Particle image velocimetry: improving fringe signal-to-noise ratio with a two-step photographic process

Abstract

In using particle image velocimetry to obtain quantitative velocity measurements in fluid flows, a double-exposure photograph of the seeded flow is analyzed optically to produce correlation fringes. Ultimately, it is the signal-to-noise ratio (SNR) of these fringe patterns that determines measurement accuracy. It is shown both experimentally and theoretically that the fringe SNR is significantly improved by using two-step processing. The original double-exposure negative transparency is copied onto a negative film to produce a positive transparency of the original recording. It is this positive transparency that is subsequently analyzed to produce fringes with increased SNR.