An experimental study was conducted to investigate the physics underlying the oscillation cycle of high-Mach-number, turbulent, open-cavity flows. The specific aim was to investigate how the interaction between the cavity acoustics and the shear-layer dynamics is affected by increasing Mach number. In the experiments the freestream Mach numbers tested were 2 and 5, the cavity length-to-depth ratio was 6, and the primary measurements made were of fluctuating surface pressures. The shear-layer-acoustics coupling was investigated by testing the cavity both with and without a plate covering over 80% of the cavity. The cover plate isolated the cavity from the free shear layer above it. In the Mach 2 cavity flow, the uncovered-cavity resonance frequencies agree well with those predicted by Rossiter's model. On the other hand, the resonance frequencies measured in the covered cavity do not agree with Rossiter's model but instead are consistent with a model that is based on closed-box acoustics. At Mach 5, both the uncovered and covered cavities have resonance frequencies that agree equally well with both Rossiter's model and closed-box acoustics. The success of a pure acoustics model suggests that the coupling between the shear-layer dynamics and the cavity acoustics is greatly reduced at high Mach numbers. Based on a consideration of the physical mechanisms implicit in Rossiter's model, it is argued that its successful prediction of the resonance frequencies in high-Mach-number cavity flows is largely coincidental and likely does not reflect the correct modeling of the flow physics.
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