Optical evaluation methods in particle image velocimetry

Application of particle image velocimetry (PIV) techniques for measurement of fluid velocities typically requires two steps. The first of these is the photography step, in which two exposures of a particle field, displaced between the exposures, are taken. The second step is the evaluation of the double-exposure particle pattern and production of appropriate particle velocities. Each of these steps involves optimization, which is usually specific to the experiment being conducted, and there is significant interaction between photographic parameters and evaluation characteristics. This paper will focus on the latter step, that of evaluation of the double-exposure photograph. In several parts of a PIV system, some performance advantage may be obtained by increasing use of optical processing over conventional digital image processing. Among the processes for which a performance advantage may be obtained are parallel or multiplex image interrogation and the evaluation of the Young's fringe pattern obtained from the scattered pattern from the double-exposure photograph. This paper will discuss parallel image interrogation and compare the performance of optical and numerical Fourier transform analysis of Young's fringes using speckle images.