A short explanation about Particle Image Velocimetry
PIV is an experimental technique to analyze images for obtaining a
velocity field from a fluid flow (gas or liquid) that has been
seeded with small tracer particles or smoke. Though, it also may be
applied for transportation and deformation analyses of (solid)
surfaces. It correlates small samples at identic location from one
or two images, mostly by using FFT techniques. This results into an
estimation of the mean displacement of the tracer particles that
are lying within the sample area of the image. As the image
magnification factor and the timing between the image recordings
are known, these displacament estimators may directly be
transformed to an estimator of the local velocity. A set of image
samples (also called interrogation area's) that are positioned on a
(rectangular) grid, then, result into a velocity
field of
the fluid flow. In this way the coherent structures of a
(turbulent) flow may be observed quantitatively. A more extended
explanation about PIV and related techniques with references can be
found at
Wikipedia.
Explanation of PIV