EFFECTS OF FREE-STREAM TURBULENCE ON SURFACE PRESSURE-FLUCTUATIONS INA SEPARATION BUBBLE

Wind-tunnel experiments were conducted to investigate the cause of lar ge pressure fluctuations near leading edges of sharp-edged bluff bodie s. Measurements obtained with a blunt flat plate showed that very low pressures occur in a narrow region located approximately 0.25X(R) from the leading edge, where X-R defines the distance from the leading edg e to the mean reattachment location. This phenomenon occurs in the und isturbed flow as well as turbulent flow, although the magnitude of pea k pressure fluctuations increases with both turbulence intensity, sigm a(u)/(u) over bar, and turbulence scale, L-X. Flow visualization exper iments conducted with a high-speed cine-camera reveal the process that causes large pressure fluctuations in separation bubbles. This proces s is initiated when a perturbation in the approaching flow causes a ro ll-up of the separated shear layer, producing a strong vortex near the surface. Conditional sampling of pressure data was used to determine the spanwise length of the vortex. A significant increase in the spanw ise correlation of pressure fluctuations occurs when the shear layer r olls up. Coherence measurements indicate that the spanwise length of v ortices in the separation bubble is not directly related to longitudin al velocity fluctuations in the free-stream.