Non-confined jet-slot-cavity oscillator: flow-acoustic interaction modeling

Disturbing whistling due to self-sustained oscillations can be produced in a wide variety of geometries where a sheared subsonic flow impinges on a downstream obstacle. When those self-sustained oscillations are coupled with an acoustic resonator, the acoustic production can reach dramatically high levels. The configuration known as jet-slot oscillator is studied in the particular case where the resonator is the flow-supply duct and the source region is unconfined. This sound production is due to flow-acoustic interactions and can be modeled by using the vortex-sound theory. It allows one to express the acoustic power generated by the interaction between the acoustic, velocity, and vorticity fields. They are obtained separately by using rather simple models together with experimental input data. The vorticity field is modeled with the assumption that the shear layer is rectilinear and the vorticity is only concentrated on discrete vortex-points convected at constant velocity. The vorticity being non null only in the shear layer, the velocity field is calculated only in this region where it is equal to the vortices convection speed. The source region dimensions being small compared to the acoustic wavelength, this field is considered, in harmonic regime, as a sinusoidal temporal fluctuation of a potential field. This field can be taken as resulting from the flow produced by the radiation of the flow supply duct's acoustic resonance. It is obtained by solving the Poisson equation in a simplified geometry obtained by Schwarz-Christoffel mapping. The configuration studied here presents some specific problems which must be addressed to correctly model its sound production. Experimentations are used to specify the missing parameters. The major one is the synchronisation between the vorticity field and the acoustic field which governs the way that the vortices interact with the resonant acoustic field. At last, comparison between experimental data and modeling results permits to validate the study, in terms of emitted frequency. Further developments of the model will allow to predict the emitted sound level.