抄録
The measurement of flow velocity vector distribution in two-dimensional flow fields has been progressed by the image processing technique. In this technique, a tracer method has been usually adopted for flow visualization and also the velocity vectors have been calculated from the tracer paths extracted during a constant period. The serious problem of this technique, however, is how to separate the overlapped tracer paths.
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 procedure is 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 cross-correlations 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 following facts were specifically confirmed:
1) The velocity vector distribution over the entire flow field was obtained without interpolation.
2) The changes of velocity vectors were clearly observed in the vortex fields.
3) The movement and development of the vortices in the flow direction were clearly observed.
