Experimental and modal decomposition studies on cavities in supersonic flow

Abstract

The supersonic flow past cavities which incorporate sub-cavities is experimentally investigated in detail. The study is carried out at Mach 1.71 for an open cavity having a length to depth ratio of 2. The sub-cavities at the front and aft wall are considered to regulate the oscillations within the cavity. Unsteady pressure measurements and high-speed Schlieren images are utilized. Fast Fourier Transform (FFT) on the unsteady pressure data and Proper Orthogonal Decomposition (POD) analysis on the Schlieren images are employed to derive further physical insights on the cavity flow physics. The POD modes and their corresponding Power Spectral Density (PSD) plots were analysed for various subcavity configurations and the respective flow features. Front wall subcavity configuration is found to be the best passive suppression device. An analysis was carried out to compare the PSD of the unsteady pressure data and the PSD of the POD modes obtained from the Schlieren images. The close relation between the frequencies obtained from power spectral density (PSD) of pressure measurements and those obtained from the PSD of POD of the Schlieren images indicated the global instability associated with the respective flow feature with different pressure frequency modes; shear layer impingement and shear oscillation was found to occur at around 5000 Hz whereas shear layer spanning the cavity were occurring at around 10,000 Hz.