High-speed flow over cavities in vehicle surfaces can produce intense tonal pressure fluctuations. This problem has been of concern for decades, especially for aircraft, but the underlying physical mechanisms have not been well understood. A fairly simple analytical model has been developed that improves understanding of the oscillation mechanisms in shallow rectangular cavities. The model describes the phenomenon, and gives reasonable agreement with experimental data. The analysis is constructed from waves that exist in a system having a finite thickness shear layer dividing two acoustic media, one at rest and one in motion. The wave types can be interpreted in physical terms. The shear layer thickness has an important effect on the speed of convective waves. Only two convective waves and three acoustic waves suffice to represent the basic phenomenon. Mass addition and removal at the rear bulkhead due to shear layer oscillation plays an important role in the process. Conditions on energetics and shear layer motion at the rear of the cavity must be satisfied simultaneously for the oscillation to occur.
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