ¢ 2 ù 2 ) ¢ ø ô ¥ a ¢ ô ¢ 2 õ ô 4 ù $ ù ù ø ô ù
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ø ù ¦ ¢ ô ø $ ù ù ¦ ô E ô ÷ ¦ ù ø ) ù0 $ 2 ` 2 $ ù ' ù ô õ ù 4 ¢ ¦ ÷ ù û ¢ ù 2
$ ù ù ` C $ ¢ ô $ ù ) ¢ ô ) ÷ ) ÷ ø ¢ ! ÷ ) ¤ ù ø ¢ ¦ ¦ C ùT 2 P ø ¢ © ) V £ ô E ¢ ¤ ô ¦ ô © `
¦ ¢ ô C $ ©v £ $ ô 2 ô ¢ ù ø ø ) $ ù ¢ C ù ø ø ¤ ¦ ù ) æ û ô þ ô × ø ) ¢ ¦ `
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C h a p t e r 8
C H O I C E O F T I M E - M A R C H I N G
M E T H O D S
I n t h i s c h a p t e r w e d i s c u s s c o n s i d e r a t i o n s i n v o l v e d i n s e l e c t i n g a t i m e - m a r c h i n g m e t h o d
f o r a s p e c i c a p p l i c a t i o n . E x a m p l e s a r e g i v e n s h o w i n g h o w t i m e - m a r c h i n g m e t h o d s
c a n b e c o m p a r e d i n a g i v e n c o n t e x t . A n i m p o r t a n t c o n c e p t u n d e r l y i n g m u c h o f t h i s
d i s c u s s i o n i s s t i n e s s , w h i c h i s d e n e d i n t h e n e x t s e c t i o n .
8 . 1 S t i n e s s D e n i t i o n f o r O D E ' s
8 . 1 . 1 R e l a t i o n t o - E i g e n v a l u e s
T h e i n t r o d u c t i o n o f t h e c o n c e p t r e f e r r e d t o a s \ s t i n e s s " c o m e s a b o u t f r o m t h e n u -
m e r i c a l a n a l y s i s o f m a t h e m a t i c a l m o d e l s c o n s t r u c t e d t o s i m u l a t e d y n a m i c p h e n o m -
e n a c o n t a i n i n g w i d e l y d i e r e n t t i m e s c a l e s . D e n i t i o n s g i v e n i n t h e l i t e r a t u r e a r e
n o t u n i q u e , b u t f o r t u n a t e l y w e n o w h a v e t h e b a c k g r o u n d m a t e r i a l t o c o n s t r u c t a
d e n i t i o n w h i c h i s e n t i r e l y s u c i e n t f o r o u r p u r p o s e s .
W e s t a r t w i t h t h e a s s u m p t i o n t h a t o u r C F D p r o b l e m i s m o d e l e d w i t h s u c i e n t
a c c u r a c y b y a c o u p l e d s e t o f O D E ' s p r o d u c i n g a n A m a t r i x t y p i e d b y E q . 7 . 1 .
A n y d e n i t i o n o f s t i n e s s r e q u i r e s a c o u p l e d s y s t e m w i t h a t l e a s t t w o e i g e n v a l u e s ,
a n d t h e d e c i s i o n t o u s e s o m e n u m e r i c a l t i m e - m a r c h i n g o r i t e r a t i v e m e t h o d t o s o l v e
i t . T h e d i e r e n c e b e t w e e n t h e d y n a m i c s c a l e s i n p h y s i c a l s p a c e i s r e p r e s e n t e d b y
t h e d i e r e n c e i n t h e m a g n i t u d e o f t h e e i g e n v a l u e s i n e i g e n s p a c e . I n t h e f o l l o w i n g
d i s c u s s i o n w e c o n c e n t r a t e o n t h e t r a n s i e n t p a r t o f t h e s o l u t i o n . T h e f o r c i n g f u n c t i o n
m a y a l s o b e t i m e v a r y i n g i n w h i c h c a s e i t w o u l d a l s o h a v e a t i m e s c a l e . H o w e v e r ,
w e a s s u m e t h a t t h i s s c a l e w o u l d b e a d e q u a t e l y r e s o l v e d b y t h e c h o s e n t i m e - m a r c h i n g
m e t h o d , a n d , s i n c e t h i s p a r t o f t h e O D E h a s n o e e c t o n t h e n u m e r i c a l s t a b i l i t y o f
1 4 9
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1 5 0 C H A P T E R 8 . C H O I C E O F T I M E - M A R C H I N G M E T H O D S
Stable
Region
Accurate
Region
1
I( h)
R( h)
λ
λ
λ h
F i g u r e 8 . 1 : S t a b l e a n d a c c u r a t e r e g i o n s f o r t h e e x p l i c i t E u l e r m e t h o d .
t h e h o m o g e n e o u s p a r t , w e e x c l u d e t h e f o r c i n g f u n c t i o n f r o m f u r t h e r d i s c u s s i o n i n t h i s
s e c t i o n .
C o n s i d e r n o w t h e f o r m o f t h e e x a c t s o l u t i o n o f a s y s t e m o f O D E ' s w i t h a c o m -
p l e t e e i g e n s y s t e m . T h i s i s g i v e n b y E q . 6 . 2 7 a n d i t s s o l u t i o n u s i n g a o n e - r o o t , t i m e -
m a r c h i n g m e t h o d i s r e p r e s e n t e d b y E q . 6 . 2 8 . F o r a g i v e n t i m e s t e p , t h e t i m e i n t e g r a -
t i o n i s a n a p p r o x i m a t i o n i n e i g e n s p a c e t h a t i s d i e r e n t f o r e v e r y e i g e n v e c t o r ~x
m
. I n
m a n y n u m e r i c a l a p p l i c a t i o n s t h e e i g e n v e c t o r s a s s o c i a t e d w i t h t h e s m a l l j
m
j a r e w e l l
r e s o l v e d a n d t h o s e a s s o c i a t e d w i t h t h e l a r g e j
m
j a r e r e s o l v e d m u c h l e s s a c c u r a t e l y ,
i f a t a l l . T h e s i t u a t i o n i s r e p r e s e n t e d i n t h e c o m p l e x h p l a n e i n F i g . 8 . 1 . I n t h i s
g u r e t h e t i m e s t e p h a s b e e n c h o s e n s o t h a t t i m e a c c u r a c y i s g i v e n t o t h e e i g e n v e c t o r s
a s s o c i a t e d w i t h t h e e i g e n v a l u e s l y i n g i n t h e s m a l l c i r c l e a n d s t a b i l i t y w i t h o u t t i m e
a c c u r a c y i s g i v e n t o t h o s e a s s o c i a t e d w i t h t h e e i g e n v a l u e s l y i n g o u t s i d e o f t h e s m a l l
c i r c l e b u t s t i l l i n s i d e t h e l a r g e c i r c l e .
T h e w h o l e c o n c e p t o f s t i n e s s i n C F D a r i s e s f r o m t h e f a c t t h a t w e o f t e n d o
n o t n e e d t h e t i m e r e s o l u t i o n o f e i g e n v e c t o r s a s s o c i a t e d w i t h t h e l a r g e j
m
j i n
t h e t r a n s i e n t s o l u t i o n , a l t h o u g h t h e s e e i g e n v e c t o r s m u s t r e m a i n c o u p l e d i n t o
t h e s y s t e m t o m a i n t a i n a h i g h a c c u r a c y o f t h e s p a t i a l r e s o l u t i o n .
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8 . 1 . S T I F F N E S S D E F I N I T I O N F O R O D E ' S 1 5 1
8 . 1 . 2 D r i v i n g a n d P a r a s i t i c E i g e n v a l u e s
F o r t h e a b o v e r e a s o n i t i s c o n v e n i e n t t o s u b d i v i d e t h e t r a n s i e n t s o l u t i o n i n t o t w o
p a r t s . F i r s t w e o r d e r t h e e i g e n v a l u e s b y t h e i r m a g n i t u d e s , t h u s
j
1
j j
2
j j
M
j ( 8 . 1 )
T h e n w e w r i t e
T r a n s i e n t
S o l u t i o n
=
p
X
m = 1
c
m
e
m
t
~
x
m
| { z }
D r i v i n g
+
M
X
m = p + 1
c
m
e
m
t
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x
m
| { z }
P a r a s i t i c
( 8 . 2 )
T h i s c o n c e p t i s c r u c i a l t o o u r d i s c u s s i o n . R e p h r a s e d , i t s t a t e s t h a t w e c a n s e p a r a t e
o u r e i g e n v a l u e s p e c t r u m i n t o t w o g r o u p s o n e
1
!
p
] c a l l e d t h e d r i v i n g e i g e n v a l u e s
( o u r c h o i c e o f a t i m e - s t e p a n d m a r c h i n g m e t h o d m u s t a c c u r a t e l y a p p r o x i m a t e t h e t i m e
v a r i a t i o n o f t h e e i g e n v e c t o r s a s s o c i a t e d w i t h t h e s e ) , a n d t h e o t h e r ,
p + 1
!
M
] , c a l l e d
t h e p a r a s i t i c e i g e n v a l u e s ( n o t i m e a c c u r a c y w h a t s o e v e r i s r e q u i r e d f o r t h e e i g e n v e c t o r s
a s s o c i a t e d w i t h t h e s e , b u t t h e i r p r e s e n c e m u s t n o t c o n t a m i n a t e t h e a c c u r a c y o f t h e
c o m p l e t e s o l u t i o n ) . U n f o r t u n a t e l y , w e n d t h a t , a l t h o u g h t i m e a c c u r a c y r e q u i r e m e n t s
a r e d i c t a t e d b y t h e d r i v i n g e i g e n v a l u e s , n u m e r i c a l s t a b i l i t y r e q u i r e m e n t s a r e d i c t a t e d
b y t h e p a r a s i t i c o n e s .
8 . 1 . 3 S t i n e s s C l a s s i c a t i o n s
T h e f o l l o w i n g d e n i t i o n s a r e s o m e w h a t u s e f u l . A n i n h e r e n t l y s t a b l e s e t o f O D E ' s i s
s t i i f
j
p
j j
M
j
I n p a r t i c u l a r w e d e n e t h e r a t i o
C
r
= j
M
j = j
p
j
a n d f o r m t h e c a t e g o r i e s
M i l d l y - s t i C
r
< 1 0
2
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3
< C
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5
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6
< C
r
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8
P a t h o l o g i c a l l y - s t i 1 0
9
< C
r
I t s h o u l d b e m e n t i o n e d t h a t t h e g a p s i n t h e s t i c a t e g o r y d e n i t i o n s a r e i n t e n t i o n a l
b e c a u s e t h e b o u n d s a r e a r b i t r a r y . I t i s i m p o r t a n t t o n o t i c e t h a t t h e s e d e n i t i o n s
m a k e n o d i s t i n c t i o n b e t w e e n r e a l , c o m p l e x , a n d i m a g i n a r y e i g e n v a l u e s .
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1 5 2 C H A P T E R 8 . C H O I C E O F T I M E - M A R C H I N G M E T H O D S
8 . 2 R e l a t i o n o f S t i n e s s t o S p a c e M e s h S i z e
M a n y o w e l d s a r e c h a r a c t e r i z e d b y a f e w r e g i o n s h a v i n g h i g h s p a t i a l g r a d i e n t s o f
t h e d e p e n d e n t v a r i a b l e s a n d o t h e r d o m a i n s h a v i n g r e l a t i v e l y l o w g r a d i e n t p h e n o m e n a .
A s a r e s u l t i t i s q u i t e c o m m o n t o c l u s t e r m e s h p o i n t s i n c e r t a i n r e g i o n s o f s p a c e a n d
s p r e a d t h e m o u t o t h e r w i s e . E x a m p l e s o f w h e r e t h i s c l u s t e r i n g m i g h t o c c u r a r e a t a
s h o c k w a v e , n e a r a n a i r f o i l l e a d i n g o r t r a i l i n g e d g e , a n d i n a b o u n d a r y l a y e r .
O n e q u i c k l y n d s t h a t t h i s g r i d c l u s t e r i n g c a n s t r o n g l y a e c t t h e e i g e n s y s t e m o f
t h e r e s u l t i n g A m a t r i x . I n o r d e r t o d e m o n s t r a t e t h i s , l e t u s e x a m i n e t h e e i g e n s y s t e m s
o f t h e m o d e l p r o b l e m s g i v e n i n S e c t i o n 4 . 3 . 2 . T h e s i m p l e s t e x a m p l e t o d i s c u s s r e l a t e s
t o t h e m o d e l d i u s i o n e q u a t i o n . I n t h i s c a s e t h e e i g e n v a l u e s a r e a l l r e a l , n e g a t i v e
n u m b e r s t h a t a u t o m a t i c a l l y o b e y t h e o r d e r i n g g i v e n i n E q . 8 . 1 . C o n s i d e r t h e c a s e
w h e n a l l o f t h e e i g e n v a l u e s a r e p a r a s i t i c , i . e . , w e a r e i n t e r e s t e d o n l y i n t h e c o n v e r g e d
s t e a d y - s t a t e s o l u t i o n . U n d e r t h e s e c o n d i t i o n s , t h e s t i n e s s i s d e t e r m i n e d b y t h e r a t i o
M
=
1
. A s i m p l e c a l c u l a t i o n s h o w s t h a t
1
= ;
4
x
2
s i n
2
2 ( M + 1 )
!
;
4
x
2
x
2
2
= ;
M
;
4
x
2
s i n
2
2
= ;
4
x
2
a n d t h e r a t i o i s
M
=
1
4
x
2
= 4
M + 1
2
T h e m o s t i m p o r t a n t i n f o r m a t i o n f o u n d f r o m t h i s e x a m p l e i s t h e f a c t t h a t t h e
s t i n e s s o f t h e t r a n s i e n t s o l u t i o n i s d i r e c t l y r e l a t e d t o t h e g r i d s p a c i n g . F u r t h e r m o r e ,
i n d i u s i o n p r o b l e m s t h i s s t i n e s s i s p r o p o r t i o n a l t o t h e r e c i p r o c a l o f t h e s p a c e m e s h
s i z e s q u a r e d . F o r a m e s h s i z e M = 4 0 , t h i s r a t i o i s a b o u t 6 8 0 . E v e n f o r a m e s h o f
t h i s m o d e r a t e s i z e t h e p r o b l e m i s a l r e a d y a p p r o a c h i n g t h e c a t e g o r y o f s t r o n g l y s t i .
F o r t h e b i c o n v e c t i o n m o d e l a s i m i l a r a n a l y s i s s h o w s t h a t
j
M
j = j
1
j
1
x
H e r e t h e s t i n e s s p a r a m e t e r i s s t i l l s p a c e - m e s h d e p e n d e n t , b u t m u c h l e s s s o t h a n f o r
d i u s i o n - d o m i n a t e d p r o b l e m s .
W e s e e t h a t i n b o t h c a s e s w e a r e f a c e d w i t h t h e r a t h e r a n n o y i n g f a c t t h a t t h e m o r e
w e t r y t o i n c r e a s e t h e r e s o l u t i o n o f o u r s p a t i a l g r a d i e n t s , t h e s t i e r o u r e q u a t i o n s t e n d
t o b e c o m e . T y p i c a l C F D p r o b l e m s w i t h o u t c h e m i s t r y v a r y b e t w e e n t h e m i l d l y a n d
s t r o n g l y s t i c a t e g o r i e s , a n d a r e g r e a t l y a e c t e d b y t h e r e s o l u t i o n o f a b o u n d a r y l a y e r
s i n c e i t i s a d i u s i o n p r o c e s s . O u r b r i e f a n a l y s i s h a s b e e n l i m i t e d t o e q u i s p a c e d
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8 . 3 . P R A C T I C A L C O N S I D E R A T I O N S F O R C O M P A R I N G M E T H O D S 1 5 3
p r o b l e m s , b u t i n g e n e r a l t h e s t i n e s s o f C F D p r o b l e m s i s p r o p o r t i o n a l t o t h e m e s h
i n t e r v a l s i n t h e m a n n e r s h o w n a b o v e w h e r e t h e c r i t i c a l i n t e r v a l i s t h e s m a l l e s t o n e i n
t h e p h y s i c a l d o m a i n .
8 . 3 P r a c t i c a l C o n s i d e r a t i o n s f o r C o m p a r i n g M e t h -
o d s
W e h a v e p r e s e n t e d r e l a t i v e l y s i m p l e a n d r e l i a b l e m e a s u r e s o f s t a b i l i t y a n d b o t h t h e
l o c a l a n d g l o b a l a c c u r a c y o f t i m e - m a r c h i n g m e t h o d s . S i n c e t h e r e a r e a n e n d l e s s
n u m b e r o f t h e s e m e t h o d s t o c h o o s e f r o m , o n e c a n w o n d e r h o w t h i s i n f o r m a t i o n i s
t o b e u s e d t o p i c k a \ b e s t " c h o i c e f o r a p a r t i c u l a r p r o b l e m . T h e r e i s n o u n i q u e
a n s w e r t o s u c h a q u e s t i o n . F o r e x a m p l e , i t i s , a m o n g o t h e r t h i n g s , h i g h l y d e p e n d e n t
u p o n t h e s p e e d , c a p a c i t y , a n d a r c h i t e c t u r e o f t h e a v a i l a b l e c o m p u t e r , a n d t e c h n o l o g y
i n u e n c i n g t h i s i s u n d e r g o i n g r a p i d a n d d r a m a t i c c h a n g e s a s t h i s i s b e i n g w r i t t e n .
N e v e r t h e l e s s , i f c e r t a i n g r o u n d r u l e s a r e a g r e e d u p o n , r e l e v a n t c o n c l u s i o n s c a n b e
r e a c h e d . L e t u s n o w e x a m i n e s o m e g r o u n d r u l e s t h a t m i g h t b e a p p r o p r i a t e . I t s h o u l d
t h e n b e c l e a r h o w t h e a n a l y s i s c a n b e e x t e n d e d t o o t h e r c a s e s .
L e t u s c o n s i d e r t h e p r o b l e m o f m e a s u r i n g t h e e c i e n c y o f a t i m e { m a r c h i n g m e t h o d
f o r c o m p u t i n g , o v e r a x e d i n t e r v a l o f t i m e , a n a c c u r a t e t r a n s i e n t s o l u t i o n o f a c o u p l e d
s e t o f O D E ' s . T h e l e n g t h o f t h e t i m e i n t e r v a l , T , a n d t h e a c c u r a c y r e q u i r e d o f t h e
s o l u t i o n a r e d i c t a t e d b y t h e p h y s i c s o f t h e p a r t i c u l a r p r o b l e m i n v o l v e d . F o r e x a m p l e ,
i n c a l c u l a t i n g t h e a m o u n t o f t u r b u l e n c e i n a h o m o g e n e o u s o w , t h e t i m e i n t e r v a l
w o u l d b e t h a t r e q u i r e d t o e x t r a c t a r e l i a b l e s t a t i s t i c a l s a m p l e , a n d t h e a c c u r a c y
w o u l d b e r e l a t e d t o h o w m u c h t h e e n e r g y o f c e r t a i n h a r m o n i c s w o u l d b e p e r m i t t e d
t o d i s t o r t f r o m a g i v e n l e v e l . S u c h a c o m p u t a t i o n w e r e f e r t o a s a n e v e n t .
T h e a p p r o p r i a t e e r r o r m e a s u r e s t o b e u s e d i n c o m p a r i n g m e t h o d s f o r c a l c u l a t i n g
a n e v e n t a r e t h e g l o b a l o n e s , E r
a
, E r
a n d E r
!
, d i s c u s s e d i n S e c t i o n 6 . 6 . 5 , r a t h e r
t h a n t h e l o c a l o n e s e r
, e r
a
, a n d e r
p
d i s c u s s e d e a r l i e r .
T h e a c t u a l f o r m o f t h e c o u p l e d O D E ' s t h a t a r e p r o d u c e d b y t h e s e m i - d i s c r e t e
a p p r o a c h i s
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~
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A t e v e r y t i m e s t e p w e m u s t e v a l u a t e t h e f u n c t i o n
~
F (
~
u t ) a t l e a s t o n c e . T h i s f u n c t i o n
i s u s u a l l y n o n l i n e a r , a n d i t s c o m p u t a t i o n u s u a l l y c o n s u m e s t h e m a j o r p o r t i o n o f t h e
c o m p u t e r t i m e r e q u i r e d t o m a k e t h e s i m u l a t i o n . W e r e f e r t o a s i n g l e c a l c u l a t i o n o f t h e
v e c t o r
~
F (
~
u t ) a s a f u n c t i o n e v a l u a t i o n a n d d e n o t e t h e t o t a l n u m b e r o f s u c h e v a l u a t i o n s
b y F
e v
.
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1 5 4 C H A P T E R 8 . C H O I C E O F T I M E - M A R C H I N G M E T H O D S
8 . 4 C o m p a r i n g t h e E c i e n c y o f E x p l i c i t M e t h o d s
8 . 4 . 1 I m p o s e d C o n s t r a i n t s
A s m e n t i o n e d a b o v e , t h e e c i e n c y o f m e t h o d s c a n b e c o m p a r e d o n l y i f o n e a c c e p t s a
s e t o f l i m i t i n g c o n s t r a i n t s w i t h i n w h i c h t h e c o m p a r i s o n s a r e c a r r i e d o u t . T h e f o l l o w
a s s u m p t i o n s b o u n d t h e c o n s i d e r a t i o n s m a d e i n t h i s S e c t i o n :
1 . T h e t i m e - m a r c h m e t h o d i s e x p l i c i t .
2 . I m p l i c a t i o n s o f c o m p u t e r s t o r a g e c a p a c i t y a n d a c c e s s t i m e a r e i g n o r e d . I n s o m e
c o n t e x t s , t h i s c a n b e a n i m p o r t a n t c o n s i d e r a t i o n .
3 . T h e c a l c u l a t i o n i s t o b e t i m e - a c c u r a t e , m u s t s i m u l a t e a n e n t i r e e v e n t w h i c h
t a k e s a t o t a l t i m e T , a n d m u s t u s e a c o n s t a n t t i m e s t e p s i z e , h , s o t h a t
T = N h
w h e r e N i s t h e t o t a l n u m b e r o f t i m e s t e p s .
8 . 4 . 2 A n E x a m p l e I n v o l v i n g D i u s i o n
L e t t h e e v e n t b e t h e n u m e r i c a l s o l u t i o n o f
d u
d t
= ; u ( 8 . 3 )
f r o m t = 0 t o T = ; l n ( 0 : 2 5 ) w i t h u ( 0 ) = 1 . E q . 8 . 3 i s o b t a i n e d f r o m o u r r e p r e s e n t a -
t i v e O D E w i t h = ; 1 , a = 0 . S i n c e t h e e x a c t s o l u t i o n i s u ( t ) = u ( 0 ) e
; t
, t h i s m a k e s
t h e e x a c t v a l u e o f u a t t h e e n d o f t h e e v e n t e q u a l t o 0 . 2 5 , i . e . , u ( T ) = 0 : 2 5 . T o t h e
c o n s t r a i n t s i m p o s e d a b o v e , l e t u s s e t t h e a d d i t i o n a l r e q u i r e m e n t
T h e e r r o r i n u a t t h e e n d o f t h e e v e n t , i . e . , t h e g l o b a l e r r o r , m u s t b e < 0 : 5 % .
W e j u d g e t h e m o s t e c i e n t m e t h o d a s t h e o n e t h a t s a t i s e s t h e s e c o n d i t i o n s a n d
h a s t h e f e w e s t n u m b e r o f e v a l u a t i o n s , F
e v
. T h r e e m e t h o d s a r e c o m p a r e d | e x p l i c i t
E u l e r , A B 2 , a n d R K 4 .
F i r s t o f a l l , t h e a l l o w a b l e e r r o r c o n s t r a i n t m e a n s t h a t t h e g l o b a l e r r o r i n t h e a m -
p l i t u d e , s e e E q . 6 . 4 8 , m u s t h a v e t h e p r o p e r t y :
E r
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T
< 0 : 0 0 5
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8 . 4 . C O M P A R I N G T H E E F F I C I E N C Y O F E X P L I C I T M E T H O D S 1 5 5
T h e n , s i n c e h = T = N = ; l n ( 0 : 2 5 ) = N , i t f o l l o w s t h a t
1 ; (
1
( l n ( : 2 5 ) = N ) )
N
= : 2 5
< 0 : 0 0 5
w h e r e
1
i s f o u n d f r o m t h e c h a r a c t e r i s t i c p o l y n o m i a l s g i v e n i n T a b l e 7 . 1 . T h e r e s u l t s
s h o w n i n T a b l e 8 . 1 w e r e c o m p u t e d u s i n g a s i m p l e i t e r a t i v e p r o c e d u r e .
M e t h o d N h
1
F
e v
E r
E u l e r 1 9 3 : 0 0 7 1 8 : 9 9 2 8 2 1 9 3 : 0 0 1 2 4 8 w o r s t
A B 2 1 6 : 0 8 6 6 : 9 1 7 2 1 6 : 0 0 1 1 3 7
R K 4 2 : 6 9 3 1 : 5 0 1 2 8 : 0 0 1 1 9 5 b e s t
T a b l e 8 . 1 : C o m p a r i s o n o f t i m e - m a r c h i n g m e t h o d s f o r a s i m p l e d i s s i p a t i o n p r o b l e m .
I n t h i s e x a m p l e w e s e e t h a t , f o r a g i v e n g l o b a l a c c u r a c y , t h e m e t h o d w i t h t h e
h i g h e s t l o c a l a c c u r a c y i s t h e m o s t e c i e n t o n t h e b a s i s o f t h e e x p e n s e i n e v a l u a t i n g
F
e v
. T h u s t h e s e c o n d - o r d e r A d a m s - B a s h f o r t h m e t h o d i s m u c h b e t t e r t h a n t h e r s t -
o r d e r E u l e r m e t h o d , a n d t h e f o u r t h - o r d e r R u n g e - K u t t a m e t h o d i s t h e b e s t o f a l l . T h e
m a i n p u r p o s e o f t h i s e x e r c i s e i s t o s h o w t h e ( u s u a l l y ) g r e a t s u p e r i o r i t y o f s e c o n d - o r d e r
o v e r r s t - o r d e r t i m e - m a r c h i n g m e t h o d s .
8 . 4 . 3 A n E x a m p l e I n v o l v i n g P e r i o d i c C o n v e c t i o n
L e t u s u s e a s a b a s i s f o r t h i s e x a m p l e t h e s t u d y o f h o m o g e n e o u s t u r b u l e n c e s i m u l a t e d
b y t h e n u m e r i c a l s o l u t i o n o f t h e i n c o m p r e s s i b l e N a v i e r - S t o k e s e q u a t i o n s i n s i d e a c u b e
w i t h p e r i o d i c b o u n d a r y c o n d i t i o n s o n a l l s i d e s . I n t h i s n u m e r i c a l e x p e r i m e n t t h e
f u n c t i o n e v a l u a t i o n s c o n t r i b u t e o v e r w h e l m i n g l y t o t h e C P U t i m e , a n d t h e n u m b e r o f
t h e s e e v a l u a t i o n s m u s t b e k e p t t o a n a b s o l u t e m i n i m u m b e c a u s e o f t h e m a g n i t u d e o f
t h e p r o b l e m . O n t h e o t h e r h a n d , a c o m p l e t e e v e n t m u s t b e e s t a b l i s h e d i n o r d e r t o
o b t a i n m e a n i n g f u l s t a t i s t i c a l s a m p l e s w h i c h a r e t h e e s s e n c e o f t h e s o l u t i o n . I n t h i s
c a s e , i n a d d i t i o n t o t h e c o n s t r a i n t s g i v e n i n S e c t i o n 8 . 4 . 1 , w e a d d t h e f o l l o w i n g :
T h e n u m b e r o f e v a l u a t i o n s o f
~
F ( ~ u t ) i s x e d .
U n d e r t h e s e c o n d i t i o n s a m e t h o d i s j u d g e d a s b e s t w h e n i t h a s t h e h i g h e s t g l o b a l
a c c u r a c y f o r r e s o l v i n g e i g e n v e c t o r s w i t h i m a g i n a r y e i g e n v a l u e s . T h e a b o v e c o n s t r a i n t
h a s l e d t o t h e i n v e n t i o n o f s c h e m e s t h a t o m i t t h e f u n c t i o n e v a l u a t i o n i n t h e c o r -
r e c t o r s t e p o f a p r e d i c t o r - c o r r e c t o r c o m b i n a t i o n , l e a d i n g t o t h e s o - c a l l e d i n c o m p l e t e
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1 5 6 C H A P T E R 8 . C H O I C E O F T I M E - M A R C H I N G M E T H O D S
p r e d i c t o r - c o r r e c t o r m e t h o d s . T h e p r e s u m p t i o n i s , o f c o u r s e , t h a t m o r e e c i e n t m e t h -
o d s w i l l r e s u l t f r o m t h e o m i s s i o n o f t h e s e c o n d f u n c t i o n e v a l u a t i o n . A n e x a m p l e i s
t h e m e t h o d o f G a z d a g , g i v e n i n S e c t i o n 6 . 8 . B a s i c a l l y t h i s i s c o m p o s e d o f a n A B 2
p r e d i c t o r a n d a t r a p e z o i d a l c o r r e c t o r . H o w e v e r , t h e d e r i v a t i v e o f t h e f u n d a m e n t a l
f a m i l y i s n e v e r f o u n d s o t h e r e i s o n l y o n e e v a l u a t i o n r e q u i r e d t o c o m p l e t e e a c h c y c l e .
T h e - r e l a t i o n f o r t h e m e t h o d i s s h o w n a s e n t r y 1 0 i n T a b l e 7 . 1 .
I n o r d e r t o d i s c u s s o u r c o m p a r i s i o n s w e i n t r o d u c e t h e f o l l o w i n g d e n i t i o n s :
L e t a k - e v a l u a t i o n m e t h o d b e d e n e d a s o n e t h a t r e q u i r e s k e v a l u a t i o n s o f
~
F ( ~ u t ) t o a d v a n c e o n e s t e p u s i n g t h a t m e t h o d ' s t i m e i n t e r v a l , h .
L e t K r e p r e s e n t t h e t o t a l n u m b e r o f a l l o w a b l e F
e v
.
L e t h
1
b e t h e t i m e i n t e r v a l a d v a n c e d i n o n e s t e p o f a o n e - e v a l u a t i o n m e t h o d .
T h e G a z d a g , l e a p f r o g , a n d A B 2 s c h e m e s a r e a l l 1 - e v a l u a t i o n m e t h o d s . T h e s e c o n d
a n d f o u r t h o r d e r R K m e t h o d s a r e 2 - a n d 4 - e v a l u a t i o n m e t h o d s , r e s p e c t i v e l y . F o r a 1 -
e v a l u a t i o n m e t h o d t h e t o t a l n u m b e r o f t i m e s t e p s , N , a n d t h e n u m b e r o f e v a l u a t i o n s ,
K , a r e t h e s a m e , o n e e v a l u a t i o n b e i n g u s e d f o r e a c h s t e p , s o t h a t f o r t h e s e m e t h o d s
h = h
1
. F o r a 2 - e v a l u a t i o n m e t h o d N = K = 2 s i n c e t w o e v a l u a t i o n s a r e u s e d f o r
e a c h s t e p . H o w e v e r , i n t h i s c a s e , i n o r d e r t o a r r i v e a t t h e s a m e t i m e T a f t e r K
e v a l u a t i o n s , t h e t i m e s t e p m u s t b e t w i c e t h a t o f a o n e { e v a l u a t i o n m e t h o d s o h = 2 h
1
.
F o r a 4 - e v a l u a t i o n m e t h o d t h e t i m e i n t e r v a l m u s t b e h = 4 h
1
, e t c . N o t i c e t h a t
a s k i n c r e a s e s , t h e t i m e s p a n r e q u i r e d f o r o n e a p p l i c a t i o n o f t h e m e t h o d i n c r e a s e s .
H o w e v e r , n o t i c e a l s o t h a t a s k i n c r e a s e s , t h e p o w e r t o w h i c h
1
i s r a i s e d t o a r r i v e
a t t h e n a l d e s t i n a t i o n d e c r e a s e s s e e t h e F i g u r e b e l o w . T h i s i s t h e k e y t o t h e t r u e
c o m p a r i s o n o f t i m e - m a r c h m e t h o d s f o r t h i s t y p e o f p r o b l e m .
0 T u
N
k = 1 j j ( h
1
) ]
8
k = 2 j 2 h
1
j ( 2 h
1
) ]
4
k = 4 j 4 h
1
j ( 4 h
1
) ]
2
S t e p s i z e s a n d p o w e r s o f f o r k - e v a l u a t i o n m e t h o d s u s e d t o g e t t o t h e s a m e v a l u e
o f T i f 8 e v a l u a t i o n s a r e a l l o w e d .
I n g e n e r a l , a f t e r K e v a l u a t i o n s , t h e g l o b a l a m p l i t u d e a n d p h a s e e r r o r f o r k -
e v a l u a t i o n m e t h o d s a p p l i e d t o s y s t e m s w i t h p u r e i m a g i n a r y - r o o t s c a n b e w r i t t e n
1
E r
a
= 1 ; j
1
( i k ! h
1
) j
K = k
( 8 . 4 )
1
S e e E q s . 6 . 3 8 a n d 6 . 3 9 .
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8 . 4 . C O M P A R I N G T H E E F F I C I E N C Y O F E X P L I C I T M E T H O D S 1 5 7
E r
!
= ! T ;
K
k
t a n
; 1
"
1
( i k ! h
1
) ]
i m a g i n a r y
1
( i k ! h
1
) ]
r e a l
#
( 8 . 5 )
C o n s i d e r a c o n v e c t i o n - d o m i n a t e d e v e n t f o r w h i c h t h e f u n c t i o n e v a l u a t i o n i s v e r y
t i m e c o n s u m i n g . W e i d e a l i z e t o t h e c a s e w h e r e = i ! a n d s e t ! e q u a l t o o n e . T h e
e v e n t m u s t p r o c e e d t o t h e t i m e t = T = 1 0 . W e c o n s i d e r t w o m a x i m u m e v a l u a t i o n
l i m i t s K = 5 0 a n d K = 1 0 0 a n d c h o o s e f r o m f o u r p o s s i b l e m e t h o d s , l e a p f r o g , A B 2 ,
G a z d a g , a n d R K 4 . T h e r s t t h r e e o f t h e s e a r e o n e - e v a l u a t i o n m e t h o d s a n d t h e l a s t
o n e i s a f o u r - e v a l u a t i o n m e t h o d . I t i s n o t d i c u l t t o s h o w t h a t o n t h e b a s i s o f l o c a l
e r r o r ( m a d e i n a s i n g l e s t e p ) t h e G a z d a g m e t h o d i s s u p e r i o r t o t h e R K 4 m e t h o d i n
b o t h a m p l i t u d e a n d p h a s e . F o r e x a m p l e , f o r ! h = 0 : 2 t h e G a z d a g m e t h o d p r o d u c e s
a j
1
j = 0 : 9 9 9 2 2 7 6 w h e r e a s f o r ! h = 0 : 8 ( w h i c h m u s t b e u s e d t o k e e p t h e n u m b e r o f
e v a l u a t i o n s t h e s a m e ) t h e R K 4 m e t h o d p r o d u c e s a j
1
j = 0 : 9 9 8 3 2 4 . H o w e v e r , w e a r e
m a k i n g o u r c o m p a r i s o n s o n t h e b a s i s o f g l o b a l e r r o r f o r a x e d n u m b e r o f e v a l u a t i o n s .
F i r s t o f a l l w e s e e t h a t f o r a o n e - e v a l u a t i o n m e t h o d h
1
= T = K . U s i n g t h i s , a n d t h e
f a c t t h a t ! = 1 , w e n d , b y s o m e r a t h e r s i m p l e c a l c u l a t i o n s
2
m a d e u s i n g E q s . 8 . 4 a n d
8 . 5 , t h e r e s u l t s s h o w n i n T a b l e 8 . 2 . N o t i c e t h a t t o n d g l o b a l e r r o r t h e G a z d a g r o o t
m u s t b e r a i s e d t o t h e p o w e r o f 5 0 w h i l e t h e R K 4 r o o t i s r a i s e d o n l y t o t h e p o w e r o f
5 0 / 4 . O n t h e b a s i s o f g l o b a l e r r o r t h e G a z d a g m e t h o d i s n o t s u p e r i o r t o R K 4 i n e i t h e r
a m p l i t u d e o r p h a s e , a l t h o u g h , i n t e r m s o f p h a s e e r r o r ( f o r w h i c h i t w a s d e s i g n e d ) i t
i s s u p e r i o r t o t h e o t h e r t w o m e t h o d s s h o w n .
K l e a p f r o g A B 2 G a z d a g R K 4
! h
1
= : 1 1 0 0 1 : 0 1 : 0 0 3 : 9 9 5 : 9 9 9
! h
1
= : 2 5 0 1 : 0 1 : 0 2 2 : 9 6 2 : 9 7 9
a . A m p l i t u d e , e x a c t = 1 . 0 .
K l e a p f r o g A B 2 G a z d a g R K 4
! h
1
= : 1 1 0 0 ; : 9 6 ; 2 : 4 : 4 5 : 1 2
! h
1
= : 2 5 0 ; 3 : 8 ; 9 : 8 1 : 5 1 : 5
b . P h a s e e r r o r i n d e g r e e s .
T a b l e 8 . 2 : C o m p a r i s o n o f g l o b a l a m p l i t u d e a n d p h a s e e r r o r s f o r f o u r m e t h o d s .
2
T h e
1
r o o t f o r t h e G a z d a g m e t h o d c a n b e f o u n d u s i n g a n u m e r i c a l r o o t n d i n g r o u t i n e t o t r a c e
t h e t h r e e r o o t s i n t h e - p l a n e , s e e F i g . 7 . 3 e .
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1 5 8 C H A P T E R 8 . C H O I C E O F T I M E - M A R C H I N G M E T H O D S
U s i n g a n a l y s i s s u c h a s t h i s ( a n d a l s o c o n s i d e r i n g t h e s t a b i l i t y b o u n d a r i e s ) t h e
R K 4 m e t h o d i s r e c o m m e n d e d a s a b a s i c r s t c h o i c e f o r a n y e x p l i c i t t i m e - a c c u r a t e
c a l c u l a t i o n o f a c o n v e c t i o n - d o m i n a t e d p r o b l e m .
8 . 5 C o p i n g W i t h S t i n e s s
8 . 5 . 1 E x p l i c i t M e t h o d s
T h e a b i l i t y o f a n u m e r i c a l m e t h o d t o c o p e w i t h s t i n e s s c a n b e i l l u s t r a t e d q u i t e n i c e l y
i n t h e c o m p l e x h p l a n e . A g o o d e x a m p l e o f t h e c o n c e p t i s p r o d u c e d b y s t u d y i n g
t h e E u l e r m e t h o d a p p l i e d t o t h e r e p r e s e n t a t i v e e q u a t i o n . T h e t r a n s i e n t s o l u t i o n i s
u
n
= ( 1 + h )
n
a n d t h e t r a c e o f t h e c o m p l e x v a l u e o f h w h i c h m a k e s j 1 + h j = 1
g i v e s t h e w h o l e s t o r y . I n t h i s c a s e t h e t r a c e f o r m s a c i r c l e o f u n i t r a d i u s c e n t e r e d a t
( ; 1 0 ) a s s h o w n i n F i g . 8 . 1 . I f h i s c h o s e n s o t h a t a l l h i n t h e O D E e i g e n s y s t e m
f a l l i n s i d e t h i s c i r c l e t h e i n t e g r a t i o n w i l l b e n u m e r i c a l l y s t a b l e . A l s o s h o w n b y t h e
s m a l l c i r c l e c e n t e r e d a t t h e o r i g i n i s t h e r e g i o n o f T a y l o r s e r i e s a c c u r a c y . I f s o m e h
f a l l o u t s i d e t h e s m a l l c i r c l e b u t s t a y w i t h i n t h e s t a b l e r e g i o n , t h e s e h a r e s t i , b u t
s t a b l e . W e h a v e d e n e d t h e s e h a s p a r a s i t i c e i g e n v a l u e s . S t a b i l i t y b o u n d a r i e s f o r
s o m e e x p l i c i t m e t h o d s a r e s h o w n i n F i g s . 7 . 5 a n d 7 . 6 .
F o r a s p e c i c e x a m p l e , c o n s i d e r t h e m i l d l y s t i s y s t e m c o m p o s e d o f a c o u p l e d
t w o - e q u a t i o n s e t h a v i n g t h e t w o e i g e n v a l u e s
1
= ; 1 0 0 a n d
2
= ; 1 . I f u n c o u p l e d
a n d e v a l u a t e d i n w a v e s p a c e , t h e t i m e h i s t o r i e s o f t h e t w o s o l u t i o n s w o u l d a p p e a r a s a
r a p i d l y d e c a y i n g f u n c t i o n i n o n e c a s e , a n d a r e l a t i v e l y s l o w l y d e c a y i n g f u n c t i o n i n t h e
o t h e r . A n a l y t i c a l e v a l u a t i o n o f t h e t i m e h i s t o r i e s p o s e s n o p r o b l e m s i n c e e
; 1 0 0 t
q u i c k l y
b e c o m e s v e r y s m a l l a n d c a n b e n e g l e c t e d i n t h e e x p r e s s i o n s w h e n t i m e b e c o m e s l a r g e .
