INSTANTANEOUS STRUCTURE OF THE NEAR-WAKE OF A CIRCULAR-CYLINDER - ON THE EFFECT OF REYNOLDS-NUMBER

The near-wake structure of a cylinder is examined for Reynolds numbers from 10(3) to 10(4), with the intent of providing insight into the large changes in mean base pressure and fluctuating life observed in previous investigations. The use of high-image-density particle image velocimetry allows the generation of instantaneous and averaged representations of velocity fields, streamline topology, and vorticity distributions. Modes of large-scale (Karman) vortex formation, in conjunction with patterns of small-scale (Kelvin-Helmoltz) vortical structures, allow physical interpretation of the vortex formation length, which is the traditional criterion for linking the flow pattern to the loading coefficients.