Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
Mesh Generation Tutorial 1Instituto Tecnológico de Veracruz 21-25 January 2008
Fathi El-YafiSenior Product Engineer
Product Department of EXA Corporation
An Introduction to Mesh Generation An Introduction to Mesh Generation AlgorithmsAlgorithms
Part 2Part 2
Mesh Generation Tutorial 2Instituto Tecnológico de Veracruz 21-25 January 2008
Overview
Remeshing
Decimation
Optimization
Adaptive Meshing:
Mesh Generation Tutorial 3Instituto Tecnológico de Veracruz 21-25 January 2008
Finite Element Context
Method:Calculating the density of the mesh from a tree QuaternaryNodal generation within each cell of the treeMeshing by advancing front method according to a
criterion of Delaunay
Objective:2D Adaptive Mesh according to the specifications sizes imposed by the user
Adaptive Meshing
Mesh Generation Tutorial 4Instituto Tecnológico de Veracruz 21-25 January 2008
Finite Element Context
In computer science, a B+ tree (also known as a Quaternary Tree) is a type of tree, which represents sorted data in a way that allows for efficient insertion, retrieval and removal of records, each of which is identified by a key. It is a dynamic, multilevel index, with maximum and minimum bounds on the number of keys in each index segment (usually called a block' or node'). In a B+ tree, in contrast to a B+ tree, all records are stored at the lowest level of the tree; only keys are stored in interior blocks.
Quaternary Tree
Mesh Generation Tutorial 5Instituto Tecnológico de Veracruz 21-25 January 2008
Finite Element Context
Remeshing a Polygon
n points
Possible Remeshing?
?
Mesh Generation Tutorial 6Instituto Tecnológico de Veracruz 21-25 January 2008
Finite Element Context
Principle
=
=
=
Mesh Generation Tutorial 7Instituto Tecnológico de Veracruz 21-25 January 2008
Finite Element Context
Principle
Tn =
n(n − 1)(n − 2)6
It shows that we can create Tndifferent triangles
n nodes lead to n-2 triangles
=
120845635201041
109876543
N=10, 1430 possible triangulations!
Mesh Generation Tutorial 8Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
State of the Art
Methods that use the surface parameterization:
Appropriate Plane + transport functions
Appropriate Plane + Metric: INRIA
Discrete Models:
Very Refined Initial Mesh + Simplifications
Reconstruction:
• Direct Method: Frontal Triangulation (R. Löhner)
• Remeshing: HDI (A. Rassineux)
Mesh Generation Tutorial 9Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Typical Examples
Mesh Generation Tutorial 10Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Adaptive Meshing using Diffuse InterpolationObjective: Building an adaptive mesh of a 3D surface according to size map, using the initial mesh as only data
Initial Mesh
Local geometric model
Diffuse Interpolation
Hermite type
Adaptive mesh
Remeshing Procedure
Mesh Generation Tutorial 11Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Remeshing
Mesh Generation Tutorial 12Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Some Remeshing Tools
Edge splitting
Swapping
Nodal Shifting
Node removing
Edge collapsing
Edge collapsing and nodal insertion
Mesh Generation Tutorial 13Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Remeshing
Remeshing according to a size map
Simplification of a mesh (70%)
Mesh Generation Tutorial 14Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Remeshing
Initial Mesh Intermediate Mesh Optimized Mesh
Mesh Generation Tutorial 15Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Surface Remeshing
Initial Mesh
Optimized Mesh
Mesh Generation Tutorial 16Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Surface Remeshing
Reduction: 60%Reduction: 75%
Initial model
Adapted to curvature
Mesh Generation Tutorial 17Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Surface Meshing
Adapted to curvature
Simplified model
Mesh Generation Tutorial 18Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Simplified Curvature
In a plane 360iθ = °∑
Θ1
Θ2
Of all the triangles 360iα ≥ °∑
αι
default angle = f (Gaussian Curvature)( ) =∆=−∑ θθα ii
Mesh Generation Tutorial 19Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Feature Lines Detection
Mesh Generation Tutorial 20Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Feature Lines Detection
Curvature using Diffuse Interpolation Model
(a) (b)
Identification of geometrical parameters (radius, axis)
Detecting neighborhood
Mesh Generation Tutorial 21Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Principal Curvatures
Segmentation into subsets
Surface Identification
Detection of Feature lines
Meshing
Mesh Generation Tutorial 22Instituto Tecnológico de Veracruz 21-25 January 2008…
Adaptive Mesh
ExampleSegmentation into subsets
Surface Identification
Detection of Feature lines
Meshing
Mesh Generation Tutorial 23Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Main Steps
…
Mesh Generation Tutorial 24Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Mesh Generation Tutorial 25Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Diffuse Model
xz
y
αα TPyxyxyxyxfz =>=<= 22 ,,,,,1),(Local model
MLS Technique : Diffuse approximationMinimisation of an interpolation criterion
( ) ( )( )21
1 ),( iiT
ni
iix zxxpxxwJ −−×= ∑
=
=αα
Mesh Generation Tutorial 26Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Moving Least Squares
( ) ( )2 21, , , , , ,T Tx y x xy y p x y p x< >= =Polynomial Base
( )( )2
1( ) ( )
i nT
x i ii
J p x x f x xα α=
=
= − − −∑
n = Total number of points ⇒ «standard» Least Squares
X
Z
R
Select a neighbor V(x) around a point(x,y)
( )( )2
( )( ) ( )T
x i ii V x
J p x x f x xα α∈
= − − −∑
⇒ The approximation is local
Mesh Generation Tutorial 27Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Weighted Moving Least Squares
( ) ( )( )2
( )( ) ( )T
x i i ii V x
J w x x p x x f x xα α∈
= − − − −∑
Weight Function
/R d l)l()-l(,x)w(x
w,x)w(x
ii
iii
xRxxi
refi
=+×=
=
−
avec 211 2
)(
Mesh Generation Tutorial 28Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Moving Least Squares Interpolation
[ ]
−
→),(1
),(),(xxw
xxwxxwi
ii
( ) ( )( )2
( )( ) ( )T
x i i ii V x
J w x x p x x f x xα α∈
= − − − −∑
If w(x,xi)→∞⇒ interpolation at point xi
Mesh Generation Tutorial 29Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
CAD-STL Face Per Face
Mesh Generation Tutorial 30Instituto Tecnológico de Veracruz 21-25 January 2008
Finite Element Context
STL/Mesh/Adaptive Mesh
STL Mesh
Mesh Face per Face Adaptive Mesh
Mesh Generation Tutorial 31Instituto Tecnológico de Veracruz 21-25 January 2008
STL/Adaptive Mesh
STL Mesh
Adaptive Mesh
Adaptive Mesh
Mesh Generation Tutorial 32Instituto Tecnológico de Veracruz 21-25 January 2008
CAD/Adaptive Meshes
Adaptive Mesh
Mesh Generation Tutorial 33Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Mesh Details
Mesh Generation Tutorial 34Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Feature Lines
Mesh Generation Tutorial 35Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Surface Remeshing
Initial model Simplified model
Mesh Generation Tutorial 36Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Surface Remeshing
Mesh Generation Tutorial 37Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Surface Remeshing
Initial Mesh
Adaptive Mesh
Mesh Generation Tutorial 38Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Surface Remeshing
Mesh Generation Tutorial 39Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Repairing Surface Mesh
Mesh Generation Tutorial 40Instituto Tecnológico de Veracruz 21-25 January 2008
Adaptive Mesh
Demonstrations