Mechanism of noise suppression due to subcavity in turbulent supersonic cavity flow

The perturbation of the shear layer at the leading edge of a cavity flow due to an upstream traveling acoustic wave (generated at the trailing edge) causes self-sustained flow oscillations and generates noise. Typically, the noise reduction in a turbulent supersonic cavity flow is achieved by disrupting the feedback loop of disturbances. One of the passive control techniques for noise suppression involves construction of a subcavity at the leading edge of the cavity. In this work, we investigate the mechanism of this passive control technique through Large Eddy Simulation (LES) of the flow over a cavity at Mach 1.71 and length (L) to depth (D) ratio of 2. A preliminary investigation by placing a subcavity of length (l) = 0.2L reveals that it suppresses the perturbation of the shear layer from upstream-propagating acoustic waves. This is achieved through the interaction between the reflected acoustic wave from the subcavity and the upstream-propagating acoustic wave. From LES simulation, the wavelet coherence of the probed pressure signals at upstream and downstream locations about the wave-interaction point reveals a destructive interference between the two waves and supports above hypothesis. Moreover, the same subcavity, when placed at the trailing edge, resulted in a constructive interference. The present study will be extended to more length ratios (l/L), and the effect of acoustic waves interaction on the attenuation of shear layer oscillations will be closely examined.