Microphone-Array Measurements of Acoustic and Hydrodynamic Wall-Pressure Fluctuations in a Low-Speed Cavity Flow

The unsteady wall-pressure field on the floor of a rectangular shallow (long) cavity has been experimentally investigated at a low Mach number of 0.086. The pressure measurements were conducted using an array of 16 microphones arranged in line along the centerline of the cavity and spanning its full length. Data were acquired for two cavity configurations with length-to-depth, L/D, ratio of 5 and 8. Results show that for L/D = 5, the pressure oscillations are driven by the wake mode, while for L/D = 8, Rossiter-type modes are observed. The latter is different from the traditional Rossiter-type resonance in short cavities, which is associated with the Kelvin-Helmholtz shear-layer modes. This new long-cavity Rossiter-type resonance is associated with the large-scale structure near the end of the reattachment zone on the cavity floor, and hence is referred to as back-step resonance. The ability to contrast the wake and Rossiter type oscillations here was made possible through the unique wall-pressure wavenumber-frequency signature of each of the modes.

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