This paper discusses the phenomena of flow-acoustic coupling between an unstable shear layer across a slot covering a cavity, thereby forming a Helmholtz resonator. The flow was caused by a turbulent boundary layer flowing over the slot. The physical phenomena implicated were responsible for the generation of high-amplitude sound level that occurred at the resonant frequencies of the system. It concerns a wide variety of applications, especially in the transport industry. Cavity oscillations have been studied steadily for some 50years and there is by now a vast and conflicting literature on the measured tonal frequencies, the physical explanations, and the predictive models for them. A theory was presented to predict the onset of the instability. It was based on the identification of the main parameters that played a role in the shear layer. Particularly it was suggested to use the frequency dependent phase velocity instead of the flow velocity. The theory was then confronted with results obtained from an experimental study carried out in a wind tunnel on several models. This validation underlined the sturdiness and the efficiency of the model, which made it useful for engineering applications.

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