Optical processing of particle tracking velocimetry under deformeddouble exposure

Abstract

A new technique which can quantitatively visualize the spatialdistribution of velocity in flows using optical processing of particletracking velocimetry (PTV) under deformed double exposure has been developed.Tracer particles seeded in the flow are irradiated by a sheet of light from anargon-ion laser under deformed double exposure. Under deformed doubleexposure, each locus of the particles displaced in the flow visualizes thedirection and the magnitude of the velocity. The loci of displaced particlesshow the spatial distribution of the flow velocity in a single image frame.The image is taken continuously in real time by a CCD camera. The deformeddouble exposure has been synchronized with the scanning frame of the CCDcamera. The spatial distributions of the loci of the particles have beendiscriminated at certain magnitudes and directions simultaneously andinstantaneously by a multiplexed matched spatial filter (MMSF). The MMSF is aholographic filter which can discriminate the shapes of the loci using opticalcross-correlation. Cross-correlation of the shapes has been calculatedoptically and the correlation peaks which are the discriminated signal appearskew-symmetrically relative to the position of the shapes in eachdiscrimination area. Using this technique, the number and the position ofloci, i.e. spatial distributions of velocity in the flow, are visualized. TheMMSF technique has been tested for synthetic images and experimental images.The synthetic images have the overlapped and deformed loci of particles inflow. The experimental images are loci of the cavity flow with a high Reynoldsnumber. From the results, it has been demonstrated that PTV images which havehigh density tracer particles and deformed streaks can be measured by thistechnique.