Experimental Investigation of Cavity-Induced Acoustic Oscillations in Confined Supersonic Flow

The focus of the present work is the acoustic oscillations exhibited by confined supersonic flow past a rectangular cavity of varying length-to-depth (L∕D) ratio, with a view to identify optimal dimensions for application in scramjet combustors. Experiments were conducted to study the acoustic oscillations induced by supersonic flow at a Mach number of 1.5 past a rectangular cavity of variable dimensions mounted on one wall of a rectangular duct. The effect of L∕D ratio of the cavity on the dominant acoustic modes registered on the wall of the duct opposite to the cavity is investigated. The range of L∕D ratio varied is 0.25–6.25. The dominant acoustic modes and the amplitudes are observed to be quite sensitive to L∕D ratio in the above range. Shifts in the dominant acoustic modes are observed predominantly for L∕D≈0.94 and L∕D≈1.5. The variation of the Strouhal number with L∕D ratio indicates a transition in the modal content in the 0.94 1. Such distinct streamwise oscillations are also observed for cavities with L∕D<1, when the length is appreciable. The presence of higher modes in the acoustic oscillations is correlated with shocks produced at the lip of the cavity at a different frequency than the compression waves inside the cavity.