Image processing instrumentation of flow by using correlation technique. Application to an unsteady flow.

In this study, each tracer particle is not tracked, but an image of the tracer particles is grasped as a spatial gray level pattern. To obtain the velocity vectors from the spatial gray level patterns, the following procedures are needed. First, two tracer digital images are detected at a constant time interval. Second, each of the two images is divided into many pixel matrices which include the overlapping ones. Third, the crosscorrelations of gray level between the respective pixel matrices of the two images are calculated. Forth, the traveling distance and direction of a group of the tracer particles are determined from the maximum value of the correlation. Finally, the velocity vector distributions are obtained. This method was applied to the two-dimensional water flow behind a circular cylinder. As a result, the movement and development of the vortices in the flow direction was clearly observed.