ATTENUATION OF CAVITY FLOW OSCILLATION THROUGH LEADING EDGE FLOW CONTROL

Abstract

The unsteady flows over a shallow rectangular cavity at Mach 1·5 and 2·5 are modified at the leading edge by using compression ramps, expansion surfaces, and mass injection. The study is performed through solutions of Short-time Reynolds-Averaged Navier-Stokes equations (TRANS) with turbulence modelled by a two-equation k–ω model. When a compression ramp is introduced, two types of responses are observed: At Mach 1·5, a strong flapping motion leads to small changes in the frequency and sound pressure level in the cavity compared with the baseline case of rectangular geometry. The roll-up of the shear layer produces convective vortices, leading to enhanced pressure fluctuations on the downstream surface; At Mach 2·5, a weak shear layer instability produces a reduction in the sound pressure level, and the increased distance between the leading edge and the trailing edge produces a reduction in frequency. An increase in the mean pressure drag coefficient is produced due to the high pressure on the ramp. When an expansion surface is employed, the mean pressure drag coefficient is also increased slightly. When the flow is attached to the surface, the major flow physics are similar to the baseline case. A reduction of the sound pressure level is observed in the cavity with the surface height. When a shock induced separation occurs on the surface, a steady flow is established in the cavity. When the mass injection is introduced, a passive pressure response is observed at the leading edge, producing local vorticity and vortex shedding. The flow mechanism remains the same at both Mach numbers, with a weak sitting vortex near the rear corner. An optimal mass injection pressure ratio is identified.