Unsteady Reynolds Averaged Navier Stokes (URANS) computations were performed to simulate flow past a rectangular cavity with free stream Mach number of 0.3. Deep cavity geometry was used with L/D ratio of 0.21. Frequency spectrum of the time accurate pressure data obtained using both URANS and LES simulations for closed cavity case. The dominant frequency of the resonance in the cavity was obtained as 16 kHz for the closed cavity case both with URANS and LES simulations. By injection of %1 of the main stream mass flow from the bottom of the cavity for the same free stream Mach number, dominant frequency reduced to the 8 kHz. The unsteady flow structures and effect of injection on the cavity flow instability mechanism were analyzed in detail in terms of flow physics. It was shown that injection effects vortex-acoustic feedback loop, by lifting up the vortices near the trailing edge of the cavity. Moreover, with the effect of injection, pressure fluctuations increased from 280 Pa to the 650 Pa.
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