Effect of a Downstream Ventilated Gas Cavity on the Spectrum of Turbulent Boundary Layer Wall Pressure Fluctuations

This paper theoretically and experimentally examines the effect of a downstream ventilated gas cavity on the spectrum of turbulent boundary layer wall pressure fluctuations. The theoretical model predicts that the ratio of the point spectrum of the turbulent boundary layer wall pressure fluctuations upstream of a ventilated gas cavity to the blocked point pressure spectrum decays rapidly to zero as the cavity origin is approached and undergoes oscillations in amplitude that relax to unity as the quantity ωx/U_c goes to infinity upstream of the cavity. Here ω is the radian frequency, x is the distance upstream from the cavity origin and U_c is the convection velocity. A water tunnel experiment was performed to investigate the theoretical predictions. Dynamic wall pressure sensors were mounted flush to the surface of a flat plate at various distances upstream from a rearward facing step. Carbon dioxide gas was injected into the separated flow region downstream of the step to form a ventilated cavity. The water tunnel measurements were unable to verify the reduction in the amplitude of the turbulent boundary layer wall pressure fluctuations as the step and cavity were approached but did verify the fundamental oscillation predicted by the theoretical model and its relaxation to unity as ωx/U_c went to infinity upstream of the step and cavity.