N u m e r i c a l e v a l u a t i o n i s a l t o g e t h e r d i e r e n t . N u m e r i c a l s o l u t i o n s , o f c o u r s e , d e p e n d
u p o n (
m
h ) ]
n
a n d n o j
m
j c a n e x c e e d o n e f o r a n y
m
i n t h e c o u p l e d s y s t e m o r e l s e
t h e p r o c e s s i s n u m e r i c a l l y u n s t a b l e .
L e t u s c h o o s e t h e s i m p l e e x p l i c i t E u l e r m e t h o d f o r t h e t i m e m a r c h . T h e c o u p l e d
e q u a t i o n s i n r e a l s p a c e a r e r e p r e s e n t e d b y
u
1
( n ) = c
1
( 1 ; 1 0 0 h )
n
x
1 1
+ c
2
( 1 ; h )
n
x
1 2
+ ( P S )
1
u
2
( n ) = c
1
( 1 ; 1 0 0 h )
n
x
2 1
+ c
2
( 1 ; h )
n
x
2 2
+ ( P S )
2
( 8 . 6 )
W e w i l l a s s u m e t h a t o u r a c c u r a c y r e q u i r e m e n t s a r e s u c h t h a t s u c i e n t a c c u r a c y i s
o b t a i n e d a s l o n g a s j h j 0 : 1 . T h i s d e n e s a t i m e s t e p l i m i t b a s e d o n a c c u r a c y
c o n s i d e r a t i o n s o f h = 0 : 0 0 1 f o r
1
a n d h = 0 : 1 f o r
2
. T h e t i m e s t e p l i m i t b a s e d
o n s t a b i l i t y , w h i c h i s d e t e r m i n e d f r o m
1
, i s h = 0 : 0 2 . W e w i l l a l s o a s s u m e t h a t
c
1
= c
2
= 1 a n d t h a t a n a m p l i t u d e l e s s t h a n 0 . 0 0 1 i s n e g l i g i b l e . W e r s t r u n 6 6
t i m e s t e p s w i t h h = 0 : 0 0 1 i n o r d e r t o r e s o l v e t h e
1
t e r m . W i t h t h i s t i m e s t e p t h e
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1 6 0 C H A P T E R 8 . C H O I C E O F T I M E - M A R C H I N G M E T H O D S
s t e p s i s e v e n l e s s . A c t u a l l y , a l t h o u g h t h i s r o o t i s o n e o f t h e p r i n c i p a l r o o t s i n t h e
s y s t e m , i t s b e h a v i o r f o r t > 0 : 0 7 i s i d e n t i c a l t o t h a t o f a s t a b l e s p u r i o u s r o o t . T h e
t o t a l s i m u l a t i o n r e q u i r e s 1 3 9 t i m e s t e p s .
8 . 5 . 3 A P e r s p e c t i v e
I t i s i m p o r t a n t t o r e t a i n a p r o p e r p e r s p e c t i v e o n a p r o b l e m r e p r e s e n t e d b y t h e a b o v e
e x a m p l e . I t i s c l e a r t h a t a n u n c o n d i t i o n a l l y s t a b l e m e t h o d c a n a l w a y s b e c a l l e d u p o n
t o s o l v e s t i p r o b l e m s w i t h a m i n i m u m n u m b e r o f t i m e s t e p s . I n t h e e x a m p l e , t h e
c o n d i t i o n a l l y s t a b l e E u l e r m e t h o d r e q u i r e d 4 0 5 t i m e s t e p s , a s c o m p a r e d t o a b o u t
1 3 9 f o r t h e t r a p e z o i d a l m e t h o d , a b o u t t h r e e t i m e s a s m a n y . H o w e v e r , t h e E u l e r
m e t h o d i s e x t r e m e l y e a s y t o p r o g r a m a n d r e q u i r e s v e r y l i t t l e a r i t h m e t i c p e r s t e p . F o r
p r e l i m i n a r y i n v e s t i g a t i o n s i t i s o f t e n t h e b e s t m e t h o d t o u s e f o r m i l d l y - s t i d i u s i o n
d o m i n a t e d p r o b l e m s . F o r r e n e d i n v e s t i g a t i o n s o f s u c h p r o b l e m s a n e x p l i c i t m e t h o d
o f s e c o n d o r d e r o r h i g h e r , s u c h a s A d a m s - B a s h f o r t h o r R u n g e - K u t t a m e t h o d s , i s
r e c o m m e n d e d . T h e s e e x p l i c i t m e t h o d s c a n b e c o n s i d e r e d a s e e c t i v e m i l d l y s t i -
s t a b l e m e t h o d s . H o w e v e r , i t s h o u l d b e c l e a r t h a t a s t h e d e g r e e o f s t i n e s s o f t h e
p r o b l e m i n c r e a s e s , t h e a d v a n t a g e b e g i n s t o t i l t t o w a r d s i m p l i c i t m e t h o d s , a s t h e
r e d u c e d n u m b e r o f t i m e s t e p s b e g i n s t o o u t w e i g h t h e i n c r e a s e d c o s t p e r t i m e s t e p .
T h e r e a d e r c a n r e p e a t t h e a b o v e e x a m p l e w i t h
1
= ; 1 0 0 0 0 ,
2
= ; 1 , w h i c h i s i n
t h e s t r o n g l y - s t i c a t e g o r y .
T h e r e i s y e t a n o t h e r t e c h n i q u e f o r c o p i n g w i t h c e r t a i n s t i s y s t e m s i n u i d d y n a m i c
a p p l i c a t i o n s . T h i s i s k n o w n a s t h e m u l t i g r i d m e t h o d . I t h a s e n j o y e d r e m a r k a b l e
s u c c e s s i n m a n y p r a c t i c a l p r o b l e m s h o w e v e r , w e n e e d a n i n t r o d u c t i o n t o t h e t h e o r y
o f r e l a x a t i o n b e f o r e i t c a n b e p r e s e n t e d .
8 . 6 S t e a d y P r o b l e m s
I n C h a p t e r 6 w e w r o t e t h e O E s o l u t i o n i n t e r m s o f t h e p r i n c i p a l a n d s p u r i o u s r o o t s
a s f o l l o w s :
u
n
= c
1 1
(
1
)
n
1
~
x
1
+ + c
m 1
(
m
)
n
1
~
x
m
+ + c
M 1
(
M
)
n
1
~
x
M
+ P : S :
+ c
1 2
(
1
)
n
2
~
x
1
+ + c
m 2
(
m
)
n
2
~
x
m
+ + c
M 2
(
M
)
n
2
~
x
M
+ c
1 3
(
1
)
n
3
~
x
1
+ + c
m 3
(
m
)
n
3
~
x
m
+ + c
M 3
(
M
)
n
3
~
x
M
+ e t c . , i f t h e r e a r e m o r e s p u r i o u s r o o t s ( 8 . 8 )
W h e n s o l v i n g a s t e a d y p r o b l e m , w e h a v e n o i n t e r e s t w h a t s o e v e r i n t h e t r a n s i e n t p o r -
t i o n o f t h e s o l u t i o n . O u r s o l e g o a l i s t o e l i m i n a t e i t a s q u i c k l y a s p o s s i b l e . T h e r e f o r e ,
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8 . 7 . P R O B L E M S 1 6 1
t h e c h o i c e o f a t i m e - m a r c h i n g m e t h o d f o r a s t e a d y p r o b l e m i s s i m i l a r t o t h a t f o r a
s t i p r o b l e m , t h e d i e r e n c e b e i n g t h a t t h e o r d e r o f a c c u r a c y i s i r r e l e v a n t . H e n c e t h e
e x p l i c i t E u l e r m e t h o d i s a c a n d i d a t e f o r s t e a d y d i u s i o n d o m i n a t e d p r o b l e m s , a n d
t h e f o u r t h - o r d e r R u n g e - K u t t a m e t h o d i s a c a n d i d a t e f o r s t e a d y c o n v e c t i o n d o m i n a t e d
p r o b l e m s , b e c a u s e o f t h e i r s t a b i l i t y p r o p e r t i e s . A m o n g i m p l i c i t m e t h o d s , t h e i m p l i c i t
E u l e r m e t h o d i s t h e o b v i o u s c h o i c e f o r s t e a d y p r o b l e m s .
W h e n w e s e e k o n l y t h e s t e a d y s o l u t i o n , a l l o f t h e e i g e n v a l u e s c a n b e c o n s i d e r e d t o
b e p a r a s i t i c . R e f e r r i n g t o F i g . 8 . 1 , n o n e o f t h e e i g e n v a l u e s a r e r e q u i r e d t o f a l l i n t h e
a c c u r a t e r e g i o n o f t h e t i m e - m a r c h i n g m e t h o d . T h e r e f o r e t h e t i m e s t e p c a n b e c h o s e n
t o e l i m i n a t e t h e t r a n s i e n t a s q u i c k l y a s p o s s i b l e w i t h n o r e g a r d f o r t i m e a c c u r a c y .
F o r e x a m p l e , w h e n u s i n g t h e i m p l i c i t E u l e r m e t h o d w i t h l o c a l t i m e l i n e a r i z a t i o n , E q .
6 . 9 6 , o n e w o u l d l i k e t o t a k e t h e l i m i t h ! 1 , w h i c h l e a d s t o N e w t o n ' s m e t h o d , E q .
6 . 9 8 . H o w e v e r , a n i t e t i m e s t e p m a y b e r e q u i r e d u n t i l t h e s o l u t i o n i s s o m e w h a t c l o s e
t o t h e s t e a d y s o l u t i o n .
8 . 7 P r o b l e m s
1 . R e p e a t t h e t i m e - m a r c h c o m p a r i s o n s f o r d i u s i o n ( S e c t i o n 8 . 4 . 2 ) a n d p e r i o d i c
c o n v e c t i o n ( S e c t i o n 8 . 4 . 3 ) u s i n g 2 n d - a n d 3 r d - o r d e r R u n g e - K u t t a m e t h o d s .
2 . R e p e a t t h e t i m e - m a r c h c o m p a r i s o n s f o r d i u s i o n ( S e c t i o n 8 . 4 . 2 ) a n d p e r i o d i c
c o n v e c t i o n ( S e c t i o n 8 . 4 . 3 ) u s i n g t h e 3 r d - a n d 4 t h - o r d e r A d a m s - B a s h f o r t h m e t h -
o d s . C o n s i d e r i n g t h e s t a b i l i t y b o u n d s f o r t h e s e m e t h o d s ( s e e p r o b l e m 4 i n
C h a p t e r 7 ) a s w e l l a s t h e i r m e m o r y r e q u i r e m e n t s , c o m p a r e a n d c o n t r a s t t h e m
w i t h t h e 3 r d - a n d 4 t h - o r d e r R u n g e - K u t t a m e t h o d s .
3 . C o n s i d e r t h e d i u s i o n e q u a t i o n ( w i t h = 1 ) d i s c r e t i z e d u s i n g 2 n d - o r d e r c e n t r a l
d i e r e n c e s o n a g r i d w i t h 1 0 ( i n t e r i o r ) p o i n t s . F i n d a n d p l o t t h e e i g e n v a l u e s
a n d t h e c o r r e s p o n d i n g m o d i e d w a v e n u m b e r s . I f w e u s e t h e e x p l i c i t E u l e r t i m e -
m a r c h i n g m e t h o d w h a t i s t h e m a x i m u m a l l o w a b l e t i m e s t e p i f a l l b u t t h e r s t
t w o e i g e n v e c t o r s a r e c o n s i d e r e d p a r a s i t i c ? A s s u m e t h a t s u c i e n t a c c u r a c y i s
o b t a i n e d a s l o n g a s j h j 0 : 1 . W h a t i s t h e m a x i m u m a l l o w a b l e t i m e s t e p i f a l l
b u t t h e r s t e i g e n v e c t o r a r e c o n s i d e r e d p a r a s i t i c ?
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1 6 2 C H A P T E R 8 . C H O I C E O F T I M E - M A R C H I N G M E T H O D S
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C h a p t e r 9
R E L A X A T I O N M E T H O D S
I n t h e p a s t t h r e e c h a p t e r s , w e d e v e l o p e d a m e t h o d o l o g y f o r d e s i g n i n g , a n a l y z i n g ,
a n d c h o o s i n g t i m e - m a r c h i n g m e t h o d s . T h e s e m e t h o d s c a n b e u s e d t o c o m p u t e t h e
t i m e - a c c u r a t e s o l u t i o n t o l i n e a r a n d n o n l i n e a r s y s t e m s o f O D E ' s i n t h e g e n e r a l f o r m
d ~u
d t
=
~
F ( ~ u t ) ( 9 . 1 )
w h i c h a r i s e a f t e r s p a t i a l d i s c r e t i z a t i o n o f a P D E . A l t e r n a t i v e l y , t h e y c a n b e u s e d t o
s o l v e f o r t h e s t e a d y s o l u t i o n o f E q . 9 . 1 , w h i c h s a t i s e s t h e f o l l o w i n g c o u p l e d s y s t e m
o f n o n l i n e a r a l g e b r a i c e q u a t i o n s :
~
F ( ~u ) = 0 ( 9 . 2 )
I n t h e l a t t e r c a s e , t h e u n s t e a d y e q u a t i o n s a r e i n t e g r a t e d u n t i l t h e s o l u t i o n c o n v e r g e s
t o a s t e a d y s o l u t i o n . T h e s a m e a p p r o a c h p e r m i t s a t i m e - m a r c h i n g m e t h o d t o b e u s e d
t o s o l v e a l i n e a r s y s t e m o f a l g e b r a i c e q u a t i o n s i n t h e f o r m
A ~x =
~
b ( 9 . 3 )
T o s o l v e t h i s s y s t e m u s i n g a t i m e - m a r c h i n g m e t h o d , a t i m e d e r i v a t i v e i s i n t r o d u c e d
a s f o l l o w s
d ~x
d t
= A ~x ;
~
b ( 9 . 4 )
a n d t h e s y s t e m i s i n t e g r a t e d i n t i m e u n t i l t h e t r a n s i e n t h a s d e c a y e d t o a s u c i e n t l y
l o w l e v e l . F o l l o w i n g a t i m e - d e p e n d e n t p a t h t o s t e a d y s t a t e i s p o s s i b l e o n l y i f a l l o f
t h e e i g e n v a l u e s o f t h e m a t r i x A ( o r ; A ) h a v e r e a l p a r t s l y i n g i n t h e l e f t h a l f - p l a n e .
A l t h o u g h t h e s o l u t i o n ~x = A
; 1
~
b e x i s t s a s l o n g a s A i s n o n s i n g u l a r , t h e O D E g i v e n b y
E q . 9 . 4 h a s a s t a b l e s t e a d y s o l u t i o n o n l y i f A m e e t s t h e a b o v e c o n d i t i o n .
1 6 3
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1 6 4 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
T h e c o m m o n f e a t u r e o f a l l t i m e - m a r c h i n g m e t h o d s i s t h a t t h e y a r e a t l e a s t r s t -
o r d e r a c c u r a t e . I n t h i s c h a p t e r , w e c o n s i d e r i t e r a t i v e m e t h o d s w h i c h a r e n o t t i m e
a c c u r a t e a t a l l . S u c h m e t h o d s a r e k n o w n a s r e l a x a t i o n m e t h o d s . W h i l e t h e y a r e
a p p l i c a b l e t o c o u p l e d s y s t e m s o f n o n l i n e a r a l g e b r a i c e q u a t i o n s i n t h e f o r m o f E q . 9 . 2 ,
o u r a n a l y s i s w i l l f o c u s o n t h e i r a p p l i c a t i o n t o l a r g e s p a r s e l i n e a r s y s t e m s o f e q u a t i o n s
i n t h e f o r m
A
b
~
u ;
~
f
b
= 0 ( 9 . 5 )
w h e r e A
b
i s n o n s i n g u l a r , a n d t h e u s e o f t h e s u b s c r i p t b w i l l b e c o m e c l e a r s h o r t l y . S u c h
s y s t e m s o f e q u a t i o n s a r i s e , f o r e x a m p l e , a t e a c h t i m e s t e p o f a n i m p l i c i t t i m e - m a r c h i n g
m e t h o d o r a t e a c h i t e r a t i o n o f N e w t o n ' s m e t h o d . U s i n g a n i t e r a t i v e m e t h o d , w e s e e k
t o o b t a i n r a p i d l y a s o l u t i o n w h i c h i s a r b i t r a r i l y c l o s e t o t h e e x a c t s o l u t i o n o f E q . 9 . 5 ,
w h i c h i s g i v e n b y
~
u
1
= A
; 1
b
~
f
b
( 9 . 6 )
9 . 1 F o r m u l a t i o n o f t h e M o d e l P r o b l e m
9 . 1 . 1 P r e c o n d i t i o n i n g t h e B a s i c M a t r i x
I t i s s t a n d a r d p r a c t i c e i n a p p l y i n g r e l a x a t i o n p r o c e d u r e s t o p r e c o n d i t i o n t h e b a s i c
e q u a t i o n . T h i s p r e c o n d i t i o n i n g h a s t h e e e c t o f m u l t i p l y i n g E q . 9 . 5 f r o m t h e l e f t
b y s o m e n o n s i n g u l a r m a t r i x . I n t h e s i m p l e s t p o s s i b l e c a s e t h e c o n d i t i o n i n g m a t r i x
i s a d i a g o n a l m a t r i x c o m p o s e d o f a c o n s t a n t D ( b ) . I f w e d e s i g n a t e t h e c o n d i t i o n i n g
m a t r i x b y C , t h e p r o b l e m b e c o m e s o n e o f s o l v i n g f o r
~
u i n
C A
b
~
u ; C
~
f
b
= 0 ( 9 . 7 )
N o t i c e t h a t t h e s o l u t i o n o f E q . 9 . 7 i s
~
u = C A
b
]
; 1
C
~
f
b
= A
; 1
b
C
; 1
C
~
f
b
= A
; 1
b
~
f
b
( 9 . 8 )
w h i c h i s i d e n t i c a l t o t h e s o l u t i o n o f E q . 9 . 5 , p r o v i d e d C
; 1
e x i s t s .
I n t h e f o l l o w i n g w e w i l l s e e t h a t o u r a p p r o a c h t o t h e i t e r a t i v e s o l u t i o n o f E q . 9 . 7
d e p e n d s c r u c i a l l y o n t h e e i g e n v a l u e a n d e i g e n v e c t o r s t r u c t u r e o f t h e m a t r i x C A
b
, a n d ,
e q u a l l y i m p o r t a n t , d o e s n o t d e p e n d a t a l l o n t h e e i g e n s y s t e m o f t h e b a s i c m a t r i x A
b
.
F o r e x a m p l e , t h e r e a r e w e l l - k n o w n t e c h n i q u e s f o r a c c e l e r a t i n g r e l a x a t i o n s c h e m e s i f
t h e e i g e n v a l u e s o f C A
b
a r e a l l r e a l a n d o f t h e s a m e s i g n . T o u s e t h e s e s c h e m e s , t h e
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9 . 1 . F O R M U L A T I O N O F T H E M O D E L P R O B L E M 1 6 5
c o n d i t i o n i n g m a t r i x C m u s t b e c h o s e n s u c h t h a t t h i s r e q u i r e m e n t i s s a t i s e d . A
c h o i c e o f C w h i c h e n s u r e s t h i s c o n d i t i o n i s t h e n e g a t i v e t r a n s p o s e o f A
b
.
F o r e x a m p l e , c o n s i d e r a s p a t i a l d i s c r e t i z a t i o n o f t h e l i n e a r c o n v e c t i o n e q u a t i o n
u s i n g c e n t e r e d d i e r e n c e s w i t h a D i r i c h l e t c o n d i t i o n o n t h e l e f t s i d e a n d n o c o n s t r a i n t
o n t h e r i g h t s i d e . U s i n g a r s t - o r d e r b a c k w a r d d i e r e n c e o n t h e r i g h t s i d e ( a s i n
S e c t i o n 3 . 6 ) , t h i s l e a d s t o t h e a p p r o x i m a t i o n
x
~
u =
1
2 x
8
>
>
>
>
>
>
<
>
>
>
>
>
>
:
2
6
6
6
6
6
6
4
0 1
; 1 0 1
; 1 0 1
; 1 0 1
; 2 2
3
7
7
7
7
7
7
5
~
u +
2
6
6
6
6
6
6
4
; u
a
0
0
0
0
3
7
7
7
7
7
7
5
9
>
>
>
>
>
>
=
>
>
>
>
>
>
( 9 . 9 )
T h e m a t r i x i n E q . 9 . 9 h a s e i g e n v a l u e s w h o s e i m a g i n a r y p a r t s a r e m u c h l a r g e r t h a n
t h e i r r e a l p a r t s . I t c a n r s t b e c o n d i t i o n e d s o t h a t t h e m o d u l u s o f e a c h e l e m e n t i s 1 .
T h i s i s a c c o m p l i s h e d u s i n g a d i a g o n a l p r e c o n d i t i o n i n g m a t r i x
D = 2 x
2
6
6
6
6
6
6
4
1 0 0 0 0
0 1 0 0 0
0 0 1 0 0
0 0 0 1 0
0 0 0 0
1
2
3
7
7
7
7
7
7
5
( 9 . 1 0 )
w h i c h s c a l e s e a c h r o w . W e t h e n f u r t h e r c o n d i t i o n w i t h m u l t i p l i c a t i o n b y t h e n e g a t i v e
t r a n s p o s e . T h e r e s u l t i s
A
2
= ; A
T
1
A
1
=
2
6
6
6
6
6
6
4
0 1
; 1 0 1
; 1 0 1
; 1 0 1
; 1 ; 1
3
7
7
7
7
7
7
5
2
6
6
6
6
6
6
4
0 1
; 1 0 1
; 1 0 1
; 1 0 1
; 1 1
3
7
7
7
7
7
7
5
=
2
6
6
6
6
6
6
4
; 1 0 1
0
; 2 0 1
1 0
; 2 0 1
1 0 ; 2 1
1 1 ; 2
3
7
7
7
7
7
7
5
( 9 . 1 1 )
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1 6 6 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
I f w e d e n e a p e r m u t a t i o n m a t r i x P
1
a n d c a r r y o u t t h e p r o c e s s P
T
; A
T
1
A
1
] P ( w h i c h
j u s t r e o r d e r s t h e e l e m e n t s o f A
1
a n d d o e s n ' t c h a n g e t h e e i g e n v a l u e s ) w e n d
2
6
6
6
6
6
6
4
0 1 0 0 0
0 0 0 1 0
0 0 0 0 1
0 0 1 0 0
1 0 0 0 0
3
7
7
7
7
7
7
5
2
6
6
6
6
6
6
4
; 1 0 1
0 ; 2 0 1
1 0 ; 2 0 1
1 0 ; 2 1
1 1 ; 2
3
7
7
7
7
7
7
5
2
6
6
6
6
6
6
4
0 0 0 0 1
1 0 0 0 0
0 0 0 1 0
0 1 0 0 0
0 0 1 0 0
3
7
7
7
7
7
7
5
=
2
6
6
6
6
6
6
4
; 2 1
1 ; 2 1
1 ; 2 1
1 ; 2 1
1 ; 1
3
7
7
7
7
7
7
5
( 9 . 1 2 )
w h i c h h a s a l l n e g a t i v e r e a l e i g e n v a l u e s , a s g i v e n i n A p p e n d i x B . T h u s e v e n w h e n
t h e b a s i c m a t r i x A
b
h a s n e a r l y i m a g i n a r y e i g e n v a l u e s , t h e c o n d i t i o n e d m a t r i x ; A
T
b
A
b
i s n e v e r t h e l e s s s y m m e t r i c n e g a t i v e d e n i t e ( i . e . , s y m m e t r i c w i t h n e g a t i v e r e a l e i g e n -
v a l u e s ) , a n d t h e c l a s s i c a l r e l a x a t i o n m e t h o d s c a n b e a p p l i e d . W e d o n o t n e c e s s a r i l y
r e c o m m e n d t h e u s e o f ; A
T
b
a s a p r e c o n d i t i o n e r w e s i m p l y w i s h t o s h o w t h a t a b r o a d
r a n g e o f m a t r i c e s c a n b e p r e c o n d i t i o n e d i n t o a f o r m s u i t a b l e f o r o u r a n a l y s i s .
9 . 1 . 2 T h e M o d e l E q u a t i o n s
P r e c o n d i t i o n i n g p r o c e s s e s s u c h a s t h o s e d e s c r i b e d i n t h e l a s t s e c t i o n a l l o w u s t o
p r e p a r e o u r a l g e b r a i c e q u a t i o n s i n a d v a n c e s o t h a t c e r t a i n e i g e n s t r u c t u r e s a r e g u a r -
a n t e e d . I n t h e r e m a i n d e r o f t h i s c h a p t e r , w e w i l l t h o r o u g h l y i n v e s t i g a t e s o m e s i m p l e
e q u a t i o n s w h i c h m o d e l t h e s e s t r u c t u r e s . W e w i l l c o n s i d e r t h e p r e c o n d i t i o n e d s y s t e m
o f e q u a t i o n s h a v i n g t h e f o r m
A
~
;
~
f = 0 ( 9 . 1 3 )
w h e r e A i s s y m m e t r i c n e g a t i v e d e n i t e .
2
T h e s y m b o l f o r t h e d e p e n d e n t v a r i a b l e h a s
b e e n c h a n g e d t o a s a r e m i n d e r t h a t t h e p h y s i c s b e i n g m o d e l e d i s n o l o n g e r t i m e
1
A p e r m u t a t i o n m a t r i x ( d e n e d a s a m a t r i x w i t h e x a c t l y o n e 1 i n e a c h r o w a n d c o l u m n a n d h a s
t h e p r o p e r t y t h a t P
T
= P
; 1
) j u s t r e a r r a n g e s t h e r o w s a n d c o l u m n s o f a m a t r i x .
2
W e u s e a s y m m e t r i c n e g a t i v e d e n i t e m a t r i x t o s i m p l i f y c e r t a i n a s p e c t s o f o u r a n a l y s i s . R e l a x -
a t i o n m e t h o d s a r e a p p l i c a b l e t o m o r e g e n e r a l m a t r i c e s . T h e c l a s s i c a l m e t h o d s w i l l u s u a l l y c o n v e r g e
i f A
b
i s d i a g o n a l l y d o m i n a n t , a s d e n e d i n A p p e n d i x A .
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9 . 1 . F O R M U L A T I O N O F T H E M O D E L P R O B L E M 1 6 7
a c c u r a t e w h e n w e l a t e r d e a l w i t h O D E f o r m u l a t i o n s . N o t e t h a t t h e s o l u t i o n o f E q .
9 . 1 3 ,
~
= A
; 1
~
f , i s g u a r a n t e e d t o e x i s t b e c a u s e A i s n o n s i n g u l a r . I n t h e n o t a t i o n o f
E q s . 9 . 5 a n d 9 . 7 ,
A = C A
b
a n d
~
f = C
~
f
b
( 9 . 1 4 )
T h e a b o v e w a s w r i t t e n t o t r e a t t h e g e n e r a l c a s e . I t i s i n s t r u c t i v e i n f o r m u l a t i n g t h e
c o n c e p t s t o c o n s i d e r t h e s p e c i a l c a s e g i v e n b y t h e d i u s i o n e q u a t i o n i n o n e d i m e n s i o n
w i t h u n i t d i u s i o n c o e c i e n t :
@ u
@ t
=
@
2
u
@ x
2
; g ( x ) ( 9 . 1 5 )
T h i s h a s t h e s t e a d y - s t a t e s o l u t i o n
@
2
u
@ x
2
= g ( x ) ( 9 . 1 6 )
w h i c h i s t h e o n e - d i m e n s i o n a l f o r m o f t h e P o i s s o n e q u a t i o n . I n t r o d u c i n g t h e t h r e e -
p o i n t c e n t r a l d i e r e n c i n g s c h e m e f o r t h e s e c o n d d e r i v a t i v e w i t h D i r i c h l e t b o u n d a r y
c o n d i t i o n s , w e n d
d
~
u
d t
=
1
x
2
B ( 1 ; 2 1 )
~
u + (
~
b c ) ;
~
g ( 9 . 1 7 )
w h e r e (
~
b c ) c o n t a i n s t h e b o u n d a r y c o n d i t i o n s a n d
~
g c o n t a i n s t h e v a l u e s o f t h e s o u r c e
t e r m a t t h e g r i d n o d e s . I n t h i s c a s e
A
b
=
1
x
2
B ( 1
; 2 1 )
~
f
b
=
~
g ; (
~
b c ) ( 9 . 1 8 )
C h o o s i n g C = x
2
I , w e o b t a i n
B ( 1 ; 2 1 )
~
=
~
f ( 9 . 1 9 )
w h e r e
~
f = x
2
~
f
b
. I f w e c o n s i d e r a D i r i c h l e t b o u n d a r y c o n d i t i o n o n t h e l e f t s i d e a n d
e i t h e r a D i r i c h l e t o r a N e u m a n n c o n d i t i o n o n t h e r i g h t s i d e , t h e n A h a s t h e f o r m
A = B
1
~
b 1
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1 6 8 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
~
b = ; 2 ; 2 ; 2 s ]
T
s = ; 2 o r ; 1 ( 9 . 2 0 )
N o t e t h a t s = ; 1 i s e a s i l y o b t a i n e d f r o m t h e m a t r i x r e s u l t i n g f r o m t h e N e u m a n n
b o u n d a r y c o n d i t i o n g i v e n i n E q . 3 . 2 4 u s i n g a d i a g o n a l c o n d i t i o n i n g m a t r i x . A t r e m e n -
d o u s a m o u n t o f i n s i g h t t o t h e b a s i c f e a t u r e s o f r e l a x a t i o n i s g a i n e d b y a n a p p r o p r i a t e
s t u d y o f t h e o n e - d i m e n s i o n a l c a s e , a n d m u c h o f t h e r e m a i n i n g m a t e r i a l i s d e v o t e d t o
t h i s c a s e . W e a t t e m p t t o d o t h i s i n s u c h a w a y , h o w e v e r , t h a t i t i s d i r e c t l y a p p l i c a b l e
t o t w o - a n d t h r e e - d i m e n s i o n a l p r o b l e m s .
9 . 2 C l a s s i c a l R e l a x a t i o n
9 . 2 . 1 T h e D e l t a F o r m o f a n I t e r a t i v e S c h e m e
W e w i l l c o n s i d e r r e l a x a t i o n m e t h o d s w h i c h c a n b e e x p r e s s e d i n t h e f o l l o w i n g d e l t a
f o r m :
H
h
~
n + 1
;
~
n
i
= A
~
n
;
~
f ( 9 . 2 1 )
w h e r e H i s s o m e n o n s i n g u l a r m a t r i x w h i c h d e p e n d s u p o n t h e i t e r a t i v e m e t h o d . T h e
m a t r i x H i s i n d e p e n d e n t o f n f o r s t a t i o n a r y m e t h o d s a n d i s a f u n c t i o n o f n f o r
n o n s t a t i o n a r y o n e s . T h e i t e r a t i o n c o u n t i s d e s i g n a t e d b y t h e s u b s c r i p t n o r t h e
s u p e r s c r i p t ( n ) . T h e c o n v e r g e d s o l u t i o n i s d e s i g n a t e d
~
1
s o t h a t
~
1
= A
; 1 ~
f ( 9 . 2 2 )
9 . 2 . 2 T h e C o n v e r g e d S o l u t i o n , t h e R e s i d u a l , a n d t h e E r r o r
S o l v i n g E q . 9 . 2 1 f o r
~
n + 1
g i v e s
~
n + 1
= I + H
; 1
A ]
~
n
; H
; 1 ~
f = G
~
n
; H
; 1 ~
f ( 9 . 2 3 )
w h e r e
G I + H
; 1
A ( 9 . 2 4 )
H e n c e i t i s c l e a r t h a t H s h o u l d l e a d t o a s y s t e m o f e q u a t i o n s w h i c h i s e a s y t o s o l v e ,
o r a t l e a s t e a s i e r t o s o l v e t h a n t h e o r i g i n a l s y s t e m . T h e e r r o r a t t h e n t h i t e r a t i o n i s
d e n e d a s
~
e
n
~
n
;
~
1
=
~
n
; A
; 1
~
f ( 9 . 2 5 )
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9 . 2 . C L A S S I C A L R E L A X A T I O N 1 6 9
w h e r e
~
1
w a s d e n e d i n E q . 9 . 2 2 . T h e r e s i d u a l a t t h e n t h i t e r a t i o n i s d e n e d a s
~
r
n
A
~
n
;
~
f ( 9 . 2 6 )
M u l t i p l y E q . 9 . 2 5 b y A f r o m t h e l e f t , a n d u s e t h e d e n i t i o n i n E q . 9 . 2 6 . T h e r e r e s u l t s
t h e r e l a t i o n b e t w e e n t h e e r r o r a n d t h e r e s i d u a l
A
~
e
n
;
~
r
n
= 0 ( 9 . 2 7 )
F i n a l l y , i t i s n o t d i c u l t t o s h o w t h a t
~
e
n + 1
= G
~
e
n
( 9 . 2 8 )
C o n s e q u e n t l y , G i s r e f e r r e d t o a s t h e b a s i c i t e r a t i o n m a t r i x , a n d i t s e i g e n v a l u e s , w h i c h
w e d e s i g n a t e a s
m
, d e t e r m i n e t h e c o n v e r g e n c e r a t e o f a m e t h o d .
I n a l l o f t h e a b o v e , w e h a v e c o n s i d e r e d o n l y w h a t a r e u s u a l l y r e f e r r e d t o a s s t a -
t i o n a r y p r o c e s s e s i n w h i c h H i s c o n s t a n t t h r o u g h o u t t h e i t e r a t i o n s . N o n s t a t i o n a r y
p r o c e s s e s i n w h i c h H ( a n d p o s s i b l y C ) i s v a r i e d a t e a c h i t e r a t i o n a r e d i s c u s s e d i n
S e c t i o n 9 . 5 .
9 . 2 . 3 T h e C l a s s i c a l M e t h o d s
P o i n t O p e r a t o r S c h e m e s i n O n e D i m e n s i o n
L e t u s c o n s i d e r t h r e e c l a s s i c a l r e l a x a t i o n p r o c e d u r e s f o r o u r m o d e l e q u a t i o n
B ( 1 ; 2 1 )
~
=
~
f ( 9 . 2 9 )
a s g i v e n i n S e c t i o n 9 . 1 . 2 . T h e P o i n t - J a c o b i m e t h o d i s e x p r e s s e d i n p o i n t o p e r a t o r
f o r m f o r t h e o n e - d i m e n s i o n a l c a s e a s
( n + 1 )
j
=
1
2
h
( n )
j ; 1
+
( n )
j + 1
; f
j
i
( 9 . 3 0 )
T h i s o p e r a t o r c o m e s a b o u t b y c h o o s i n g t h e v a l u e o f
( n + 1 )
j
s u c h t h a t t o g e t h e r w i t h
t h e o l d v a l u e s o f
j ; 1
a n d
j + 1
, t h e j t h r o w o f E q . 9 . 2 9 i s s a t i s e d . T h e G a u s s - S e i d e l
m e t h o d i s
( n + 1 )
j
=
1
2
h
( n + 1 )
j ; 1
+
( n )
j + 1
; f
j
i
( 9 . 3 1 )
T h i s o p e r a t o r i s a s i m p l e e x t e n s i o n o f t h e p o i n t - J a c o b i m e t h o d w h i c h u s e s t h e m o s t
r e c e n t u p d a t e o f
j ; 1
. H e n c e t h e j t h r o w o f E q . 9 . 2 9 i s s a t i s e d u s i n g t h e n e w v a l u e s o f
j
a n d
j ; 1
a n d t h e o l d v a l u e o f
j + 1
. T h e m e t h o d o f s u c c e s s i v e o v e r r e l a x a t i o n ( S O R )
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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1 7 0 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
i s b a s e d o n t h e i d e a t h a t i f t h e c o r r e c t i o n p r o d u c e d b y t h e G a u s s - S e i d e l m e t h o d t e n d s
t o m o v e t h e s o l u t i o n t o w a r d
~
1
, t h e n p e r h a p s i t w o u l d b e b e t t e r t o m o v e f u r t h e r i n
t h i s d i r e c t i o n . I t i s u s u a l l y e x p r e s s e d i n t w o s t e p s a s
~
j
=
1
2
h
( n + 1 )
j ; 1
+
( n )
j + 1
; f
j
i
( n + 1 )
j
=
( n )
j
+ !
h
~
j
;
( n )
j
i
( 9 . 3 2 )
w h e r e ! g e n e r a l l y l i e s b e t w e e n 1 a n d 2 , b u t i t c a n a l s o b e w r i t t e n i n t h e s i n g l e l i n e
( n + 1 )
j
=
!
2
( n + 1 )
j ; 1
+ ( 1 ; ! )
( n )
j
+
!
2
( n )
j + 1
;
!
2
f
j
( 9 . 3 3 )
T h e G e n e r a l F o r m
T h e g e n e r a l f o r m o f t h e c l a s s i c a l m e t h o d s i s o b t a i n e d b y s p l i t t i n g t h e m a t r i x A i n
E q . 9 . 1 3 i n t o i t s d i a g o n a l , D , t h e p o r t i o n o f t h e m a t r i x b e l o w t h e d i a g o n a l , L , a n d
t h e p o r t i o n a b o v e t h e d i a g o n a l , U , s u c h t h a t
A = L + D + U ( 9 . 3 4 )
T h e n t h e p o i n t - J a c o b i m e t h o d i s o b t a i n e d w i t h H = ; D , w h i c h c e r t a i n l y m e e t s
t h e c r i t e r i o n t h a t i t i s e a s y t o s o l v e . T h e G a u s s - S e i d e l m e t h o d i s o b t a i n e d w i t h
H = ; ( L + D ) , w h i c h i s a l s o e a s y t o s o l v e , b e i n g l o w e r t r i a n g u l a r .
9 . 3 T h e O D E A p p r o a c h t o C l a s s i c a l R e l a x a t i o n
9 . 3 . 1 T h e O r d i n a r y D i e r e n t i a l E q u a t i o n F o r m u l a t i o n
T h e p a r t i c u l a r t y p e o f d e l t a f o r m g i v e n b y E q . 9 . 2 1 l e a d s t o a n i n t e r p r e t a t i o n o f
r e l a x a t i o n m e t h o d s i n t e r m s o f s o l u t i o n t e c h n i q u e s f o r c o u p l e d r s t - o r d e r O D E ' s ,
a b o u t w h i c h w e h a v e a l r e a d y l e a r n e d a g r e a t d e a l . O n e c a n e a s i l y s e e t h a t E q . 9 . 2 1
r e s u l t s f r o m t h e a p p l i c a t i o n o f t h e e x p l i c i t E u l e r t i m e - m a r c h i n g m e t h o d ( w i t h h = 1 )
t o t h e f o l l o w i n g s y s t e m o f O D E ' s :
H
d
~
d t
= A
~
;
~
f ( 9 . 3 5 )
T h i s i s e q u i v a l e n t t o
d
~
d t
= H
; 1
C
A
b
~
;
~
f
b
= H
; 1
A
~
;
~
f ] ( 9 . 3 6 )
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9 . 3 . T H E O D E A P P R O A C H T O C L A S S I C A L R E L A X A T I O N 1 7 1
I n t h e s p e c i a l c a s e w h e r e H
; 1
A d e p e n d s o n n e i t h e r
~
u n o r t , H
; 1 ~
f i s a l s o i n d e p e n d e n t
o f t , a n d t h e e i g e n v e c t o r s o f H
; 1
A a r e l i n e a r l y i n d e p e n d e n t , t h e s o l u t i o n c a n b e
w r i t t e n a s
~
= c
1
e
1
t
~
x
1
+ + c
M
e
M
t
~
x
M
| { z }
e r r o r
+
~
1
( 9 . 3 7 )
w h e r e w h a t i s r e f e r r e d t o i n t i m e - a c c u r a t e a n a l y s i s a s t h e t r a n s i e n t s o l u t i o n , i s n o w
r e f e r r e d t o i n r e l a x a t i o n a n a l y s i s a s t h e e r r o r . I t i s c l e a r t h a t , i f a l l o f t h e e i g e n v a l u e s
o f H
; 1
A h a v e n e g a t i v e r e a l p a r t s ( w h i c h i m p l i e s t h a t H
; 1
A i s n o n s i n g u l a r ) , t h e n t h e
s y s t e m o f O D E ' s h a s a s t e a d y - s t a t e s o l u t i o n w h i c h i s a p p r o a c h e d a s t ! 1 , g i v e n b y
~
1
= A
; 1
~
f ( 9 . 3 8 )
w h i c h i s t h e s o l u t i o n o f E q . 9 . 1 3 . W e s e e t h a t t h e g o a l o f a r e l a x a t i o n m e t h o d i s t o
r e m o v e t h e t r a n s i e n t s o l u t i o n f r o m t h e g e n e r a l s o l u t i o n i n t h e m o s t e c i e n t w a y p o s -
s i b l e . T h e e i g e n v a l u e s a r e x e d b y t h e b a s i c m a t r i x i n E q . 9 . 3 6 , t h e p r e c o n d i t i o n i n g
m a t r i x i n 9 . 7 , a n d t h e s e c o n d a r y c o n d i t i o n i n g m a t r i x i n 9 . 3 5 . T h e e i g e n v a l u e s a r e
x e d f o r a g i v e n h b y t h e c h o i c e o f t i m e - m a r c h i n g m e t h o d . T h r o u g h o u t t h e r e m a i n -
i n g d i s c u s s i o n w e w i l l r e f e r t o t h e i n d e p e n d e n t v a r i a b l e t a s \ t i m e " , e v e n t h o u g h n o
t r u e t i m e a c c u r a c y i s i n v o l v e d .
