Oscillatory characteristics of cavities in supersonic flow

Abstract

We examine the flow structures, acoustic emissions, and oscillation types for supersonic flow past a baseline cavity and cavity with a bump. To study these, time-resolved shadowgraph images and unsteady pressure measurements are used. According to the results, the baseline cavity oscillates at various dominant frequencies. By adding a bump inside the cavity, many of its dominant oscillations are inactivated which are also corroborated by cross-correlation. Using time-resolved images, we can show the differences in wave propagation mechanisms between the two cavities. Through pressure spectra, it is demonstrated that the baseline cavity displays both fluid-dynamic and fluid-resonant types of oscillations, while the cavity with bump undergoes only the fluid-resonant type. The Rossiter prediction for the baseline cavity is very accurate but greatly deviates when applied to the cavity with a bump. As well, the wavelet analysis reveals that only the baseline cavity undergoes mode-switching phenomena. Additionally, the shear layer large-scale structures influencing the cavity oscillations are identified by using proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) techniques. Furthermore, an analogy is established between POD and SPOD mode shapes along with the shadowgraph images for dominant modes of cavity oscillation. Finally, the results demonstrate that these modal decomposition techniques complement each other.