Effects of free-stream turbulence on surface pressure fluctuations in a separation bubble

Wind-tunnel experiments were conducted to investigate the cause of large pressure fluctuations near leading edges of sharp-edged bluff bodies. 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 edge to the mean reattachment location. This phenomenon occurs in the undisturbed flow as well as turbulent flow, although the magnitude of peak pressure fluctuations increases with both turbulence intensity, sigma(u)/(u) over bar, and turbulence scale, L-X. Flow visualization experiments conducted with a high-speed cine-camera reveal the process that causes large pressure fluctuations in separation bubbles. This process is initiated when a perturbation in the approaching flow causes a roll-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 spanwise correlation of pressure fluctuations occurs when the shear layer rolls up. Coherence measurements indicate that the spanwise length of vortices in the separation bubble is not directly related to longitudinal velocity fluctuations in the free-stream.