I n a s t a t i o n a r y m e t h o d , H a n d C i n E q . 9 . 3 6 a r e i n d e p e n d e n t o f t , t h a t i s , t h e y
a r e n o t c h a n g e d t h r o u g h o u t t h e i t e r a t i o n p r o c e s s . T h e g e n e r a l i z a t i o n o f t h i s i n o u r
a p p r o a c h i s t o m a k e h , t h e \ t i m e " s t e p , a c o n s t a n t f o r t h e e n t i r e i t e r a t i o n .
S u p p o s e t h e e x p l i c i t E u l e r m e t h o d i s u s e d f o r t h e t i m e i n t e g r a t i o n . F o r t h i s m e t h o d
m
= 1 +
m
h . H e n c e t h e n u m e r i c a l s o l u t i o n a f t e r n s t e p s o f a s t a t i o n a r y r e l a x a t i o n
m e t h o d c a n b e e x p r e s s e d a s ( s e e E q . 6 . 2 8 )
~
n
= c
1
~
x
1
( 1 +
1
h )
n
+ + c
m
~
x
m
( 1 +
m
h )
n
+ + c
M
~
x
M
( 1 +
M
h )
n
| { z }
e r r o r
+
~
1
( 9 . 3 9 )
T h e i n i t i a l a m p l i t u d e s o f t h e e i g e n v e c t o r s a r e g i v e n b y t h e m a g n i t u d e s o f t h e c
m
.
T h e s e a r e x e d b y t h e i n i t i a l g u e s s . I n g e n e r a l i t i s a s s u m e d t h a t a n y o r a l l o f t h e
e i g e n v e c t o r s c o u l d h a v e b e e n g i v e n a n e q u a l l y \ b a d " e x c i t a t i o n b y t h e i n i t i a l g u e s s ,
s o t h a t w e m u s t d e v i s e a w a y t o r e m o v e t h e m a l l f r o m t h e g e n e r a l s o l u t i o n o n a n
e q u a l b a s i s . A s s u m i n g t h a t H
; 1
A h a s b e e n c h o s e n ( t h a t i s , a n i t e r a t i o n p r o c e s s h a s
b e e n d e c i d e d u p o n ) , t h e o n l y f r e e c h o i c e r e m a i n i n g t o a c c e l e r a t e t h e r e m o v a l o f t h e
e r r o r t e r m s i s t h e c h o i c e o f h . A s w e s h a l l s e e , t h e t h r e e c l a s s i c a l m e t h o d s h a v e a l l
b e e n c o n d i t i o n e d b y t h e c h o i c e o f H t o h a v e a n o p t i m u m h e q u a l t o 1 f o r a s t a t i o n a r y
i t e r a t i o n p r o c e s s .
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1 7 2 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
9 . 3 . 2 O D E F o r m o f t h e C l a s s i c a l M e t h o d s
T h e t h r e e i t e r a t i v e p r o c e d u r e s d e n e d b y E q s . 9 . 3 0 , 9 . 3 1 a n d 9 . 3 2 o b e y n o a p p a r e n t
p a t t e r n e x c e p t t h a t t h e y a r e e a s y t o i m p l e m e n t i n a c o m p u t e r c o d e s i n c e a l l o f t h e
d a t a r e q u i r e d t o u p d a t e t h e v a l u e o f o n e p o i n t a r e e x p l i c i t l y a v a i l a b l e a t t h e t i m e
o f t h e u p d a t e . N o w l e t u s s t u d y t h e s e m e t h o d s a s s u b s e t s o f O D E a s f o r m u l a t e d i n
S e c t i o n 9 . 3 . 1 . I n s e r t t h e m o d e l e q u a t i o n 9 . 2 9 i n t o t h e O D E f o r m 9 . 3 5 . T h e n
H
d
~
d t
= B ( 1 ; 2 1 )
~
;
~
f ( 9 . 4 0 )
A s a s t a r t , l e t u s u s e f o r t h e n u m e r i c a l i n t e g r a t i o n t h e e x p l i c i t E u l e r m e t h o d
n + 1
=
n
+ h
0
n
( 9 . 4 1 )
w i t h a s t e p s i z e , h , e q u a l t o 1 . W e a r r i v e a t
H (
~
n + 1
;
~
n
) = B ( 1 ; 2 1 )
~
n
;
~
f ( 9 . 4 2 )
I t i s c l e a r t h a t t h e b e s t c h o i c e o f H f r o m t h e p o i n t o f v i e w o f m a t r i x a l g e b r a i s
; B ( 1 ; 2 1 ) s i n c e t h e n m u l t i p l i c a t i o n f r o m t h e l e f t b y ; B
; 1
( 1 ; 2 1 ) g i v e s t h e c o r -
r e c t a n s w e r i n o n e s t e p . H o w e v e r , t h i s i s n o t i n t h e s p i r i t o f o u r s t u d y , s i n c e m u l t i -
p l i c a t i o n b y t h e i n v e r s e a m o u n t s t o s o l v i n g t h e p r o b l e m b y a d i r e c t m e t h o d w i t h o u t
i t e r a t i o n . T h e c o n s t r a i n t o n H t h a t i s i n k e e p i n g w i t h t h e f o r m u l a t i o n o f t h e t h r e e
m e t h o d s d e s c r i b e d i n S e c t i o n 9 . 2 . 3 i s t h a t a l l t h e e l e m e n t s a b o v e t h e d i a g o n a l ( o r
b e l o w t h e d i a g o n a l i f t h e s w e e p s a r e f r o m r i g h t t o l e f t ) a r e z e r o . I f w e i m p o s e t h i s
c o n s t r a i n t a n d f u r t h e r r e s t r i c t o u r s e l v e s t o b a n d e d t r i d i a g o n a l s w i t h a s i n g l e c o n s t a n t
i n e a c h b a n d , w e a r e l e d t o
B (
;
2
!
0 ) (
~
n + 1
;
~
n
) = B ( 1
; 2 1 )
~
n
;
~
f ( 9 . 4 3 )
w h e r e a n d ! a r e a r b i t r a r y . W i t h t h i s c h o i c e o f n o t a t i o n t h e t h r e e m e t h o d s p r e s e n t e d
i n S e c t i o n 9 . 2 . 3 c a n b e i d e n t i e d u s i n g t h e e n t r i e s i n T a b l e 9 . 1 .
T A B L E 9 . 1 : V A L U E S O F a n d ! I N E Q . 9 . 4 3 T H A T
L E A D T O C L A S S I C A L R E L A X A T I O N M E T H O D S
! M e t h o d E q u a t i o n
0 1 P o i n t - J a c o b i 6 : 2 : 3
1 1 G a u s s - S e i d e l 6 : 2 : 4
1 2 =
h
1 + s i n
M + 1
i
O p t i m u m S O R 6 : 2 : 5
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9 . 4 . E I G E N S Y S T E M S O F T H E C L A S S I C A L M E T H O D S 1 7 3
T h e f a c t t h a t t h e v a l u e s i n t h e t a b l e s l e a d t o t h e m e t h o d s i n d i c a t e d c a n b e v e r i e d
b y s i m p l e a l g e b r a i c m a n i p u l a t i o n . H o w e v e r , o u r p u r p o s e i s t o e x a m i n e t h e w h o l e
p r o c e d u r e a s a s p e c i a l s u b s e t o f t h e t h e o r y o f o r d i n a r y d i e r e n t i a l e q u a t i o n s . I n t h i s
l i g h t , t h e t h r e e m e t h o d s a r e a l l c o n t a i n e d i n t h e f o l l o w i n g s e t o f O D E ' s
d
~
d t
= B
; 1
( ;
2
!
0 )
B ( 1 ; 2 1 )
~
;
~
f
( 9 . 4 4 )
a n d a p p e a r f r o m i t i n t h e s p e c i a l c a s e w h e n t h e e x p l i c i t E u l e r m e t h o d i s u s e d f o r i t s
n u m e r i c a l i n t e g r a t i o n . T h e p o i n t o p e r a t o r t h a t r e s u l t s f r o m t h e u s e o f t h e e x p l i c i t
E u l e r s c h e m e i s
( n + 1 )
j
=
!
2
( n + 1 )
j ; 1
+
!
2
( h ; )
( n )
j ; 1
!
;
( ! h ; 1 )
( n )
j
+
! h
2
( n )
j + 1
!
;
! h
2
f
j
( 9 . 4 5 )
T h i s r e p r e s e n t s a g e n e r a l i z a t i o n o f t h e c l a s s i c a l r e l a x a t i o n t e c h n i q u e s .
9 . 4 E i g e n s y s t e m s o f t h e C l a s s i c a l M e t h o d s
T h e O D E a p p r o a c h t o r e l a x a t i o n c a n b e s u m m a r i z e d a s f o l l o w s . T h e b a s i c e q u a t i o n
t o b e s o l v e d c a m e f r o m s o m e t i m e - a c c u r a t e d e r i v a t i o n
A
b
~
u ;
~
f
b
= 0 ( 9 . 4 6 )
T h i s e q u a t i o n i s p r e c o n d i t i o n e d i n s o m e m a n n e r w h i c h h a s t h e e e c t o f m u l t i p l i c a t i o n
b y a c o n d i t i o n i n g m a t r i x C g i v i n g
A
~
;
~
f = 0 ( 9 . 4 7 )
A n i t e r a t i v e s c h e m e i s d e v e l o p e d t o n d t h e c o n v e r g e d , o r s t e a d y - s t a t e , s o l u t i o n o f
t h e s e t o f O D E ' s
H
d
~
d t
= A
~
;
~
f ( 9 . 4 8 )
T h i s s o l u t i o n h a s t h e a n a l y t i c a l f o r m
~
n
=
~
e
n
+
~
1
( 9 . 4 9 )
w h e r e
~
e
n
i s t h e t r a n s i e n t , o r e r r o r , a n d
~
1
A
; 1 ~
f i s t h e s t e a d y - s t a t e s o l u t i o n . T h e
t h r e e c l a s s i c a l m e t h o d s , P o i n t - J a c o b i , G a u s s - S e i d e l , a n d S O R , a r e i d e n t i e d f o r t h e
o n e - d i m e n s i o n a l c a s e b y E q . 9 . 4 4 a n d T a b l e 9 . 1 .
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1 7 4 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
G i v e n o u r a s s u m p t i o n t h a t t h e c o m p o n e n t o f t h e e r r o r a s s o c i a t e d w i t h e a c h e i g e n -
v e c t o r i s e q u a l l y l i k e l y t o b e e x c i t e d , t h e a s y m p t o t i c c o n v e r g e n c e r a t e i s d e t e r m i n e d
b y t h e e i g e n v a l u e
m
o f G ( I + H
; 1
A ) h a v i n g m a x i m u m a b s o l u t e v a l u e . T h u s
C o n v e r g e n c e r a t e
j
m
j
m a x
m = 1 2
M ( 9 . 5 0 )
I n t h i s s e c t i o n , w e u s e t h e O D E a n a l y s i s t o n d t h e c o n v e r g e n c e r a t e s o f t h e t h r e e
c l a s s i c a l m e t h o d s r e p r e s e n t e d b y E q s . 9 . 3 0 , 9 . 3 1 , a n d 9 . 3 2 . I t i s a l s o i n s t r u c t i v e t o
i n s p e c t t h e e i g e n v e c t o r s a n d e i g e n v a l u e s i n t h e H
; 1
A m a t r i x f o r t h e t h r e e m e t h o d s .
T h i s a m o u n t s t o s o l v i n g t h e g e n e r a l i z e d e i g e n v a l u e p r o b l e m
A
~
x
m
=
m
H
~
x
m
( 9 . 5 1 )
f o r t h e s p e c i a l c a s e
B ( 1 ; 2 1 )
~
x
m
=
m
B ( ;
2
!
0 )
~
x
m
( 9 . 5 2 )
T h e g e n e r a l i z e d e i g e n s y s t e m f o r s i m p l e t r i d i g o n a l s i s g i v e n i n A p p e n d i x B . 2 . T h e
t h r e e s p e c i a l c a s e s c o n s i d e r e d b e l o w a r e o b t a i n e d w i t h a = 1 , b = ; 2 , c = 1 , d = ; ,
e = 2 = ! , a n d f = 0 . T o i l l u s t r a t e t h e b e h a v i o r , w e t a k e M = 5 f o r t h e m a t r i x o r d e r .
T h i s s p e c i a l c a s e m a k e s t h e g e n e r a l r e s u l t q u i t e c l e a r .
9 . 4 . 1 T h e P o i n t - J a c o b i S y s t e m
I f = 0 a n d ! = 1 i n E q . 9 . 4 4 , t h e O D E m a t r i x H
; 1
A r e d u c e s t o s i m p l y B (
1
2
; 1
1
2
) .
T h e e i g e n s y s t e m c a n b e d e t e r m i n e d f r o m A p p e n d i x B . 1 s i n c e b o t h d a n d f a r e z e r o .
T h e e i g e n v a l u e s a r e g i v e n b y t h e e q u a t i o n
m
= ; 1 + c o s
m
M + 1
m = 1 2 : : : M ( 9 . 5 3 )
T h e - r e l a t i o n f o r t h e e x p l i c i t E u l e r m e t h o d i s
m
= 1 +
m
h . T h i s r e l a t i o n c a n
b e p l o t t e d f o r a n y h . T h e p l o t f o r h = 1 , t h e o p t i m u m s t a t i o n a r y c a s e , i s s h o w n i n
F i g . 9 . 1 . F o r h < 1 , t h e m a x i m u m j
m
j i s o b t a i n e d w i t h m = 1 , F i g . 9 . 2 a n d f o r
h > 1 , t h e m a x i m u m j
m
j i s o b t a i n e d w i t h m = M , F i g . 9 . 3 . N o t e t h a t f o r h > 1 : 0
( d e p e n d i n g o n M ) t h e r e i s t h e p o s s i b i l i t y o f i n s t a b i l i t y , i . e . j
m
j > 1 : 0 . T o o b t a i n t h e
o p t i m a l s c h e m e w e w i s h t o m i n i m i z e t h e m a x i m u m j
m
j w h i c h o c c u r s w h e n h = 1 ,
j
1
j = j
M
j a n d t h e b e s t p o s s i b l e c o n v e r g e n c e r a t e i s a c h i e v e d :
j
m
j
m a x
= c o s
M + 1
( 9 . 5 4 )
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1 7 6 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
σ m
1.0
0.0
-2.0 -1.0 0.0
h = 1.0
Μ = 5
Values are equal
m=1
m=2
m=3
m=4
m=5
F i g u r e 9 . 1 : T h e r e l a t i o n f o r P o i n t - J a c o b i , h = 1 M = 5 .
+ c
3
1 ; ( 1 ) h ]
n
~
x
3
+ c
4
1 ; ( 1 +
1
2
) h ]
n
~
x
4
+ c
5
1 ; ( 1 +
p
3
2
) h ]
n
~
x
5
( 9 . 5 8 )
9 . 4 . 2 T h e G a u s s - S e i d e l S y s t e m
I f a n d ! a r e e q u a l t o 1 i n E q . 9 . 4 4 , t h e m a t r i x e i g e n s y s t e m e v o l v e s f r o m t h e r e l a t i o n
B ( 1 ; 2 1 )
~
x
m
=
m
B ( ; 1 2 0 )
~
x
m
( 9 . 5 9 )
w h i c h c a n b e s t u d i e d u s i n g t h e r e s u l t s i n A p p e n d i x B . 2 . O n e c a n s h o w t h a t t h e H
; 1
A
m a t r i x f o r t h e G a u s s - S e i d e l m e t h o d , A
G S
, i s
A
G S
B
; 1
( ; 1 2 0 ) B ( 1 ; 2 1 ) =
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9 . 4 . E I G E N S Y S T E M S O F T H E C L A S S I C A L M E T H O D S 1 7 7
σ m
λ m h
1.0
0.0
-2.0 -1.0 0.0
h < 1.0
D e c r e a s i n g h
Approaches 1.0
D e c
r e a s i n g
h
F i g u r e 9 . 2 : T h e r e l a t i o n f o r P o i n t - J a c o b i , h = 0 : 9 M = 5 .
σ m
Exceeds 1.0 at h 1.072 / Ustable h > 1.072∼∼
1.0
0.0
-2.0 -1.0 0.0
h > 1.0
I n c r e a s i n g h
λ m h
M = 5
I n
c r e a s i n g
h
F i g u r e 9 . 3 : T h e r e l a t i o n f o r P o i n t - J a c o b i , h = 1 : 1 M = 5 .
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1 7 8 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
2
6
6
6
6
6
6
6
6
6
6
6
4
; 1 1 = 2
0 ; 3 = 4 1 = 2
0 1 = 8 ; 3 = 4 1 = 2
0 1 = 1 6 1 = 8 ; 3 = 4 1 = 2
0 1 = 3 2 1 = 1 6 1 = 8 ; 3 = 4 1 = 2
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0 1 = 2
M
3
7
7
7
7
7
7
7
7
7
7
7
5
1
2
3
4
5
.
.
.
M
( 9 . 6 0 )
T h e e i g e n v e c t o r s t r u c t u r e o f t h e G a u s s - S e i d e l O D E m a t r i x i s q u i t e i n t e r e s t i n g . I f M
i s o d d t h e r e a r e ( M + 1 ) = 2 d i s t i n c t e i g e n v a l u e s w i t h c o r r e s p o n d i n g l i n e a r l y i n d e p e n -
d e n t e i g e n v e c t o r s , a n d t h e r e a r e ( M ; 1 ) = 2 d e f e c t i v e e i g e n v a l u e s w i t h c o r r e s p o n d i n g
p r i n c i p a l v e c t o r s . T h e e q u a t i o n f o r t h e n o n d e f e c t i v e e i g e n v a l u e s i n t h e O D E m a t r i x
i s ( f o r o d d M )
m
= ; 1 + c o s
2
(
m
M + 1
) m = 1 2 : : :
M + 1
2
( 9 . 6 1 )
a n d t h e c o r r e s p o n d i n g e i g e n v e c t o r s a r e g i v e n b y
~
x
m
=
c o s
m
M + 1
j ; 1
s i n
j
m
M + 1
m = 1 2 : : :
M + 1
2
( 9 . 6 2 )
T h e - r e l a t i o n f o r h = 1 , t h e o p t i m u m s t a t i o n a r y c a s e , i s s h o w n i n F i g . 9 . 4 . T h e
m
w i t h t h e l a r g e s t a m p l i t u d e i s o b t a i n e d w i t h m = 1 . H e n c e t h e c o n v e r g e n c e r a t e i s
j
m
j
m a x
=
c o s
M + 1
2
( 9 . 6 3 )
S i n c e t h i s i s t h e s q u a r e o f t h a t o b t a i n e d f o r t h e P o i n t - J a c o b i m e t h o d , t h e e r r o r a s s o c i -
a t e d w i t h t h e \ w o r s t " e i g e n v e c t o r i s r e m o v e d i n h a l f a s m a n y i t e r a t i o n s . F o r M = 4 0 ,
j
m
j
m a x
= 0 : 9 9 4 2 . 2 5 0 i t e r a t i o n s a r e r e q u i r e d t o r e d u c e t h e e r r o r c o m p o n e n t o f t h e
w o r s t e i g e n v e c t o r b y a f a c t o r o f r o u g h l y 0 . 2 3 .
T h e e i g e n v e c t o r s a r e q u i t e u n l i k e t h e P o i n t - J a c o b i s e t . T h e y a r e n o l o n g e r s y m -
m e t r i c a l , p r o d u c i n g w a v e s t h a t a r e h i g h e r i n a m p l i t u d e o n o n e s i d e ( t h e u p d a t e d s i d e )
t h a n t h e y a r e o n t h e o t h e r . F u r t h e r m o r e , t h e y d o n o t r e p r e s e n t a c o m m o n f a m i l y f o r
d i e r e n t v a l u e s o f M .
T h e J o r d a n c a n o n i c a l f o r m f o r M = 5 i s
X
; 1
A
G S
X = J
G S
=
2
6
6
6
6
6
6
6
6
4
h
~
1
i
h
~
2
i
2
6
6
4
~
3
1
~
3
1
~
3
3
7
7
5
3
7
7
7
7
7
7
7
7
5
( 9 . 6 4 )
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1 8 0 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
+
"
c
3
( 1
; h )
n
+ c
4
h
n
1 !
( 1
; h )
n ; 1
+ c
5
h
2
n ( n ; 1 )
2 !
( 1
; h )
n ; 2
#
~
x
3
+
c
4
( 1 ; h )
n
+ c
5
h
n
1 !
( 1 ; h )
n ; 1
~
x
4
+ c
5
( 1 ; h )
n
~
x
5
( 9 . 6 7 )
9 . 4 . 3 T h e S O R S y s t e m
I f = 1 a n d 2 = ! = x i n E q . 9 . 4 4 , t h e O D E m a t r i x i s B
; 1
( ; 1 x 0 ) B ( 1 ; 2 1 ) .
O n e c a n s h o w t h a t t h i s c a n b e w r i t t e n i n t h e f o r m g i v e n b e l o w f o r M = 5 . T h e
g e n e r a l i z a t i o n t o a n y M i s f a i r l y c l e a r . T h e H
; 1
A m a t r i x f o r t h e S O R m e t h o d ,
A
S O R
B
; 1
( ; 1 x 0 ) B ( 1 ; 2 1 ) , i s
1
x
5
2
6
6
6
6
6
6
4
; 2 x
4
x
4
0 0 0
; 2 x
3
+ x
4
x
3
; 2 x
4
x
4
0 0
; 2 x
2
+ x
3
x
2
; 2 x
3
+ x
4
x
3
; 2 x
4
x
4
0
; 2 x + x
2
x
; 2 x
2
+ x
3
x
2
; 2 x
3
+ x
4
x
3
; 2 x
4
x
4
; 2 + x 1 ; 2 x + x
2
x ; 2 x
2
+ x
3
x
2
; 2 x
3
+ x
4
x
3
; 2 x
4
3
7
7
7
7
7
7
5
( 9 . 6 8 )
E i g e n v a l u e s o f t h e s y s t e m a r e g i v e n b y
m
= ; 1 +
! p
m
+ z
m
2
2
m = 1 2 : : : M ( 9 . 6 9 )
w h e r e
z
m
= 4 ( 1
; ! ) + !
2
p
2
m
]
1 = 2
p
m
= c o s m = ( M + 1 ) ]
I f ! = 1 , t h e s y s t e m i s G a u s s - S e i d e l . I f 4 ( 1
; ! ) + !
2
p
m
< 0 z
m
a n d
m
a r e c o m p l e x .
I f ! i s c h o s e n s u c h t h a t 4 ( 1 ; ! ) + !
2
p
2
1
= 0 ! i s o p t i m u m f o r t h e s t a t i o n a r y c a s e ,
a n d t h e f o l l o w i n g c o n d i t i o n s h o l d :
1 . T w o e i g e n v a l u e s a r e r e a l , e q u a l a n d d e f e c t i v e .
2 . I f M i s e v e n , t h e r e m a i n i n g e i g e n v a l u e s a r e c o m p l e x a n d o c c u r i n c o n j u g a t e
p a i r s .
3 . I f M i s o d d , o n e o f t h e r e m a i n i n g e i g e n v a l u e s i s r e a l a n d t h e o t h e r s a r e c o m p l e x
o c c u r r i n g i n c o n j u g a t e p a i r s .
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9 . 4 . E I G E N S Y S T E M S O F T H E C L A S S I C A L M E T H O D S 1 8 1
O n e c a n e a s i l y s h o w t h a t t h e o p t i m u m ! f o r t h e s t a t i o n a r y c a s e i s
!
o p t
= 2 =
1 + s i n
M + 1
( 9 . 7 0 )
a n d f o r ! = !
o p t
m
=
2
m
; 1
~
x
m
=
j ; 1
m
s i n
j
m
M + 1
( 9 . 7 1 )
w h e r e
m
=
!
o p t
2
p
m
+ i
q
p
2
1
; p
2
m
U s i n g t h e e x p l i c i t E u l e r m e t h o d t o i n t e g r a t e t h e O D E ' s ,
m
= 1 ; h + h
2
m
, a n d i f
h = 1 , t h e o p t i m u m v a l u e f o r t h e s t a t i o n a r y c a s e , t h e - r e l a t i o n r e d u c e s t o t h a t
s h o w n i n F i g . 9 . 5 . T h i s i l l u s t r a t e s t h e f a c t t h a t f o r o p t i m u m s t a t i o n a r y S O R a l l t h e
j
m
j a r e i d e n t i c a l a n d e q u a l t o !
o p t
; 1 . H e n c e t h e c o n v e r g e n c e r a t e i s
j
m
j
m a x
= !
o p t
; 1 ( 9 . 7 2 )
!
o p t
= 2 =
1 + s i n
M + 1
F o r M = 4 0 , j
m
j
m a x
= 0 : 8 5 7 8 . H e n c e t h e w o r s t e r r o r c o m p o n e n t i s r e d u c e d t o l e s s
t h a n 0 . 2 3 t i m e s i t s i n i t i a l v a l u e i n o n l y 1 0 i t e r a t i o n s , m u c h f a s t e r t h a n b o t h G a u s s -
S e i d e l a n d P o i n t - J a c o b i . I n p r a c t i c a l a p p l i c a t i o n s , t h e o p t i m u m v a l u e o f ! m a y h a v e
t o b e d e t e r m i n e d b y t r i a l a n d e r r o r , a n d t h e b e n e t m a y n o t b e a s g r e a t .
F o r o d d M , t h e r e a r e t w o r e a l e i g e n v e c t o r s a n d o n e r e a l p r i n c i p a l v e c t o r . T h e
r e m a i n i n g l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s a r e a l l c o m p l e x . F o r M = 5 t h e y c a n b e
w r i t t e n
~
x
1
=
2
6
6
6
6
6
6
4
1 = 2
1 = 2
1 = 3
1 = 6
1 = 1 8
3
7
7
7
7
7
7
5
~
x
2
=
2
6
6
6
6
6
6
4
; 6
9
1 6
1 3
6
3
7
7
7
7
7
7
5
~
x
3 4
=
2
6
6
6
6
6
6
4
p
3 ( 1 ) = 2
p
3 ( 1 i
p
2 ) = 6
0
p
3 ( 5 i
p
2 ) = 5 4
p
3 ( 7 4 i
p
2 ) = 1 6 2
3
7
7
7
7
7
7
5
~
x
5
=
2
6
6
6
6
6
6
4
1
0
1 = 3
0
1 = 9
3
7
7
7
7
7
7
5
( 9 . 7 3 )
T h e c o r r e s p o n d i n g e i g e n v a l u e s a r e
1
= ; 2 = 3
( 2 ) D e f e c t i v e l i n k e d t o
1
3
= ; ( 1 0 ; 2
p
2 i ) = 9
4
= ; ( 1 0 + 2
p
2 i ) = 9
5
= ; 4 = 3 ( 9 . 7 4 )
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1 8 2 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
σ m
λ m
λ m
λ m
1.0
0.0
-2.0 -1.0 0.0
h = 1.0
Μ = 5
-1.0-1.0
2 real defective
1 real
2 complex
hλ m
F i g u r e 9 . 5 : T h e r e l a t i o n f o r o p t i m u m s t a t i o n a r y S O R , M = 5 , h = 1 .
T h e n u m e r i c a l s o l u t i o n w r i t t e n i n f u l l i s
~
n
;
~
1
= c
1
( 1 ; 2 h = 3 )
n
+ c
2
n h ( 1 ; 2 h = 3 )
n ; 1
]
~
x
1
+ c
2
( 1 ; 2 h = 3 )
n
~
x
2
+ c
3
1 ; ( 1 0 ; 2
p
2 i ) h = 9 ]
n
~
x
3
+ c
4
1 ; ( 1 0 + 2
p
2 i ) h = 9 ]
n
~
x
4
+ c
5
( 1 ; 4 h = 3 )
n
~
x
5
( 9 . 7 5 )
9 . 5 N o n s t a t i o n a r y P r o c e s s e s
I n c l a s s i c a l t e r m i n o l o g y a m e t h o d i s s a i d t o b e n o n s t a t i o n a r y i f t h e c o n d i t i o n i n g
m a t r i c e s , H a n d C , a r e v a r i e d a t e a c h t i m e s t e p . T h i s d o e s n o t c h a n g e t h e s t e a d y -
s t a t e s o l u t i o n A
; 1
b
~
f
b
, b u t i t c a n g r e a t l y a e c t t h e c o n v e r g e n c e r a t e . I n o u r O D E
a p p r o a c h t h i s c o u l d a l s o b e c o n s i d e r e d a n d w o u l d l e a d t o a s t u d y o f e q u a t i o n s w i t h
n o n c o n s t a n t c o e c i e n t s . I t i s m u c h s i m p l e r , h o w e v e r , t o s t u d y t h e c a s e o f x e d
H a n d C b u t v a r i a b l e s t e p s i z e , h . T h i s p r o c e s s c h a n g e s t h e P o i n t - J a c o b i m e t h o d
t o R i c h a r d s o n ' s m e t h o d i n s t a n d a r d t e r m i n o l o g y . F o r t h e G a u s s - S e i d e l a n d S O R
m e t h o d s i t l e a d s t o p r o c e s s e s t h a t c a n b e s u p e r i o r t o t h e s t a t i o n a r y m e t h o d s .
T h e n o n s t a t i o n a r y f o r m o f E q . 9 . 3 9 i s
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9 . 5 . N O N S T A T I O N A R Y P R O C E S S E S 1 8 3
~
N
= c
1
~
x
1
N
Y
n = 1
( 1 +
1
h
n
) + + c
m
~
x
m
N
Y
n = 1
( 1 +
m
h
n
)
+ + c
M
~
x
M
N
Y
n = 1
( 1 +
M
h
n
) +
~
1
( 9 . 7 6 )
w h e r e t h e s y m b o l s t a n d s f o r p r o d u c t . S i n c e h
n
c a n n o w b e c h a n g e d a t e a c h s t e p ,
t h e e r r o r t e r m c a n t h e o r e t i c a l l y b e c o m p l e t e l y e l i m i n a t e d i n M s t e p s b y t a k i n g h
m
=
; 1 =
m
, f o r m = 1 2 M . H o w e v e r , t h e e i g e n v a l u e s
m
a r e g e n e r a l l y u n k n o w n a n d
c o s t l y t o c o m p u t e . I t i s t h e r e f o r e u n n e c e s s a r y a n d i m p r a c t i c a l t o s e t h
m
= ; 1 =
m
f o r m = 1 2 : : : M . W e w i l l s e e t h a t a f e w w e l l c h o s e n h ' s c a n r e d u c e w h o l e c l u s t e r s
o f e i g e n v e c t o r s a s s o c i a t e d w i t h n e a r b y ' s i n t h e
m
s p e c t r u m . T h i s l e a d s t o t h e
c o n c e p t o f s e l e c t i v e l y a n n i h i l a t i n g c l u s t e r s o f e i g e n v e c t o r s f r o m t h e e r r o r t e r m s a s
p a r t o f a t o t a l i t e r a t i o n p r o c e s s . T h i s i s t h e b a s i s f o r t h e m u l t i g r i d m e t h o d s d i s c u s s e d
i n C h a p t e r 1 0 .
L e t u s c o n s i d e r t h e v e r y i m p o r t a n t c a s e w h e n a l l o f t h e
m
a r e r e a l a n d n e g a -
t i v e ( r e m e m b e r t h a t t h e y a r i s e f r o m a c o n d i t i o n e d m a t r i x s o t h i s c o n s t r a i n t i s n o t
u n r e a l i s t i c f o r q u i t e p r a c t i c a l c a s e s ) . C o n s i d e r o n e o f t h e e r r o r t e r m s t a k e n f r o m
~
e
N
~
N
;
~
1
=
M
X
m = 1
c
m
~
x
m
N
Y
n = 1
( 1 +
m
h
n
) ( 9 . 7 7 )
a n d w r i t e i t i n t h e f o r m
c
m
~
x
m
P
e
(
m
) c
m
~
x
m
N
Y
n = 1
( 1 +
m
h
n
) ( 9 . 7 8 )
w h e r e P
e
s i g n i e s a n \ E u l e r " p o l y n o m i a l . N o w f o c u s a t t e n t i o n o n t h e p o l y n o m i a l
( P
e
)
N
( ) = ( 1 + h
1
) ( 1 + h
2
) ( 1 + h
N
) ( 9 . 7 9 )
t r e a t i n g i t a s a c o n t i n u o u s f u n c t i o n o f t h e i n d e p e n d e n t v a r i a b l e . I n t h e a n n i h i l a t i o n
p r o c e s s m e n t i o n e d a f t e r E q . 9 . 7 6 , w e c o n s i d e r e d m a k i n g t h e e r r o r e x a c t l y z e r o b y
t a k i n g a d v a n t a g e o f s o m e k n o w l e d g e a b o u t t h e d i s c r e t e v a l u e s o f
m
f o r a p a r t i c u l a r
c a s e . N o w w e p o s e a l e s s d e m a n d i n g p r o b l e m . L e t u s c h o o s e t h e h
n
s o t h a t t h e
m a x i m u m v a l u e o f ( P
e
)
N
( ) i s a s s m a l l a s p o s s i b l e f o r a l l l y i n g b e t w e e n
a
a n d
b
s u c h t h a t
b
a
0 . M a t h e m a t i c a l l y s t a t e d , w e s e e k
m a x
b
a
j ( P
e
)
N
( ) j = m i n i m u m w i t h ( P
e
)
N
( 0 ) = 1 ( 9 . 8 0 )
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9 . 5 . N O N S T A T I O N A R Y P R O C E S S E S 1 8 5
a n d t h e a m p l i t u d e o f t h e e i g e n v e c t o r i s r e d u c e d t o
( P
e
)
3
( ) = T
3
( 2 + 3 ) = T
3
( 3 ) ( 9 . 8 7 )
w h e r e
T
3
( 3 ) =
n
3 +
p
8 ]
3
+ 3 ;
p
8 ]
3
o
= 2 9 9 ( 9 . 8 8 )
A p l o t o f E q . 9 . 8 7 i s g i v e n i n F i g . 9 . 6 a n d w e s e e t h a t t h e a m p l i t u d e s o f a l l t h e
e i g e n v e c t o r s a s s o c i a t e d w i t h t h e e i g e n v a l u e s i n t h e r a n g e ; 2 ; 1 h a v e b e e n
r e d u c e d t o l e s s t h a n a b o u t 1 % o f t h e i r i n i t i a l v a l u e s . T h e v a l u e s o f h u s e d i n F i g . 9 . 6
a r e
h
1
= 4 = ( 6 ;
p
3 )
h
2
= 4 = ( 6 ; 0 )
h
3
= 4 = ( 6 +
p
3 )
R e t u r n n o w t o E q . 9 . 7 6 . T h i s w a s d e r i v e d f r o m E q . 9 . 3 7 o n t h e c o n d i t i o n t h a t t h e
e x p l i c i t E u l e r m e t h o d , E q . 9 . 4 1 , w a s u s e d t o i n t e g r a t e t h e b a s i c O D E ' s . I f i n s t e a d
t h e i m p l i c i t t r a p e z o i d a l r u l e
n + 1
=
n
+
1
2
h (
0
n + 1
+
0
n
) ( 9 . 8 9 )
i s u s e d , t h e n o n s t a t i o n a r y f o r m u l a
~
N
=
M
X
m = 1
c
m
~
x
m
N
Y
n = 1
0
B
B
@
1 +
1
2
h
n
m
1 ;
1
2
h
n
m
1
C
C
A
+
~
1
( 9 . 9 0 )
w o u l d r e s u l t . T h i s c a l l s f o r a s t u d y o f t h e r a t i o n a l \ t r a p e z o i d a l " p o l y n o m i a l , P
t
:
( P
t
)
N
( ) =
N
Y
n = 1
0
B
B
@
1 +
1
2
h
n
1 ;
1
2
h
n
1
C
C
A
( 9 . 9 1 )
u n d e r t h e s a m e c o n s t r a i n t s a s b e f o r e , n a m e l y t h a t
m a x
b
a
j ( P
t
)
N
( ) j = m i n i m u m , ( 9 . 9 2 )
w i t h ( P
t
)
N
( 0 ) = 1
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1 8 6 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
−2 −1.8 −1.6 −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
λ
( P
e
) 3
( λ )
−2 −1.8 −1.6 −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0−0.02
−0.015
−0.01
−0.005
0
0.005
0.01
0.015
0.02
λ
( P
e
) 3
( λ )
F i g u r e 9 . 6 : R i c h a r d s o n ' s m e t h o d f o r 3 s t e p s , m i n i m i z a t i o n o v e r ; 2 ; 1 .
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1 8 8 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
−2 −1.8 −1.6 −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
λ
( P
t )
3
( λ )
−2 −1.8 −1.6 −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0−2
−1.5
−1
−0.5
0
0.5
1
1.5
2x 10
−3
λ
( P
t )
3
( λ )
F i g u r e 9 . 7 : W a c h s p r e s s m e t h o d f o r 3 s t e p s , m i n i m i z a t i o n o v e r ; 2 ; 1 .
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9 . 6 . P R O B L E M S 1 8 9
4 . S o l v e t h e f o l l o w i n g e q u a t i o n o n t h e d o m a i n 0 x 1 w i t h b o u n d a r y c o n d i t i o n s
u ( 0 ) = 0 , u ( 1 ) = 1 :
@
2
u
@ x
2
; 6 x = 0
F o r t h e i n i t i a l c o n d i t i o n , u s e u ( x ) = 0 . U s e s e c o n d - o r d e r c e n t e r e d d i e r e n c e s
o n a g r i d w i t h 4 0 c e l l s ( M = 3 9 ) . I t e r a t e t o s t e a d y s t a t e u s i n g
( a ) t h e p o i n t - J a c o b i m e t h o d ,
( b ) t h e G a u s s - S e i d e l m e t h o d ,
( c ) t h e S O R m e t h o d w i t h t h e o p t i m u m v a l u e o f ! , a n d
( d ) t h e 3 - s t e p R i c h a r d s o n m e t h o d d e r i v e d i n S e c t i o n 9 . 5 .
P l o t t h e s o l u t i o n a f t e r t h e r e s i d u a l i s r e d u c e d b y 2 , 3 , a n d 4 o r d e r s o f m a g -
n i t u d e . P l o t t h e l o g a r i t h m o f t h e L
2
- n o r m o f t h e r e s i d u a l v s . t h e n u m b e r o f
i t e r a t i o n s . D e t e r m i n e t h e a s y m p t o t i c c o n v e r g e n c e r a t e . C o m p a r e w i t h t h e t h e -
o r e t i c a l a s y m p t o t i c c o n v e r g e n c e r a t e .
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1 9 0 C H A P T E R 9 . R E L A X A T I O N M E T H O D S
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C h a p t e r 1 0
M U L T I G R I D
T h e i d e a o f s y s t e m a t i c a l l y u s i n g s e t s o f c o a r s e r g r i d s t o a c c e l e r a t e t h e c o n v e r g e n c e o f
i t e r a t i v e s c h e m e s t h a t a r i s e f r o m t h e n u m e r i c a l s o l u t i o n t o p a r t i a l d i e r e n t i a l e q u a -
t i o n s w a s m a d e p o p u l a r b y t h e w o r k o f B r a n d t . T h e r e a r e m a n y v a r i a t i o n s o f t h e
p r o c e s s a n d m a n y v i e w p o i n t s o f t h e u n d e r l y i n g t h e o r y . T h e v i e w p o i n t p r e s e n t e d h e r e
i s a n a t u r a l e x t e n s i o n o f t h e c o n c e p t s d i s c u s s e d i n C h a p t e r 9 .
1 0 . 1 M o t i v a t i o n
1 0 . 1 . 1 E i g e n v e c t o r a n d E i g e n v a l u e I d e n t i c a t i o n w i t h S p a c e
F r e q u e n c i e s
C o n s i d e r t h e e i g e n s y s t e m o f t h e m o d e l m a t r i x B ( 1 ; 2 1 ) . T h e e i g e n v a l u e s a n d
e i g e n v e c t o r s a r e g i v e n i n S e c t i o n s 4 . 3 . 2 a n d 4 . 3 . 3 , r e s p e c t i v e l y . N o t i c e t h a t a s t h e
m a g n i t u d e s o f t h e e i g e n v a l u e s i n c r e a s e , t h e s p a c e - f r e q u e n c y ( o r w a v e n u m b e r ) o f t h e
c o r r e s p o n d i n g e i g e n v e c t o r s a l s o i n c r e a s e . T h a t i s , i f t h e e i g e n v a l u e s a r e o r d e r e d s u c h
t h a t
j
1
j j
2
j j
M
j ( 1 0 . 1 )
t h e n t h e c o r r e s p o n d i n g e i g e n v e c t o r s a r e o r d e r e d f r o m l o w t o h i g h s p a c e f r e q u e n c i e s .
