Optical analysis of particle image velocimetry data

Full field velocity data can be of significant value for transient fluid flows. A recently developed technique utilizing double exposure photographs of particles seeded into a flow can provide this kind of data. Analysis of the resulting particle seed photographs is a problem due to the number of particles and spatial resolution requirements of the measurement. A current technique for analyzing the particle photographs is based on a point by point illumination of the particle image, and two subsequent 2-D fast Fourier transforms (FFT) to produce an autocorrelation pattern of the particles. The particle displacement can be determined from the autocorrelation pattern. The digital 2-D FFT of image data may take a considerable amount of computer time depending on the computer involved. We present a technique for performing the equivalent of the Fourier transform optically, producing an autocorrelation function from an input function. Preliminary results are presented as well as a discussion on possible system improvements and system potential.