Measurements of Fluctuating Wall Pressure for Separated/Reattached Boundary Layer Flows

Measurements were made of the wall pressure field beneath separated/reattached boundary layer flows. These flows consisted of two types; flow over a forward-facing step and flow over a backward-facing step. Wall pressure fluctuations from an equilibrium flat plate boundary layer flow were also measured and used as a baseline for comparative purposes. Values of the RMS fluctuating pressure as well as the frequency spectral density, phase velocity, and coherence of the surface pressure field were measured at various locations upstream and downstream of the steps. The experimental results show that the separation-reattachment process produces large-amplitude, low-frequency pressure fluctuations. The measured spectral statistics of the wall pressure fluctuations are consistent with the view that at reattachment there exists a region of coherent highly energized velocity fluctuations located near the wall which, as it convects downstream, decays and diffuses away from the wall. This energized region remains identifiable in the wall pressure statistics as far as 72 step heights downstream of the backward-facing step.