T h i s h a s a r a t i o n a l e x p l a n a t i o n f r o m t h e o r i g i n o f t h e b a n d e d m a t r i x . N o t e t h a t
@
2
@ x
2
s i n ( m x ) = ; m
2
s i n ( m x ) ( 1 0 . 2 )
a n d r e c a l l t h a t
x x
~
=
1
x
2
B ( 1 ; 2 1 )
~
= X
1
x
2
D (
~
)
X
; 1
~
( 1 0 . 3 )
1 9 1
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1 9 2 C H A P T E R 1 0 . M U L T I G R I D
w h e r e D (
~
) i s a d i a g o n a l m a t r i x c o n t a i n i n g t h e e i g e n v a l u e s . W e h a v e s e e n t h a t
X
; 1
~
r e p r e s e n t s a s i n e t r a n s f o r m , a n d X
~
, a s i n e s y n t h e s i s . T h e r e f o r e , t h e o p e r a -
t i o n
1
x
2
D (
~
) r e p r e s e n t s t h e n u m e r i c a l a p p r o x i m a t i o n o f t h e m u l t i p l i c a t i o n o f t h e
a p p r o p r i a t e s i n e w a v e b y t h e n e g a t i v e s q u a r e o f i t s w a v e n u m b e r , ; m
2
. O n e n d s
t h a t
1
x
2
m
=
M + 1
2
; 2 + 2 c o s
m
M + 1
; m
2
m < < M ( 1 0 . 4 )
H e n c e , t h e c o r r e l a t i o n o f l a r g e m a g n i t u d e s o f
m
w i t h h i g h s p a c e - f r e q u e n c i e s i s t o b e
e x p e c t e d f o r t h e s e p a r t i c u l a r m a t r i x o p e r a t o r s . T h i s i s c o n s i s t e n t w i t h t h e p h y s i c s o f
d i u s i o n a s w e l l . H o w e v e r , t h i s c o r r e l a t i o n i s n o t n e c e s s a r y i n g e n e r a l . I n f a c t , t h e
c o m p l e t e c o u n t e r e x a m p l e o f t h e a b o v e a s s o c i a t i o n i s c o n t a i n e d i n t h e e i g e n s y s t e m
f o r B (
1
2
1
1
2
) . F o r t h i s m a t r i x o n e n d s , f r o m A p p e n d i x B , e x a c t l y t h e o p p o s i t e
b e h a v i o r .
1 0 . 1 . 2 P r o p e r t i e s o f t h e I t e r a t i v e M e t h o d
T h e s e c o n d k e y m o t i v a t i o n f o r m u l t i g r i d i s t h e f o l l o w i n g :
M a n y i t e r a t i v e m e t h o d s r e d u c e e r r o r c o m p o n e n t s c o r r e s p o n d i n g t o e i g e n v a l u e s
o f l a r g e a m p l i t u d e m o r e e e c t i v e l y t h a n t h o s e c o r r e s p o n d i n g t o e i g e n v a l u e s o f
s m a l l a m p l i t u d e .
T h i s i s t o b e e x p e c t e d o f a n i t e r a t i v e m e t h o d w h i c h i s t i m e a c c u r a t e . I t i s a l s o
t r u e , f o r e x a m p l e , o f t h e G a u s s - S e i d e l m e t h o d a n d , b y d e s i g n , o f t h e R i c h a r d s o n
m e t h o d d e s c r i b e d i n S e c t i o n 9 . 5 . T h e c l a s s i c a l p o i n t - J a c o b i m e t h o d d o e s n o t s h a r e
t h i s p r o p e r t y . A s w e s a w i n S e c t i o n 9 . 4 . 1 , t h i s m e t h o d p r o d u c e s t h e s a m e v a l u e o f j j
f o r
m i n
a n d
m a x
. H o w e v e r , t h e p r o p e r t y c a n b e r e s t o r e d b y u s i n g h < 1 , a s s h o w n
i n F i g . 9 . 2 .
W h e n a n i t e r a t i v e m e t h o d w i t h t h i s p r o p e r t y i s a p p l i e d t o a m a t r i x w i t h t h e
a b o v e c o r r e l a t i o n b e t w e e n t h e m o d u l u s o f t h e e i g e n v a l u e s a n d t h e s p a c e f r e q u e n c y o f
t h e e i g e n v e c t o r s , e r r o r c o m p o n e n t s c o r r e s p o n d i n g t o h i g h s p a c e f r e q u e n c i e s w i l l b e
r e d u c e d m o r e q u i c k l y t h a n t h o s e c o r r e s p o n d i n g t o l o w s p a c e f r e q u e n c i e s . T h i s i s t h e
k e y c o n c e p t u n d e r l y i n g t h e m u l t i g r i d p r o c e s s .
1 0 . 2 T h e B a s i c P r o c e s s
F i r s t o f a l l w e a s s u m e t h a t t h e d i e r e n c e e q u a t i o n s r e p r e s e n t i n g t h e b a s i c p a r t i a l
d i e r e n t i a l e q u a t i o n s a r e i n a f o r m t h a t c a n b e r e l a t e d t o a m a t r i x w h i c h h a s c e r t a i n
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1 0 . 2 . T H E B A S I C P R O C E S S 1 9 3
b a s i c p r o p e r t i e s . T h i s f o r m c a n b e a r r i v e d a t \ n a t u r a l l y " b y s i m p l y r e p l a c i n g t h e
d e r i v a t i v e s i n t h e P D E w i t h d i e r e n c e s c h e m e s , a s i n t h e e x a m p l e g i v e n b y E q . 3 . 2 7 ,
o r i t c a n b e \ c o n t r i v e d " b y f u r t h e r c o n d i t i o n i n g , a s i n t h e e x a m p l e s g i v e n b y E q . 9 . 1 1 .
T h e b a s i c a s s u m p t i o n s r e q u i r e d f o r o u r d e s c r i p t i o n o f t h e m u l t i g r i d p r o c e s s a r e :
1 . T h e p r o b l e m i s l i n e a r .
2 . T h e e i g e n v a l u e s ,
m
, o f t h e m a t r i x a r e a l l r e a l a n d n e g a t i v e .
3 . T h e
m
a r e f a i r l y e v e n l y d i s t r i b u t e d b e t w e e n t h e i r m a x i m u m a n d m i n i m u m
v a l u e s .
4 . T h e e i g e n v e c t o r s a s s o c i a t e d w i t h t h e e i g e n v a l u e s h a v i n g l a r g e s t m a g n i t u d e s c a n
b e c o r r e l a t e d w i t h h i g h f r e q u e n c i e s o n t h e d i e r e n c i n g m e s h .
5 . T h e i t e r a t i v e p r o c e d u r e u s e d g r e a t l y r e d u c e s t h e a m p l i t u d e s o f t h e e i g e n v e c t o r s
a s s o c i a t e d w i t h e i g e n v a l u e s i n t h e r a n g e b e t w e e n
1
2
j j
m a x
a n d j j
m a x
.
T h e s e c o n d i t i o n s a r e s u c i e n t t o e n s u r e t h e v a l i d i t y o f t h e p r o c e s s d e s c r i b e d n e x t .
H a v i n g p r e c o n d i t i o n e d ( i f n e c e s s a r y ) t h e b a s i c n i t e d i e r e n c i n g s c h e m e b y a p r o -
c e d u r e e q u i v a l e n t t o t h e m u l t i p l i c a t i o n b y a m a t r i x C , w e a r e l e d t o t h e s t a r t i n g
f o r m u l a t i o n
C A
b
~
1
;
~
f
b
] = 0 ( 1 0 . 5 )
w h e r e t h e m a t r i x f o r m e d b y t h e p r o d u c t C A
b
h a s t h e p r o p e r t i e s g i v e n a b o v e . I n E q .
1 0 . 5 , t h e v e c t o r
~
f
b
r e p r e s e n t s t h e b o u n d a r y c o n d i t i o n s a n d t h e f o r c i n g f u n c t i o n , i f
a n y , a n d
~
1
i s a v e c t o r r e p r e s e n t i n g t h e d e s i r e d e x a c t s o l u t i o n . W e s t a r t w i t h s o m e
i n i t i a l g u e s s f o r
~
1
a n d p r o c e e d t h r o u g h n i t e r a t i o n s m a k i n g u s e o f s o m e i t e r a t i v e
p r o c e s s t h a t s a t i s e s p r o p e r t y 5 a b o v e . W e d o n o t a t t e m p t t o d e v e l o p a n o p t i m u m
p r o c e d u r e h e r e , b u t f o r c l a r i t y w e s u p p o s e t h a t t h e t h r e e - s t e p R i c h a r d s o n m e t h o d
i l l u s t r a t e d i n F i g . 9 . 6 i s u s e d . A t t h e e n d o f t h e t h r e e s t e p s w e n d
~
r , t h e r e s i d u a l ,
w h e r e
~
r = C A
b
~
;
~
f
b
] ( 1 0 . 6 )
R e c a l l t h a t t h e
~
u s e d t o c o m p u t e
~
r i s c o m p o s e d o f t h e e x a c t s o l u t i o n
~
1
a n d t h e
e r r o r
~
e i n s u c h a w a y t h a t
A
~
e ;
~
r = 0 ( 1 0 . 7 )
w h e r e
A C A
b
( 1 0 . 8 )
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1 9 4 C H A P T E R 1 0 . M U L T I G R I D
I f o n e c o u l d s o l v e E q . 1 0 . 7 f o r
~
e t h e n
~
1
=
~
;
~
e ( 1 0 . 9 )
T h u s o u r g o a l n o w i s t o s o l v e f o r
~
e . W e c a n w r i t e t h e e x a c t s o l u t i o n f o r
~
e i n t e r m s o f
t h e e i g e n v e c t o r s o f A , a n d t h e e i g e n v a l u e s o f t h e R i c h a r d s o n p r o c e s s i n t h e f o r m :
~
e =
M = 2
X
m = 1
c
m
~
x
m
3
Y
n = 1
(
m
h
n
) ] +
M
X
m = M = 2 + 1
c
m
~
x
m
3
Y
n = 1
(
m
h
n
) ]
| { z }
v e r y l o w a m p l i t u d e
( 1 0 . 1 0 )
C o m b i n i n g o u r b a s i c a s s u m p t i o n s , w e c a n b e s u r e t h a t t h e h i g h f r e q u e n c y c o n t e n t o f
~
e h a s b e e n g r e a t l y r e d u c e d ( a b o u t 1 % o r l e s s o f i t s o r i g i n a l v a l u e i n t h e i n i t i a l g u e s s ) .
I n a d d i t i o n , a s s u m p t i o n 4 e n s u r e s t h a t t h e e r r o r h a s b e e n s m o o t h e d .
N e x t w e c o n s t r u c t a p e r m u t a t i o n m a t r i x w h i c h s e p a r a t e s a v e c t o r i n t o t w o p a r t s ,
o n e c o n t a i n i n g t h e o d d e n t r i e s , a n d t h e o t h e r t h e e v e n e n t r i e s o f t h e o r i g i n a l v e c t o r
( o r a n y o t h e r a p p r o p r i a t e s o r t i n g w h i c h i s c o n s i s t e n t w i t h t h e i n t e r p o l a t i o n a p p r o x i -
m a t i o n t o b e d i s c u s s e d b e l o w ) . F o r a 7 - p o i n t e x a m p l e
2
6
6
6
6
6
6
6
6
6
6
6
4
e
2
e
4
e
6
e
1
e
3
e
5
e
7
3
7
7
7
7
7
7
7
7
7
7
7
5
=
2
6
6
6
6
6
6
6
6
6
6
6
4
0 1 0 0 0 0 0
0 0 0 1 0 0 0
0 0 0 0 0 1 0
1 0 0 0 0 0 0
0 0 1 0 0 0 0
0 0 0 0 1 0 0
0 0 0 0 0 0 1
3
7
7
7
7
7
7
7
7
7
7
7
5
2
6
6
6
6
6
6
6
6
6
6
6
4
e
1
e
2
e
3
e
4
e
5
e
6
e
7
3
7
7
7
7
7
7
7
7
7
7
7
5
"
~
e
e
~
e
o
#
= P
~
e ( 1 0 . 1 1 )
M u l t i p l y E q . 1 0 . 7 f r o m t h e l e f t b y P a n d , s i n c e a p e r m u t a t i o n m a t r i x h a s a n i n v e r s e
w h i c h i s i t s t r a n s p o s e , w e c a n w r i t e
P A P
; 1
P ]
~
e = P
~
r ( 1 0 . 1 2 )
T h e o p e r a t i o n P A P
; 1
p a r t i t i o n s t h e A m a t r i x t o f o r m
2
6
4
A
1
A
2
A
3
A
4
3
7
5
"
~
e
e
~
e
o
#
=
"
~
r
e
~
r
o
#
( 1 0 . 1 3 )
N o t i c e t h a t
A
1
~
e
e
+ A
2
~
e
o
=
~
r
e
( 1 0 . 1 4 )
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1 0 . 2 . T H E B A S I C P R O C E S S 1 9 5
i s a n e x a c t e x p r e s s i o n .
A t t h i s p o i n t w e m a k e o u r o n e c r u c i a l a s s u m p t i o n . I t i s t h a t t h e r e i s s o m e c o n n e c -
t i o n b e t w e e n
~
e
e
a n d
~
e
o
b r o u g h t a b o u t b y t h e s m o o t h i n g p r o p e r t y o f t h e R i c h a r d s o n
r e l a x a t i o n p r o c e d u r e . S i n c e t h e t o p h a l f o f t h e f r e q u e n c y s p e c t r u m h a s b e e n r e m o v e d ,
i t i s r e a s o n a b l e t o s u p p o s e t h a t t h e o d d p o i n t s a r e t h e a v e r a g e o f t h e e v e n p o i n t s .
F o r e x a m p l e
e
1
1
2
( e
a
+ e
2
)
e
3
1
2
( e
2
+ e
4
)
e
5
1
2
( e
4
+ e
6
) o r
~
e
o
= A
0
2
~
e
e
( 1 0 . 1 5 )
e
7
1
2
( e
6
+ e
b
)
I t i s i m p o r t a n t t o n o t i c e t h a t e
a
a n d e
b
r e p r e s e n t e r r o r s o n t h e b o u n d a r i e s w h e r e t h e
e r r o r i s z e r o i f t h e b o u n d a r y c o n d i t i o n s a r e g i v e n . I t i s a l s o i m p o r t a n t t o n o t i c e t h a t w e
a r e d e a l i n g w i t h t h e r e l a t i o n b e t w e e n
~
e a n d
~
r s o t h e o r i g i n a l b o u n d a r y c o n d i t i o n s a n d
f o r c i n g f u n c t i o n ( w h i c h a r e c o n t a i n e d i n
~
f i n t h e b a s i c f o r m u l a t i o n ) n o l o n g e r a p p e a r
i n t h e p r o b l e m . H e n c e , n o a l i a s i n g o f t h e s e f u n c t i o n s c a n o c c u r i n s u b s e q u e n t s t e p s .
F i n a l l y , n o t i c e t h a t , i n t h i s f o r m u l a t i o n , t h e a v e r a g i n g o f
~
e i s o u r o n l y a p p r o x i m a t i o n ,
n o o p e r a t i o n s o n
~
r a r e r e q u i r e d o r j u s t i e d .
I f t h e b o u n d a r y c o n d i t i o n s a r e D i r i c h l e t , e
a
a n d e
b
a r e z e r o , a n d o n e c a n w r i t e f o r
t h e e x a m p l e c a s e
A
0
2
=
1
2
2
6
6
6
4
1 0 0
1 1 0
0 1 1
0 0 1
3
7
7
7
5
( 1 0 . 1 6 )
W i t h t h i s a p p r o x i m a t i o n E q . 1 0 . 1 4 r e d u c e s t o
A
1
~
e
e
+ A
2
A
0
2
~
e
e
=
~
r
e
( 1 0 . 1 7 )
o r
A
c
~
e
e
;
~
r
e
= 0 ( 1 0 . 1 8 )
w h e r e
A
c
= A
1
+ A
2
A
0
2
] ( 1 0 . 1 9 )
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1 9 6 C H A P T E R 1 0 . M U L T I G R I D
T h e f o r m o f A
c
, t h e m a t r i x o n t h e c o a r s e m e s h , i s c o m p l e t e l y d e t e r m i n e d b y t h e
c h o i c e o f t h e p e r m u t a t i o n m a t r i x a n d t h e i n t e r p o l a t i o n a p p r o x i m a t i o n . I f t h e o r i g i n a l
A h a d b e e n B ( 7 : 1 ; 2 1 ) , o u r 7 - p o i n t e x a m p l e w o u l d p r o d u c e
P A P
; 1
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
4
; 2 1 1
; 2 1 1
; 2 1 1
1 ; 2
1 1 ; 2
1 1 ; 2
1 ; 2
3
7
7
7
7
7
7
7
7
7
7
7
7
7
5
=
2
6
4
A
1
A
2
A
3
A
4
3
7
5
( 1 0 . 2 0 )
a n d E q . 1 0 . 1 8 g i v e s
A
1
z } | {
2
6
4
; 2
; 2
; 2
3
7
5
+
A
2
z } | {
2
6
4
1 1
1 1
1 1
3
7
5
1
2
A
0
2
z } | {
2
6
6
6
4
1
1 1
1 1
1
3
7
7
7
5
=
A
c
z } | {
2
6
4
; 1 1 = 2
1 = 2 ; 1 1 = 2
1 = 2 ; 1
3
7
5
( 1 0 . 2 1 )
I f t h e b o u n d a r y c o n d i t i o n s a r e m i x e d D i r i c h l e t - N e u m a n n , A i n t h e 1 - D m o d e l
e q u a t i o n i s B ( 1
~
b 1 ) w h e r e
~
b = ; 2 ; 2 : : : ; 2 ; 1 ]
T
. T h e e i g e n s y s t e m i s g i v e n b y
E q . B . 1 9 . I t i s e a s y t o s h o w t h a t t h e h i g h s p a c e - f r e q u e n c i e s s t i l l c o r r e s p o n d t o t h e
e i g e n v a l u e s w i t h h i g h m a g n i t u d e s , a n d , i n f a c t , a l l o f t h e p r o p e r t i e s g i v e n i n S e c t i o n
1 0 . 1 a r e m e t . H o w e v e r , t h e e i g e n v e c t o r s t r u c t u r e i s d i e r e n t f r o m t h a t g i v e n i n
E q . 9 . 5 5 f o r D i r i c h l e t c o n d i t i o n s . I n t h e p r e s e n t c a s e t h e y a r e g i v e n b y
x
j m
= s i n
"
j
( 2 m ; 1 )
2 M + 1
! #
m = 1 2 M ( 1 0 . 2 2 )
a n d a r e i l l u s t r a t e d i n F i g . 1 0 . 1 . A l l o f t h e m g o t h r o u g h z e r o o n t h e l e f t ( D i r i c h l e t )
s i d e , a n d a l l o f t h e m r e e c t o n t h e r i g h t ( N e u m a n n ) s i d e .
F o r N e u m a n n c o n d i t i o n s , t h e i n t e r p o l a t i o n f o r m u l a i n E q . 1 0 . 1 5 m u s t b e c h a n g e d .
I n t h e p a r t i c u l a r c a s e i l l u s t r a t e d i n F i g . 1 0 . 1 , e
b
i s e q u a l t o e
M
. I f N e u m a n n c o n d i t i o n s
a r e o n t h e l e f t , e
a
= e
1
. W h e n e
b
= e
M
, t h e e x a m p l e i n E q . 1 0 . 1 6 c h a n g e s t o
A
0
2
=
1
2
2
6
6
6
4
1 0 0
1 1 0
0 1 1
0 0 2
3
7
7
7
5
( 1 0 . 2 3 )
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1 0 . 2 . T H E B A S I C P R O C E S S 1 9 7
X
F i g u r e 1 0 . 1 : E i g e n v e c t o r s f o r t h e m i x e d D i r i c h l e t { N e u m a n n c a s e .
T h e p e r m u t a t i o n m a t r i x r e m a i n s t h e s a m e a n d b o t h A
1
a n d A
2
i n t h e p a r t i t i o n e d
m a t r i x P A P
; 1
a r e u n c h a n g e d ( o n l y A
4
i s m o d i e d b y p u t t i n g ; 1 i n t h e l o w e r r i g h t
e l e m e n t ) . T h e r e f o r e , w e c a n c o n s t r u c t t h e c o a r s e m a t r i x f r o m
A
1
z } | {
2
6
4
; 2
; 2
; 2
3
7
5
+
A
2
z } | {
2
6
4
1 1
1 1
1 1
3
7
5
1
2
A
0
2
z } | {
2
6
6
6
4
1
1 1
1 1
2
3
7
7
7
5
=
A
c
z } | {
2
6
4
; 1 1 = 2
1 = 2 ; 1 1 = 2
1 = 2 ; 1 = 2
3
7
5
( 1 0 . 2 4 )
w h i c h g i v e s u s w h a t w e m i g h t h a v e \ e x p e c t e d . "
W e w i l l c o n t i n u e w i t h D i r i c h l e t b o u n d a r y c o n d i t i o n s f o r t h e r e m a i n d e r o f t h i s
S e c t i o n . A t t h i s s t a g e , w e h a v e r e d u c e d t h e p r o b l e m f r o m B ( 1 ; 2 1 )
~
e =
~
r o n t h e
n e m e s h t o
1
2
B ( 1 ; 2 1 )
~
e
e
=
~
r
e
o n t h e n e x t c o a r s e r m e s h . R e c a l l t h a t o u r g o a l i s
t o s o l v e f o r
~
e , w h i c h w i l l p r o v i d e u s w i t h t h e s o l u t i o n
~
1
u s i n g E q . 1 0 . 9 . G i v e n
~
e
e
c o m p u t e d o n t h e c o a r s e g r i d ( p o s s i b l y u s i n g e v e n c o a r s e r g r i d s ) , w e c a n c o m p u t e
~
e
o
u s i n g E q . 1 0 . 1 5 , a n d t h u s
~
e . I n o r d e r t o c o m p l e t e t h e p r o c e s s , w e m u s t n o w d e t e r m i n e
t h e r e l a t i o n s h i p b e t w e e n
~
e
e
a n d
~
e .
I n o r d e r t o e x a m i n e t h i s r e l a t i o n s h i p , w e n e e d t o c o n s i d e r t h e e i g e n s y s t e m s o f A
a n d A
c
:
A = X X
; 1
A
c
= X
c
c
X
; 1
c
( 1 0 . 2 5 )
F o r A = B ( M : 1 ; 2 1 ) t h e e i g e n v a l u e s a n d e i g e n v e c t o r s a r e
m
= ; 2
1 ; c o s
m
M + 1
~
x
m
= s i n
j
m
M + 1
j = 1 2 M
m = 1 2 M
( 1 0 . 2 6 )
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1 9 8 C H A P T E R 1 0 . M U L T I G R I D
B a s e d o n o u r a s s u m p t i o n s , t h e m o s t d i c u l t e r r o r m o d e t o e l i m i n a t e i s t h a t w i t h
m = 1 , c o r r e s p o n d i n g t o
1
= ; 2
1 ; c o s
M + 1
~
x
1
= s i n
j
M + 1
j = 1 2 M ( 1 0 . 2 7 )
F o r e x a m p l e , w i t h M = 5 1 ,
1
= ; 0 : 0 0 3 6 4 9 . I f w e r e s t r i c t o u r a t t e n t i o n t o o d d M ,
t h e n M
c
= ( M ; 1 ) = 2 i s t h e s i z e o f A
c
. T h e e i g e n v a l u e a n d e i g e n v e c t o r c o r r e s p o n d i n g
t o m = 1 f o r t h e m a t r i x A
c
=
1
2
B ( M
c
1 ; 2 1 ) a r e
(
c
)
1
= ;
1 ; c o s
2
M + 1
(
~
x
c
)
1
= s i n
j
2
M + 1
j = 1 2 M
c
( 1 0 . 2 8 )
F o r M = 5 1 ( M
c
= 2 5 ) , w e o b t a i n (
c
)
1
= ; 0 : 0 0 7 2 9 1 = 1 : 9 9 8
1
. A s M i n c r e a s e s ,
(
c
)
1
a p p r o a c h e s 2
1
. I n a d d i t i o n , o n e c a n e a s i l y s e e t h a t (
~
x
c
)
1
c o i n c i d e s w i t h
~
x
1
a t
e v e r y s e c o n d p o i n t o f t h e l a t t e r v e c t o r , t h a t i s , i t c o n t a i n s t h e e v e n e l e m e n t s o f
~
x
1
.
N o w l e t u s c o n s i d e r t h e c a s e i n w h i c h a l l o f t h e e r r o r c o n s i s t s o f t h e e i g e n v e c t o r
c o m p o n e n t
~
x
1
, i . e . ,
~
e =
~
x
1
. T h e n t h e r e s i d u a l i s
~
r = A
~
x
1
=
1
~
x
1
( 1 0 . 2 9 )
a n d t h e r e s i d u a l o n t h e c o a r s e g r i d i s
~
r
e
=
1
(
~
x
c
)
1
( 1 0 . 3 0 )
s i n c e (
~
x
c
)
1
c o n t a i n s t h e e v e n e l e m e n t s o f
~
x
1
. T h e e x a c t s o l u t i o n o n t h e c o a r s e g r i d
s a t i s e s
~
e
e
= A
; 1
c
~
r
e
= X
c
; 1
c
X
; 1
c
1
(
~
x
c
)
1
( 1 0 . 3 1 )
=
1
X
c
; 1
c
2
6
6
6
6
4
1
0
.
.
.
0
3
7
7
7
7
5
( 1 0 . 3 2 )
=
1
X
c
2
6
6
6
6
4
1 = (
c
)
1
0
.
.
.
0
3
7
7
7
7
5
( 1 0 . 3 3 )
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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1 0 . 2 . T H E B A S I C P R O C E S S 1 9 9
=
1
(
c
)
1
(
~
x
c
)
1
( 1 0 . 3 4 )
1
2
(
~
x
c
)
1
( 1 0 . 3 5 )
S i n c e o u r g o a l i s t o c o m p u t e
~
e =
~
x
1
, i n a d d i t i o n t o i n t e r p o l a t i n g
~
e
e
t o t h e n e g r i d
( u s i n g E q . 1 0 . 1 5 ) , w e m u s t m u l t i p l y t h e r e s u l t b y 2 . T h i s i s e q u i v a l e n t t o s o l v i n g
1
2
A
c
~
e
e
=
~
r
e
( 1 0 . 3 6 )
o r
1
4
B ( M
c
: 1 ; 2 1 )
~
e
e
=
~
r
e
( 1 0 . 3 7 )
I n o u r c a s e , t h e m a t r i x A = B ( M : 1 ; 2 1 ) c o m e s f r o m a d i s c r e t i z a t i o n o f t h e
d i u s i o n e q u a t i o n , w h i c h g i v e s
A
b
=
x
2
B ( M : 1 ; 2 1 ) ( 1 0 . 3 8 )
a n d t h e p r e c o n d i t i o n i n g m a t r i x C i s s i m p l y
C =
x
2
I ( 1 0 . 3 9 )
A p p l y i n g t h e d i s c r e t i z a t i o n o n t h e c o a r s e g r i d w i t h t h e s a m e p r e c o n d i t i o n i n g m a t r i x
a s u s e d o n t h e n e g r i d g i v e s , s i n c e x
c
= 2 x ,
C
x
2
c
B ( M
c
: 1 ; 2 1 ) =
x
2
x
2
c
B ( M
c
: 1 ; 2 1 ) =
1
4
B ( M
c
: 1 ; 2 1 ) ( 1 0 . 4 0 )
w h i c h i s p r e c i s e l y t h e m a t r i x a p p e a r i n g i n E q . 1 0 . 3 7 . T h u s w e s e e t h a t t h e p r o c e s s i s
r e c u r s i v e . T h e p r o b l e m t o b e s o l v e d o n t h e c o a r s e g r i d i s t h e s a m e a s t h a t s o l v e d o n
t h e n e g r i d .
T h e r e m a i n i n g s t e p s r e q u i r e d t o c o m p l e t e a n e n t i r e m u l t i g r i d p r o c e s s a r e r e l a t i v e l y
s t r a i g h t f o r w a r d , b u t t h e y v a r y d e p e n d i n g o n t h e p r o b l e m a n d t h e u s e r . T h e r e d u c t i o n
c a n b e , a n d u s u a l l y i s , c a r r i e d t o e v e n c o a r s e r g r i d s b e f o r e r e t u r n i n g t o t h e n e s t l e v e l .
H o w e v e r , i n e a c h c a s e t h e a p p r o p r i a t e p e r m u t a t i o n m a t r i x a n d t h e i n t e r p o l a t i o n
a p p r o x i m a t i o n d e n e b o t h t h e d o w n - a n d u p - g o i n g p a t h s . T h e d e t a i l s o f n d i n g
o p t i m u m t e c h n i q u e s a r e , o b v i o u s l y , q u i t e i m p o r t a n t b u t t h e y a r e n o t d i s c u s s e d h e r e .
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2 0 0 C H A P T E R 1 0 . M U L T I G R I D
1 0 . 3 A T w o - G r i d P r o c e s s
W e n o w d e s c r i b e a t w o - g r i d p r o c e s s f o r t h e l i n e a r p r o b l e m A
~
=
~
f , w h i c h c a n b e e a s i l y
g e n e r a l i z e d t o a p r o c e s s w i t h a n a r b i t r a r y n u m b e r o f g r i d s d u e t o t h e r e c u r s i v e n a t u r e
o f m u l t i g r i d . E x t e n s i o n t o n o n l i n e a r p r o b l e m s r e q u i r e s t h a t b o t h t h e s o l u t i o n a n d t h e
r e s i d u a l b e t r a n s f e r r e d t o t h e c o a r s e g r i d i n a p r o c e s s k n o w n a s f u l l a p p r o x i m a t i o n
s t o r a g e m u l t i g r i d .
1 . P e r f o r m n
1
i t e r a t i o n s o f t h e s e l e c t e d r e l a x a t i o n m e t h o d o n t h e n e g r i d , s t a r t i n g
w i t h
~
=
~
n
. C a l l t h e r e s u l t
~
( 1 )
. T h i s g i v e s
1
~
( 1 )
= G
n
1
1
~
n
+ ( I ; G
n
1
1
) A
; 1
~
f ( 1 0 . 4 1 )
w h e r e
G
1
= I + H
; 1
1
A
1
( 1 0 . 4 2 )
a n d H
1
i s d e n e d a s i n C h a p t e r 9 ( e . g . , E q . 9 . 2 1 ) . N e x t c o m p u t e t h e r e s i d u a l b a s e d
o n
~
( 1 )
:
~r
( 1 )
= A
~
( 1 )
;
~
f = A G
n
1
1
~
n
+ A ( I ; G
n
1
1
) A
; 1
~
f ;
~
f
= A G
n
1
1
~
n
; A G
n
1
1
A
; 1
~
f ( 1 0 . 4 3 )
2 . T r a n s f e r ( o r r e s t r i c t ) ~r
( 1 )
t o t h e c o a r s e g r i d :
~r
( 2 )
= R
2
1
~r
( 1 )
( 1 0 . 4 4 )
I n o u r e x a m p l e i n t h e p r e c e d i n g s e c t i o n , t h e r e s t r i c t i o n m a t r i x i s
R
2
1
=
2
6
4
0 1 0 0 0 0 0
0 0 0 1 0 0 0
0 0 0 0 0 1 0
3
7
5
( 1 0 . 4 5 )
t h a t i s , t h e r s t t h r e e r o w s o f t h e p e r m u t a t i o n m a t r i x P i n E q . 1 0 . 1 1 . T h i s t y p e o f
r e s t r i c t i o n i s k n o w n a s \ s i m p l e i n j e c t i o n . " S o m e f o r m o f w e i g h t e d r e s t r i c t i o n c a n a l s o
b e u s e d .
3 . S o l v e t h e p r o b l e m A
2
~e
( 2 )
= ~r
( 2 )
o n t h e c o a r s e g r i d e x a c t l y :
2
~e
( 2 )
= A
; 1
2
~r
( 2 )
( 1 0 . 4 6 )
1
S e e p r o b l e m 1 o f C h a p t e r 9 .
2
N o t e t h a t t h e c o a r s e g r i d m a t r i x d e n o t e d A
2
h e r e w a s d e n o t e d A
c
i n t h e p r e c e d i n g s e c t i o n .
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1 0 . 3 . A T W O - G R I D P R O C E S S 2 0 1
H e r e A
2
c a n b e f o r m e d b y a p p l y i n g t h e d i s c r e t i z a t i o n o n t h e c o a r s e g r i d . I n t h e
p r e c e d i n g e x a m p l e ( e q . 1 0 . 4 0 ) , A
2
=
1
4
B ( M
c
: 1 ; 2 1 ) . I t i s a t t h i s s t a g e t h a t t h e
g e n e r a l i z a t i o n t o a m u l t i g r i d p r o c e d u r e w i t h m o r e t h a n t w o g r i d s o c c u r s . I f t h i s i s t h e
c o a r s e s t g r i d i n t h e s e q u e n c e , s o l v e e x a c t l y . O t h e r w i s e , a p p l y t h e t w o - g r i d p r o c e s s
r e c u r s i v e l y .
4 . T r a n s f e r ( o r p r o l o n g ) t h e e r r o r b a c k t o t h e n e g r i d a n d u p d a t e t h e s o l u t i o n :
~
n + 1
=
~
( 1 )
; I
1
2
~e
( 2 )
( 1 0 . 4 7 )
I n o u r e x a m p l e , t h e p r o l o n g a t i o n m a t r i x i s
I
1
2
=
2
6
6
6
6
6
6
6
6
6
6
6
4
1 = 2 0 0
1 0 0
1 = 2 1 = 2 0
0 1 0
0 1 = 2 1 = 2
0 0 1
0 0 1 = 2
3
7
7
7
7
7
7
7
7
7
7
7
5
( 1 0 . 4 8 )
w h i c h f o l l o w s f r o m E q . 1 0 . 1 5 .
C o m b i n i n g t h e s e s t e p s , o n e o b t a i n s
~
n + 1
= I ; I
1
2
A
; 1
2
R
2
1
A ] G
n
1
1
~
n
; I ; I
1
2
A
; 1
2
R
2
1
A ] G
n
1
1
A
; 1
~
f + A
; 1
~
f ( 1 0 . 4 9 )
T h u s t h e b a s i c i t e r a t i o n m a t r i x i s
I ; I
1
2
A
; 1
2
R
2
1
A ] G
n
1
1
( 1 0 . 5 0 )
T h e e i g e n v a l u e s o f t h i s m a t r i x d e t e r m i n e t h e c o n v e r g e n c e r a t e o f t h e t w o - g r i d p r o c e s s .
T h e b a s i c i t e r a t i o n m a t r i x f o r a t h r e e - g r i d p r o c e s s i s f o u n d f r o m E q . 1 0 . 5 0 b y
r e p l a c i n g A
; 1
2
w i t h ( I ; G
2
3
) A
; 1
2
, w h e r e
G
2
3
= I ; I
2
3
A
; 1
3
R
3
2
A
2
] G
n
2
2
( 1 0 . 5 1 )
I n t h i s e x p r e s s i o n n
2
i s t h e n u m b e r o f r e l a x a t i o n s t e p s o n g r i d 2 , I
2
3
a n d R
3
2
a r e t h e
t r a n s f e r o p e r a t o r s b e t w e e n g r i d s 2 a n d 3 , a n d A
3
i s o b t a i n e d b y d i s c r e t i z i n g o n g r i d
3 . E x t e n s i o n t o f o u r o r m o r e g r i d s p r o c e e d s i n s i m i l a r f a s h i o n .
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2 0 2 C H A P T E R 1 0 . M U L T I G R I D
1 0 . 4 P r o b l e m s
1 . D e r i v e E q . 1 0 . 5 1 .
2 . R e p e a t p r o b l e m 4 o f C h a p t e r 9 u s i n g a f o u r - g r i d m u l t i g r i d m e t h o d t o g e t h e r
w i t h
( a ) t h e G a u s s - S e i d e l m e t h o d ,
( b ) t h e 3 - s t e p R i c h a r d s o n m e t h o d d e r i v e d i n S e c t i o n 9 . 5 .
S o l v e e x a c t l y o n t h e c o a r s e s t g r i d . P l o t t h e s o l u t i o n a f t e r t h e r e s i d u a l i s r e d u c e d
b y 2 , 3 , a n d 4 o r d e r s o f m a g n i t u d e . P l o t t h e l o g a r i t h m o f t h e L
2
- n o r m o f t h e
r e s i d u a l v s . t h e n u m b e r o f i t e r a t i o n s . D e t e r m i n e t h e a s y m p t o t i c c o n v e r g e n c e
r a t e . C a l c u l a t e t h e t h e o r e t i c a l a s y m p t o t i c c o n v e r g e n c e r a t e a n d c o m p a r e .
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C h a p t e r 1 1
N U M E R I C A L D I S S I P A T I O N
U p t o t h i s p o i n t , w e h a v e e m p h a s i z e d t h e s e c o n d - o r d e r c e n t e r e d - d i e r e n c e a p p r o x i m a -
t i o n s t o t h e s p a t i a l d e r i v a t i v e s i n o u r m o d e l e q u a t i o n s . W e h a v e s e e n t h a t a c e n t e r e d
a p p r o x i m a t i o n t o a r s t d e r i v a t i v e i s n o n d i s s i p a t i v e , i . e . , t h e e i g e n v a l u e s o f t h e a s -
s o c i a t e d c i r c u l a n t m a t r i x ( w i t h p e r i o d i c b o u n d a r y c o n d i t i o n s ) a r e p u r e i m a g i n a r y .
I n p r o c e s s e s g o v e r n e d b y n o n l i n e a r e q u a t i o n s , s u c h a s t h e E u l e r a n d N a v i e r - S t o k e s
e q u a t i o n s , t h e r e c a n b e a c o n t i n u a l p r o d u c t i o n o f h i g h - f r e q u e n c y c o m p o n e n t s o f t h e
s o l u t i o n , l e a d i n g , f o r e x a m p l e , t o t h e p r o d u c t i o n o f s h o c k w a v e s . I n a r e a l p h y s -
i c a l p r o b l e m , t h e p r o d u c t i o n o f h i g h f r e q u e n c i e s i s e v e n t u a l l y l i m i t e d b y v i s c o s i t y .
H o w e v e r , w h e n w e s o l v e t h e E u l e r e q u a t i o n s n u m e r i c a l l y , w e h a v e n e g l e c t e d v i s c o u s
e e c t s . T h u s t h e n u m e r i c a l a p p r o x i m a t i o n m u s t c o n t a i n s o m e i n h e r e n t d i s s i p a t i o n t o
l i m i t t h e p r o d u c t i o n o f h i g h - f r e q u e n c y m o d e s . A l t h o u g h n u m e r i c a l a p p r o x i m a t i o n s
t o t h e N a v i e r - S t o k e s e q u a t i o n s c o n t a i n d i s s i p a t i o n t h r o u g h t h e v i s c o u s t e r m s , t h i s
c a n b e i n s u c i e n t , e s p e c i a l l y a t h i g h R e y n o l d s n u m b e r s , d u e t o t h e l i m i t e d g r i d r e s -
o l u t i o n w h i c h i s p r a c t i c a l . T h e r e f o r e , u n l e s s t h e r e l e v a n t l e n g t h s c a l e s a r e r e s o l v e d ,
s o m e f o r m o f a d d e d n u m e r i c a l d i s s i p a t i o n i s r e q u i r e d i n t h e n u m e r i c a l s o l u t i o n o f
t h e N a v i e r - S t o k e s e q u a t i o n s a s w e l l . S i n c e t h e a d d i t i o n o f n u m e r i c a l d i s s i p a t i o n i s
t a n t a m o u n t t o i n t e n t i o n a l l y i n t r o d u c i n g n o n p h y s i c a l b e h a v i o r , i t m u s t b e c a r e f u l l y
c o n t r o l l e d s u c h t h a t t h e e r r o r i n t r o d u c e d i s n o t e x c e s s i v e . I n t h i s C h a p t e r , w e d i s c u s s
s o m e d i e r e n t w a y s o f a d d i n g n u m e r i c a l d i s s i p a t i o n t o t h e s p a t i a l d e r i v a t i v e s i n t h e
l i n e a r c o n v e c t i o n e q u a t i o n a n d h y p e r b o l i c s y s t e m s o f P D E ' s .
