Influence of Aspect Ratio and Flow Compressibility on Flow Dynamics in a Confined Cavity

TL;DR

This study uses Large-Eddy Simulations to analyze how aspect ratio and flow compressibility affect oscillations in a confined cavity, revealing the role of shock-induced Kelvin-Helmholtz instability in flow dynamics.

Contribution

It provides new insights into the impact of cavity geometry and Mach number on flow oscillations and instability mechanisms using advanced numerical simulations.

Findings

Shock impingement enhances Kelvin-Helmholtz instability.

Flow oscillation frequency is influenced by shock location and shear layer convection.

Flow structures and dominant frequencies vary with aspect ratio and Mach number.

Abstract

Cavities possess self-sustaining oscillations driven by the interaction of hydrodynamic and acoustic characteristics. These oscillations have applications in fuel-air mixing, heat exchangers, and landing gears, but resonance can damage the structures that house the cavities. Consequently, understanding cavity oscillations under varying geometries and flow conditions is essential for optimizing their benefits while minimizing the adverse effects. The present study investigates flow variations in a supersonic cavity confined by a top wall with a fixed deflection angle of $2.29^\circ$. We examine two aspect ratios of the cavity across freestream Mach numbers from 1.71 to 3 using Large-Eddy Simulations (LES) in OpenFOAM. Numerical Schlieren reveals the key flow structures, while spectral analysis and reduced-order modeling help identify dominant frequency modes and the corresponding flow structures. The results show that the shock from the deflection corner induces high gradients in the flow properties as it impinges on the shear layer. This amplifies the Kelvin-Helmholtz (KH) instability, which enhances mixing and mitigates the expected increase in oscillation frequency with Mach number. The KH instability develops spatially. Hence, the location of the shock impingement on the shear layer and the distance that the disturbances in the shear layer convect before reaching the cavity wall significantly influence the prominence of the instability, thereby influencing the frequency of cavity oscillations.