Particle image velocimetry

The method relies on the fact that small particles introduced in a fluid stream would move with the local fluid velocity. These particles, ideally, are neutrally buoyant with respect to the fluid medium and would not respond to buoyancy forces. This is particulalrly true for particles of very small diameters where surface forces (that scale with the square of the particle diameter) are much larger than body forces (that scale as diameter cube).

The basic measurements in particle image velocimetry (PIV) relate particle displacement over a time period in such a way that velocity is measured as the ratio of displacement and the time interval. The former being a vector, velocity components in the plane of illumination are jointly determined.

Since particle sizes are very small, a small interrogation area selected by the camera for determination for velocity would have several particles. These, in turn, are indistinguishable. The displacement measured is a statistical quantity, applicable for the collection of particles as a whole. The local velocity thus obtained is a group velocity of these particles. Statistical methods preferred are usually based on cross-correlation between a pair of images that are separated by a time interval of . Clearly, smaller the time interval, better is the time estimate of velocity. Intervals as small as a few hundred nanoseconds are possible with pulsed lasers; conventional light sources severely fail in this regard. It should also be clear that the cameras used for imaging should record two images of the particle positions separated on the time axis by such a small interval.

The important components of a PIV system would then be (a) a pulsed light source, (b) an imaging system synchronized with the laser, (c) seeding arrangement for creating particles, and (d) software for calculating the cross-correlation function between the image pairs.

The details of a simple PIV system are presented the following sections.