PARTICLE IMAGE VELOCIMETRY

Subject : Fluid MechanicsSemester : IV semesterSupervisor : Dr. V. K. PatelPresenter : Raj Zunke Hemant Rawat Amar Prem

What we are going to discuss ?

• What is PIV AND its brief history• Principle of working• Construction of experimental setup• Components and their description• Post processing of data• Other PIV techniques • Advantages and Disadvantages• Application

It is a double-pulsed laser technique used to measure instantaneous velocity distribution in a plane of flow by determining the displacement of particles in that plane during a short time interval.

HISTORICAL ASPECTS

• Origins: Flow visualizations• 1970’s: Laser Speckle Velocimetry• 1980’s: LSV,PTV, PIV, • Further development of LASER , CCD cameras , Computers lead to the

development of PIV • Computers First scientific paper on PIV was published in 1984 • First commercial PIV systems came in 1988 .

Experimental setup of the PIV

Physical representation of PIV

• The PIV technique for velocity measurement is based on flow visualization and image processing .

• The first step is to trace the flow with suitable seed particles in order to obtain the path lines of fluid motion.

• A pulse of laser light illuminates certain region of flow field at any desired plane and the photographic view is recorded digitally by using a video camera positioned at right angle to the plane.

• After a short interval of time , the particles are illuminated again through the laser light and the new positions are recorded.

• Using the information of both the images, the particle displacement is determined and subsequently the magnitude of velocity of the particle in the plane is calculated.

• Acceleration of particle = Viscous resistance +Acceleration due to Pressure gradient + Resistance of inviscid fluid due to acceleration of sphere + Resistance due to flow field unsteadiness

• Body force and centrifugal force (swirling flow) can also be other important forces

• Response of the velocity field to different velocity Fluctuations provides the criteria on ideal particle tracking .

COMPONENTSSEEDING PARTICLES- These are an inherently critical component of the PIV system.

Depending on the fluid under investigation, the particles must be able to match the fluid properties reasonably well. Otherwise they will not follow the flow satisfactorily enough for the PIV analysis to be considered accurate.

Refractive index for the seeding particles should be different from the fluid which they are seeding.

The particles are typically of a diameter in the order of 10 to 100 micrometres.

Silicon carbide, titanium dioxide and polystyrene latex particles are few categories of seed particles that are used depending on the type of fluid (liquid/gas).

CAMERATo perform PIV analysis on the flow, two

exposures of laser light are required upon the camera from the flow. 

 Originally, with the inability of cameras to capture multiple frames at high speeds, both exposures were captured on the same frame and this single frame was used to determine the flow.

LASER AND OPTICS For macro PIV setups, lasers are predominant due to

their ability to produce high-power light beams with short pulse durations.

The optics consist of a spherical lens and cylindrical lens combination. The cylindrical lens expands the laser into a plane while the spherical lens compresses the plane into a thin sheet.

The correct lens for the camera should also be selected to properly focus on and visualize the particles within the investigation area.

Laser Light: Almost constant thickness and without any aberration

Continuous Laser: He - Neon, Argon Laser

SYNCHRONIZERThe synchronizer acts as an external trigger

for both the cameras and the laser.

 Controlled by a computer, the synchronizer can dictate the timing of each frame of the CCD camera's sequence in conjunction with the firing of the laser to within 1 ns precision. 

POST PROCESSING

Spurious vectors !!!!!• -Low particles density • -Inhomogeneous particles seeding • -Particles within a vortex • -Low S/N • - 3D movement of the particles

Why the spurious vectors haveto be eliminated ?

Induce errors in velocity derivatives. Spurious vector

Other PIV techniquesMicro PIVParticles size: from nanometers to several microns.• The particles should be large

enough to dampen the effects of Brownian motion:

• Brownian motion results from the interaction between the particles. This prevents the particles to follow the flow.

The relative error in the measured particle displacement is:

tD

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21

Advantages of PIV1-Entire velocity field can be calculated.2-Capability of measuring flows in 3-D space3-Generally, the equipment is nonintrusive to flow4-High degree of accuracy

Disadvantages of PIV

1-Requires proper selection of particles2-Size of flow structures are limited by resolution of image3-Costly

ApplicationsPIV has been applied to a wide range of flow problems, varying from the flow over an aircraft wing in a wind tunnel to vortex formation in prosthetic heart valves. 3-Dimensional techniques have been sought to analyse turbulent flow and jets.

Rudimentary PIV algorithms based on cross-correlation can be implemented in a matter of hours, while more sophisticated algorithms may require a significant investment of time. Several open source implementations are available. Application of PIV in the US education system has been limited due to high price and safety concerns of industrial research grade PIV systems.