STRUCTURE OF TURBULENT LEADING-EDGE SEPARATION BUBBLE OF A BLUNT CIRCULAR-CYLINDER AND ITS RESPONSE TO SINUSOIDAL DISTURBANCES

The leading edge separation bubble of a blunt circular cylinder was fo rced by a sinusoidally oscillating jet issued from a thin slit along t he separation edge. Reynolds number based on the main-flow velocity U- infinity and the diameter of the cylinder d was in a range of 4000- 95 00. The level of the forcing was defined at a position near the edge w here the time-mean velocity attained a maximum value. The time-mean an d r.m.s. velocities in the separation bubble were measured by a two-co lour four-beam Laser Doppler Velocimeter. A secondary separation zone was found in the separation bubble, and its length decreased with incr easing Reynolds number. The reattachment length of the forced separati on bubble decreased with decreasing forcing frequency in a range (2.5- 8.5)U-infinity /d, probably attaining a minimum at a smaller forcing f requency.