Digital image plane holography as a three-dimensional flow velocimetry technique

Abstract

This paper shows the feasibility of applying digital image plane holography (DIPH) as a fluid velocimetry technique for simultaneous measurements of all three components of the velocity field. As a first approach DIPH has been set up to measure a single fluid plane. The recording apparatus is a digital speckle interferometer (DSPI) with spatial phase shifting (SPS). The speckle interferometer has an out-of-plane sensitivity and the off-axis reference beam produces a spatial modulation in the pattern (hologram) recorded by the CCD camera. From the interferometric and photographic analysis of the reconstructed object wave, the three velocity components in the fluid plane are obtained. The complex amplitude of the object wave is calculated by the application of a Fourier-transform method to the hologram. The phase change between two subsequent frames yields an out-of-plane component of the velocity field. The two in-plane components are obtained, as in digital speckle photography, by cross correlation of the reconstructed object wave’s intensity. Some quantitative results in a Rayleigh-Benard convective flow are presented. In the final setup, angular multiplexing with coherence length control has been introduced in order to simultaneously measure the velocity fields in two fluid planes. Some preliminary results from the convective flow are presented.