Slides_particle Image Velocimetry

8/18/2019 Slides_particle Image Velocimetry

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Measurement Techniques

Digital Particle Image Velocimetry

Heat and Mass Transfer Laboratory (LTCM)

Sepideh Khodaparast

Marco Milan

Navid Borhani

Spring semester 2011


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! Introduction

! Particle Image Velocimetry features

!

System components for Particle Image Velocimetry

!

Principles of Particle Image Velocimetry

!

Practical test (TPA) description

Content



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Origins of Particle Image Velocimetry

Ludwig Prandtl water tunnel (1904) Poohsticks (1928)


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Particle Image Velocimetry applications


Fluid velocity measurement for fluid dynamic

characterization: Air flowing around a car or anaircraft, water running through hydroelectric turbine,

blood flow, etc.


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Particle Image Velocimetry features


Advantages:

"

Non-intrusive technique: no modification of the flowproperty at the scale of interest

" Good resolution and accuracy: instantaneous velocityvector maps in a cross-section of the flow

"

2D-3D Velocity field reconstruction: 3D componentsmay be obtained with the use of a stereoscopicarrangement

Drawbacks:

"

Setup Time: need to optimize a number of parameters" Cost


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System components for Particle Image Velocimetry


"

Test section, seeding particles

" Laser sheet

" Camera


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Principles of Particle Image Velocimetry


FlowDirection

SeedingparticlesTube Wall

Time: tPosition: (x1, y1)

Time: t + !tPosition: (x2, y2)

Velocity vector at:

!"#$

%&

'(

'(=!

"#$

%& ++

'+

t

y y

t

x x y y x x

t t

1212

2

2121 ,2

,2


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Cross-correlation function


! += dx s x I x I s R )()()( 21

" I1 and I2 are sub-area (interrogation windows) of the total frame

" x is the interrogation location

"

s is the shift between the images

( ) ( ) ( ) y y x x I y x I y x R x y

!+!+=!! "" ,,, 21

Two-dimensional discrete correlation function:

R(!"# !"%& '())*+,-(. /,0


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Particle Image Velocimetry protocol


1 - Divide the images into a regular grid of

smaller regions: interrogation windows (IW)

2 - Each IW of the first image is correlated with

the corresponding IW of the second image

3 - Find the location of the displacement peak

and compute the velocity vector

4 - Reconstruct the flow velocity field


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Particle selection


Capturing fluid motion

" Small enough to follow fluid motion

" Large enough to be visible

" Homogeneously distributed

" Tracer should not alter fluid / flow properties

Stokes number: F

V

v

flowof sticcharacteritime

timeresponse particleSt

!

!

==

_ _ _

_ _

Stv1: The particles will be unaffected by the fluid

Optimal particle image diameter: DI =2.5 pixels


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Seeding particle density(number of particle per interrogation window)

2

2

0

0

I I D M

z C N

!=

C Particle concentration!12 Light sheet ticknessDI Interrogation window sizeM0 Magnification

More particles: better signal to noise ratioUnambiguous detection of peak from noise

NI = 10

(10 particles per IW are sufficient to perform PIV)

NI = 5 NI = 10 NI = 15


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Maximum in-plane particle displacement

!345I = 0.00FI=1.00

0.28 0.56 0.850.64 0.36 0.16

!3 Particle displacement

DI Interrogation window sizeFI In plane loss-of-correlation

X,Y Displacements < quarter of the interrogation window size

Choose the sampling interval and the optical magnification factor so thatthe maximum image displacement is less than a quarter of the

interrogation window size


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Maximum out-of-plane particle displacement

!64 !10 = 0.00Fo=1.00

0.25 0.50 0.750.75 0.50 0.25

!6 Particle displacement normal to the observed plane

!12 Light sheet thicknessFo Out-of-plane loss-of-correlation

Z Displacement < quarter of the light sheet thickness

Define a suitable observation plane and choose a sampling interval sothat the particle shift normal to the observed plane is less than a

quarter of the light sheet thickness


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Spatial gradient - Interrogation window size

The PIV resolution is proportional to

the IW size. It has to be defined to

resolve local flow gradients

Keeping constant the particle size and density, altering the IW size will

change the number of particles used to calculate the local velocity in an

interrogation window. The total number of vector for a given image pair

depends directly on the IW size


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Interrogation window overlap

Overlap of the interrogation windows increase the number of particle

that are used in calculating the flow field

" Better spatial resolution

- Longer computational time

IW 1 IW 2

50% IW Overlap

Interrogation window offset

Offset increase the PIV accuracy for flow with a dominant direction. The

IW is shifted between the first and the second image

"

Better accuracy

- Longer computational timeIW Offset

Image 1 Image 2

IW 1 IW 1


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PIV Design rules


Optimal particle image diameter

Image density

In-plane motion

Out-of-plane motion

DI =2.5 pixels

NI = 10

!!X ! < 0.25 IW Size

!!z ! < 0.25 light sheet thickness

The IW size and the sampling interval define the spatial and thetemporal resolution


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Practical test (TPA) description

1: Laminar flow

2: guidingvanes

3: elbow

PIV system components:

" Test section

"

Laser sheet" Camera

TP procedure:

1. Tune and calibrate the PIV system

2.

Record images at 3 differentlocations

3. Process the images on the PC

4.

Save and export the results

TP objectives:

"

PIV system tuning (Record nice images)

" Understand the principles of the PIV technique (ex: correlation function)

and the influence of the measurement parameters (n° of particles, size of

the IW, overlap, etc.)


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Enjoy the TPA !!!

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Slides_particle Image Velocimetry