Physics of Resonance in a Supersonic Forward-Facing Cavity

An experimental study of a blunt nose with a forward-facing, cylindrical cavity was conducted in a Mach 4, quiet-flow wind tunnel. The length of the cavity was varied, whereas the diameter was fixed. A fast-response pressure transducer was located at the cavity base. Under conventional (noisy) wind-tunnel conditions, the rms pressure fluctuations were nearly 0.4 percent of the mean pressure. Under quiet-flow conditions, the rms fluctuations were on the order of 0.04 percent of the mean. A laser-perturbation system created controlled, repeatable, and localized disturbances upstream of the bow shock. These disturbances convected with the flow, impinged on the nose, and caused damped cavity oscillations. As the cavity length was increased, the resonant frequencies and the damping both decreased. The small damped oscillations present under quiet-flow conditions seem to explain the low heat transfer rates measured previously in flight. The large fluctuation levels observed in conventional wind tunnels are an artifact of the high levels of noise present there and should not be expected under typical flight conditions. (Author)