2 0 3
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2 0 4 C H A P T E R 1 1 . N U M E R I C A L D I S S I P A T I O N
1 1 . 1 O n e - S i d e d F i r s t - D e r i v a t i v e S p a c e D i e r e n c -
i n g
W e i n v e s t i g a t e t h e p r o p e r t i e s o f o n e - s i d e d s p a t i a l d i e r e n c e o p e r a t o r s i n t h e c o n t e x t
o f t h e b i c o n v e c t i o n m o d e l e q u a t i o n g i v e n b y
@ u
@ t
= ; a
@ u
@ x
( 1 1 . 1 )
w i t h p e r i o d i c b o u n d a r y c o n d i t i o n s . C o n s i d e r t h e f o l l o w i n g p o i n t o p e r a t o r f o r t h e
s p a t i a l d e r i v a t i v e t e r m
; a (
x
u )
j
=
; a
2 x
; ( 1 + ) u
j ; 1
+ 2 u
j
+ ( 1 ; ) u
j + 1
]
=
; a
2 x
( ; u
j ; 1
+ u
j + 1
) + ( ; u
j ; 1
+ 2 u
j
; u
j + 1
) ] ( 1 1 . 2 )
T h e s e c o n d f o r m s h o w n d i v i d e s t h e o p e r a t o r i n t o a n a n t i s y m m e t r i c c o m p o n e n t ( ; u
j ; 1
+
u
j + 1
) = 2 x a n d a s y m m e t r i c c o m p o n e n t ( ; u
j ; 1
+ 2 u
j
; u
j + 1
) = 2 x . T h e a n t i s y m -
m e t r i c c o m p o n e n t i s t h e s e c o n d - o r d e r c e n t e r e d d i e r e n c e o p e r a t o r . W i t h 6= 0 , t h e
o p e r a t o r i s o n l y r s t - o r d e r a c c u r a t e . A b a c k w a r d d i e r e n c e o p e r a t o r i s g i v e n b y = 1
a n d a f o r w a r d d i e r e n c e o p e r a t o r i s g i v e n b y = ; 1 .
F o r p e r i o d i c b o u n d a r y c o n d i t i o n s t h e c o r r e s p o n d i n g m a t r i x o p e r a t o r i s
; a
x
=
; a
2 x
B
p
( ; 1 ; 2 1 ; )
T h e e i g e n v a l u e s o f t h i s m a t r i x a r e
m
=
; a
x
1 ; c o s
2 m
M
+ i s i n
2 m
M
f o r m = 0 1 : : : M ; 1
I f a i s p o s i t i v e , t h e f o r w a r d d i e r e n c e o p e r a t o r ( = ; 1 ) p r o d u c e s R e (
m
) > 0 ,
t h e c e n t e r e d d i e r e n c e o p e r a t o r ( = 0 ) p r o d u c e s R e (
m
) = 0 , a n d t h e b a c k w a r d
d i e r e n c e o p e r a t o r p r o d u c e s R e (
m
) < 0 . H e n c e t h e f o r w a r d d i e r e n c e o p e r a t o r i s
i n h e r e n t l y u n s t a b l e w h i l e t h e c e n t e r e d a n d b a c k w a r d o p e r a t o r s a r e i n h e r e n t l y s t a b l e .
I f a i s n e g a t i v e , t h e r o l e s a r e r e v e r s e d . W h e n R e (
m
) = 0 , t h e s o l u t i o n w i l l e i t h e r
g r o w o r d e c a y w i t h t i m e . I n e i t h e r c a s e , o u r c h o i c e o f d i e r e n c i n g s c h e m e p r o d u c e s
n o n p h y s i c a l b e h a v i o r . W e p r o c e e d n e x t t o s h o w w h y t h i s o c c u r s .
7/28/2019 Fundamentals of Computational Fluid Dynamics - Lomax, Pulliam
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1 1 . 2 . T H E M O D I F I E D P A R T I A L D I F F E R E N T I A L E Q U A T I O N 2 0 5
1 1 . 2 T h e M o d i e d P a r t i a l D i e r e n t i a l E q u a t i o n
F i r s t c a r r y o u t a T a y l o r s e r i e s e x p a n s i o n o f t h e t e r m s i n E q . 1 1 . 2 . W e a r e l e a d t o t h e
e x p r e s s i o n
(
x
u )
j
=
1
2 x
2
4
2 x
@ u
@ x
!
j
; x
2
@
2
u
@ x
2
!
j
+
x
3
3
@
3
u
@ x
3
!
j
;
x
4
1 2
@
4
u
@ x
4
!
j
+ : : :
3
5
W e s e e t h a t t h e a n t i s y m m e t r i c p o r t i o n o f t h e o p e r a t o r i n t r o d u c e s o d d d e r i v a t i v e
t e r m s i n t h e t r u n c a t i o n e r r o r w h i l e t h e s y m m e t r i c p o r t i o n i n t r o d u c e s e v e n d e r i v a t i v e s .
S u b s t i t u t i n g t h i s i n t o E q . 1 1 . 1 g i v e s
@ u
@ t
= ; a
@ u
@ x
+
a x
2
@
2
u
@ x
2
;
a x
2
6
@
3
u
@ x
3
+
a x
3
2 4
@
4
u
@ x
4
+ : : : ( 1 1 . 3 )
T h i s i s t h e p a r t i a l d i e r e n t i a l e q u a t i o n w e a r e r e a l l y s o l v i n g w h e n w e a p p l y t h e
a p p r o x i m a t i o n g i v e n b y E q . 1 1 . 2 t o E q . 1 1 . 1 . N o t i c e t h a t E q . 1 1 . 3 i s c o n s i s t e n t w i t h
E q . 1 1 . 1 , s i n c e t h e t w o e q u a t i o n s a r e i d e n t i c a l w h e n x ! 0 . H o w e v e r , w h e n w e u s e
a c o m p u t e r t o n d a n u m e r i c a l s o l u t i o n o f t h e p r o b l e m , x c a n b e s m a l l b u t i t i s
n o t z e r o . T h i s m e a n s t h a t e a c h t e r m i n t h e e x p a n s i o n g i v e n b y E q . 1 1 . 3 i s e x c i t e d t o
s o m e d e g r e e . W e r e f e r t o E q . 1 1 . 3 a s t h e m o d i e d p a r t i a l d i e r e n t i a l e q u a t i o n . W e
p r o c e e d n e x t t o i n v e s t i g a t e t h e i m p l i c a t i o n s o f t h i s c o n c e p t .
C o n s i d e r t h e s i m p l e l i n e a r p a r t i a l d i e r e n t i a l e q u a t i o n
@ u
@ t
=
; a
@ u
@ x
+
@
2
u
@ x
2
+
@
3
u
@ x
3
+
@
4
u
@ x
4
( 1 1 . 4 )
C h o o s e p e r i o d i c b o u n d a r y c o n d i t i o n s a n d i m p o s e a n i n i t i a l c o n d i t i o n u = e
i x
. U n d e r
t h e s e c o n d i t i o n s t h e r e i s a w a v e - l i k e s o l u t i o n t o E q . 1 1 . 4 o f t h e f o r m
u ( x t ) = e
i x
e
( r + i s ) t
p r o v i d e d r a n d s s a t i s f y t h e c o n d i t i o n
r + i s = ; i a ;
2
; i
3
+
4
o r
r = ;
2
( ;
2
) s = ; ( a +
2
)
T h e s o l u t i o n i s c o m p o s e d o f b o t h a m p l i t u d e a n d p h a s e t e r m s . T h u s
u = e
;
2
( ;
2
)
| { z }
a m p l i t u d e
e
i x ; ( a +
2
) t ]
| { z }
p h a s e
( 1 1 . 5 )
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2 0 6 C H A P T E R 1 1 . N U M E R I C A L D I S S I P A T I O N
I t i s i m p o r t a n t t o n o t i c e t h a t t h e a m p l i t u d e o f t h e s o l u t i o n d e p e n d s o n l y u p o n a n d
, t h e c o e c i e n t s o f t h e e v e n d e r i v a t i v e s i n E q . 1 1 . 4 , a n d t h e p h a s e d e p e n d s o n l y o n
a a n d , t h e c o e c i e n t s o f t h e o d d d e r i v a t i v e s .
I f t h e w a v e s p e e d a i s p o s i t i v e , t h e c h o i c e o f a b a c k w a r d d i e r e n c e s c h e m e ( = 1 )
p r o d u c e s a m o d i e d P D E w i t h
;
2
> 0 a n d h e n c e t h e a m p l i t u d e o f t h e s o l u t i o n
d e c a y s . T h i s i s t a n t a m o u n t t o d e l i b e r a t e l y a d d i n g d i s s i p a t i o n t o t h e P D E . U n d e r t h e
s a m e c o n d i t i o n , t h e c h o i c e o f a f o r w a r d d i e r e n c e s c h e m e ( = ; 1 ) i s e q u i v a l e n t t o
d e l i b e r a t e l y a d d i n g a d e s t a b i l i z i n g t e r m t o t h e P D E .
B y e x a m i n i n g t h e t e r m g o v e r n i n g t h e p h a s e o f t h e s o l u t i o n i n E q . 1 1 . 5 , w e s e e
t h a t t h e s p e e d o f p r o p a g a t i o n i s a +
2
. R e f e r r i n g t o t h e m o d i e d P D E , E q . 1 1 . 3
w e h a v e = ; a x
2
= 6 . T h e r e f o r e , t h e p h a s e s p e e d o f t h e n u m e r i c a l s o l u t i o n i s l e s s
t h a n t h e a c t u a l p h a s e s p e e d . F u r t h e r m o r e , t h e n u m e r i c a l p h a s e s p e e d i s d e p e n d e n t
u p o n t h e w a v e n u m b e r . T h i s w e r e f e r t o a s d i s p e r s i o n .
O u r p u r p o s e h e r e i s t o i n v e s t i g a t e t h e p r o p e r t i e s o f o n e - s i d e d s p a t i a l d i e r e n c i n g
o p e r a t o r s r e l a t i v e t o c e n t e r e d d i e r e n c e o p e r a t o r s . W e h a v e s e e n t h a t t h e t h r e e -
p o i n t c e n t e r e d d i e r e n c e a p p r o x i m a t i o n o f t h e s p a t i a l d e r i v a t i v e p r o d u c e s a m o d i e d
P D E t h a t h a s n o d i s s i p a t i o n ( o r a m p l i c a t i o n ) . O n e c a n e a s i l y s h o w , b y u s i n g t h e
a n t i s y m m e t r y o f t h e m a t r i x d i e r e n c e o p e r a t o r s , t h a t t h e s a m e i s t r u e f o r a n y c e n -
t e r e d d i e r e n c e a p p r o x i m a t i o n o f a r s t d e r i v a t i v e . A s a c o r o l l a r y , a n y d e p a r t u r e
f r o m a n t i s y m m e t r y i n t h e m a t r i x d i e r e n c e o p e r a t o r m u s t i n t r o d u c e d i s s i p a t i o n ( o r
a m p l i c a t i o n ) i n t o t h e m o d i e d P D E .
N o t e t h a t t h e u s e o f o n e - s i d e d d i e r e n c i n g s c h e m e s i s n o t t h e o n l y w a y t o i n -
t r o d u c e d i s s i p a t i o n . A n y s y m m e t r i c c o m p o n e n t i n t h e s p a t i a l o p e r a t o r i n t r o d u c e s
d i s s i p a t i o n ( o r a m p l i c a t i o n ) . T h e r e f o r e , o n e c o u l d c h o o s e = 1 = 2 i n E q . 1 1 . 2 . T h e
r e s u l t i n g s p a t i a l o p e r a t o r i s n o t o n e - s i d e d b u t i t i s d i s s i p a t i v e . B i a s e d s c h e m e s u s e
m o r e i n f o r m a t i o n o n o n e s i d e o f t h e n o d e t h a n t h e o t h e r . F o r e x a m p l e , a t h i r d - o r d e r
b a c k w a r d - b i a s e d s c h e m e i s g i v e n b y
(
x
u )
j
=
1
6 x
( u
j ; 2
; 6 u
j ; 1
+ 3 u
j
+ 2 u
j + 1
)
=
1
1 2 x
( u
j ; 2
; 8 u
j ; 1
+ 8 u
j + 1
; u
j + 2
)
+ ( u
j ; 2
; 4 u
j ; 1
+ 6 u
j
; 4 u
j + 1
+ u
j + 2
) ] ( 1 1 . 6 )
T h e a n t i s y m m e t r i c c o m p o n e n t o f t h i s o p e r a t o r i s t h e f o u r t h - o r d e r c e n t e r e d d i e r e n c e
o p e r a t o r . T h e s y m m e t r i c c o m p o n e n t a p p r o x i m a t e s x
3
u
x x x x
= 1 2 . T h e r e f o r e , t h i s
o p e r a t o r p r o d u c e s f o u r t h - o r d e r a c c u r a c y i n p h a s e w i t h a t h i r d - o r d e r d i s s i p a t i v e t e r m .
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1 1 . 3 . T H E L A X - W E N D R O F F M E T H O D 2 0 7
1 1 . 3 T h e L a x - W e n d r o M e t h o d
I n o r d e r t o i n t r o d u c e n u m e r i c a l d i s s i p a t i o n u s i n g o n e - s i d e d d i e r e n c i n g , b a c k w a r d
d i e r e n c i n g m u s t b e u s e d i f t h e w a v e s p e e d i s p o s i t i v e , a n d f o r w a r d d i e r e n c i n g m u s t
b e u s e d i f t h e w a v e s p e e d i s n e g a t i v e . N e x t w e c o n s i d e r a m e t h o d w h i c h i n t r o d u c e s
d i s s i p a t i o n i n d e p e n d e n t o f t h e s i g n o f t h e w a v e s p e e d , k n o w n a s t h e L a x - W e n d r o
m e t h o d . T h i s e x p l i c i t m e t h o d d i e r s c o n c e p t u a l l y f r o m t h e m e t h o d s c o n s i d e r e d p r e -
v i o u s l y i n w h i c h s p a t i a l d i e r e n c i n g a n d t i m e - m a r c h i n g a r e t r e a t e d s e p a r a t e l y .
C o n s i d e r t h e f o l l o w i n g T a y l o r - s e r i e s e x p a n s i o n i n t i m e :
u ( x t + h ) = u + h
@ u
@ t
+
1
2
h
2
@
2
u
@ t
2
+ O ( h
3
) ( 1 1 . 7 )
F i r s t r e p l a c e t h e t i m e d e r i v a t i v e s w i t h s p a c e d e r i v a t i v e s a c c o r d i n g t o t h e P D E ( i n
t h i s c a s e , t h e l i n e a r c o n v e c t i o n e q u a t i o n
@ u
@ t
+ a
@ u
@ x
= 0 ) . T h u s
@ u
@ t
= ; a
@ u
@ x
@
2
u
@ t
2
= a
2
@
2
u
@ x
2
( 1 1 . 8 )
N o w r e p l a c e t h e s p a c e d e r i v a t i v e s w i t h t h r e e - p o i n t c e n t e r e d d i e r e n c e o p e r a t o r s , g i v -
i n g
u
( n + 1 )
j
= u
( n )
j
;
1
2
a h
x
( u
( n )
j + 1
; u
( n )
j ; 1
) +
1
2
a h
x
!
2
( u
( n )
j + 1
; 2 u
( n )
j
+ u
( n )
j ; 1
) ( 1 1 . 9 )
T h i s i s t h e L a x - W e n d r o m e t h o d a p p l i e d t o t h e l i n e a r c o n v e c t i o n e q u a t i o n . I t i s a
f u l l y - d i s c r e t e n i t e - d i e r e n c e s c h e m e . T h e r e i s n o i n t e r m e d i a t e s e m i - d i s c r e t e s t a g e .
F o r p e r i o d i c b o u n d a r y c o n d i t i o n s , t h e c o r r e s p o n d i n g f u l l y - d i s c r e t e m a t r i x o p e r a t o r
i s
~u
n + 1
= B
p
0
@
1
2
2
4
a h
x
+
a h
x
!
2
3
5
1 ;
a h
x
!
2
1
2
2
4
;
a h
x
+
a h
x
!
2
3
5
1
A
~u
n
T h e e i g e n v a l u e s o f t h i s m a t r i x a r e
m
= 1 ;
a h
x
!
2
1 ; c o s
2 m
M
; i
a h
x
s i n
2 m
M
f o r m = 0 1 : : : M ; 1
F o r j
a h
x
j 1 a l l o f t h e e i g e n v a l u e s h a v e m o d u l u s l e s s t h a n o r e q u a l t o u n i t y a n d h e n c e
t h e m e t h o d i s s t a b l e i n d e p e n d e n t o f t h e s i g n o f a . T h e q u a n t i t y j
a h
x
j i s k n o w n a s t h e
C o u r a n t ( o r C F L ) n u m b e r . I t i s e q u a l t o t h e r a t i o o f t h e d i s t a n c e t r a v e l l e d b y a w a v e
i n o n e t i m e s t e p t o t h e m e s h s p a c i n g .
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2 0 8 C H A P T E R 1 1 . N U M E R I C A L D I S S I P A T I O N
T h e n a t u r e o f t h e d i s s i p a t i v e p r o p e r t i e s o f t h e L a x - W e n d r o s c h e m e c a n b e s e e n
b y e x a m i n i n g t h e m o d i e d p a r t i a l d i e r e n t i a l e q u a t i o n , w h i c h i s g i v e n b y
@ u
@ t
+ a
@ u
@ x
= ;
a
6
( x
2
; a
2
h
2
)
@
3
u
@ x
3
;
a
2
h
8
( x
2
; a
2
h
2
)
@
4
u
@ x
4
+ : : :
T h i s i s d e r i v e d b y s u b s t i t u t i n g T a y l o r s e r i e s e x p a n s i o n s f o r a l l t e r m s i n E q . 1 1 . 9 a n d
c o n v e r t i n g t h e t i m e d e r i v a t i v e s t o s p a c e d e r i v a t i v e s u s i n g E q . 1 1 . 8 . T h e t w o l e a d i n g
e r r o r t e r m s a p p e a r o n t h e r i g h t s i d e o f t h e e q u a t i o n . R e c a l l t h a t t h e o d d d e r i v a t i v e s o n
t h e r i g h t s i d e l e a d t o u n w a n t e d d i s p e r s i o n a n d t h e e v e n d e r i v a t i v e s l e a d t o d i s s i p a t i o n
( o r a m p l i c a t i o n , d e p e n d i n g o n t h e s i g n ) . T h e r e f o r e , t h e l e a d i n g e r r o r t e r m i n t h e
L a x - W e n d r o m e t h o d i s d i s p e r s i v e a n d p r o p o r t i o n a l t o
;
a
6
( x
2
; a
2
h
2
)
@
3
u
@ x
3
= ;
a x
2
6
( 1 ; C
2
n
)
@
3
u
@ x
3
T h e d i s s i p a t i v e t e r m i s p r o p o r t i o n a l t o
;
a
2
h
8
( x
2
; a
2
h
2
)
@
4
u
@ x
4
= ;
a
2
h x
2
8
( 1 ; C
2
n
)
@
4
u
@ x
4
T h i s t e r m h a s t h e a p p r o p r i a t e s i g n a n d h e n c e t h e s c h e m e i s t r u l y d i s s i p a t i v e a s l o n g
a s C
n
1 .
A c l o s e l y r e l a t e d m e t h o d i s t h a t o f M a c C o r m a c k . R e c a l l M a c C o r m a c k ' s t i m e -
m a r c h i n g m e t h o d , p r e s e n t e d i n C h a p t e r 6 :
~ u
n + 1
= u
n
+ h u
0
n
u
n + 1
=
1
2
u
n
+ ~ u
n + 1
+ h ~ u
0
n + 1
] ( 1 1 . 1 0 )
I f w e u s e r s t - o r d e r b a c k w a r d d i e r e n c i n g i n t h e r s t s t a g e a n d r s t - o r d e r f o r w a r d
d i e r e n c i n g i n t h e s e c o n d s t a g e ,
1
a d i s s i p a t i v e s e c o n d - o r d e r m e t h o d i s o b t a i n e d . F o r
t h e l i n e a r c o n v e c t i o n e q u a t i o n , t h i s a p p r o a c h l e a d s t o
~ u
( n + 1 )
j
= u
( n )
j
;
a h
x
( u
( n )
j
; u
( n )
j ; 1
)
u
( n + 1 )
j
=
1
2
u
( n )
j
+ ~ u
( n + 1 )
j
;
a h
x
( ~ u
( n + 1 )
j + 1
; ~ u
( n + 1 )
j
) ] ( 1 1 . 1 1 )
w h i c h c a n b e s h o w n t o b e i d e n t i c a l t o t h e L a x - W e n d r o m e t h o d . H e n c e M a c C o r -
m a c k ' s m e t h o d h a s t h e s a m e d i s s i p a t i v e a n d d i s p e r s i v e p r o p e r t i e s a s t h e L a x - W e n d r o
m e t h o d . T h e t w o m e t h o d s d i e r w h e n a p p l i e d t o n o n l i n e a r h y p e r b o l i c s y s t e m s , h o w -
e v e r .
1
O r v i c e - v e r s a f o r n o n l i n e a r p r o b l e m s , t h e s e s h o u l d b e a p p l i e d a l t e r n a t e l y .
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1 1 . 4 . U P W I N D S C H E M E S 2 0 9
1 1 . 4 U p w i n d S c h e m e s
I n S e c t i o n 1 1 . 1 , w e s a w t h a t n u m e r i c a l d i s s i p a t i o n c a n b e i n t r o d u c e d i n t h e s p a t i a l
d i e r e n c e o p e r a t o r u s i n g o n e - s i d e d d i e r e n c e s c h e m e s o r , m o r e g e n e r a l l y , b y a d d i n g
a s y m m e t r i c c o m p o n e n t t o t h e s p a t i a l o p e r a t o r . W i t h t h i s a p p r o a c h , t h e d i r e c t i o n
o f t h e o n e - s i d e d o p e r a t o r ( i . e . , w h e t h e r i t i s a f o r w a r d o r a b a c k w a r d d i e r e n c e )
o r t h e s i g n o f t h e s y m m e t r i c c o m p o n e n t d e p e n d s o n t h e s i g n o f t h e w a v e s p e e d .
W h e n a h y p e r b o l i c s y s t e m o f e q u a t i o n s i s b e i n g s o l v e d , t h e w a v e s p e e d s c a n b e b o t h
p o s i t i v e a n d n e g a t i v e . F o r e x a m p l e , t h e e i g e n v a l u e s o f t h e u x J a c o b i a n f o r t h e o n e -
d i m e n s i o n a l E u l e r e q u a t i o n s a r e u u + a u ; a . W h e n t h e o w i s s u b s o n i c , t h e s e a r e
o f m i x e d s i g n . I n o r d e r t o a p p l y o n e - s i d e d d i e r e n c i n g s c h e m e s t o s u c h s y s t e m s , s o m e
f o r m o f s p l i t t i n g i s r e q u i r e d . T h i s i s a v o i d e d i n t h e L a x - W e n d r o s c h e m e . H o w e v e r ,
a s a r e s u l t o f t h e i r s u p e r i o r e x i b i l i t y , s c h e m e s i n w h i c h t h e n u m e r i c a l d i s s i p a t i o n
i s i n t r o d u c e d i n t h e s p a t i a l o p e r a t o r a r e g e n e r a l l y p r e f e r r e d o v e r t h e L a x - W e n d r o
a p p r o a c h .
C o n s i d e r a g a i n t h e l i n e a r c o n v e c t i o n e q u a t i o n :
@ u
@ t
+ a
@ u
@ x
= 0 ( 1 1 . 1 2 )
w h e r e w e d o n o t m a k e a n y a s s u m p t i o n s a s t o t h e s i g n o f a . W e c a n r e w r i t e E q . 1 1 . 1 2
a s
@ u
@ t
+ ( a
+
+ a
;
)
@ u
@ x
= 0 a
=
a j a j
2
I f a 0 , t h e n a
+
= a 0 a n d a
;
= 0 . A l t e r n a t i v e l y , i f a 0 , t h e n a
+
= 0 a n d
a
;
= a 0 . N o w f o r t h e a
+
( 0 ) t e r m w e c a n s a f e l y b a c k w a r d d i e r e n c e a n d f o r t h e
a
;
( 0 ) t e r m f o r w a r d d i e r e n c e . T h i s i s t h e b a s i c c o n c e p t b e h i n d u p w i n d m e t h o d s ,
t h a t i s , s o m e d e c o m p o s i t i o n o r s p l i t t i n g o f t h e u x e s i n t o t e r m s w h i c h h a v e p o s i t i v e
a n d n e g a t i v e c h a r a c t e r i s t i c s p e e d s s o t h a t a p p r o p r i a t e d i e r e n c i n g s c h e m e s c a n b e
c h o s e n . I n t h e n e x t t w o s e c t i o n s , w e p r e s e n t t w o s p l i t t i n g t e c h n i q u e s c o m m o n l y u s e d
w i t h u p w i n d m e t h o d s . T h e s e a r e b y n o m e a n s u n i q u e .
T h e a b o v e a p p r o a c h t o o b t a i n i n g a s t a b l e d i s c r e t i z a t i o n i n d e p e n d e n t o f t h e s i g n
o f a c a n b e w r i t t e n i n a d i e r e n t , b u t e n t i r e l y e q u i v a l e n t , m a n n e r . F r o m E q . 1 1 . 2 , w e
s e e t h a t a s t a b l e d i s c r e t i z a t i o n i s o b t a i n e d w i t h = 1 i f a
0 a n d w i t h =
; 1 i f
a 0 . T h i s i s a c h i e v e d b y t h e f o l l o w i n g p o i n t o p e r a t o r :
; a (
x
u )
j
=
; 1
2 x
a ( ; u
j ; 1
+ u
j + 1
) + j a j ( ; u
j ; 1
+ 2 u
j
; u
j + 1
) ] ( 1 1 . 1 3 )
T h i s a p p r o a c h i s e x t e n d e d t o s y s t e m s o f e q u a t i o n s i n S e c t i o n 1 1 . 5 .
I n t h i s s e c t i o n , w e p r e s e n t t h e b a s i c i d e a s o f u x - v e c t o r a n d u x - d i e r e n c e s p l i t t i n g .
F o r m o r e s u b t l e a s p e c t s o f i m p l e m e n t a t i o n a n d a p p l i c a t i o n o f s u c h t e c h n i q u e s t o
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2 1 0 C H A P T E R 1 1 . N U M E R I C A L D I S S I P A T I O N
n o n l i n e a r h y p e r b o l i c s y s t e m s s u c h a s t h e E u l e r e q u a t i o n s , t h e r e a d e r i s r e f e r r e d t o
t h e l i t e r a t u r e o n t h i s s u b j e c t .
1 1 . 4 . 1 F l u x - V e c t o r S p l i t t i n g
R e c a l l f r o m S e c t i o n 2 . 5 t h a t a l i n e a r , c o n s t a n t - c o e c i e n t , h y p e r b o l i c s y s t e m o f p a r t i a l
d i e r e n t i a l e q u a t i o n s g i v e n b y
@ u
@ t
+
@ f
@ x
=
@ u
@ t
+ A
@ u
@ x
= 0 ( 1 1 . 1 4 )
c a n b e d e c o u p l e d i n t o c h a r a c t e r i s t i c e q u a t i o n s o f t h e f o r m
@ w
i
@ t
+
i
@ w
i
@ x
= 0 ( 1 1 . 1 5 )
w h e r e t h e w a v e s p e e d s ,
i
, a r e t h e e i g e n v a l u e s o f t h e J a c o b i a n m a t r i x , A , a n d t h e
w
i
' s a r e t h e c h a r a c t e r i s t i c v a r i a b l e s . I n o r d e r t o a p p l y a o n e - s i d e d ( o r b i a s e d ) s p a t i a l
d i e r e n c i n g s c h e m e , w e n e e d t o a p p l y a b a c k w a r d d i e r e n c e i f t h e w a v e s p e e d ,
i
, i s
p o s i t i v e , a n d a f o r w a r d d i e r e n c e i f t h e w a v e s p e e d i s n e g a t i v e . T o a c c o m p l i s h t h i s ,
l e t u s s p l i t t h e m a t r i x o f e i g e n v a l u e s , , i n t o t w o c o m p o n e n t s s u c h t h a t
=
+
+
;
( 1 1 . 1 6 )
w h e r e
+
=
+ j j
2
;
=
; j j
2
( 1 1 . 1 7 )
W i t h t h e s e d e n i t i o n s ,
+
c o n t a i n s t h e p o s i t i v e e i g e n v a l u e s a n d
;
c o n t a i n s t h e n e g -
a t i v e e i g e n v a l u e s . W e c a n n o w r e w r i t e t h e s y s t e m i n t e r m s o f c h a r a c t e r i s t i c v a r i a b l e s
a s
@ w
@ t
+
@ w
@ x
=
@ w
@ t
+
+
@ w
@ x
+
;
@ w
@ x
= 0 ( 1 1 . 1 8 )
T h e s p a t i a l t e r m s h a v e b e e n s p l i t i n t o t w o c o m p o n e n t s a c c o r d i n g t o t h e s i g n o f t h e
w a v e s p e e d s . W e c a n u s e b a c k w a r d d i e r e n c i n g f o r t h e
+
@ w
@ x
t e r m a n d f o r w a r d
d i e r e n c i n g f o r t h e
;
@ w
@ x
t e r m . P r e m u l t i p l y i n g b y X a n d i n s e r t i n g t h e p r o d u c t
X
; 1
X i n t h e s p a t i a l t e r m s g i v e s
@ X w
@ t
+
@ X
+
X
; 1
X w
@ x
+
@ X
;
X
; 1
X w
@ x
= 0 ( 1 1 . 1 9 )
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1 1 . 4 . U P W I N D S C H E M E S 2 1 1
W i t h t h e d e n i t i o n s
2
A
+
= X
+
X
; 1
A
;
= X
;
X
; 1
( 1 1 . 2 0 )
a n d r e c a l l i n g t h a t u = X w , w e o b t a i n
@ u
@ t
+
@ A
+
u
@ x
+
@ A
;
u
@ x
= 0 ( 1 1 . 2 1 )
F i n a l l y t h e s p l i t u x v e c t o r s a r e d e n e d a s
f
+
= A
+
u f
;
= A
;
u ( 1 1 . 2 2 )
a n d w e c a n w r i t e
@ u
@ t
+
@ f
+
@ x
+
@ f
;
@ x
= 0 ( 1 1 . 2 3 )
I n t h e l i n e a r c a s e , t h e d e n i t i o n o f t h e s p l i t u x e s f o l l o w s d i r e c t l y f r o m t h e d e n i -
t i o n o f t h e u x , f = A u . F o r t h e E u l e r e q u a t i o n s , f i s a l s o e q u a l t o A u a s a r e s u l t o f
t h e i r h o m o g e n e o u s p r o p e r t y , a s d i s c u s s e d i n A p p e n d i x C . N o t e t h a t
f = f
+
+ f
;
( 1 1 . 2 4 )
T h u s b y a p p l y i n g b a c k w a r d d i e r e n c e s t o t h e f
+
t e r m a n d f o r w a r d d i e r e n c e s t o t h e
f
;
t e r m , w e a r e i n e e c t s o l v i n g t h e c h a r a c t e r i s t i c e q u a t i o n s i n t h e d e s i r e d m a n n e r .
T h i s a p p r o a c h i s k n o w n a s u x - v e c t o r s p l i t t i n g .
W h e n a n i m p l i c i t t i m e - m a r c h i n g m e t h o d i s u s e d , t h e J a c o b i a n s o f t h e s p l i t u x
v e c t o r s a r e r e q u i r e d . I n t h e n o n l i n e a r c a s e ,
@ f
+
@ u
6= A
+
@ f
;
@ u
6= A
;
( 1 1 . 2 5 )
T h e r e f o r e , o n e m u s t n d a n d u s e t h e n e w J a c o b i a n s g i v e n b y
A
+ +
=
@ f
+
@ u
A
; ;
=
@ f
;
@ u
( 1 1 . 2 6 )
F o r t h e E u l e r e q u a t i o n s , A
+ +
h a s e i g e n v a l u e s w h i c h a r e a l l p o s i t i v e , a n d A
; ;
h a s a l l
n e g a t i v e e i g e n v a l u e s .
2
W i t h t h e s e d e n i t i o n s A
+
h a s a l l p o s i t i v e e i g e n v a l u e s , a n d A
;
h a s a l l n e g a t i v e e i g e n v a l u e s .
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2 1 2 C H A P T E R 1 1 . N U M E R I C A L D I S S I P A T I O N
1 1 . 4 . 2 F l u x - D i e r e n c e S p l i t t i n g
A n o t h e r a p p r o a c h , m o r e s u i t e d t o n i t e - v o l u m e m e t h o d s , i s k n o w n a s u x - d i e r e n c e
s p l i t t i n g . I n a n i t e - v o l u m e m e t h o d , t h e u x e s m u s t b e e v a l u a t e d a t c e l l b o u n d -
a r i e s . W e a g a i n b e g i n w i t h t h e d i a g o n a l i z e d f o r m o f t h e l i n e a r , c o n s t a n t - c o e c i e n t ,
h y p e r b o l i c s y s t e m o f e q u a t i o n s
@ w
@ t
+
@ w
@ x
= 0 ( 1 1 . 2 7 )
T h e u x v e c t o r a s s o c i a t e d w i t h t h i s f o r m i s g = w . N o w , a s i n C h a p t e r 5 , w e
c o n s i d e r t h e n u m e r i c a l u x a t t h e i n t e r f a c e b e t w e e n n o d e s j a n d j + 1 , ^ g
j + 1 = 2
, a s a
f u n c t i o n o f t h e s t a t e s t o t h e l e f t a n d r i g h t o f t h e i n t e r f a c e , w
L
a n d w
R
, r e s p e c t i v e l y .
T h e c e n t e r e d a p p r o x i m a t i o n t o g
j + 1 = 2
, w h i c h i s n o n d i s s i p a t i v e , i s g i v e n b y
g
j + 1 = 2
=
1
2
( g ( w
L
) + g ( w
R
) ) ( 1 1 . 2 8 )
I n o r d e r t o o b t a i n a o n e - s i d e d u p w i n d a p p r o x i m a t i o n , w e r e q u i r e
( g
i
)
j + 1 = 2
=
(
i
( w
i
)
L
i f
i
> 0
i
( w
i
)
R
i f
i
< 0
( 1 1 . 2 9 )
w h e r e t h e s u b s c r i p t i i n d i c a t e s i n d i v i d u a l c o m p o n e n t s o f w a n d g . T h i s i s a c h i e v e d
w i t h
( g
i
)
j + 1 = 2
=
1
2
i
( w
i
)
L
+ ( w
i
)
R
] +
1
2
j
i
j ( w
i
)
L
; ( w
i
)
R
] ( 1 1 . 3 0 )
o r
g
j + 1 = 2
=
1
2
( w
L
+ w
R
) +
1
2
j j ( w
L
; w
R
) ( 1 1 . 3 1 )
N o w , a s i n E q . 1 1 . 1 9 , w e p r e m u l t i p l y b y X t o r e t u r n t o t h e o r i g i n a l v a r i a b l e s a n d
i n s e r t t h e p r o d u c t X
; 1
X a f t e r a n d j j t o o b t a i n
X g
j + 1 = 2
=
1
2
X X
; 1
X ( w
L
+ w
R
) +
1
2
X
j
j X
; 1
X ( w
L
; w
R
) ( 1 1 . 3 2 )
a n d t h u s
f
j + 1 = 2
=
1
2
( f
L
+ f
R
) +
1
2
j A j ( u
L
; u
R
) ( 1 1 . 3 3 )
w h e r e
j A j = X j j X
; 1
( 1 1 . 3 4 )
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1 1 . 5 . A R T I F I C I A L D I S S I P A T I O N 2 1 3
a n d w e h a v e a l s o u s e d t h e r e l a t i o n s f = X g , u = X w , a n d A = X X
; 1
.
I n t h e l i n e a r , c o n s t a n t - c o e c i e n t c a s e , t h i s l e a d s t o a n u p w i n d o p e r a t o r w h i c h i s
i d e n t i c a l t o t h a t o b t a i n e d u s i n g u x - v e c t o r s p l i t t i n g . H o w e v e r , i n t h e n o n l i n e a r c a s e ,
t h e r e i s s o m e a m b i g u i t y r e g a r d i n g t h e d e n i t i o n o f j A j a t t h e c e l l i n t e r f a c e j + 1 = 2 .
I n o r d e r t o r e s o l v e t h i s , c o n s i d e r a s i t u a t i o n i n w h i c h t h e e i g e n v a l u e s o f A a r e a l l o f
t h e s a m e s i g n . I n t h i s c a s e , w e w o u l d l i k e o u r d e n i t i o n o f
f
j + 1 = 2
t o s a t i s f y
f
j + 1 = 2
=
(
f
L
i f a l l
i
0
s > 0
f
R
i f a l l
i
0
s < 0
( 1 1 . 3 5 )
g i v i n g p u r e u p w i n d i n g . I f t h e e i g e n v a l u e s o f A a r e a l l p o s i t i v e , j A j = A i f t h e y a r e
a l l n e g a t i v e , j A j = ; A . H e n c e s a t i s f a c t i o n o f E q . 1 1 . 3 5 i s o b t a i n e d b y t h e d e n i t i o n
f
j + 1 = 2
=
1
2
( f
L
+ f
R
) +
1
2
j A
j + 1 = 2
j ( u
L
; u
R
) ( 1 1 . 3 6 )
i f A
j + 1 = 2
s a t i s e s
f
L
; f
R
= A
j + 1 = 2
( u
L
; u
R
) ( 1 1 . 3 7 )
F o r t h e E u l e r e q u a t i o n s f o r a p e r f e c t g a s , E q . 1 1 . 3 7 i s s a t i s e d b y t h e u x J a c o b i a n
e v a l u a t e d a t t h e R o e - a v e r a g e s t a t e g i v e n b y
u
j + 1 = 2
=
p
L
u
L
+
p
R
u
R
p
L
+
p
R
( 1 1 . 3 8 )
H
j + 1 = 2
=
p
L
H
L
+
p
R
H
R
p
L
+
p
R
( 1 1 . 3 9 )
w h e r e u a n d H = ( e + p ) = a r e t h e v e l o c i t y a n d t h e t o t a l e n t h a l p y p e r u n i t m a s s ,
r e s p e c t i v e l y .
3
1 1 . 5 A r t i c i a l D i s s i p a t i o n
W e h a v e s e e n t h a t n u m e r i c a l d i s s i p a t i o n c a n b e i n t r o d u c e d b y u s i n g o n e - s i d e d d i f -
f e r e n c i n g s c h e m e s t o g e t h e r w i t h s o m e f o r m o f u x s p l i t t i n g . W e h a v e a l s o s e e n t h a t
s u c h d i s s i p a t i o n c a n b e i n t r o d u c e d b y a d d i n g a s y m m e t r i c c o m p o n e n t t o a n a n t i s y m -
m e t r i c ( d i s s i p a t i o n - f r e e ) o p e r a t o r . T h u s w e c a n g e n e r a l i z e t h e c o n c e p t o f u p w i n d i n g
t o i n c l u d e a n y s c h e m e i n w h i c h t h e s y m m e t r i c p o r t i o n o f t h e o p e r a t o r i s t r e a t e d i n
s u c h a m a n n e r a s t o b e t r u l y d i s s i p a t i v e .
3
N o t e t h a t t h e u x J a c o b i a n c a n b e w r i t t e n i n t e r m s o f u a n d H o n l y s e e p r o b l e m 6 a t t h e e n d
o f t h i s c h a p t e r .
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2 1 4 C H A P T E R 1 1 . N U M E R I C A L D I S S I P A T I O N
F o r e x a m p l e , l e t
(
a
x
u )
j
=
u
j + 1
; u
j ; 1
2 x
(
s
x
u )
j
=
; u
j + 1
+ 2 u
j
; u
j ; 1
2 x
( 1 1 . 4 0 )
A p p l y i n g
x
=
a
x
+
s
x
t o t h e s p a t i a l d e r i v a t i v e i n E q . 1 1 . 1 5 i s s t a b l e i f
i
0 a n d
u n s t a b l e i f
i
< 0 . S i m i l a r l y , a p p l y i n g
x
=
a
x
;
s
x
i s s t a b l e i f
i
0 a n d u n s t a b l e i f
i
> 0 . T h e a p p r o p r i a t e i m p l e m e n t a t i o n i s t h u s
i
x
=
i
a
x
+ j
i
j
s
x
( 1 1 . 4 1 )
E x t e n s i o n t o a h y p e r b o l i c s y s t e m b y a p p l y i n g t h e a b o v e a p p r o a c h t o t h e c h a r a c t e r i s t i c
v a r i a b l e s , a s i n t h e p r e v i o u s t w o s e c t i o n s , g i v e s
x
( A u ) =
a
x
( A u ) +
s
x
( j A j u ) ( 1 1 . 4 2 )
o r
x
f =
a
x
f +
s
x
( j A j u ) ( 1 1 . 4 3 )
w h e r e j A j i s d e n e d i n E q . 1 1 . 3 4 . T h e s e c o n d s p a t i a l t e r m i s k n o w n a s a r t i c i a l
d i s s i p a t i o n . I t i s a l s o s o m e t i m e s r e f e r r e d t o a s a r t i c i a l d i u s i o n o r a r t i c i a l v i s c o s i t y .
W i t h a p p r o p r i a t e c h o i c e s o f
a
x
a n d
s
x
, t h i s a p p r o a c h c a n b e r e l a t e d t o t h e u p w i n d
a p p r o a c h . T h i s i s p a r t i c u l a r l y e v i d e n t f r o m a c o m p a r i s o n o f E q s . 1 1 . 3 6 a n d 1 1 . 4 3 .
