Wind-tunnel experiments were conducted to investigate the cause
fluctuations near leading edges of sharp-edged bluff bodies. Measurements
with a blunt flat plate showed that very low pressures occur in a narrow
approximately 0.25XR from the leading edge,
XR defines the distance from the
leading edge to the mean reattachment location. This phenomenon occurs
undisturbed flow as well as turbulent flow, although the magnitude of peak
fluctuations increases with both turbulence intensity,
and turbulence scale, LX.
Flow visualization experiments conducted with a high-speed cine-camera
process that causes large pressure fluctuations in separation bubbles.
initiated when a perturbation in the approaching flow causes a roll-up
shear layer, producing a strong vortex near the surface. Conditional sampling
pressure data was used to determine the spanwise length of the vortex.
increase in the spanwise correlation of pressure fluctuations occurs when
layer rolls up. Coherence measurements indicate that the spanwise length
in the separation bubble is not directly related to longitudinal velocity
fluctuations in the free-stream.