The mechanisms for passive suppression of fluctuating surface pressure in a supersonic cavity flow

The study of the passive suppression of supersonic cavity flow using a rod immersed in the upstream boundary layer is a unique and challenging fluid mechanics problem. The flowfield includes a compressible shear layer interacting with a complex pattern of compression and expansion waves. The turbulent fluctuations inside the shear layer may be amplified through a feedback-receptivity cycle resulting in increased pressure loading on the surfaces of the cavity. Studying the mechanisms dictating the suppression of these amplified turbulent fluctuations when control is present makes for an enlightening and challenging problem. A combined experimental and time accurate numerical study using detached-eddy simulation was conducted to study the suppression of pressure fluctuations due to supersonic cavity flow at Minfinity = 1.4 over an open rectangular cavity with a length-to-depth ratio of six. In this study, the focus is confined to suppression due to a rod spoiler. The experimental measurements included temporally resolved fluctuating surface pressure measurements coupled with spatially resolved particle image velocimetry.;Analysis of the fluctuating pressures on the cavity surfaces included investigations of the root-mean-square fluctuating pressure, spectral analysis, correlation and coherence analysis and joint time-frequency spectrograms. The shear layer flowfield and turbulence was studied using ensemble averaged turbulent statistics including two-point spatial turbulent velocity correlations and Proper Orthogonal Decomposition. Results indicate that the most effective suppression of the fluctuating pressures was achieved when a rod sized roughly 40% of the boundary layer was placed such that the top of the rod was near the top edge of the boundary layer. It was shown that the rod leads to a thicker shear layer that initially spreads more rapidly. The turbulent structures in the wake of the rod interact with the cavity shear layer with a time periodic excitation which lifts the shear layer near the cavity leading edge. The structures are smaller and less organized which is believed to lead to a shear layer that is less receptive to the disturbances propagating upstream inside the cavity. The controlled cavity exhibits an altered aft wall impingement point which is due to the lifting and altered flapping nature of the shear layer. The upstream propagating disturbance emanating from the aft wall is thus weakened due in part to the lower speed flow impinging on the aft wall. These coupled events lead to drastically reduced tonal components (which are lowered to near broadband levels) and notable lowering of the broadband levels of the fluctuating pressure measured on the cavity surfaces.