I t i s c o m m o n t o u s e t h e f o l l o w i n g o p e r a t o r f o r
s
x
(
s
x
u )
j
=
x
( u
j ; 2
; 4 u
j ; 1
+ 6 u
j
; 4 u
j + 1
+ u
j + 2
) ( 1 1 . 4 4 )
w h e r e i s a p r o b l e m - d e p e n d e n t c o e c i e n t . T h i s s y m m e t r i c o p e r a t o r a p p r o x i m a t e s
x
3
u
x x x x
a n d t h u s i n t r o d u c e s a t h i r d - o r d e r d i s s i p a t i v e t e r m . W i t h a n a p p r o p r i a t e
v a l u e o f , t h i s o f t e n p r o v i d e s s u c i e n t d a m p i n g o f h i g h f r e q u e n c y m o d e s w i t h o u t
g r e a t l y a e c t i n g t h e l o w f r e q u e n c y m o d e s . F o r d e t a i l s o f h o w t h i s c a n b e i m p l e m e n t e d
f o r n o n l i n e a r h y p e r b o l i c s y s t e m s , t h e r e a d e r s h o u l d c o n s u l t t h e l i t e r a t u r e . A m o r e
c o m p l i c a t e d t r e a t m e n t o f t h e n u m e r i c a l d i s s i p a t i o n i s a l s o r e q u i r e d n e a r s h o c k w a v e s
a n d o t h e r d i s c o n t i n u i t i e s , b u t i s b e y o n d t h e s c o p e o f t h i s b o o k .
1 1 . 6 P r o b l e m s
1 . A s e c o n d - o r d e r b a c k w a r d d i e r e n c e a p p r o x i m a t i o n t o a 1 s t d e r i v a t i v e i s g i v e n
a s a p o i n t o p e r a t o r b y
(
x
u )
j
=
1
2 x
( u
j ; 2
; 4 u
j ; 1
+ 3 u
j
)
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1 1 . 6 . P R O B L E M S 2 1 5
( a ) E x p r e s s t h i s o p e r a t o r i n b a n d e d m a t r i x f o r m ( f o r p e r i o d i c b o u n d a r y c o n d i -
t i o n s ) , t h e n d e r i v e t h e s y m m e t r i c a n d s k e w - s y m m e t r i c m a t r i c e s t h a t h a v e
t h e m a t r i x o p e r a t o r a s t h e i r s u m . ( S e e A p p e n d i x A . 3 t o s e e h o w t o c o n -
s t r u c t t h e s y m m e t r i c a n d s k e w - s y m m e t r i c c o m p o n e n t s o f a m a t r i x . )
( b ) U s i n g a T a y l o r t a b l e , n d t h e d e r i v a t i v e w h i c h i s a p p r o x i m a t e d b y t h e
c o r r e s p o n d i n g s y m m e t r i c a n d s k e w - s y m m e t r i c o p e r a t o r s a n d t h e l e a d i n g
e r r o r t e r m f o r e a c h .
2 . F i n d t h e m o d i e d w a v e n u m b e r f o r t h e r s t - o r d e r b a c k w a r d d i e r e n c e o p e r a t o r .
P l o t t h e r e a l a n d i m a g i n a r y p a r t s o f
x v s . x f o r 0 x . U s i n g
F o u r i e r a n a l y s i s a s i n S e c t i o n 6 . 6 . 2 , n d j j f o r t h e c o m b i n a t i o n o f t h i s s p a t i a l
o p e r a t o r w i t h 4 t h - o r d e r R u n g e - K u t t a t i m e m a r c h i n g a t a C o u r a n t n u m b e r o f
u n i t y a n d p l o t v s . x f o r 0 x .
3 . F i n d t h e m o d i e d w a v e n u m b e r f o r t h e o p e r a t o r g i v e n i n E q . 1 1 . 6 . P l o t t h e r e a l
a n d i m a g i n a r y p a r t s o f
x v s . x f o r 0
x
. U s i n g F o u r i e r a n a l y s i s
a s i n S e c t i o n 6 . 6 . 2 , n d j j f o r t h e c o m b i n a t i o n o f t h i s s p a t i a l o p e r a t o r w i t h
4 t h - o r d e r R u n g e - K u t t a t i m e m a r c h i n g a t a C o u r a n t n u m b e r o f u n i t y a n d p l o t
v s . x f o r 0 x .
4 . C o n s i d e r t h e s p a t i a l o p e r a t o r o b t a i n e d b y c o m b i n i n g s e c o n d - o r d e r c e n t e r e d d i f -
f e r e n c e s w i t h t h e s y m m e t r i c o p e r a t o r g i v e n i n E q . 1 1 . 4 4 . F i n d t h e m o d i e d
w a v e n u m b e r f o r t h i s o p e r a t o r w i t h = 0 1 = 1 2 1 = 2 4 , a n d 1 = 4 8 . P l o t t h e r e a l
a n d i m a g i n a r y p a r t s o f
x v s . x f o r 0 x . U s i n g F o u r i e r a n a l y s i s
a s i n S e c t i o n 6 . 6 . 2 , n d j j f o r t h e c o m b i n a t i o n o f t h i s s p a t i a l o p e r a t o r w i t h
4 t h - o r d e r R u n g e - K u t t a t i m e m a r c h i n g a t a C o u r a n t n u m b e r o f u n i t y a n d p l o t
v s . x f o r 0 x .
5 . C o n s i d e r t h e h y p e r b o l i c s y s t e m d e r i v e d i n p r o b l e m 8 o f C h a p t e r 2 . F i n d t h e
m a t r i x j A j . F o r m t h e p l u s - m i n u s s p l i t u x v e c t o r s a s i n S e c t i o n 1 1 . 4 . 1 .
6 . S h o w t h a t t h e u x J a c o b i a n f o r t h e 1 - D E u l e r e q u a t i o n s c a n b e w r i t t e n i n t e r m s
o f u a n d H . S h o w t h a t t h e u s e o f t h e R o e a v e r a g e s t a t e g i v e n i n E q s . 1 1 . 3 8 a n d
1 1 . 3 9 l e a d s t o s a t i s f a c t i o n o f E q . 1 1 . 3 7 .
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2 1 6 C H A P T E R 1 1 . N U M E R I C A L D I S S I P A T I O N
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C h a p t e r 1 2
S P L I T A N D F A C T O R E D F O R M S
I n t h e n e x t t w o c h a p t e r s , w e p r e s e n t a n d a n a l y z e s p l i t a n d f a c t o r e d a l g o r i t h m s . T h i s
g i v e s t h e r e a d e r a f e e l f o r s o m e o f t h e m o d i c a t i o n s w h i c h c a n b e m a d e t o t h e b a s i c
a l g o r i t h m s i n o r d e r t o o b t a i n e c i e n t s o l v e r s f o r p r a c t i c a l m u l t i d i m e n s i o n a l a p p l i c a -
t i o n s , a n d a m e a n s f o r a n a l y z i n g s u c h m o d i e d f o r m s .
1 2 . 1 T h e C o n c e p t
F a c t o r e d f o r m s o f n u m e r i c a l o p e r a t o r s a r e u s e d e x t e n s i v e l y i n c o n s t r u c t i n g a n d a p -
p l y i n g n u m e r i c a l m e t h o d s t o p r o b l e m s i n u i d m e c h a n i c s . T h e y a r e t h e b a s i s f o r a
w i d e v a r i e t y o f m e t h o d s v a r i o u s l y k n o w n b y t h e l a b e l s \ h y b r i d " , \ t i m e s p l i t " , a n d
\ f r a c t i o n a l s t e p " . F a c t o r e d f o r m s a r e e s p e c i a l l y u s e f u l f o r t h e d e r i v a t i o n o f p r a c t i c a l
a l g o r i t h m s t h a t u s e i m p l i c i t m e t h o d s . W h e n w e a p p r o a c h n u m e r i c a l a n a l y s i s i n t h e
l i g h t o f m a t r i x d e r i v a t i v e o p e r a t o r s , t h e c o n c e p t o f f a c t o r i n g i s q u i t e s i m p l e t o p r e s e n t
a n d g r a s p . L e t u s s t a r t w i t h t h e f o l l o w i n g o b s e r v a t i o n s :
1 . M a t r i c e s c a n b e s p l i t i n q u i t e a r b i t r a r y w a y s .
2 . A d v a n c i n g t o t h e n e x t t i m e l e v e l a l w a y s r e q u i r e s s o m e r e f e r e n c e t o a p r e v i o u s
o n e .
3 . T i m e m a r c h i n g m e t h o d s a r e v a l i d o n l y t o s o m e o r d e r o f a c c u r a c y i n t h e s t e p
s i z e , h .
N o w r e c a l l t h e g e n e r i c O D E ' s p r o d u c e d b y t h e s e m i - d i s c r e t e a p p r o a c h
d
~
u
d t
= A
~
u ;
~
f ( 1 2 . 1 )
2 1 7
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2 1 8 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
a n d c o n s i d e r t h e a b o v e o b s e r v a t i o n s . F r o m o b s e r v a t i o n 1 ( a r b i t r a r y s p l i t t i n g o f A ) :
d
~
u
d t
= A
1
+ A
2
]
~
u ;
~
f ( 1 2 . 2 )
w h e r e A = A
1
+ A
2
] b u t A
1
a n d A
2
a r e n o t u n i q u e . F o r t h e t i m e m a r c h l e t u s c h o o s e
t h e s i m p l e , r s t - o r d e r ,
1
e x p l i c i t E u l e r m e t h o d . T h e n , f r o m o b s e r v a t i o n 2 ( n e w d a t a
~
u
n + 1
i n t e r m s o f o l d
~
u
n
) :
~
u
n + 1
= I + h A
1
+ h A
2
]
~
u
n
; h
~
f + O ( h
2
) ( 1 2 . 3 )
o r i t s e q u i v a l e n t
~
u
n + 1
=
h
I + h A
1
] I + h A
2
] ; h
2
A
1
A
2
i
~
u
n
; h
~
f + O ( h
2
)
F i n a l l y , f r o m o b s e r v a t i o n 3 ( a l l o w i n g u s t o d r o p h i g h e r o r d e r t e r m s ; h
2
A
1
A
2
~
u
n
) :
~
u
n + 1
= I + h A
1
] I + h A
2
]
~
u
n
; h
~
f + O ( h
2
) ( 1 2 . 4 )
N o t i c e t h a t E q s . 1 2 . 3 a n d 1 2 . 4 h a v e t h e s a m e f o r m a l o r d e r o f a c c u r a c y a n d , i n
t h i s s e n s e , n e i t h e r o n e i s t o b e p r e f e r r e d o v e r t h e o t h e r . H o w e v e r , t h e i r n u m e r i c a l
s t a b i l i t y c a n b e q u i t e d i e r e n t , a n d t e c h n i q u e s t o c a r r y o u t t h e i r n u m e r i c a l e v a l u a t i o n
c a n h a v e a r i t h m e t i c o p e r a t i o n c o u n t s t h a t v a r y b y o r d e r s o f m a g n i t u d e . B o t h o f t h e s e
c o n s i d e r a t i o n s a r e i n v e s t i g a t e d l a t e r . H e r e w e s e e k o n l y t o a p p l y t o s o m e s i m p l e c a s e s
t h e c o n c e p t o f f a c t o r i n g .
1 2 . 2 F a c t o r i n g P h y s i c a l R e p r e s e n t a t i o n s | T i m e
S p l i t t i n g
S u p p o s e w e h a v e a P D E t h a t r e p r e s e n t s b o t h t h e p r o c e s s e s o f c o n v e c t i o n a n d d i s s i -
p a t i o n . T h e s e m i - d i s c r e t e a p p r o a c h t o i t s s o l u t i o n m i g h t b e p u t i n t h e f o r m
d
~
u
d t
= A
c
~
u + A
d
~
u +
~
( b c ) ( 1 2 . 5 )
w h e r e A
c
a n d A
d
a r e m a t r i c e s r e p r e s e n t i n g t h e c o n v e c t i o n a n d d i s s i p a t i o n t e r m s ,
r e s p e c t i v e l y a n d t h e i r s u m f o r m s t h e A m a t r i x w e h a v e c o n s i d e r e d i n t h e p r e v i o u s
s e c t i o n s . C h o o s e a g a i n t h e e x p l i c i t E u l e r t i m e m a r c h s o t h a t
~
u
n + 1
= I + h A
d
+ h A
c
]
~
u
n
+ h
~
( b c ) + O ( h
2
) ( 1 2 . 6 )
1
S e c o n d - o r d e r t i m e - m a r c h i n g m e t h o d s a r e c o n s i d e r e d l a t e r .
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1 2 . 2 . F A C T O R I N G P H Y S I C A L R E P R E S E N T A T I O N S | T I M E S P L I T T I N G 2 1 9
N o w c o n s i d e r t h e f a c t o r e d f o r m
~
u
n + 1
= I + h A
d
]
I + h A
c
]
~
u
n
+ h
~
( b c )
= I + h A
d
+ h A
c
]
~
u
n
+ h
~
( b c )
| { z }
O r i g i n a l U n f a c t o r e d T e r m s
+ h
2
A
d
A
c
~
u
n
+
~
( b c )
| { z }
H i g h e r O r d e r T e r m s
+ O ( h
2
) ( 1 2 . 7 )
a n d w e s e e t h a t E q . 1 2 . 7 a n d t h e o r i g i n a l u n f a c t o r e d f o r m E q . 1 2 . 6 h a v e i d e n t i c a l
o r d e r s o f a c c u r a c y i n t h e t i m e a p p r o x i m a t i o n . T h e r e f o r e , o n t h i s b a s i s , t h e i r s e l e c t i o n
i s a r b i t r a r y . I n p r a c t i c a l a p p l i c a t i o n s
2
e q u a t i o n s s u c h a s 1 2 . 7 a r e o f t e n a p p l i e d i n a
p r e d i c t o r - c o r r e c t o r s e q u e n c e . I n t h i s c a s e o n e c o u l d w r i t e
~ u
n + 1
= I + h A
c
]
~
u
n
+ h
~
( b c )
~
u
n + 1
= I + h A
d
] ~ u
n + 1
( 1 2 . 8 )
F a c t o r i n g c a n a l s o b e u s e f u l t o f o r m s p l i t c o m b i n a t i o n s o f i m p l i c i t a n d e x p l i c i t
t e c h n i q u e s . F o r e x a m p l e , a n o t h e r w a y t o a p p r o x i m a t e E q . 1 2 . 6 w i t h t h e s a m e o r d e r
o f a c c u r a c y i s g i v e n b y t h e e x p r e s s i o n
~
u
n + 1
= I ; h A
d
]
; 1
I + h A
c
]
~
u
n
+ h
~
( b c )
= I + h A
d
+ h A
c
]
~
u
n
+ h
~
( b c )
| { z }
O r i g i n a l U n f a c t o r e d T e r m s
+ O ( h
2
) ( 1 2 . 9 )
w h e r e i n t h i s a p p r o x i m a t i o n w e h a v e u s e d t h e f a c t t h a t
I ; h A
d
]
; 1
= I + h A
d
+ h
2
A
2
d
+
i f h j j A
d
j j < 1 , w h e r e j j A
d
j j i s s o m e n o r m o f A
d
] . T h i s t i m e a p r e d i c t o r - c o r r e c t o r
i n t e r p r e t a t i o n l e a d s t o t h e s e q u e n c e
~ u
n + 1
= I + h A
c
]
~
u
n
+ h
~
( b c )
I ; h A
d
]
~
u
n + 1
= ~ u
n + 1
( 1 2 . 1 0 )
T h e c o n v e c t i o n o p e r a t o r i s a p p l i e d e x p l i c i t l y , a s b e f o r e , b u t t h e d i u s i o n o p e r a t o r i s
n o w i m p l i c i t , r e q u i r i n g a t r i d i a g o n a l s o l v e r i f t h e d i u s i o n t e r m i s c e n t r a l d i e r e n c e d .
S i n c e n u m e r i c a l s t i n e s s i s g e n e r a l l y m u c h m o r e s e v e r e f o r t h e d i u s i o n p r o c e s s , t h i s
f a c t o r e d f o r m w o u l d a p p e a r t o b e s u p e r i o r t o t h a t p r o v i d e d b y E q . 1 2 . 8 . H o w e v e r ,
t h e i m p o r t a n t a s p e c t o f s t a b i l i t y h a s y e t t o b e d i s c u s s e d .
2
W e d o n o t s u g g e s t t h a t t h i s p a r t i c u l a r m e t h o d i s s u i t a b l e f o r u s e . W e h a v e y e t t o d e t e r m i n e i t s
s t a b i l i t y , a n d a r s t - o r d e r t i m e - m a r c h m e t h o d i s u s u a l l y u n s a t i s f a c t o r y .
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2 2 0 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
W e s h o u l d m e n t i o n h e r e t h a t E q . 1 2 . 9 c a n b e d e r i v e d f o r a d i e r e n t p o i n t o f v i e w
b y w r i t i n g E q . 1 2 . 6 i n t h e f o r m
u
n + 1
; u
n
h
= A
c
u
n
+ A
d
u
n + 1
+
~
( b c ) + O ( h
2
)
T h e n
I ; h A
d
] u
n + 1
= I + h A
c
] u
n
+ h
~
( b c )
w h i c h i s i d e n t i c a l t o E q . 1 2 . 1 0 .
1 2 . 3 F a c t o r i n g S p a c e M a t r i x O p e r a t o r s i n 2 { D
1 2 . 3 . 1 M e s h I n d e x i n g C o n v e n t i o n
F a c t o r i n g i s w i d e l y u s e d i n c o d e s d e s i g n e d f o r t h e n u m e r i c a l s o l u t i o n o f e q u a t i o n s
g o v e r n i n g u n s t e a d y t w o - a n d t h r e e - d i m e n s i o n a l o w s . L e t u s s t u d y t h e b a s i c c o n c e p t
o f f a c t o r i n g b y i n s p e c t i n g i t s u s e o n t h e l i n e a r 2 - D s c a l a r P D E t h a t m o d e l s d i u s i o n :
@ u
@ t
=
@
2
u
@ x
2
+
@
2
u
@ y
2
( 1 2 . 1 1 )
W e b e g i n b y r e d u c i n g t h i s P D E t o a c o u p l e d s e t o f O D E ' s b y d i e r e n c i n g t h e s p a c e
d e r i v a t i v e s a n d i n s p e c t i n g t h e r e s u l t i n g m a t r i x o p e r a t o r .
A c l e a r d e s c r i p t i o n o f a m a t r i x n i t e - d i e r e n c e o p e r a t o r i n 2 - a n d 3 - D r e q u i r e s s o m e
r e f e r e n c e t o a m e s h . W e c h o o s e t h e 3 4 p o i n t m e s h
3
s h o w n i n t h e S k e t c h 1 2 . 1 2 .
I n t h i s e x a m p l e M
x
, t h e n u m b e r o f ( i n t e r i o r ) x p o i n t s , i s 4 a n d M
y
, t h e n u m b e r o f
( i n t e r i o r ) y p o i n t s i s 3 . T h e n u m b e r s 1 1 1 2 4 3 r e p r e s e n t t h e l o c a t i o n i n t h e
m e s h o f t h e d e p e n d e n t v a r i a b l e b e a r i n g t h a t i n d e x . T h u s u
3 2
r e p r e s e n t s t h e v a l u e o f
u a t j = 3 a n d k = 2 .
M
y
1 3 2 3 3 3 4 3
k 1 2 2 2 3 2 4 2
1 1 1 2 1 3 1 4 1
1 j M
x
M e s h i n d e x i n g i n 2 - D .
( 1 2 . 1 2 )
3
T h i s c o u l d a l s o b e c a l l e d a 5 6 p o i n t m e s h i f t h e b o u n d a r y p o i n t s ( l a b e l e d i n t h e s k e t c h )
w e r e i n c l u d e d , b u t i n t h e s e n o t e s w e d e s c r i b e t h e s i z e o f a m e s h b y t h e n u m b e r o f i n t e r i o r p o i n t s .
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1 2 . 3 . F A C T O R I N G S P A C E M A T R I X O P E R A T O R S I N 2 { D 2 2 1
1 2 . 3 . 2 D a t a B a s e s a n d S p a c e V e c t o r s
T h e d i m e n s i o n e d a r r a y i n a c o m p u t e r c o d e t h a t a l l o t s t h e s t o r a g e l o c a t i o n s o f t h e
d e p e n d e n t v a r i a b l e ( s ) i s r e f e r r e d t o a s a d a t a - b a s e . T h e r e a r e m a n y w a y s t o l a y o u t
a d a t a - b a s e . O f t h e s e , w e c o n s i d e r o n l y t w o : ( 1 ) , c o n s e c u t i v e l y a l o n g r o w s t h a t a r e
t h e m s e l v e s c o n s e c u t i v e f r o m k = 1 t o M
y
, a n d ( 2 ) , c o n s e c u t i v e l y a l o n g c o l u m n s t h a t
a r e c o n s e c u t i v e f r o m j = 1 t o M
x
. W e r e f e r t o e a c h r o w o r c o l u m n g r o u p a s a
s p a c e v e c t o r ( t h e y r e p r e s e n t d a t a a l o n g l i n e s t h a t a r e c o n t i n u o u s i n s p a c e ) a n d l a b e l
t h e i r s u m w i t h t h e s y m b o l U . I n p a r t i c u l a r , ( 1 ) a n d ( 2 ) a b o v e a r e r e f e r r e d t o a s x -
v e c t o r s a n d y - v e c t o r s , r e s p e c t i v e l y . T h e s y m b o l U b y i t s e l f i s n o t e n o u g h t o i d e n t i f y
t h e s t r u c t u r e o f t h e d a t a - b a s e a n d i s u s e d o n l y w h e n t h e s t r u c t u r e i s i m m a t e r i a l o r
u n d e r s t o o d .
T o b e s p e c i c a b o u t t h e s t r u c t u r e , w e l a b e l a d a t a { b a s e c o m p o s e d o f x - v e c t o r s w i t h
U
( x )
, a n d o n e c o m p o s e d o f y - v e c t o r s w i t h U
( y )
. E x a m p l e s o f t h e o r d e r o f i n d e x i n g
f o r t h e s e s p a c e v e c t o r s a r e g i v e n i n E q . 1 2 . 1 6 p a r t a a n d b .
1 2 . 3 . 3 D a t a B a s e P e r m u t a t i o n s
T h e t w o v e c t o r s ( a r r a y s ) a r e r e l a t e d b y a p e r m u t a t i o n m a t r i x P s u c h t h a t
U
( x )
= P
x y
U
( y )
a n d U
( y )
= P
y x
U
( x )
( 1 2 . 1 3 )
w h e r e
P
y x
= P
T
x y
= P
; 1
x y
N o w c o n s i d e r t h e s t r u c t u r e o f a m a t r i x n i t e - d i e r e n c e o p e r a t o r r e p r e s e n t i n g 3 -
p o i n t c e n t r a l - d i e r e n c i n g s c h e m e s f o r b o t h s p a c e d e r i v a t i v e s i n t w o d i m e n s i o n s . W h e n
t h e m a t r i x i s m u l t i p l y i n g a s p a c e v e c t o r U , t h e u s u a l ( b u t a m b i g u o u s ) r e p r e s e n t a t i o n
i s g i v e n b y A
x + y
. I n t h i s n o t a t i o n t h e O D E f o r m o f E q . 1 2 . 1 1 c a n b e w r i t t e n
4
d U
d t
= A
x + y
U +
~
( b c ) ( 1 2 . 1 4 )
I f i t i s i m p o r t a n t t o b e s p e c i c a b o u t t h e d a t a - b a s e s t r u c t u r e , w e u s e t h e n o t a t i o n
A
( x )
x + y
o r A
( y )
x + y
, d e p e n d i n g o n t h e d a t a { b a s e c h o s e n f o r t h e U i t m u l t i p l i e s . E x a m p l e s
a r e i n E q . 1 2 . 1 6 p a r t a a n d b . N o t i c e t h a t t h e m a t r i c e s a r e n o t t h e s a m e a l t h o u g h
t h e y r e p r e s e n t t h e s a m e d e r i v a t i v e o p e r a t i o n . T h e i r s t r u c t u r e s a r e s i m i l a r , h o w e v e r ,
a n d t h e y a r e r e l a t e d b y t h e s a m e p e r m u t a t i o n m a t r i x t h a t r e l a t e s U
( x )
t o U
( y )
. T h u s
A
( x )
x + y
= P
x y
A
( y )
x + y
P
y x
( 1 2 . 1 5 )
4
N o t i c e t h a t A
x + y
a n d U , w h i c h a r e n o t a t i o n s u s e d i n t h e s p e c i a l c a s e o f s p a c e v e c t o r s , a r e
s u b s e t s o f A a n d
~
u , u s e d i n t h e p r e v i o u s s e c t i o n s .
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2 2 2 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
A
( x )
x + y
U
( x )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
x j o j
x x j o j
x x j o j
x j o j
o j x j o
o j x x j o
o j x x j o
o j x j o
j o j x
j o j x x
j o j x x
j o j x
3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
1 1
2 1
3 1
4 1
; ;
1 2
2 2
3 2
4 2
; ;
1 3
2 3
3 3
4 3
a : E l e m e n t s i n 2 - d i m e n s i o n a l , c e n t r a l - d i e r e n c e , m a t r i x
o p e r a t o r , A
x + y
, f o r 3 4 m e s h s h o w n i n S k e t c h 1 2 . 1 2 .
D a t a b a s e c o m p o s e d o f M
y
x { v e c t o r s s t o r e d i n U
( x )
.
E n t r i e s f o r x ! x , f o r y ! o , f o r b o t h ! .
A
( y )
x + y
U
( y )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
o j x j j
o o j x j j
o j x j j
x j o j x j
x j o o j x j
x j o j x j
j x j o j x
j x j o o j x
j x j o j x
j j x j o
j j x j o o
j j x j o
3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
1 1
1 2
1 3
; ;
2 1
2 2
2 3
; ;
3 1
3 2
3 3
; ;
4 1
4 2
4 3
b : E l e m e n t s i n 2 - d i m e n s i o n a l , c e n t r a l - d i e r e n c e , m a t r i x
o p e r a t o r , A
x + y
, f o r 3 4 m e s h s h o w n i n S k e t c h 1 2 . 1 2 .
D a t a b a s e c o m p o s e d o f M
x
y { v e c t o r s s t o r e d i n U
( y )
.
E n t r i e s f o r x ! x , f o r y ! o , f o r b o t h ! .
( 1 2 . 1 6 )
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1 2 . 3 . F A C T O R I N G S P A C E M A T R I X O P E R A T O R S I N 2 { D 2 2 3
1 2 . 3 . 4 S p a c e S p l i t t i n g a n d F a c t o r i n g
W e a r e n o w p r e p a r e d t o d i s c u s s s p l i t t i n g i n t w o d i m e n s i o n s . I t s h o u l d b e c l e a r t h a t
t h e m a t r i x A
( x )
x + y
c a n b e s p l i t i n t o t w o m a t r i c e s s u c h t h a t
A
( x )
x + y
= A
( x )
x
+ A
( x )
y
( 1 2 . 1 7 )
w h e r e A
( x )
x
a n d A
( x )
y
a r e s h o w n i n E q . 1 2 . 2 2 . S i m i l a r i l y
A
( y )
x + y
= A
( y )
x
+ A
( y )
y
( 1 2 . 1 8 )
w h e r e t h e s p l i t m a t r i c e s a r e s h o w n i n E q . 1 2 . 2 3 .
T h e p e r m u t a t i o n r e l a t i o n a l s o h o l d s f o r t h e s p l i t m a t r i c e s s o
A
( x )
y
= P
x y
A
( y )
y
P
y x
a n d
A
( x )
x
= P
x y
A
( y )
x
P
y x
T h e s p l i t t i n g s i n E q s . 1 2 . 1 7 a n d 1 2 . 1 8 c a n b e c o m b i n e d w i t h f a c t o r i n g i n t h e
m a n n e r d e s c r i b e d i n S e c t i o n 1 2 . 2 . A s a n e x a m p l e ( r s t - o r d e r i n t i m e ) , a p p l y i n g t h e
i m p l i c i t E u l e r m e t h o d t o E q . 1 2 . 1 4 g i v e s
U
( x )
n + 1
= U
( x )
n
+ h
h
A
( x )
x
+ A
( x )
y
i
U
( x )
n + 1
+ h
~
( b c )
o r
h
I ; h A
( x )
x
; h A
( x )
y
i
U
( x )
n + 1
= U
( x )
n
+ h
~
( b c ) + O ( h
2
) ( 1 2 . 1 9 )
A s i n S e c t i o n 1 2 . 2 , w e r e t a i n t h e s a m e r s t o r d e r a c c u r a c y w i t h t h e a l t e r n a t i v e
h
I ; h A
( x )
x
i h
I ; h A
( x )
y
i
U
( x )
n + 1
= U
( x )
n
+ h
~
( b c ) + O ( h
2
) ( 1 2 . 2 0 )
W r i t e t h i s i n p r e d i c t o r - c o r r e c t o r f o r m a n d p e r m u t e t h e d a t a b a s e o f t h e s e c o n d r o w .
T h e r e r e s u l t s
h
I ; h A
( x )
x
i
~
U
( x )
= U
( x )
n
+ h
~
( b c )
h
I ; h A
( y )
y
i
U
( y )
n + 1
=
~
U
( y )
( 1 2 . 2 1 )
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2 2 4 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
A
( x )
x
U
( x )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
x x j j
x x x j j
x x x j j
x x j j
j x x j
j x x x j
j x x x j
j x x j
j j x x
j j x x x
j j x x x
j j x x
3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
U
( x )
A
( x )
y
U
( x )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
o j o j
o j o j
o j o j
o j o j
o j o j o
o j o j o
o j o j o
o j o j o
j o j o
j o j o
j o j o
j o j o
3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
U
( x )
T h e s p l i t t i n g o f A
( x )
x + y
.
( 1 2 . 2 2 )
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1 2 . 3 . F A C T O R I N G S P A C E M A T R I X O P E R A T O R S I N 2 { D 2 2 5
A
( y )
x
U
( y )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
x j x j j
x j x j j
x j x j j
x j x j x j
x j x j x j
x j x j x j
j x j x j x
j x j x j x
j x j x j x
j j x j x
j j x j x
j j x j x
3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
U
( y )
A
( y )
y
U
( y )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
o o j j j
o o o j j j
o o j j j
j o o j j
j o o o j j
j o o j j
j j o o j
j j o o o j
j j o o j
j j j o o
j j j o o o
j j j o o
3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
U
( y )
T h e s p l i t t i n g o f A
( y )
x + y
.
( 1 2 . 2 3 )
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2 2 6 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
1 2 . 4 S e c o n d - O r d e r F a c t o r e d I m p l i c i t M e t h o d s
S e c o n d - o r d e r a c c u r a c y i n t i m e c a n b e m a i n t a i n e d i n a c e r t a i n f a c t o r e d i m p l i c i t m e t h -
o d s . F o r e x a m p l e , a p p l y t h e t r a p e z o i d a l m e t h o d t o E q . 1 2 . 1 4 w h e r e t h e d e r i v a t i v e
o p e r a t o r s h a v e b e e n s p l i t a s i n E q . 1 2 . 1 7 o r 1 2 . 1 8 . L e t t h e d a t a b a s e b e i m m a t e r i a l
a n d t h e
~
( b c ) b e t i m e i n v a r i a n t . T h e r e r e s u l t s
I ;
1
2
h A
x
;
1
2
h A
y
U
n + 1
=
I +
1
2
h A
x
+
1
2
h A
y
U
n
+ h
~
( b c ) + O ( h
3
) ( 1 2 . 2 4 )
F a c t o r b o t h s i d e s g i v i n g
I ;
1
2
h A
x
I ;
1
2
h A
y
;
1
4
h
2
A
x
A
y
U
n + 1
=
I +
1
2
h A
x
I +
1
2
h A
y
;
1
4
h
2
A
x
A
y
U
n
+ h
~
( b c ) + O ( h
3
) ( 1 2 . 2 5 )
T h e n n o t i c e t h a t t h e c o m b i n a t i o n
1
4
h
2
A
x
A
y
] ( U
n + 1
; U
n
) i s p r o p o r t i o n a l t o h
3
s i n c e
t h e l e a d i n g t e r m i n t h e e x p a n s i o n o f ( U
n + 1
; U
n
) i s p r o p o r t i o n a l t o h . T h e r e f o r e , w e
c a n w r i t e
I ;
1
2
h A
x
I ;
1
2
h A
y
U
n + 1
=
I +
1
2
h A
x
I +
1
2
h A
y
U
n
+ h
~
( b c ) + O ( h
3
) ( 1 2 . 2 6 )
a n d b o t h t h e f a c t o r e d a n d u n f a c t o r e d f o r m o f t h e t r a p e z o i d a l m e t h o d a r e s e c o n d - o r d e r
a c c u r a t e i n t h e t i m e m a r c h .
A n a l t e r n a t i v e f o r m o f t h i s k i n d o f f a c t o r i z a t i o n i s t h e c l a s s i c a l A D I ( a l t e r n a t i n g
d i r e c t i o n i m p l i c i t ) m e t h o d
5
u s u a l l y w r i t t e n
I ;
1
2
h A
x
~
U =
I +
1
2
h A
y
U
n
+
1
2
h F
n
I ;
1
2
h A
y
U
n + 1
=
I +
1
2
h A
x
~
U +
1
2
h F
n + 1
+ O ( h
3
) ( 1 2 . 2 7 )
F o r i d e a l i z e d c o m m u t i n g s y s t e m s t h e m e t h o d s g i v e n b y E q s . 1 2 . 2 6 a n d 1 2 . 2 7 d i e r
o n l y i n t h e i r e v a l u a t i o n o f a t i m e - d e p e n d e n t f o r c i n g t e r m .
1 2 . 5 I m p o r t a n c e o f F a c t o r e d F o r m s i n 2 a n d 3 D i -
m e n s i o n s
W h e n t h e t i m e - m a r c h e q u a t i o n s a r e s t i a n d i m p l i c i t m e t h o d s a r e r e q u i r e d t o p e r m i t
r e a s o n a b l y l a r g e t i m e s t e p s , t h e u s e o f f a c t o r e d f o r m s b e c o m e s a v e r y v a l u a b l e t o o l
5
A f o r m o f t h e D o u g l a s o r P e a c e m a n - R a c h f o r d m e t h o d s .
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1 2 . 5 . I M P O R T A N C E O F F A C T O R E D F O R M S I N 2 A N D 3 D I M E N S I O N S 2 2 7
f o r r e a l i s t i c p r o b l e m s . C o n s i d e r , f o r e x a m p l e , t h e p r o b l e m o f c o m p u t i n g t h e t i m e
a d v a n c e i n t h e u n f a c t o r e d f o r m o f t h e t r a p e z o i d a l m e t h o d g i v e n b y E q . 1 2 . 2 4
I ;
1
2
h A
x + y
U
n + 1
=
I +
1
2
h A
x + y
U
n
+ h
~
( b c )
F o r m i n g t h e r i g h t h a n d s i d e p o s e s n o p r o b l e m , b u t n d i n g U
n + 1
r e q u i r e s t h e s o l u t i o n
o f a s p a r s e , b u t v e r y l a r g e , s e t o f c o u p l e d s i m u l t a n e o u s e q u a t i o n s h a v i n g t h e m a t r i x
f o r m s h o w n i n E q . 1 2 . 1 6 p a r t a a n d b . F u r t h e r m o r e , i n r e a l c a s e s i n v o l v i n g t h e E u l e r
o r N a v i e r - S t o k e s e q u a t i o n s , e a c h s y m b o l ( o x ) r e p r e s e n t s a 4 4 b l o c k m a t r i x w i t h
e n t r i e s t h a t d e p e n d o n t h e p r e s s u r e , d e n s i t y a n d v e l o c i t y e l d . S u p p o s e w e w e r e t o
s o l v e t h e e q u a t i o n s d i r e c t l y . T h e f o r w a r d s w e e p o f a s i m p l e G a u s s i a n e l i m i n a t i o n l l s
6
a l l o f t h e 4 4 b l o c k s b e t w e e n t h e m a i n a n d o u t e r m o s t d i a g o n a l
7
( e . g . b e t w e e n
a n d o i n E q . 1 2 . 1 6 p a r t b . ) . T h i s m u s t b e s t o r e d i n c o m p u t e r m e m o r y t o b e u s e d t o
n d t h e n a l s o l u t i o n i n t h e b a c k w a r d s w e e p . I f N
e
r e p r e s e n t s t h e o r d e r o f t h e s m a l l
b l o c k m a t r i x ( 4 i n t h e 2 - D E u l e r c a s e ) , t h e a p p r o x i m a t e m e m o r y r e q u i r e m e n t i s
( N
e
M
y
) ( N
e
M
y
) M
x
o a t i n g p o i n t w o r d s . H e r e i t i s a s s u m e d t h a t M
y
< M
x
. I f M
y
> M
x
, M
y
a n d M
x
w o u l d b e i n t e r c h a n g e d . A m o d e r a t e m e s h o f 6 0 2 0 0 p o i n t s w o u l d r e q u i r e o v e r 1 1
m i l l i o n w o r d s t o n d t h e s o l u t i o n . A c t u a l l y c u r r e n t c o m p u t e r p o w e r i s a b l e t o c o p e
r a t h e r e a s i l y w i t h s t o r a g e r e q u i r e m e n t s o f t h i s o r d e r o f m a g n i t u d e . W i t h c o m p u t i n g
s p e e d s o f o v e r o n e g i g a o p ,
8
d i r e c t s o l v e r s m a y b e c o m e u s e f u l f o r n d i n g s t e a d y - s t a t e
s o l u t i o n s o f p r a c t i c a l p r o b l e m s i n t w o d i m e n s i o n s . H o w e v e r , a t h r e e - d i m e n s i o n a l
s o l v e r w o u l d r e q u i r e a m e m o r y o f a p p r o x i m a t l y
N
2
e
M
2
y
M
2
z
M
x
w o r d s a n d , f o r w e l l r e s o l v e d o w e l d s , t h i s p r o b a b l y e x c e e d s m e m o r y a v a i l a b i l i t y f o r
s o m e t i m e t o c o m e .
O n t h e o t h e r h a n d , c o n s i d e r c o m p u t i n g a s o l u t i o n u s i n g t h e f a c t o r e d i m p l i c i t e q u a -
t i o n 1 2 . 2 5 . A g a i n c o m p u t i n g t h e r i g h t h a n d s i d e p o s e s n o p r o b l e m . A c c u m u l a t e t h e
r e s u l t o f s u c h a c o m p u t a t i o n i n t h e a r r a y ( R H S ) . O n e c a n t h e n w r i t e t h e r e m a i n i n g
t e r m s i n t h e t w o - s t e p p r e d i c t o r - c o r r e c t o r f o r m
I ;
1
2
h A
( x )
x
~
U
( x )
= ( R H S )
( x )
I ;
1
2
h A
( y )
y
U
( y )
n + 1
=
~
U
( y )
( 1 2 . 2 8 )
6
F o r m a t r i c e s a s s m a l l a s t h o s e s h o w n t h e r e a r e m a n y g a p s i n t h i s \ l l " , b u t f o r m e s h e s o f
p r a c t i c a l s i z e t h e l l i s m o s t l y d e n s e .
7
T h e l o w e r b a n d i s a l s o c o m p u t e d b u t d o e s n o t h a v e t o b e s a v e d u n l e s s t h e s o l u t i o n i s t o b e
r e p e a t e d f o r a n o t h e r v e c t o r .
8
O n e b i l l i o n o a t i n g - p o i n t o p e r a t i o n s p e r s e c o n d .
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2 2 8 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
w h i c h h a s t h e s a m e a p p e a r a n c e a s E q . 1 2 . 2 1 b u t i s s e c o n d - o r d e r t i m e a c c u r a t e . T h e
r s t s t e p w o u l d b e s o l v e d u s i n g M
y
u n c o u p l e d b l o c k t r i d i a g o n a l s o l v e r s
9
. I n s p e c t i n g
t h e t o p o f E q . 1 2 . 2 2 , w e s e e t h a t t h i s i s e q u i v a l e n t t o s o l v i n g M
y
o n e - d i m e n s i o n a l
p r o b l e m s , e a c h w i t h M
x
b l o c k s o f o r d e r N
e
. T h e t e m p o r a r y s o l u t i o n
~
U
( x )
w o u l d t h e n
b e p e r m u t e d t o
~
U
( y )
a n d a n i n s p e c t i o n o f t h e b o t t o m o f E q . 1 2 . 2 3 s h o w s t h a t t h e n a l
s t e p c o n s i s t s o f s o l v i n g M
x
o n e - d i m e n s i o n a l i m p l i c i t p r o b l e m s e a c h w i t h d i m e n s i o n
M
y
.
1 2 . 6 T h e D e l t a F o r m
C l e a r l y m a n y w a y s c a n b e d e v i s e d t o s p l i t t h e m a t r i c e s a n d g e n e r a t e f a c t o r e d f o r m s .
O n e w a y t h a t i s e s p e c i a l l y u s e f u l , f o r e n s u r i n g a c o r r e c t s t e a d y - s t a t e s o l u t i o n i n a
c o n v e r g e d t i m e - m a r c h , i s r e f e r r e d t o a s t h e \ d e l t a f o r m " a n d w e d e v e l o p i t n e x t .
