Particle Image Velocimetry Demo Outline (For reference) ‏ Topic NumberTopic NamePage Type 1Flow of PIVAnimated page

Particle Image Velocimetry The Particle Imaging Velocimetry (PIV) is a quantitative velocity measurement technique, with visualizing the flow field by small tracer particles and with analyzing the visualized digital images. In this topic discusses about Direction of fluid flow, direction of light travel and randomly selected image patterns. Particle Image Velocimetry

START Laser Lasers are predominant due to their ability to produce high-power light beams with short pulse durations. This yields short exposure times for each frame. Click START. Flow of Particle Image Velocimetry

Particle Image Velocimetry START Optical Equipment 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. Click START. Flow of Particle Image Velocimetry

Particle Image Velocimetry START Seeding The seeding particles are an inherently critical and reflective component of the PIV system. So that the laser sheet incident on the fluid flow will reflect off of the particles and be scattered towards the camera. Click START. Flow of Particle Image Velocimetry

Particle Image Velocimetry START Camera To perform PIV analysis on the flow, two exposures of laser light are required upon the camera from the flow. Click START. Flow of Particle Image Velocimetry

Particle Image Velocimetry PIV overview

Particle Image Velocimetry To compute particle displacements, the image plane is divided into several small disjoint or overlapping interrogation windows and corresponding window pairs in consecutive recordings are cross correlated. The spatial displacements that produces the maximum cross-correlation statistically approximates the average displacement of the particle in interrogation region. This displacement divided by the time between the lazer pulses, yields the velocity that is associated with each interrogation area. The cross corelation function for two discretely sampled interrogation windows is defined by :

Particle Image Velocimetry With f i,j and g i,j indicating image density distribution of the first and second image M and n are pixel offset distances between the two images. C f,g (m,n) is the Two dimensional cross corelation function. Given the size of the interrogation area is M, then O(M 4 ) operations have to be computed. The cross corelation between the two image pairs are normalized to prevent false correlation peaks arising from changes in the search area local means. To remove additive local bias differences and other noises the mean is removed from both interrogation windows and each correlation sample is divided by

Particle Image Velocimetry Where μ F and μ G are the means of the first and second interrogation windows. The wienier khinchin thoerm states that cross corelation between f*g between the two signals can be computed in the frequency domain as Where F f, and F* g are fourier transform of first interrogation window and complex conjugate of fourier transform of second interrogation window. F -1 represent inverse fourier transform. The overall complexity of the system now reduces to O[M 2 ln(M)] operations.

Particle Image Velocimetry In signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them. This is also known as a sliding dot product or inner-product. Cross correlation of Images Click the Quiz button Particle Image field-1 Particle Image field-2 Displacemen t vector

Particle Image Velocimetry

Errors in PIV Random Errors : Wich occur due to noise in the recoreded images Bias errors: Arising from the process of computing signal peak location to sub pixel accuracy. Gradient Error: arising due to rotation and deformation of flow within an interrogation spot leading to loss of correlation. Tracking errors :Arising due to Inability of the particle to follow the flowwithout slip. Accelerating Errors:occuring caused due to approximating the local eulerian velocity from the lagrangian motion of the tracer particles

Particle Image Velocimetry Which of these is/are used as seeding material ? Glass particles PVC Mica Fluid itself Particle Image Velocimetry

The PIV technique provides ________ ? Measurement of average velocity with high accuracy Measurement of Instantaneous velocity with high accuracy Measurement of instantaneous velocity with low accuracy. Measurement of overall velocity with low accuracy. Particle Image Velocimetry

The order of time difference between two image snaps by the CCD Camera is? 1 second 1 millisecond 1 Microsecond 1 nanosecond Particle Image Velocimetry

Which controls time between first Image & Second image? Laser Synchronizer Seeding material all Particle Image Velocimetry

The velocity of fluid can be measured accurately by ? o o Flow meter o Venturimeter o Prandtl pitot tube o Particle Image velocimetry

Particle Image Velocimetry ci_2000.pdf