Abstract
Three-dimensional characteristics of the coherent structure behind a three-dimensional (hemisphere) or a two-dimensional (hemicylinder) obstacle on a flat plate were studied using flow visualization and image processing techniques. The three-dimensional time-averaged velocity field inside the separation zone behind a hemisphere and the turbulence structure of the vortices shed to the downstream were elucidated in detail. It is seen from the similarity and difference in the expects of the vortex shedding behind a hemicylinder and behind a hemisphere found experimentally, that a strong upward current occurs after the coalescence of shed vortices behind a three-dimensional obstacle and even behind a two-dimensional one. The difference in the generation mechanism of the upward current before and after the coalescence was explained in terms of the variation of the instantaneous Reynolds stress field or the fluctuation of the reattachment point.