C o n s i d e r t h e u n f a c t o r e d f o r m o f t h e t r a p e z o i d a l m e t h o d g i v e n b y E q . 1 2 . 2 4 , a n d
l e t t h e
~
( b c ) b e t i m e i n v a r i a n t :
I ;
1
2
h A
x
;
1
2
h A
y
U
n + 1
=
I +
1
2
h A
x
+
1
2
h A
y
U
n
+ h
~
( b c ) + O ( h
3
)
F r o m b o t h s i d e s s u b t r a c t
I ;
1
2
h A
x
;
1
2
h A
y
U
n
l e a v i n g t h e e q u a l i t y u n c h a n g e d . T h e n , u s i n g t h e s t a n d a r d d e n i t i o n o f t h e d i e r e n c e
o p e r a t o r ,
U
n
= U
n + 1
; U
n
o n e n d s
I ;
1
2
h A
x
;
1
2
h A
y
U
n
= h
h
A
x + y
U
n
+
~
( b c )
i
+ O ( h
3
) ( 1 2 . 2 9 )
N o t i c e t h a t t h e r i g h t s i d e o f t h i s e q u a t i o n i s t h e p r o d u c t o f h a n d a t e r m t h a t i s
i d e n t i c a l t o t h e r i g h t s i d e o f E q . 1 2 . 1 4 , o u r o r i g i n a l O D E . T h u s , i f E q . 1 2 . 2 9 c o n v e r g e s ,
i t i s g u a r a n t e e d t o c o n v e r g e t o t h e c o r r e c t s t e a d y - s t a t e s o l u t i o n o f t h e O D E . N o w w e
c a n f a c t o r E q . 1 2 . 2 9 a n d m a i n t a i n O ( h
2
) a c c u r a c y . W e a r r i v e a t t h e e x p r e s s i o n
I ;
1
2
h A
x
I ;
1
2
h A
y
U
n
= h
h
A
x + y
U
n
+
~
( b c )
i
+ O ( h
3
) ( 1 2 . 3 0 )
T h i s i s t h e d e l t a f o r m o f a f a c t o r e d , 2 n d - o r d e r , 2 - D e q u a t i o n .
9
A b l o c k t r i d i a g o n a l s o l v e r i s s i m i l a r t o a s c a l a r s o l v e r e x c e p t t h a t s m a l l b l o c k m a t r i x o p e r a t i o n s
r e p l a c e t h e s c a l a r o n e s , a n d m a t r i x m u l t i p l i c a t i o n s d o n o t c o m m u t e .
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1 2 . 7 . P R O B L E M S 2 2 9
T h e p o i n t a t w h i c h t h e f a c t o r i n g i s m a d e m a y n o t a e c t t h e o r d e r o f t i m e - a c c u r a c y ,
b u t i t c a n h a v e a p r o f o u n d e e c t o n t h e s t a b i l i t y a n d c o n v e r g e n c e p r o p e r t i e s o f a
m e t h o d . F o r e x a m p l e , t h e u n f a c t o r e d f o r m o f a r s t - o r d e r m e t h o d d e r i v e d f r o m t h e
i m p l i c i t E u l e r t i m e m a r c h i s g i v e n b y E q . 1 2 . 1 9 , a n d i f i t i s i m m e d i a t e l y f a c t o r e d ,
t h e f a c t o r e d f o r m i s p r e s e n t e d i n E q . 1 2 . 2 0 . O n t h e o t h e r h a n d , t h e d e l t a f o r m o f t h e
u n f a c t o r e d E q . 1 2 . 1 9 i s
I ; h A
x
; h A
y
] U
n
= h
h
A
x + y
U
n
+
~
( b c )
i
a n d i t s f a c t o r e d f o r m b e c o m e s
1 0
I ; h A
x
] I ; h A
y
] U
n
= h
h
A
x + y
U
n
+
~
( b c )
i
( 1 2 . 3 1 )
I n s p i t e o f t h e s i m i l a r i t i e s i n d e r i v a t i o n , w e w i l l s e e i n t h e n e x t c h a p t e r t h a t t h e
c o n v e r g e n c e p r o p e r t i e s o f E q . 1 2 . 2 0 a n d E q . 1 2 . 3 1 a r e v a s t l y d i e r e n t .
1 2 . 7 P r o b l e m s
1 . C o n s i d e r t h e 1 - D h e a t e q u a t i o n :
@ u
@ t
=
@
2
u
@ x
2
0 x 9
L e t u ( 0 t ) = 0 a n d u ( 9 t ) = 0 , s o t h a t w e c a n s i m p l i f y t h e b o u n d a r y c o n d i t i o n s .
A s s u m e t h a t s e c o n d o r d e r c e n t r a l d i e r e n c i n g i s u s e d , i . e . ,
(
x x
u )
j
=
1
x
2
( u
j ; 1
; 2 u
j
+ u
j + 1
)
T h e u n i f o r m g r i d h a s x = 1 a n d 8 i n t e r i o r p o i n t s .
( a ) S p a c e v e c t o r d e n i t i o n
i . W h a t i s t h e s p a c e v e c t o r f o r t h e n a t u r a l o r d e r i n g ( m o n o t o n i c a l l y i n -
c r e a s i n g i n i n d e x ) , u
( 1 )
? O n l y i n c l u d e t h e i n t e r i o r p o i n t s .
i i . I f w e r e o r d e r t h e p o i n t s w i t h t h e o d d p o i n t s r s t a n d t h e n t h e e v e n
p o i n t s , w r i t e t h e s p a c e v e c t o r , u
( 2 )
?
i i i . W r i t e d o w n t h e p e r m u t a t i o n m a t r i c e s , ( P
1 2
, P
2 1
) .
1 0
N o t i c e t h a t t h e o n l y d i e r e n c e b e t w e e n t h e O ( h
2
) m e t h o d g i v e n b y E q . 1 2 . 3 0 a n d t h e O ( h )
m e t h o d g i v e n b y E q . 1 2 . 3 1 i s t h e a p p e a r a n c e o f t h e f a c t o r
1
2
o n t h e l e f t s i d e o f t h e O ( h
2
) m e t h o d .
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2 3 0 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
i v . T h e g e n e r i c O D E r e p r e s e n t i n g t h e d i s c r e t e f o r m o f t h e h e a t e q u a t i o n
i s
d u
( 1 )
d t
= A
1
u
( 1 )
+ f
W r i t e d o w n t h e m a t r i x A
1
. ( N o t e f = 0 , d u e t o t h e b o u n d a r y c o n d i -
t i o n s ) N e x t n d t h e m a t r i x A
2
s u c h t h a t
d u
( 2 )
d t
= A
2
u
( 2 )
N o t e t h a t A
2
c a n b e w r i t t e n a s
A
2
=
2
6
4
D U
T
U D
3
7
5
D e n e D a n d U .
v . A p p l y i n g i m p l i c i t E u l e r t i m e m a r c h i n g , w r i t e t h e d e l t a f o r m o f t h e
i m p l i c i t a l g o r i t h m . C o m m e n t o n t h e f o r m o f t h e r e s u l t i n g i m p l i c i t
m a t r i x o p e r a t o r .
( b ) S y s t e m d e n i t i o n
I n p r o b l e m 1 a , w e d e n e d u
( 1 )
u
( 2 )
A
1
A
2
P
1 2
, a n d P
2 1
w h i c h p a r t i t i o n
t h e o d d p o i n t s f r o m t h e e v e n p o i n t s . W e c a n p u t s u c h a p a r t i t i o n i n g t o
u s e . F i r s t d e n e e x t r a c t i o n o p e r a t o r s
I
( o )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
4
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
3
7
7
7
7
7
7
7
7
7
7
7
7
5
=
2
6
4
I
4
0
4
0
4
0
4
3
7
5
I
( e )
=
2
6
6
6
6
6
6
6
6
6
6
6
6
4
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
3
7
7
7
7
7
7
7
7
7
7
7
7
5
=
2
6
4
0
4
0
4
0
4
I
4
3
7
5
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1 2 . 7 . P R O B L E M S 2 3 1
w h i c h e x t r a c t t h e o d d e v e n p o i n t s f r o m u
( 2 )
a s f o l l o w s : u
( o )
= I
( o )
u
( 2 )
a n d
u
( e )
= I
( e )
u
( 2 )
.
i . B e g i n n i n g w i t h t h e O D E w r i t t e n i n t e r m s o f u
( 2 )
, d e n e a s p l i t t i n g
A
2
= A
o
+ A
e
, s u c h t h a t A
o
o p e r a t e s o n l y o n t h e o d d t e r m s , a n d A
e
o p e r a t e s o n l y o n t h e e v e n t e r m s . W r i t e o u t t h e m a t r i c e s A
o
a n d A
e
.
A l s o , w r i t e t h e m i n t e r m s o f D a n d U d e n e d a b o v e .
i i . A p p l y i m p l i c i t E u l e r t i m e m a r c h i n g t o t h e s p l i t O D E . W r i t e d o w n t h e
d e l t a f o r m o f t h e a l g o r i t h m a n d t h e f a c t o r e d d e l t a f o r m . C o m m e n t o n
t h e o r d e r o f t h e e r r o r t e r m s .
i i i . E x a m i n e t h e i m p l i c i t o p e r a t o r s f o r t h e f a c t o r e d d e l t a f o r m . C o m m e n t
o n t h e i r f o r m . Y o u s h o u l d b e a b l e t o a r g u e t h a t t h e s e a r e n o w t r a n g u -
l a r m a t r i c e s ( a l o w e r a n d a n u p p e r ) . C o m m e n t o n t h e s o l u t i o n p r o c e s s
t h i s g i v e s u s r e l a t i v e t o t h e d i r e c t i n v e r s i o n o f t h e o r i g i n a l s y s t e m .
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2 3 2 C H A P T E R 1 2 . S P L I T A N D F A C T O R E D F O R M S
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C h a p t e r 1 3
L I N E A R A N A L Y S I S O F S P L I T
A N D F A C T O R E D F O R M S
I n S e c t i o n 4 . 4 w e i n t r o d u c e d t h e c o n c e p t o f t h e r e p r e s e n t a t i v e e q u a t i o n , a n d u s e d
i t i n C h a p t e r 7 t o s t u d y t h e s t a b i l i t y , a c c u r a c y , a n d c o n v e r g e n c e p r o p e r t i e s o f t i m e -
m a r c h i n g s c h e m e s . T h e q u e s t i o n i s : C a n w e n d a s i m i l a r e q u a t i o n t h a t w i l l a l l o w
u s t o e v a l u a t e t h e s t a b i l i t y a n d c o n v e r g e n c e p r o p e r t i e s o f s p l i t a n d f a c t o r e d s c h e m e s ?
T h e a n s w e r i s y e s | f o r c e r t a i n f o r m s o f l i n e a r m o d e l e q u a t i o n s .
T h e a n a l y s i s i n t h i s c h a p t e r i s u s e f u l f o r e s t i m a t i n g t h e s t a b i l i t y a n d s t e a d y - s t a t e
p r o p e r t i e s o f a w i d e v a r i e t y o f t i m e - m a r c h i n g s c h e m e s t h a t a r e v a r i o u s l y r e f e r r e d
t o a s t i m e - s p l i t , f r a c t i o n a l - s t e p , h y b r i d , a n d ( a p p r o x i m a t e l y ) f a c t o r e d . W h e n t h e s e
m e t h o d s a r e a p p l i e d t o p r a c t i c a l p r o b l e m s , t h e r e s u l t s f o u n d f r o m t h i s a n a l y s i s a r e
n e i t h e r n e c e s s a r y n o r s u c i e n t t o g u a r a n t e e s t a b i l i t y . H o w e v e r , i f t h e r e s u l t s i n d i c a t e
t h a t a m e t h o d h a s a n i n s t a b i l i t y , t h e m e t h o d i s p r o b a b l y n o t s u i t a b l e f o r p r a c t i c a l
u s e .
1 3 . 1 T h e R e p r e s e n t a t i v e E q u a t i o n f o r C i r c u l a n t
O p e r a t o r s
C o n s i d e r l i n e a r P D E ' s w i t h c o e c i e n t s t h a t a r e x e d i n b o t h s p a c e a n d t i m e a n d w i t h
b o u n d a r y c o n d i t i o n s t h a t a r e p e r i o d i c . W e h a v e s e e n t h a t u n d e r t h e s e c o n d i t i o n s
a s e m i - d i s c r e t e a p p r o a c h c a n l e a d t o c i r c u l a n t m a t r i x d i e r e n c e o p e r a t o r s , a n d w e
d i s c u s s e d c i r c u l a n t e i g e n s y s t e m s
1
i n S e c t i o n 4 . 3 . I n t h i s a n d t h e f o l l o w i n g s e c t i o n
w e a s s u m e c i r c u l a n t s y s t e m s a n d o u r a n a l y s i s d e p e n d s c r i t i c a l l y o n t h e f a c t t h a t a l l
c i r c u l a n t m a t r i c e s c o m m u t e a n d h a v e a c o m m o n s e t o f e i g e n v e c t o r s .
1
S e e a l s o t h e d i s c u s s i o n o n F o u r i e r s t a b i l i t y a n a l y s i s i n S e c t i o n 7 . 7 .
2 3 3
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1 3 . 2 . E X A M P L E A N A L Y S I S O F C I R C U L A N T S Y S T E M S 2 3 5
I f t h e s p a c e d i e r e n c i n g t a k e s t h e f o r m
d
~
u
d t
= ;
a
2 x
B
p
( ; 1 0 1 )
~
u +
x
2
B
p
( 1 ; 2 1 )
~
u ( 1 3 . 5 )
t h e c o n v e c t i o n m a t r i x o p e r a t o r a n d t h e d i u s i o n m a t r i x o p e r a t o r , c a n b e r e p r e s e n t e d
b y t h e e i g e n v a l u e s
c
a n d
d
, r e s p e c t i v e l y , w h e r e ( s e e S e c t i o n 4 . 3 . 2 ) :
(
c
)
m
=
i a
x
s i n
m
(
d
)
m
= ;
4
x
2
s i n
2
m
2
( 1 3 . 6 )
I n t h e s e e q u a t i o n s
m
= 2 m = M , m = 0 1 M ; 1 , s o t h a t 0
m
2 .
U s i n g t h e s e v a l u e s a n d t h e r e p r e s e n t a t i v e e q u a t i o n 1 3 . 4 , w e c a n a n a l y z e t h e s t a b i l i t y
o f t h e t w o f o r m s o f s i m p l e t i m e - s p l i t t i n g d i s c u s s e d i n S e c t i o n 1 2 . 2 . I n t h i s s e c t i o n w e
r e f e r t o t h e s e a s
1 . t h e e x p l i c i t - i m p l i c i t E u l e r m e t h o d , E q . 1 2 . 1 0 .
2 . t h e e x p l i c i t - e x p l i c i t E u l e r m e t h o d , E q . 1 2 . 8 .
1 . T h e E x p l i c i t - I m p l i c i t M e t h o d
W h e n a p p l i e d t o E q . 1 3 . 4 , t h e c h a r a c t e r i s t i c p o l y n o m i a l o f t h i s m e t h o d i s
P ( E ) = ( 1 ; h
d
) E ; ( 1 + h
c
)
T h i s l e a d s t o t h e p r i n c i p a l r o o t
=
1 + i
a h
x
s i n
m
1 + 4
h
x
2
s i n
2
m
2
w h e r e w e h a v e m a d e u s e o f E q . 1 3 . 6 t o q u a n t i f y t h e e i g e n v a l u e s . N o w i n t r o d u c e t h e
d i m e n s i o n l e s s n u m b e r s
C
n
=
a h
x
C o u r a n t n u m b e r
R
=
a x
m e s h R e y n o l d s n u m b e r
a n d w e c a n w r i t e f o r t h e a b s o l u t e v a l u e o f
j j =
q
1 + C
2
n
s i n
2
m
1 + 4
C
n
R
s i n
2
m
2
0
m
2 ( 1 3 . 7 )
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2 3 6 C H A P T E R 1 3 . L I N E A R A N A L Y S I S O F S P L I T A N D F A C T O R E D F O R M S
0 4 8
R∆
Cn
0
0.4
0.8
1.2
Cn
R∆
= 2/
Explicit-Implicit
0 4 8
R∆
Cn
0
0.4
0.8
1.2
Explicit-Explicit
Cn
R∆
= 2/ Cn
R∆
= /2
F i g u r e 1 3 . 1 :
_
S t a b i l i t y r e g i o n s f o r t w o s i m p l e t i m e - s p l i t m e t h o d s .
A s i m p l e n u m e r i c a l p a r a m e t r i c s t u d y o f E q . 1 3 . 7 s h o w s t h a t t h e c r i t i c a l r a n g e o f
m
f o r a n y c o m b i n a t i o n o f C
n
a n d R
o c c u r s w h e n
m
i s n e a r 0 ( o r 2 ) . F r o m t h i s
w e n d t h a t t h e c o n d i t i o n o n C
n
a n d R
t h a t m a k e j j 1 i s
h
1 + C
2
n
s i n
2
i
=
1 + 4
C
n
R
s i n
2
2
2
A s ! 0 t h i s g i v e s t h e s t a b i l i t y r e g i o n
C
n
<
2
R
w h i c h i s b o u n d e d b y a h y p e r b o l a a n d s h o w n i n F i g . 1 3 . 1 .
2 . T h e E x p l i c i t - E x p l i c i t M e t h o d
A n a n a l y s i s s i m i l a r t o t h e o n e g i v e n a b o v e s h o w s t h a t t h i s m e t h o d p r o d u c e s
j j =
q
1 + C
2
n
s i n
2
m
"
1 ; 4
C
n
R
s i n
2
m
2
#
0
m
2
A g a i n a s i m p l e n u m e r i c a l p a r a m e t r i c s t u d y s h o w s t h a t t h i s h a s t w o c r i t i c a l r a n g e s
o f
m
, o n e n e a r 0 , w h i c h y i e l d s t h e s a m e r e s u l t a s i n t h e p r e v i o u s e x a m p l e , a n d t h e
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1 3 . 2 . E X A M P L E A N A L Y S I S O F C I R C U L A N T S Y S T E M S 2 3 7
o t h e r n e a r 1 8 0
o
, w h i c h p r o d u c e s t h e c o n s t r a i n t t h a t
C
n
<
1
2
R
f o r R
2
T h e r e s u l t i n g s t a b i l i t y b o u n d a r y i s a l s o s h o w n i n F i g . 1 3 . 1 . T h e t o t a l y e x p l i c i t ,
f a c t o r e d m e t h o d h a s a m u c h s m a l l e r r e g i o n o f s t a b i l i t y w h e n R
i s s m a l l , a s w e
s h o u l d h a v e e x p e c t e d .
1 3 . 2 . 2 A n a l y s i s o f a S e c o n d - O r d e r T i m e - S p l i t M e t h o d
N e x t l e t u s a n a l y z e a m o r e p r a c t i c a l m e t h o d t h a t h a s b e e n u s e d i n s e r i o u s c o m p u -
t a t i o n a l a n a l y s i s o f t u r b u l e n t o w s . T h i s m e t h o d a p p l i e s t o a o w i n w h i c h t h e r e i s
a c o m b i n a t i o n o f d i u s i o n a n d p e r i o d i c c o n v e c t i o n . T h e c o n v e c t i o n t e r m i s t r e a t e d
e x p l i c i t l y u s i n g t h e s e c o n d - o r d e r A d a m s - B a s h f o r t h m e t h o d . T h e d i u s i o n t e r m i s
i n t e g r a t e d i m p l i c i t l y u s i n g t h e t r a p e z o i d a l m e t h o d . O u r m o d e l e q u a t i o n i s a g a i n t h e
l i n e a r c o n v e c t i o n - d i u s i o n e q u a t i o n 1 3 . 4 w h i c h w e s p l i t i n t h e f a s h i o n o f E q . 1 3 . 5 . I n
o r d e r t o e v a l u a t e t h e a c c u r a c y , a s w e l l a s t h e s t a b i l i t y , w e i n c l u d e t h e f o r c i n g f u n c -
t i o n i n t h e r e p r e s e n t a t i v e e q u a t i o n a n d s t u d y t h e e e c t o f o u r h y b r i d , t i m e - m a r c h i n g
m e t h o d o n t h e e q u a t i o n
u
0
=
c
u +
d
u + a e
t
F i r s t l e t u s n d e x p r e s s i o n s f o r t h e t w o p o l y n o m i a l s , P ( E ) a n d Q ( E ) . T h e c h a r -
a c t e r i s t i c p o l y n o m i a l f o l l o w s f r o m t h e a p p l i c a t i o n o f t h e m e t h o d t o t h e h o m o g e n e o u s
e q u a t i o n , t h u s
u
n + 1
= u
n
+
1
2
h
c
( 3 u
n
; u
n ; 1
) +
1
2
h
d
( u
n + 1
+ u
n
)
T h i s p r o d u c e s
P ( E ) = ( 1 ;
1
2
h
d
) E
2
; ( 1 +
3
2
h
c
+
1
2
h
d
) E +
1
2
h
c
T h e f o r m o f t h e p a r t i c u l a r p o l y n o m i a l d e p e n d s u p o n w h e t h e r t h e f o r c i n g f u n c t i o n i s
c a r r i e d b y t h e A B 2 m e t h o d o r b y t h e t r a p e z o i d a l m e t h o d . I n t h e f o r m e r c a s e i t i s
Q ( E ) =
1
2
h ( 3 E ; 1 ) ( 1 3 . 8 )
a n d i n t h e l a t t e r
Q ( E ) =
1
2
h ( E
2
+ E ) ( 1 3 . 9 )
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2 3 8 C H A P T E R 1 3 . L I N E A R A N A L Y S I S O F S P L I T A N D F A C T O R E D F O R M S
A c c u r a c y
F r o m t h e c h a r a c t e r i s t i c p o l y n o m i a l w e s e e t h a t t h e r e a r e t w o { r o o t s a n d t h e y a r e
g i v e n b y t h e e q u a t i o n
=
1 +
3
2
h
c
+
1
2
h
d
s
1 +
3
2
h
c
+
1
2
h
d
2
; 2 h
c
1 ;
1
2
h
d
2
1 ;
1
2
h
d
( 1 3 . 1 0 )
T h e p r i n c i p a l - r o o t f o l l o w s f r o m t h e p l u s s i g n a n d o n e c a n s h o w
1
= 1 + (
c
+
d
) h +
1
2
(
c
+
d
)
2
h
2
+
1
4
3
d
+
c
2
d
;
2
c
d
;
3
c
h
3
F r o m t h i s e q u a t i o n i t i s c l e a r t h a t
1
6
3
=
1
6
(
c
+
d
)
3
d o e s n o t m a t c h t h e c o e c i e n t
o f h
3
i n
1
, s o
e r
= O ( h
3
)
U s i n g P ( e
h
) a n d Q ( e
h
) t o e v a l u a t e e r
i n S e c t i o n 6 . 6 . 3 , o n e c a n s h o w
e r
= O ( h
3
)
u s i n g e i t h e r E q . 1 3 . 8 o r E q . 1 3 . 9 . T h e s e r e s u l t s s h o w t h a t , f o r t h e m o d e l e q u a t i o n ,
t h e h y b r i d m e t h o d r e t a i n s t h e s e c o n d - o r d e r a c c u r a c y o f i t s i n d i v i d u a l c o m p o n e n t s .
S t a b i l i t y
T h e s t a b i l i t y o f t h e m e t h o d c a n b e f o u n d f r o m E q . 1 3 . 1 0 b y a p a r a m e t r i c s t u d y o f c
n
a n d R
d e n e d i n E q . 1 3 . 7 . T h i s w a s c a r r i e d o u t i n a m a n n e r s i m i l a r t o t h a t u s e d
t o n d t h e s t a b i l i t y b o u n d a r y o f t h e r s t - o r d e r e x p l i c i t - i m p l i c i t m e t h o d i n S e c t i o n
1 3 . 2 . 1 . T h e r e s u l t s a r e p l o t t e d i n F i g . 1 3 . 2 . F o r v a l u e s o f R
2 t h i s s e c o n d - o r d e r
m e t h o d h a s a m u c h g r e a t e r r e g i o n o f s t a b i l i t y t h a n t h e r s t - o r d e r e x p l i c i t - i m p l i c i t
m e t h o d g i v e n b y E q . 1 2 . 1 0 a n d s h o w n i n F i g . 1 3 . 1 .
1 3 . 3 T h e R e p r e s e n t a t i v e E q u a t i o n f o r S p a c e - S p l i t
O p e r a t o r s
C o n s i d e r t h e 2 - D m o d e l
3
e q u a t i o n s
@ u
@ t
=
@
2
u
@ x
2
+
@
2
u
@ y
2
( 1 3 . 1 1 )
3
T h e e x t e n s i o n o f t h e f o l l o w i n g t o 3 - D i s s i m p l e a n d s t r a i g h t f o r w a r d .
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2 4 0 C H A P T E R 1 3 . L I N E A R A N A L Y S I S O F S P L I T A N D F A C T O R E D F O R M S
m a t r i x t h a t d i a g o n a l i z e s B a n d u s e i t t o d i a g o n a l i z e A
( x )
x
. T h u s
2
6
4
X
; 1
X
; 1
X
; 1
3
7
5
2
6
4
B
B
B
3
7
5
2
6
4
X
X
X
3
7
5
=
2
6
4
3
7
5
w h e r e
=
2
6
6
6
4
1
2
3
4
3
7
7
7
5
N o t i c e t h a t t h e m a t r i x A
( x )
y
i s t r a n s p a r e n t t o t h i s t r a n s f o r m a t i o n . T h a t i s , i f w e
s e t X d i a g ( X )
X
; 1
2
6
4
~
b
0
I
~
b
1
I
~
b
; 1
I
~
b
0
I
~
b
1
I
~
b
; 1
I
~
b
0
I
3
7
5
X =
2
6
4
~
b
0
I
~
b
1
I
~
b
; 1
I
~
b
0
I
~
b
1
I
~
b
; 1
I
~
b
0
I
3
7
5
O n e n o w p e r m u t e s t h e t r a n s f o r m e d s y s t e m t o t h e y - v e c t o r d a t a - b a s e u s i n g t h e p e r -
m u t a t i o n m a t r i x d e n e d b y E q . 1 2 . 1 3 . T h e r e r e s u l t s
P
y x
X
; 1
h
A
( x )
x
+ A
( x )
y
i
X P
x y
=
2
6
6
6
4
1
I
2
I
3
I
4
I
3
7
7
7
5
+
2
6
6
6
6
4
~
B
~
B
~
B
~
B
3
7
7
7
7
5
( 1 3 . 1 4 )
w h e r e
~
B i s t h e b a n d e d t r i d i a g o n a l m a t r i x B (
~
b
; 1
~
b
0
~
b
1
) , s e e t h e b o t t o m o f E q . 1 2 . 2 3 .
N e x t n d t h e e i g e n v e c t o r s
~
X t h a t d i a g o n a l i z e t h e
~
B b l o c k s . L e t
~
B d i a g (
~
B ) a n d
~
X d i a g (
~
X ) a n d f o r m t h e s e c o n d t r a n s f o r m a t i o n
~
X
; 1
~
B
~
X =
2
6
6
6
4
~
~
~
~
3
7
7
7
5
~
=
2
6
4
~
1
~
2
~
3
3
7
5
T h i s t i m e , b y t h e s a m e a r g u m e n t a s b e f o r e , t h e r s t m a t r i x o n t h e r i g h t s i d e o f
E q . 1 3 . 1 4 i s t r a n s p a r e n t t o t h e t r a n s f o r m a t i o n , s o t h e n a l r e s u l t i s t h e c o m p l e t e
d i a g o n a l i z a t i o n o f t h e m a t r i x A
x + y
:
h
~
X
; 1
P
y x
X
; 1
i h
A
( x )
x + y
i h
X P
x y
~
X
i
=
2
6
6
6
6
4
1
I +
~
2
I +
~
3
I +
~
4
I +
~
3
7
7
7
7
5
( 1 3 . 1 5 )
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1 3 . 3 . T H E R E P R E S E N T A T I V E E Q U A T I O N F O R S P A C E - S P L I T O P E R A T O R S 2 4 1
I t i s i m p o r t a n t t o n o t i c e t h a t :
T h e d i a g o n a l m a t r i x o n t h e r i g h t s i d e o f E q . 1 3 . 1 5 c o n t a i n s e v e r y p o s s i b l e c o m -
b i n a t i o n o f t h e i n d i v i d u a l e i g e n v a l u e s o f B a n d
~
B .
N o w w e a r e r e a d y t o p r e s e n t t h e r e p r e s e n t a t i v e e q u a t i o n f o r t w o d i m e n s i o n a l s y s -
t e m s . F i r s t r e d u c e t h e P D E t o O D E b y s o m e c h o i c e
4
o f s p a c e d i e r e n c i n g . T h i s
r e s u l t s i n a s p a t i a l l y s p l i t A m a t r i x f o r m e d f r o m t h e s u b s e t s
A
( x )
x
= d i a g ( B ) A
( y )
y
= d i a g (
~
B ) ( 1 3 . 1 6 )
w h e r e B a n d
~
B a r e a n y t w o m a t r i c e s t h a t h a v e l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s ( t h i s
p u t s s o m e c o n s t r a i n t s o n t h e c h o i c e o f d i e r e n c i n g s c h e m e s ) .
A l t h o u g h A
x
a n d A
y
d o c o m m u t e , t h i s f a c t , b y i t s e l f , d o e s n o t e n s u r e t h e p r o p -
e r t y o f \ a l l p o s s i b l e c o m b i n a t i o n s " . T o o b t a i n t h e l a t t e r p r o p e r t y t h e s t r u c t u r e o f
t h e m a t r i c e s i s i m p o r t a n t . T h e b l o c k m a t r i c e s B a n d
~
B c a n b e e i t h e r c i r c u l a n t o r
n o n c i r c u l a n t i n b o t h c a s e s w e a r e l e d t o t h e n a l r e s u l t :
T h e 2 { D r e p r e s e n t a t i v e e q u a t i o n f o r m o d e l l i n e a r s y s t e m s i s
d u
d t
=
x
+
y
] u + a e
t
w h e r e
x
a n d
y
a r e a n y c o m b i n a t i o n o f e i g e n v a l u e s f r o m A
x
a n d A
y
, a a n d a r e
( p o s s i b l y c o m p l e x ) c o n s t a n t s , a n d w h e r e A
x
a n d A
y
s a t i s f y t h e c o n d i t i o n s i n 1 3 . 1 6 .
O f t e n w e a r e i n t e r e s t e d i n n d i n g t h e v a l u e o f , a n d t h e c o n v e r g e n c e r a t e t o , t h e
s t e a d y - s t a t e s o l u t i o n o f t h e r e p r e s e n t a t i v e e q u a t i o n . I n t h a t c a s e w e s e t = 0 a n d
u s e t h e s i m p l e r f o r m
d u
d t
=
x
+
y
] u + a ( 1 3 . 1 7 )
w h i c h h a s t h e e x a c t s o l u t i o n
u ( t ) = c e
(
x
+
y
) t
;
a
x
+
y
( 1 3 . 1 8 )
4
W e h a v e u s e d 3 - p o i n t c e n t r a l d i e r e n c i n g i n o u r e x a m p l e , b u t t h i s c h o i c e w a s f o r c o n v e n i e n c e
o n l y , a n d i t s u s e i s n o t n e c e s s a r y t o a r r i v e a t E q . 1 3 . 1 5 .
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1 3 . 4 . E X A M P L E A N A L Y S I S O F 2 - D M O D E L E Q U A T I O N S 2 4 3
1 3 . 4 . 2 T h e F a c t o r e d N o n d e l t a F o r m o f t h e I m p l i c i t E u l e r
M e t h o d
N o w a p p l y t h e f a c t o r e d E u l e r m e t h o d g i v e n b y E q . 1 2 . 2 0 t o t h e 2 - D r e p r e s e n t a t i v e
e q u a t i o n . T h e r e r e s u l t s
( 1 ; h
x
) ( 1 ; h
y
) u
n + 1
= u
n
+ h a
f r o m w h i c h
P ( E ) = ( 1 ; h
x
) ( 1 ; h
y
) E ; 1
Q ( E ) = h ( 1 3 . 2 0 )
g i v i n g t h e s o l u t i o n
u
n
= c
"
1
( 1 ; h
x
) ( 1 ; h
y
)
#
n
;
a
x
+
y
; h
x
y
W e s e e t h a t t h i s m e t h o d :
1 . I s u n c o n d i t i o n a l l y s t a b l e .
2 . P r o d u c e s a s t e a d y s t a t e s o l u t i o n t h a t d e p e n d s o n t h e c h o i c e o f h .
3 . C o n v e r g e s r a p i d l y t o a s t e a d y - s t a t e f o r l a r g e h , b u t t h e c o n v e r g e d s o l u t i o n i s
c o m p l e t e l y w r o n g .
T h e m e t h o d r e q u i r e s f a r l e s s s t o r a g e t h e n t h e u n f a c t o r e d f o r m . H o w e v e r , i t i s n o t
v e r y u s e f u l s i n c e i t s t r a n s i e n t s o l u t i o n i s o n l y r s t - o r d e r a c c u r a t e a n d , i f o n e t r i e s t o
t a k e a d v a n t a g e o f i t s r a p i d c o n v e r g e n c e r a t e , t h e c o n v e r g e d v a l u e i s m e a n i n g l e s s .
1 3 . 4 . 3 T h e F a c t o r e d D e l t a F o r m o f t h e I m p l i c i t E u l e r M e t h o d
N e x t a p p l y E q . 1 2 . 3 1 t o t h e 2 - D r e p r e s e n t a t i v e e q u a t i o n . O n e n d s
( 1 ; h
x
) ( 1 ; h
y
) ( u
n + 1
; u
n
) = h (
x
u
n
+
y
u
n
+ a )
w h i c h r e d u c e s t o
( 1 ; h
x
) ( 1 ; h
y
) u
n + 1
=
1 + h
2
x
y
u
n
+ h a
a n d t h i s h a s t h e s o l u t i o n
u
n
= c
"
1 + h
2
x
y
( 1 ; h
x
) ( 1 ; h
y
)
#
n
;
a
x
+
y
T h i s m e t h o d :
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2 4 4 C H A P T E R 1 3 . L I N E A R A N A L Y S I S O F S P L I T A N D F A C T O R E D F O R M S
1 . I s u n c o n d i t i o n a l l y s t a b l e .
2 . P r o d u c e s t h e e x a c t s t e a d y - s t a t e s o l u t i o n f o r a n y c h o i c e o f h .
3 . C o n v e r g e s v e r y s l o w l y t o t h e s t e a d y { s t a t e s o l u t i o n f o r l a r g e v a l u e s o f h , s i n c e
j j ! 1 a s h ! 1 .
L i k e t h e f a c t o r e d n o n d e l t a f o r m , t h i s m e t h o d d e m a n d s f a r l e s s s t o r a g e t h a n t h e u n -
f a c t o r e d f o r m , a s d i s c u s s e d i n S e c t i o n 1 2 . 5 . T h e c o r r e c t s t e a d y s o l u t i o n i s o b t a i n e d ,
b u t c o n v e r g e n c e i s n o t n e a r l y a s r a p i d a s t h a t o f t h e u n f a c t o r e d f o r m .
1 3 . 4 . 4 T h e F a c t o r e d D e l t a F o r m o f t h e T r a p e z o i d a l M e t h o d
F i n a l l y c o n s i d e r t h e d e l t a f o r m o f a s e c o n d - o r d e r t i m e - a c c u r a t e m e t h o d . A p p l y E q .
1 2 . 3 0 t o t h e r e p r e s e n t a t i v e e q u a t i o n a n d o n e n d s
1 ;
1
2
h
x
1 ;
1
2
h
y
( u
n + 1
; u
n
) = h (
x
u
n
+
y
u
n
+ a )
w h i c h r e d u c e s t o
1 ;
1
2
h
x
1 ;
1
2
h
y
u
n + 1
=
1 +
1
2
h
x
1 +
1
2
h
y
u
n
+ h a
a n d t h i s h a s t h e s o l u t i o n
u
n
= c
2
6
6
4
1 +
1
2
h
x
1 +
1
2
h
y
1 ;
1
2
h
x
1 ;
1
2
h
y
3
7
7
5
n
;
a
x
+
y
T h i s m e t h o d :
1 . I s u n c o n d i t i o n a l l y s t a b l e .
2 . P r o d u c e s t h e e x a c t s t e a d y { s t a t e s o l u t i o n f o r a n y c h o i c e o f h .
3 . C o n v e r g e s v e r y s l o w l y t o t h e s t e a d y { s t a t e s o l u t i o n f o r l a r g e v a l u e s o f h , s i n c e
j j ! 1 a s h ! 1 .
A l l o f t h e s e p r o p e r t i e s a r e i d e n t i c a l t o t h o s e f o u n d f o r t h e f a c t o r e d d e l t a f o r m o f t h e
i m p l i c i t E u l e r m e t h o d . S i n c e i t i s s e c o n d o r d e r i n t i m e , i t c a n b e u s e d w h e n t i m e
a c c u r a c y i s d e s i r e d , a n d t h e f a c t o r e d d e l t a f o r m o f t h e i m p l i c i t E u l e r m e t h o d c a n b e
u s e d w h e n a c o n v e r g e d s t e a d y - s t a t e i s a l l t h a t i s r e q u i r e d .
5
A b r i e f i n s p e c t i o n o f e q s .
1 2 . 2 6 a n d 1 2 . 2 7 s h o u l d b e e n o u g h t o c o n v i n c e t h e r e a d e r t h a t t h e ' s p r o d u c e d b y
t h o s e m e t h o d s a r e i d e n t i c a l t o t h e p r o d u c e d b y t h i s m e t h o d .
5
I n p r a c t i c a l c o d e s , t h e v a l u e o f h o n t h e l e f t s i d e o f t h e i m p l i c i t e q u a t i o n i s l i t e r a l l y s w i t c h e d
f r o m h t o
1
2
h .
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1 3 . 5 . E X A M P L E A N A L Y S I S O F T H E 3 - D M O D E L E Q U A T I O N 2 4 5
1 3 . 5 E x a m p l e A n a l y s i s o f t h e 3 - D M o d e l E q u a t i o n
T h e a r g u m e n t s i n S e c t i o n 1 3 . 3 g e n e r a l i z e t o t h r e e d i m e n s i o n s a n d , u n d e r t h e c o n d i -
t i o n s g i v e n i n 1 3 . 1 6 w i t h a n A
( z )
z
i n c l u d e d , t h e m o d e l 3 - D c a s e s
6
h a v e t h e f o l l o w i n g
r e p r e s e n t a t i v e e q u a t i o n ( w i t h = 0 ) :
d u
d t
=
x
+
y
+
z
] u + a ( 1 3 . 2 1 )
L e t u s a n a l y z e a 2 n d - o r d e r a c c u r a t e , f a c t o r e d , d e l t a f o r m u s i n g t h i s e q u a t i o n . F i r s t
a p p l y t h e t r a p e z o i d a l m e t h o d :
u
n + 1
= u
n
+
1
2
h (
x
+
y
+
z
) u
n + 1
+ (
x
+
y
+
z
) u
n
+ 2 a ]
R e a r r a n g e t e r m s :
1 ;
1
2
h (
x
+
y
+
z
)
u
n + 1
=
1 +
1
2
h (
x
+
y
+
z
)
u
n
+ h a
P u t t h i s i n d e l t a f o r m :
1 ;
1
2
h (
x
+
y
+
z
)
u
n
= h (
x
+
y
+
z
) u
n
+ a ]
N o w f a c t o r t h e l e f t s i d e :
1 ;
1
2
h
x
1 ;
1
2
h
y
1 ;
1
2
h
z
u
n
= h (
x
+
y
+
z
) u
n
+ a ] ( 1 3 . 2 2 )
T h i s p r e s e r v e s s e c o n d o r d e r a c c u r a c y s i n c e t h e e r r o r t e r m s
1
4
h
2
(
x
y
+
x
z
+
y
z
) u
n
a n d
1
8
h
3
x
y
z
a r e b o t h O ( h
3
) . O n e c a n d e r i v e t h e c h a r a c t e r i s t i c p o l y n o m i a l f o r E q . 1 3 . 2 2 , n d t h e
r o o t , a n d w r i t e t h e s o l u t i o n e i t h e r i n t h e f o r m
u
n
= c
2
6
6
4
1 +
1
2
h (
x
+
y
+
z
) +
1
4
h
2
(
x
y
+
x
z
+
y
z
) ;
1
8
h
3
x
y
z
1 ;
1
2
h (
x
+
y
+
z
) +
1
4
h
2
(
x
y
+
x
z
+
y
z
) ;
1
8
h
3
x
y
z
3
7
7
5
n
;
a
x
+
y
+
z
( 1 3 . 2 3 )
6
E q s . 1 3 . 1 1 a n d 1 3 . 1 2 , e a c h w i t h a n a d d i t i o n a l t e r m .
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2 4 6 C H A P T E R 1 3 . L I N E A R A N A L Y S I S O F S P L I T A N D F A C T O R E D F O R M S
o r i n t h e f o r m
u
n
= c
2
6
6
4
1 +
1
2
h
x
1 +
1
2
h
y
1 +
1
2
h
z
;
1
4
h
3
x
y
z
1 ;
1
2
h
x
1 ;
1
2
h
y
1 ;
1
2
h
z
3
7
7
5
n
;
a
x
+
y
+
z
( 1 3 . 2 4 )
I t i s i n t e r e s t i n g t o n o t i c e t h a t a T a y l o r s e r i e s e x p a n s i o n o f E q . 1 3 . 2 4 r e s u l t s i n
= 1 + h (
x
+
y
+
z
) +
1
2
h
2
(
x
+
y
+
z
)
2
( 1 3 . 2 5 )
+
1
4
h
3
h
3
z
+ ( 2
y
+ 2
x
) +
2
2
y
+ 3
x
y
+ 2
2
y
+
3
y
+ 2
x
2
y
+ 2
2
x
y
+
3
x
i
+
w h i c h v e r i e s t h e s e c o n d o r d e r a c c u r a c y o f t h e f a c t o r e d f o r m . F u r t h e r m o r e , c l e a r l y ,
i f t h e m e t h o d c o n v e r g e s , i t c o n v e r g e s t o t h e p r o p e r s t e a d y - s t a t e .
