Particle Image Velocimetry measurement principles

Introduction

Particle Image Velocimetry (PIV) is a whole-flow-field technique providing instantaneous velocity vector measurements in a cross-section of a flow. Two velocity components are measured, but use of a stereoscopic approach permits all three velocity components to be recorded, resulting in instantaneous 3D velocity vectors for the whole area. The use of modern digital cameras and dedicated computing hardware, results in real-time velocity maps.

Features

• The technique is non-intrusive and measures the velocities of micron-sized particles following the flow.

• Velocity range from zero to supersonic.

• Instantaneous velocity vector maps in a cross-section of the flow.

• All three components may be obtained with the use of a stereoscopic arrangement.

• With sequences of velocity vector maps, statistics, spatial correlations and other relevant data are available.

Results are similar to computational fluid dynamics, i.e. large eddy simulations, and real-time velocity maps are an invaluable tool for fluid dynamics researchers.

Principles

In PIV, the velocity vectors are derived from sub-sections of the target area of the particle-seeded flow by measuring the movement of particles between two light pulses:

The flow is illuminated in the target area with a light sheet.
The camera lens images the target area onto the sensor array of a digital camera. The camera is able to capture each light pulse in separate image frames.

Once a sequence of two light pulses is recorded, the images are divided into small subsections called interrogation areas (IA). The interrogation areas from each image frame, I₁ and I₂, are cross-correlated with each other, pixel by pixel.

The correlation produces a signal peak, identifying the common particle displacement, DX. An accurate measure of the displacement - and thus also the velocity - is achieved with sub-pixel interpolation.

A velocity vector map over the whole target area is obtained by repeating the cross-correlation for each interrogation area over the two image frames captured by the camera.

PIV images are visual, just follow the seeding

Recording both light pulses in the same image frame to track the movements of the particles gives a clear visual sense of the flow structu