particle image velocimetry_slides

Particle Image Velocimetry

Kul-34.4551 Postgraduate Seminar on Fluid Mechanics

Jarmo Kalilainen

Outline

8.5.2012 2Kul-34.4551 Postgraduate Seminar on Fluid Mechanics

1. Introduction

2. PIV operation principlesa) Illuminationb) camerasc) Seedingd) Velocity calculation e) Post processing

3. Advanced PIV methodsa) Stereographic PIVb) Tomographic PIVc) Micro-PIV

4. Summary

Introduction


Particle Image Velocimetry (PIV) is a setof methods where the velocity of theflow is determined by investigatingmotion of a large number of particlesfollowing the flow

In PIV, the velocity is directly calculatedfrom the displacement of the flowelement at given time

• Advantage when compared toother experimental methods, suchas Laser Doppler Anomometry orhot wires, where intermediatephenomena is measured forvelocity calculation

Introduction


With PIV:

• Velocity vectors from relatively large area of a flow can be measuredintantaniously

• Visualization of the flow field

Closely related to Particle Tracking Velocimetry (PTV)

• Flow field determined by following a individual particles

Basic planar PIV


Operation principle of planar PIV:

• Tracer particles seeded to the flow

• Part of the flow that is under investigation is illuminated with light source (for example laser)

• Two successive images taken form the light sheet

• Flow velocity determined form the movement of particles at the light sheet using some correlation method Picture: Raffel, Willert, Komphans: PIV, a practical Guide

Illumination


Typically, illumination in PIV handled with double-pulsedlaser

• Solid state lasers (like Nd:YAG) commonly used

• With frequency doubling crystal, wavelength 532 nm

• Two lasers for two illuminations

Light sheet optics used to form a laser sheet from thepulse

Illumination


Q-switch is used to produce a high energy (10-1000 mJ) laser pulse

When flash lamp is enabled, Pockels cell is OFF and light emitted from the Nd:YAG rod due to spontanious emissions is reflected by the Glan-laser polarizer

After the exitation of laser rod is completed, Q-switch is enabled -> Pockels cell ON -> light passes through the polarizer and cause stimulated emissions in the laser rod -> high energy light pulse (5-10 ns)

Picture: Adrian, Westerweel: Particle Image velocimetry

Illumination


Laser pulse energy can be controlled with

• Q-switch delay time: shorter delay -> lower energy

• Flash lamp voltage: lower voltage -> lower laser pulse energy


Illumination


Camera


Camera records the particle image to a video chip

• Charge-Coupled Device (CCD), electric charge heldin electron storage wells

• Complementary Metal Oxide Semiconductor(CMOS), arrays convert light into electrical signal

Modern PIV camera has 1-22 million pixels

Particle image diameter > 5 pixels

• Smaller particle image can cause pixel locking,where particle displacement cannot be accuratelydetermined

Particles


Seed particles must be small enough to follow the flow with good accuracy

• 1 µm diameter particles for gas flows

• 10 µm diameter particles for liquid flows

On the other hand, particles should have good light scattering abilities

Mono-disperse particles

Particles


For gas flows, also liquid droplets, such as oils (olive oil, DEHS etc.) can be used


Image displacement


Light intensity of image plane with multiple particle images:

For image displacement calculation, the image is divided into interrogation spot (for example, one image contains 128 x 128 or 64 x 64 square interrogation spots)

• If the image density (related to the number of particles inside an interrogation spot) is low -> PTV

• High image density -> PIV

Image displacement


If the image density is high the displacement of interrogation spot must bedetermined using some correlation method

Two different ways to obtain image displacement with high image density

1. Single-pulsed, double-frame where successive pictures are saved into twoframes

2. Single-frame, double-pulsed -> pictures into a single frame

• Image displacement with cross-correlation

• Image displacement with auto-correlation

Image displacement


Cross-correlation

Auto-correlation

With cross-correlation, only one displacement peak

Auto-correlation produces to mirror displacement peaks -> direction of image displacement must be known


Image displacement


Improved velocity calculations and Post processing


Besides individual square interrogation windows

• Overlapping windows

• Rectangular shape (128 x 64 pixels …)

Advanced interrogation methods

• Multipass: multiple image displacement calculations. Velocity obtained from the first pass used to offset the second image for successive passes

• Multigrid: same as multipass, but second pass done with smaller interrogation window (first pass: 128 x 128 window, second pass 64 x 64 window, etc.)

• Reduces in-plane particle displacements

Improved velocity calculations and Post processing


After a measurement, obtained velocity field contains invalid vectors

• Produced by correlation peaks that do not represent the vector displacement

• Noise level too high or problem with the images (glaring wall, etc.)

• Invalid vectors can be taken out using a post processing methods, such as median filters where each vectors is compared to its neighbors median value -> vector is removed if difference too large

Out-of-plane (vector moved away form the 2nd image to z direction) and in-plane (vector moved away form the 2nd image to x or y direction) particle displacement can cause invalid vectors

The amount of invalid vectors in PIV image < 10 %

PIV operation principle


Picture: Adrian, Westerweel: Particle Image velocimetryVelocity vector fiel on a plane perpendicular to the wall in a flat-plate turbulent Boundary layer

Stereographic PIV


With stereographic PIV (SPIV), all components of a velocity vector in a planar domain can be measured

By comparing the image displacement measurement of two cameras, out of plane velocity can be calculated


Tomographic PIV


Tomographic PIV can be used to measure 3-D velocity of a volume of fluid

In principle, tomographic PIV operational with two cameras

However, more cameras are recommended, typical setup has 4


Tomographic PIV


Position of particles in a measurement volume at a given time t determined by comparing the images of all cameras

After the two illuminations, the particle displacement is calculated using a 3-D correlation procedure, closely related to one used in planar 2-D PIV discussed earlier


Tomographic PIV



Micro-PIV


Used to measure small scale flows

High image magnification

Fluerescent particles with camera filter used toprevent overexposure

Low image density -> particle tracking needs tobe used

Small tracer particles effected by Brownianmotion-> particle track won’t represent flowaccurately

Correlation average where the mean velocity isobtained from a large number of PIV images(accurate result since Re is small) Picture: Adrian, Westerweel: Particle Image velocimetry

Summary


Particle image velocimetry used to measure velocity field of liquid orgas flow

Basic PIV system can measure two planar velocity components of aflow, with more advanced measurement setup also the out of planevelocity component can be measured

Laser is used to illuminate the measurement are of the flow

Two successive images of the flow are taken and the displacement ofparticle images are calculated using auto or cross-correlation

After the velocity calculation, invalid vectors are removed usingproper post processing tools

With more advanced PIV setups, velocity field on a volume or smallscale flow field can be measured using tomographic PIV or micro-PIV,respectively

particle image velocimetry_slides

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