7
W i t h r e g a r d s t o s t a b i l i t y , i t f o l l o w s f r o m E q . 1 3 . 2 3 t h a t , i f a l l t h e ' s a r e r e a l a n d
n e g a t i v e , t h e m e t h o d i s s t a b l e f o r a l l h . T h i s m a k e s t h e m e t h o d u n c o n d i t i o n a l l y s t a b l e
f o r t h e 3 - D d i u s i o n m o d e l w h e n i t i s c e n t r a l l y d i e r e n c e d i n s p a c e .
N o w c o n s i d e r w h a t h a p p e n s w h e n w e a p p l y t h i s m e t h o d t o t h e b i c o n v e c t i o n m o d e l ,
t h e 3 - D f o r m o f E q . 1 3 . 1 2 w i t h p e r i o d i c b o u n d a r y c o n d i t i o n s . I n t h i s c a s e , c e n t r a l
d i e r e n c i n g c a u s e s a l l o f t h e ' s t o b e i m a g i n a r y w i t h s p e c t r u m s t h a t i n c l u d e b o t h
p o s i t i v e a n d n e g a t i v e v a l u e s . R e m e m b e r t h a t i n o u r a n a l y s i s w e m u s t c o n s i d e r e v e r y
p o s s i b l e c o m b i n a t i o n o f t h e s e e i g e n v a l u e s . F i r s t w r i t e t h e r o o t i n E q . 1 3 . 2 3 i n t h e
f o r m
=
1 + i ; + i
1 ; i ; + i
w h e r e , a n d a r e r e a l n u m b e r s t h a t c a n h a v e a n y s i g n . N o w w e c a n a l w a y s n d
o n e c o m b i n a t i o n o f t h e ' s f o r w h i c h , a n d a r e b o t h p o s i t i v e . I n t h a t c a s e s i n c e
t h e a b s o l u t e v a l u e o f t h e p r o d u c t i s t h e p r o d u c t o f t h e a b s o l u t e v a l u e s
j j
2
=
( 1 ; )
2
+ ( + )
2
( 1 ; )
2
+ ( ; )
2
> 1
a n d t h e m e t h o d i s u n c o n d i t i o n a l l y u n s t a b l e f o r t h e m o d e l c o n v e c t i o n p r o b l e m .
F r o m t h e a b o v e a n a l y s i s o n e w o u l d c o m e t o t h e c o n c l u s i o n t h a t t h e m e t h o d r e p -
r e s e n t e d b y E q . 1 3 . 2 2 s h o u l d n o t b e u s e d f o r t h e 3 - D E u l e r e q u a t i o n s . I n p r a c t i c a l
c a s e s , h o w e v e r , s o m e f o r m o f d i s s i p a t i o n i s a l m o s t a l w a y s a d d e d t o m e t h o d s t h a t a r e
u s e d t o s o l v e t h e E u l e r e q u a t i o n s a n d o u r e x p e r i e n c e t o d a t e i s t h a t , i n t h e p r e s e n c e
o f t h i s d i s s i p a t i o n , t h e i n s t a b i l i t y d i s c l o s e d a b o v e i s t o o w e a k t o c a u s e t r o u b l e .
7
H o w e v e r , w e a l r e a d y k n e w t h i s b e c a u s e w e c h o s e t h e d e l t a f o r m .
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1 3 . 6 . P R O B L E M S 2 4 7
1 3 . 6 P r o b l e m s
1 . S t a r t i n g w i t h t h e g e n e r i c O D E ,
d u
d t
= A u + f
w e c a n s p l i t A a s f o l l o w s : A = A
1
+ A
2
+ A
3
+ A
4
. A p p l y i n g i m p l i c i t E u l e r t i m e
m a r c h i n g g i v e s
u
n + 1
; u
n
h
= A
1
u
n + 1
+ A
2
u
n + 1
+ A
3
u
n + 1
+ A
4
u
n + 1
+ f
( a ) W r i t e t h e f a c t o r e d d e l t a f o r m . W h a t i s t h e e r r o r t e r m ?
( b ) I n s t e a d o f m a k i n g a l l o f t h e s p l i t t e r m s i m p l i c i t , l e a v e t w o e x p l i c i t :
u
n + 1
; u
n
h
= A
1
u
n + 1
+ A
2
u
n
+ A
3
u
n + 1
+ A
4
u
n
+ f
W r i t e t h e r e s u l t i n g f a c t o r e d d e l t a f o r m a n d d e n e t h e e r r o r t e r m s .
( c ) T h e s c a l a r r e p r e s e n t a t i v e e q u a t i o n i s
d u
d t
= (
1
+
2
+
3
+
4
) u + a
F o r t h e f u l l y i m p l i c i t s c h e m e o f p r o b l e m 1 a , n d t h e e x a c t s o l u t i o n t o t h e
r e s u l t i n g s c a l a r d i e r e n c e e q u a t i o n a n d c o m m e n t o n t h e s t a b i l i t y , c o n v e r -
g e n c e , a n d a c c u r a c y o f t h e c o n v e r g e d s t e a d y - s t a t e s o l u t i o n .
( d ) R e p e a t 1 c f o r t h e e x p l i c i t - i m p l i c i t s c h e m e o f p r o b l e m 1 b .
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2 4 8 C H A P T E R 1 3 . L I N E A R A N A L Y S I S O F S P L I T A N D F A C T O R E D F O R M S
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A p p e n d i x A
U S E F U L R E L A T I O N S A N D
D E F I N I T I O N S F R O M L I N E A R
A L G E B R A
A b a s i c u n d e r s t a n d i n g o f t h e f u n d a m e n t a l s o f l i n e a r a l g e b r a i s c r u c i a l t o o u r d e v e l o p -
m e n t o f n u m e r i c a l m e t h o d s a n d i t i s a s s u m e d t h a t t h e r e a d e r i s a t l e a s t f a m i l a r w i t h
t h i s s u b j e c t a r e a . G i v e n b e l o w i s s o m e n o t a t i o n a n d s o m e o f t h e i m p o r t a n t r e l a t i o n s
b e t w e e n m a t r i c e s a n d v e c t o r s .
A . 1 N o t a t i o n
1 . I n t h e p r e s e n t c o n t e x t a v e c t o r i s a v e r t i c a l c o l u m n o r s t r i n g . T h u s
~
v =
2
6
6
6
4
v
1
v
2
.
.
.
v
m
3
7
7
7
5
a n d i t s t r a n s p o s e
~
v
T
i s t h e h o r i z o n t a l r o w
~
v
T
= v
1
v
2
v
3
: : : v
m
]
~
v = v
1
v
2
v
3
: : : v
m
]
T
2 . A g e n e r a l m m m a t r i x A c a n b e w r i t t e n
A = ( a
i j
) =
2
6
6
6
4
a
1 1
a
1 2
a
1 m
a
2 1
a
2 2
a
2 m
.
.
.
a
m 1
a
m 2
a
m m
3
7
7
7
5
2 4 9
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2 5 0 A P P E N D I X A . U S E F U L R E L A T I O N S A N D D E F I N I T I O N S F R O M L I N E A R A L G E B R A
3 . A n a l t e r n a t i v e n o t a t i o n f o r A i s
A =
h
~
a
1
~
a
2
: : :
~
a
m
i
a n d i t s t r a n s p o s e A
T
i s
A
T
=
2
6
6
6
6
6
4
~
a
T
1
~
a
T
2
.
.
.
~
a
T
m
3
7
7
7
7
7
5
4 . T h e i n v e r s e o f a m a t r i x ( i f i t e x i s t s ) i s w r i t t e n A
; 1
a n d h a s t h e p r o p e r t y t h a t
A
; 1
A = A A
; 1
= I , w h e r e I i s t h e i d e n t i t y m a t r i x .
A . 2 D e n i t i o n s
1 . A i s s y m m e t r i c i f A
T
= A .
2 . A i s s k e w - s y m m e t r i c o r a n t i s y m m e t r i c i f A
T
= ; A .
3 . A i s d i a g o n a l l y d o m i n a n t i f a
i i
P
j 6= i
j a
i j
j i = 1 2 : : : m a n d a
i i
>
P
j 6= i
j a
i j
j
f o r a t l e a s t o n e i .
4 . A i s o r t h o g o n a l i f a
i j
a r e r e a l a n d A
T
A = A A
T
= I
5 .
A i s t h e c o m p l e x c o n j u g a t e o f A .
6 . P i s a p e r m u t a t i o n m a t r i x i f P
~
v i s a s i m p l e r e o r d e r i n g o f
~
v .
7 . T h e t r a c e o f a m a t r i x i s
P
i
a
i i
.
8 . A i s n o r m a l i f A
T
A = A A
T
.
9 . d e t A ] i s t h e d e t e r m i n a n t o f A .
1 0 . A
H
i s t h e c o n j u g a t e t r a n s p o s e o f A , ( H e r m i t i a n ) .
1 1 . I f
A =
a b
c d
t h e n
d e t A ] = a d ; b c
a n d
A
; 1
=
1
d e t A ]
d ; b
; c a
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2 5 2 A P P E N D I X A . U S E F U L R E L A T I O N S A N D D E F I N I T I O N S F R O M L I N E A R A L G E B R A
3 . G e r s h g o r i n ' s t h e o r e m : T h e e i g e n v a l u e s o f a m a t r i x l i e i n t h e c o m p l e x p l a n e i n
t h e u n i o n o f c i r c l e s h a v i n g c e n t e r s l o c a t e d b y t h e d i a g o n a l s w i t h r a d i i e q u a l t o
t h e s u m o f t h e a b s o l u t e v a l u e s o f t h e c o r r e s p o n d i n g o - d i a g o n a l r o w e l e m e n t s .
4 . I n g e n e r a l , a n m m m a t r i x A h a s n
~
x
l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s w i t h
n
~
x
m a n d n
d i s t i n c t e i g e n v a l u e s (
i
) w i t h n
n
~
x
m .
5 . A s e t o f e i g e n v e c t o r s i s s a i d t o b e l i n e a r l y i n d e p e n d e n t i f
a
~
x
m
+ b
~
x
n
6=
~
x
k
m 6= n 6= k
f o r a n y c o m p l e x a a n d b a n d f o r a l l c o m b i n a t i o n s o f v e c t o r s i n t h e s e t .
6 . I f A p o s s e s e s m l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s t h e n A i s d i a g o n a l i z a b l e , i . e . ,
X
; 1
A X =
w h e r e X i s a m a t r i x w h o s e c o l u m n s a r e t h e e i g e n v e c t o r s ,
X =
h
~
x
1
~
x
2
: : :
~
x
m
i
a n d i s t h e d i a g o n a l m a t r i x
=
2
6
6
6
6
4
1
0 0
0
2
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0
0 0
m
3
7
7
7
7
5
I f A c a n b e d i a g o n a l i z e d , i t s e i g e n v e c t o r s c o m p l e t e l y s p a n t h e s p a c e , a n d A i s
s a i d t o h a v e a c o m p l e t e e i g e n s y s t e m .
7 . I f A h a s m d i s t i n c t e i g e n v a l u e s , t h e n A i s a l w a y s d i a g o n a l i z a b l e , a n d w i t h
e a c h d i s t i n c t e i g e n v a l u e t h e r e i s o n e a s s o c i a t e d e i g e n v e c t o r , a n d t h i s e i g e n v e c t o r
c a n n o t b e f o r m e d f r o m a l i n e a r c o m b i n a t i o n o f a n y o f t h e o t h e r e i g e n v e c t o r s .
8 . I n g e n e r a l , t h e e i g e n v a l u e s o f a m a t r i x m a y n o t b e d i s t i n c t , i n w h i c h c a s e t h e
p o s s i b i l i t y e x i s t s t h a t i t c a n n o t b e d i a g o n a l i z e d . I f t h e e i g e n v a l u e s o f a m a t r i x
a r e n o t d i s t i n c t , b u t a l l o f t h e e i g e n v e c t o r s a r e l i n e a r l y i n d e p e n d e n t , t h e m a t r i x
i s s a i d t o b e d e r o g a t o r y , b u t i t c a n s t i l l b e d i a g o n a l i z e d .
9 . I f a m a t r i x d o e s n o t h a v e a c o m p l e t e s e t o f l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s ,
i t c a n n o t b e d i a g o n a l i z e d . T h e e i g e n v e c t o r s o f s u c h a m a t r i x c a n n o t s p a n t h e
s p a c e a n d t h e m a t r i x i s s a i d t o h a v e a d e f e c t i v e e i g e n s y s t e m .
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A . 4 . E I G E N S Y S T E M S 2 5 3
1 0 . D e f e c t i v e m a t r i c e s c a n n o t b e d i a g o n a l i z e d b u t t h e y c a n s t i l l b e p u t i n t o a c o m -
p a c t f o r m b y a s i m i l a r i t y t r a n s f o r m , S , s u c h t h a t
J = S
; 1
A S =
2
6
6
6
6
4
J
1
0 0
0 J
2
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0
0 0 J
k
3
7
7
7
7
5
w h e r e t h e r e a r e k l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s a n d J
i
i s e i t h e r a J o r d a n
s u b b l o c k o r
i
.
1 1 . A J o r d a n s u b m a t r i x h a s t h e f o r m
J
i
=
2
6
6
6
6
6
6
6
4
i
1 0 0
0
i
1
.
.
.
.
.
.
0 0
i
.
.
.
0
.
.
.
.
.
.
.
.
.
1
0 0 0
i
3
7
7
7
7
7
7
7
5
1 2 . U s e o f t h e t r a n s f o r m S i s k n o w n a s p u t t i n g A i n t o i t s J o r d a n C a n o n i c a l f o r m .
A r e p e a t e d r o o t i n a J o r d a n b l o c k i s r e f e r r e d t o a s a d e f e c t i v e e i g e n v a l u e . F o r
e a c h J o r d a n s u b m a t r i x w i t h a n e i g e n v a l u e
i
o f m u l t i p l i c i t y r , t h e r e e x i s t s o n e
e i g e n v e c t o r . T h e o t h e r r ; 1 v e c t o r s a s s o c i a t e d w i t h t h i s e i g e n v a l u e a r e r e f e r r e d
t o a s p r i n c i p a l v e c t o r s . T h e c o m p l e t e s e t o f p r i n c i p a l v e c t o r s a n d e i g e n v e c t o r s
a r e a l l l i n e a r l y i n d e p e n d e n t .
1 3 . N o t e t h a t i f P i s t h e p e r m u t a t i o n m a t r i x
P =
2
6
4
0 0 1
0 1 0
1 0 0
3
7
5
P
T
= P
; 1
= P
t h e n
P
; 1
2
6
4
1 0
0 1
0 0
3
7
5
P =
2
6
4
0 0
1 0
0 1
3
7
5
1 4 . S o m e o f t h e J o r d a n s u b b l o c k s m a y h a v e t h e s a m e e i g e n v a l u e . F o r e x a m p l e , t h e
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2 5 4 A P P E N D I X A . U S E F U L R E L A T I O N S A N D D E F I N I T I O N S F R O M L I N E A R A L G E B R A
m a t r i x
2
6
6
6
6
6
6
6
6
6
6
6
6
6
4
2
6
4
1
1
1
1
1
3
7
5
1
1
1
1
2
1
2
3
3
7
7
7
7
7
7
7
7
7
7
7
7
7
5
i s b o t h d e f e c t i v e a n d d e r o g a t o r y , h a v i n g :
9 e i g e n v a l u e s
3 d i s t i n c t e i g e n v a l u e s
3 J o r d a n b l o c k s
5 l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s
3 p r i n c i p a l v e c t o r s w i t h
1
1 p r i n c i p a l v e c t o r w i t h
2
A . 5 V e c t o r a n d M a t r i x N o r m s
1 . T h e s p e c t r a l r a d i u s o f a m a t r i x A i s s y m b o l i z e d b y ( A ) s u c h t h a t
( A ) = j
m
j
m a x
w h e r e
m
a r e t h e e i g e n v a l u e s o f t h e m a t r i x A .
2 . A p - n o r m o f t h e v e c t o r ~v i s d e n e d a s
j j v j j
p
=
0
@
M
X
j = 1
j v
j
j
p
1
A
1 = p
3 . A p - n o r m o f a m a t r i x A i s d e n e d a s
j j A j j
p
= m a x
x 6= 0
j j A v j j
p
j j v j j
p
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A . 5 . V E C T O R A N D M A T R I X N O R M S 2 5 5
4 . L e t A a n d B b e s q u a r e m a t r i c e s o f t h e s a m e o r d e r . A l l m a t r i x n o r m s m u s t h a v e
t h e p r o p e r t i e s
j j A j j 0 j j A j j = 0 i m p l i e s A = 0
j j c A j j = j c j j j A j j
j j A + B j j j j A j j + j j B j j
j j A B j j j j A j j j j B j j
5 . S p e c i a l p - n o r m s a r e
j j A j j
1
= m a x
j = 1 M
P
M
i = 1
j a
i j
j m a x i m u m c o l u m n s u m
j j A j j
2
=
q
( A
T
A )
j j A j j
1
= m a x
i = 1 2 M
P
M
j = 1
j a
i j
j m a x i m u m r o w s u m
w h e r e j j A j j
p
i s r e f e r r e d t o a s t h e L
p
n o r m o f A .
6 . I n g e n e r a l ( A ) d o e s n o t s a t i s f y t h e c o n d i t i o n s i n 4 , s o i n g e n e r a l ( A ) i s n o t a
t r u e n o r m .
7 . W h e n A i s n o r m a l , ( A ) i s a t r u e n o r m , i n f a c t , i n t h i s c a s e i t i s t h e L
2
n o r m .
8 . T h e s p e c t r a l r a d i u s o f A , ( A ) , i s t h e l o w e r b o u n d o f a l l t h e n o r m s o f A .
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2 5 6 A P P E N D I X A . U S E F U L R E L A T I O N S A N D D E F I N I T I O N S F R O M L I N E A R A L G E B R A
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A p p e n d i x B
S O M E P R O P E R T I E S O F
T R I D I A G O N A L M A T R I C E S
B . 1 S t a n d a r d E i g e n s y s t e m f o r S i m p l e T r i d i a g o n a l s
I n t h i s w o r k t r i d i a g o n a l b a n d e d m a t r i c e s a r e p r e v a l e n t . I t i s u s e f u l t o l i s t s o m e o f
t h e i r p r o p e r t i e s . M a n y o f t h e s e c a n b e d e r i v e d b y s o l v i n g t h e s i m p l e l i n e a r d i e r e n c e
e q u a t i o n s t h a t a r i s e i n d e r i v i n g r e c u r s i o n r e l a t i o n s .
L e t u s c o n s i d e r a s i m p l e t r i d i a g o n a l m a t r i x , i . e . , a t r i d i a g o n a l w i t h c o n s t a n t s c a l a r
e l e m e n t s a , b , a n d c , s e e S e c t i o n 3 . 4 . I f w e e x a m i n e t h e c o n d i t i o n s u n d e r w h i c h t h e
d e t e r m i n a n t o f t h i s m a t r i x i s z e r o , w e n d ( b y a r e c u r s i o n e x e r c i s e )
d e t B ( M : a b c ) ] = 0
i f
b + 2
p
a c c o s
m
M + 1
= 0 m = 1 2 M
F r o m t h i s i t f o l l o w s a t o n c e t h a t t h e e i g e n v a l u e s o f B ( a b c ) a r e
m
= b + 2
p
a c c o s
m
M + 1
m = 1 2 M ( B . 1 )
T h e r i g h t - h a n d e i g e n v e c t o r o f B ( a b c ) t h a t i s a s s o c i a t e d w i t h t h e e i g e n v a l u e
m
s a t i s e s t h e e q u a t i o n
B ( a b c )
~
x
m
=
m
~
x
m
( B . 2 )
a n d i s g i v e n b y
~
x
m
= ( x
j
)
m
=
a
c
j ; 1
2
s i n
j
m
M + 1
m = 1 2 M ( B . 3 )
2 5 7
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2 5 8 A P P E N D I X B . S O M E P R O P E R T I E S O F T R I D I A G O N A L M A T R I C E S
T h e s e v e c t o r s a r e t h e c o l u m n s o f t h e r i g h t - h a n d e i g e n v e c t o r m a t r i x , t h e e l e m e n t s o f
w h i c h a r e
X = ( x
j m
) =
a
c
j ; 1
2
s i n
j m
M + 1
j = 1 2 M
m = 1 2 M
( B . 4 )
N o t i c e t h a t i f a = ; 1 a n d c = 1 ,
a
c
j ; 1
2
= e
i ( j ; 1 )
2
( B . 5 )
T h e l e f t - h a n d e i g e n v e c t o r m a t r i x o f B ( a b c ) c a n b e w r i t t e n
X
; 1
=
2
M + 1
c
a
m ; 1
2
s i n
m j
M + 1
m = 1 2 M
j = 1 2 M
I n t h i s c a s e n o t i c e t h a t i f a = ; 1 a n d c = 1
c
a
m ; 1
2
= e
; i ( m ; 1 )
2
( B . 6 )
B . 2 G e n e r a l i z e d E i g e n s y s t e m f o r S i m p l e T r i d i a g -
o n a l s
T h i s s y s t e m i s d e n e d a s f o l l o w s
2
6
6
6
6
6
6
4
b c
a b c
a b
.
.
.
c
a b
3
7
7
7
7
7
7
5
2
6
6
6
6
6
6
4
x
1
x
2
x
3
.
.
.
x
M
3
7
7
7
7
7
7
5
=
2
6
6
6
6
6
6
4
e f
d e f
d e
.
.
.
f
d e
3
7
7
7
7
7
7
5
2
6
6
6
6
6
6
4
x
1
x
2
x
3
.
.
.
x
M
3
7
7
7
7
7
7
5
I n t h i s c a s e o n e c a n s h o w a f t e r s o m e a l g e b r a t h a t
d e t B ( a
; d b
; e c
; f ] = 0 ( B . 7 )
i f
b ;
m
e + 2
q
( a ;
m
d ) ( c ;
m
f ) c o s
m
M + 1
= 0 m = 1 2 M ( B . 8 )
I f w e d e n e
m
=
m
M + 1
p
m
= c o s
m
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B . 3 . T H E I N V E R S E O F A S I M P L E T R I D I A G O N A L 2 5 9
m
=
e b ; 2 ( c d + a f ) p
2
m
+ 2 p
m
q
( e c ; f b ) ( e a ; b d ) + ( c d ; a f ) p
m
]
2
e
2
; 4 f d p
2
m
T h e r i g h t - h a n d e i g e n v e c t o r s a r e
~
x
m
=
"
a ;
m
d
c ;
m
f
#
j ; 1
2
s i n j
m
]
m = 1 2 M
j = 1 2 M
T h e s e r e l a t i o n s a r e u s e f u l i n s t u d y i n g r e l a x a t i o n m e t h o d s .
B . 3 T h e I n v e r s e o f a S i m p l e T r i d i a g o n a l
T h e i n v e r s e o f B ( a b c ) c a n a l s o b e w r i t t e n i n a n a l y t i c f o r m . L e t D
M
r e p r e s e n t t h e
d e t e r m i n a n t o f B ( M : a b c )
D
M
d e t B ( M : a b c ) ]
D e n i n g D
0
t o b e 1 , i t i s s i m p l e t o d e r i v e t h e r s t f e w d e t e r m i n a n t s , t h u s
D
0
= 1
D
1
= b
D
2
= b
2
; a c
D
3
= b
3
; 2 a b c ( B . 9 )
O n e c a n a l s o n d t h e r e c u r s i o n r e l a t i o n
D
M
= b D
M ; 1
; a c D
M ; 2
( B . 1 0 )
E q . B . 1 0 i s a l i n e a r O E t h e s o l u t i o n o f w h i c h w a s d i s c u s s e d i n S e c t i o n 4 . 2 . I t s
c h a r a c t e r i s t i c p o l y n o m i a l P ( E ) i s P ( E
2
; b E + a c ) a n d t h e t w o r o o t s t o P ( ) = 0
r e s u l t i n t h e s o l u t i o n
D
M
=
1
p
b
2
; 4 a c
8
<
:
"
b +
p
b
2
; 4 a c
2
#
M + 1
;
"
b ;
p
b
2
; 4 a c
2
#
M + 1
9
=
M = 0 1 2 ( B . 1 1 )
w h e r e w e h a v e m a d e u s e o f t h e i n i t i a l c o n d i t i o n s D
0
= 1 a n d D
1
= b . I n t h e l i m i t i n g
c a s e w h e n b
2
; 4 a c = 0 , o n e c a n s h o w t h a t
D
M
= ( M + 1 )
b
2
!
M
b
2
= 4 a c
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2 6 0 A P P E N D I X B . S O M E P R O P E R T I E S O F T R I D I A G O N A L M A T R I C E S
T h e n f o r M = 4
B
; 1
=
1
D
4
2
6
6
6
4
D
3
; c D
2
c
2
D
1
; c
3
D
0
; a D
2
D
1
D
2
; c D
1
D
1
c
2
D
1
a
2
D
1
; a D
1
D
1
D
2
D
1
; c D
2
; a
3
D
0
a
2
D
1
; a D
2
D
3
3
7
7
7
5
a n d f o r M = 5
B
; 1
=
1
D
5
2
6
6
6
6
6
4
D
4
; c D
3
c
2
D
2
; c
3
D
1
c
4
D
0
; a D
3
D
1
D
3
; c D
1
D
2
c
2
D
1
D
1
; c
3
D
1
a
2
D
2
; a D
1
D
2
D
2
D
2
; c D
2
D
1
c
2
D
2
; a
3
D
1
a
2
D
1
D
1
; a D
2
D
1
D
3
D
1
; c D
3
a
4
D
0
; a
3
D
1
a
2
D
2
; a D
3
D
4
3
7
7
7
7
7
5
T h e g e n e r a l e l e m e n t d
m n
i s
U p p e r t r i a n g l e :
m = 1 2 M ; 1 n = m + 1 m + 2 M
d
m n
= D
m ; 1
D
M ; n
( ; c )
n ; m
= D
M
D i a g o n a l :
n = m = 1 2 M
d
m m
= D
M ; 1
D
M ; m
= D
M
L o w e r t r i a n g l e :
m = n + 1 n + 2 M n = 1 2 M ; 1
d
m n
= D
M ; m
D
n ; 1
( ; a )
m ; n
= D
M
B . 4 E i g e n s y s t e m s o f C i r c u l a n t M a t r i c e s
B . 4 . 1 S t a n d a r d T r i d i a g o n a l s
C o n s i d e r t h e c i r c u l a n t ( s e e S e c t i o n 3 . 4 . 4 ) t r i d i a g o n a l m a t r i x
B
p
( M : a b c ) ( B . 1 2 )
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B . 4 . E I G E N S Y S T E M S O F C I R C U L A N T M A T R I C E S 2 6 1
T h e e i g e n v a l u e s a r e
m
= b + ( a + c ) c o s
2 m
M
; i ( a ; c ) s i n
2 m
M
m = 0 1 2 M ; 1
( B . 1 3 )
T h e r i g h t - h a n d e i g e n v e c t o r t h a t s a t i s e s B
p
( a b c )
~
x
m
=
m
~
x
m
i s
~
x
m
= ( x
j
)
m
= e
i j ( 2 m = M )
j = 0 1 M ; 1 ( B . 1 4 )
w h e r e i
p
; 1 , a n d t h e r i g h t - h a n d e i g e n v e c t o r m a t r i x h a s t h e f o r m
X = ( x
j m
) = e
i j
2 m
M
j = 0 1 M ; 1
m = 0 1 M ; 1
T h e l e f t - h a n d e i g e n v e c t o r m a t r i x w i t h e l e m e n t s x
0
i s
X
; 1
= ( x
0
m j
) =
1
M
e
; i m
2 j
M
m = 0 1 M ; 1
j = 0 1
M
; 1
N o t e t h a t b o t h X a n d X
; 1
a r e s y m m e t r i c a n d t h a t X
; 1
=
1
M
X
, w h e r e X i s t h e
c o n j u g a t e t r a n s p o s e o f X .
B . 4 . 2 G e n e r a l C i r c u l a n t S y s t e m s
N o t i c e t h e r e m a r k a b l e f a c t t h a t t h e e l e m e n t s o f t h e e i g e n v e c t o r m a t r i c e s X a n d X
; 1
f o r t h e t r i d i a g o n a l c i r c u l a n t m a t r i x g i v e n b y e q . B . 1 2 d o n o t d e p e n d o n t h e e l e m e n t s
a b c i n t h e m a t r i x . I n f a c t , a l l c i r c u l a n t m a t r i c e s o f o r d e r M h a v e t h e s a m e s e t o f
l i n e a r l y i n d e p e n d e n t e i g e n v e c t o r s , e v e n i f t h e y a r e c o m p l e t e l y d e n s e . A n e x a m p l e o f
a d e n s e c i r c u l a n t m a t r i x o f o r d e r M = 4 i s
2
6
6
6
4
b
0
b
1
b
2
b
3
b
3
b
0
b
1
b
2
b
2
b
3
b
0
b
1
b
1
b
2
b
3
b
0
3
7
7
7
5
( B . 1 5 )
T h e e i g e n v e c t o r s a r e a l w a y s g i v e n b y e q . B . 1 4 , a n d f u r t h e r e x a m i n a t i o n s h o w s t h a t
t h e e l e m e n t s i n t h e s e e i g e n v e c t o r s c o r r e s p o n d t o t h e e l e m e n t s i n a c o m p l e x h a r m o n i c
a n a l y s i s o r c o m p l e x d i s c r e t e F o u r i e r s e r i e s .
A l t h o u g h t h e e i g e n v e c t o r s o f a c i r c u l a n t m a t r i x a r e i n d e p e n d e n t o f i t s e l e m e n t s ,
t h e e i g e n v a l u e s a r e n o t . F o r t h e e l e m e n t i n d e x i n g s h o w n i n e q . B . 1 5 t h e y h a v e t h e
g e n e r a l f o r m
m
=
M ; 1
X
j = 0
b
j
e
i ( 2 j m = M )
o f w h i c h e q . B . 1 3 i s a s p e c i a l c a s e .
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2 6 2 A P P E N D I X B . S O M E P R O P E R T I E S O F T R I D I A G O N A L M A T R I C E S
B . 5 S p e c i a l C a s e s F o u n d F r o m S y m m e t r i e s
C o n s i d e r a m e s h w i t h a n e v e n n u m b e r o f i n t e r i o r p o i n t s s u c h a s t h a t s h o w n i n F i g .
B . 1 . O n e c a n s e e k f r o m t h e t r i d i a g o n a l m a t r i x B ( 2 M : a b a ) t h e e i g e n v e c t o r s u b s e t
t h a t h a s e v e n s y m m e t r y w h e n s p a n n i n g t h e i n t e r v a l 0 x . F o r e x a m p l e , w e s e e k
t h e s e t o f e i g e n v e c t o r s
~
x
m
f o r w h i c h
2
6
6
6
6
6
6
6
6
4
b a
a b a
a
.
.
.
.
.
.
a
a b a
a b
3
7
7
7
7
7
7
7
7
5
2
6
6
6
6
6
6
6
6
6
4
x
1
x
2
.
.
.
.
.
.
x
2
x
1
3
7
7
7
7
7
7
7
7
7
5
=
m
2
6
6
6
6
6
6
6
6
6
4
x
1
x
2
.
.
.
.
.
.
x
2
x
1
3
7
7
7
7
7
7
7
7
7
5
T h i s l e a d s t o t h e s u b s y s t e m o f o r d e r M w h i c h h a s t h e f o r m
B ( M : a
~
b a )
~
x
m
=
2
6
6
6
6
6
6
6
6
6
6
4
b a
a b a
a
.
.
.
.
.
.
a
a b a
a b + a
3
7
7
7
7
7
7
7
7
7
7
5
~
x
m
=
m
~
x
m
( B . 1 6 )
B y f o l d i n g t h e k n o w n e i g e n v e c t o r s o f B ( 2 M : a b a ) a b o u t t h e c e n t e r , o n e c a n s h o w
f r o m p r e v i o u s r e s u l t s t h a t t h e e i g e n v a l u e s o f e q . B . 1 6 a r e
m
= b + 2 a c o s
( 2 m ; 1 )
2 M + 1
!
m = 1 2
M ( B . 1 7 )
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B . 6 . S P E C I A L C A S E S I N V O L V I N G B O U N D A R Y C O N D I T I O N S 2 6 3
a n d t h e c o r r e s p o n d i n g e i g e n v e c t o r s a r e
~
x
m
= s i n
j ( 2 m ; 1 )
2 M + 1
j = 1 2 M
I m p o s i n g s y m m e t r y a b o u t t h e s a m e i n t e r v a l
b u t f o r a m e s h w i t h a n o d d n u m b e r o f p o i n t s ,
s e e F i g . B . 1 , l e a d s t o t h e m a t r i x
B ( M :
~
a
b a ) =
2
6
6
6
6
6
6
6
6
4
b a
a b a
a
.
.
.
.
.
.
a
a b a
2 a b
3
7
7
7
7
7
7
7
7
5
B y f o l d i n g t h e k n o w n e i g e n v a l u e s o f B ( 2 M
;
1 : a b a ) a b o u t t h e c e n t e r , o n e c a n s h o w
f r o m p r e v i o u s r e s u l t s t h a t t h e e i g e n v a l u e s o f
e q . B . 1 7 a r e
L i n e o f S y m m e t r y
x = 0 x =
j
0
= 1 2 3 4 5 6
M
0
j = 1 2 3
M
a . A n e v e n - n u m b e r e d m e s h
L i n e o f S y m m e t r y
x = 0 x =
j
0
= 1 2 3 4 5
M
0
j = 1 2 3
M
b . A n o d d { n u m b e r e d m e s h
F i g u r e B . 1 { S y m m e t r i c a l f o l d s f o r
s p e c i a l c a s e s
m
= b + 2 a c o s
( 2 m ; 1 )
2 M
!
m = 1 2 M
a n d t h e c o r r e s p o n d i n g e i g e n v e c t o r s a r e
~
x
m
= s i n
j ( 2 m ; 1 )
2 M
!
j = 1 2 M
B . 6 S p e c i a l C a s e s I n v o l v i n g B o u n d a r y C o n d i t i o n s
W e c o n s i d e r t w o s p e c i a l c a s e s f o r t h e m a t r i x o p e r a t o r r e p r e s e n t i n g t h e 3 - p o i n t c e n t r a l
d i e r e n c e a p p r o x i m a t i o n f o r t h e s e c o n d d e r i v a t i v e @
2
= @ x
2
a t a l l p o i n t s a w a y f r o m t h e
b o u n d a r i e s , c o m b i n e d w i t h s p e c i a l c o n d i t i o n s i m p o s e d a t t h e b o u n d a r i e s .
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2 6 4 A P P E N D I X B . S O M E P R O P E R T I E S O F T R I D I A G O N A L M A T R I C E S
N o t e : I n b o t h c a s e s
m = 1 2 M
j = 1 2 M
; 2 + 2 c o s ( ) = ; 4 s i n
2
( = 2 )
W h e n t h e b o u n d a r y c o n d i t i o n s a r e D i r i c h l e t o n b o t h s i d e s ,
2
6
6
6
6
6
6
4
; 2 1
1
; 2 1
1 ; 2 1
1 ; 2 1
1 ; 2
3
7
7
7
7
7
7
5
m
= ; 2 + 2 c o s
m
M + 1
~
x
m
= s i n
h
j
m
M + 1
i
( B . 1 8 )
W h e n o n e b o u n d a r y c o n d i t i o n i s D i r i c h l e t a n d t h e o t h e r i s N e u m a n n ( a n d a d i a g o n a l
p r e c o n d i t i o n e r i s a p p l i e d t o s c a l e t h e l a s t e q u a t i o n ) ,
2
6
6
6
6
6
6
4
; 2 1
1 ; 2 1
1 ; 2 1
1 ; 2 1
1 ; 1
3
7
7
7
7
7
7
5
m
= ; 2 + 2 c o s
( 2 m ; 1 )
2 M + 1
~
x
m
= s i n
j
( 2 m
; 1 )
2 M + 1
( B . 1 9 )
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A p p e n d i x C
T H E H O M O G E N E O U S
P R O P E R T Y O F T H E E U L E R
E Q U A T I O N S
T h e E u l e r e q u a t i o n s h a v e a s p e c i a l p r o p e r t y t h a t i s s o m e t i m e s u s e f u l i n c o n s t r u c t i n g
n u m e r i c a l m e t h o d s . I n o r d e r t o e x a m i n e t h i s p r o p e r t y , l e t u s r s t i n s p e c t E u l e r ' s
t h e o r e m o n h o m o g e n e o u s f u n c t i o n s . C o n s i d e r r s t t h e s c a l a r c a s e . I f F ( u v ) s a t i s e s
t h e i d e n t i t y
F ( u v ) =
n
F ( u v ) ( C . 1 )
f o r a x e d n , F i s c a l l e d h o m o g e n e o u s o f d e g r e e n . D i e r e n t i a t i n g b o t h s i d e s w i t h
r e s p e c t t o a n d s e t t i n g = 1 ( s i n c e t h e i d e n t i t y h o l d s f o r a l l ) , w e n d
u
@ F
@ u
+ v
@ F
@ v
= n F ( u v ) ( C . 2 )
C o n s i d e r n e x t t h e t h e o r e m a s i t a p p l i e s t o s y s t e m s o f e q u a t i o n s . I f t h e v e c t o r
F ( Q ) s a t i s e s t h e i d e n t i t y
F ( Q ) =
n
F ( Q ) ( C . 3 )
f o r a x e d n , F i s s a i d t o b e h o m o g e n e o u s o f d e g r e e n a n d w e n d
"
@ F
@ q
#
Q = n F ( Q ) ( C . 4 )
2 6 5
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2 6 6 A P P E N D I X C . T H E H O M O G E N E O U S P R O P E R T Y O F T H E E U L E R E Q U A T I O N S
N o w i t i s e a s y t o s h o w , b y d i r e c t u s e o f e q . C . 3 , t h a t b o t h E a n d F i n e q s . 2 . 1 1 a n d
2 . 1 2 a r e h o m o g e n e o u s o f d e g r e e 1 , a n d t h e i r J a c o b i a n s , A a n d B , a r e h o m o g e n e o u s
o f d e g r e e 0 ( a c t u a l l y t h e l a t t e r i s a d i r e c t c o n s e q u e n c e o f t h e f o r m e r ) .
1
T h i s b e i n g
t h e c a s e , w e n o t i c e t h a t t h e e x p a n s i o n o f t h e u x v e c t o r i n t h e v i c i n i t y o f t
n
w h i c h ,
a c c o r d i n g t o e q . 6 . 1 0 5 c a n b e w r i t t e n i n g e n e r a l a s ,
E = E
n
+ A
n
( Q ; Q
n
) + O ( h
2
)
F = F
n
+ B
n
( Q ; Q
n
) + O ( h
2
) ( C . 5 )
c a n b e w r i t t e n
E = A
n
Q + O ( h
2
)
F = B
n
Q + O ( h
2
) ( C . 6 )
s i n c e t h e t e r m s E
n
; A
n
Q
n
a n d F
n
; B
n
Q
n
a r e i d e n t i c a l l y z e r o f o r h o m o g e n e o u s
v e c t o r s o f d e g r e e 1 , s e e e q . C . 4 . N o t i c e a l s o t h a t , u n d e r t h i s c o n d i t i o n , t h e c o n s t a n t
t e r m d r o p s o u t o f e q . 6 . 1 0 6 .
A s a n a l r e m a r k , w e n o t i c e f r o m t h e c h a i n r u l e t h a t f o r a n y v e c t o r s F a n d Q
@ F ( Q )
@ x
=
"
@ F
@ Q
#
@ Q
@ x
= A
@ Q
@ x
( C . 7 )
W e n o t i c e a l s o t h a t f o r a h o m o g e n e o u s F o f d e g r e e 1 , F = A Q a n d
@ F
@ x
= A
@ Q
@ x
+
"
@ A
@ x
#
Q ( C . 8 )
T h e r e f o r e , i f F i s h o m o g e n e o u s o f d e g r e e 1 ,
"
@ A
@ x
#
Q = 0 ( C . 9 )
i n s p i t e o f t h e f a c t t h a t i n d i v i d u a l l y @ A = @ x ] a n d Q a r e n o t e q u a l t o z e r o .
1
N o t e t h a t t h i s d e p e n d s o n t h e f o r m o f t h e e q u a t i o n o f s t a t e . T h e E u l e r e q u a t i o n s a r e h o m o g e -
n e o u s i f t h e e q u a t i o n o f s t a t e c a n b e w r i t t e n i n t h e f o r m p = f ( ) , w h e r e i s t h e i n t e r n a l e n e r g y
p e r u n i t m a s s .
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