The effect of Mach number on open cavity flows with thick or thin incoming boundary layers

TL;DR

This study investigates how Mach number influences the stability and flow dynamics of open cavity flows with different boundary layer thicknesses, revealing complex interactions between compressibility, Rossiter modes, and nonlinear effects.

Contribution

It provides a comprehensive analysis combining global linear stability and nonlinear simulations to understand Mach effects on cavity flow instabilities, highlighting the role of acoustic feedback and mode interactions.

Findings

Compressibility destabilizes low Mach flows significantly.

Higher order Rossiter modes become unstable with thin boundary layers.

Nonlinear simulations show dominant frequencies align with R1 mode and empirical predictions.

Abstract

The Rossiter modes of an open cavity were studied using global linear analysis and nonlinear numerical simulations. The length over depth ratio was two and the Reynolds numbers based on cavity depth were close to 1000. The effect of Mach on such cavities was studied. The global analysis revealed that, in the Mach range 0.1 to 0.9, for thick boundary layers, only R1 and R2 modes could become unstable, whereas for thin boundary layer up to R4 could be unstable. Compressibility was very destabilizing at low Mach. At moderate Mach the instability either saturated with Mach or had an irregular dependence. Analysis of the Rossiter mode eigenfunctions indicated that the acoustic feedback scaled to Ma^3, and explained the strong destabilizing effect of compressibility. The irregular dependence was associated with resonances between Rossiter modes and acoustic cavity modes. The analysis explained why the irregular Mach dependence occurred only for higher order Rossiter modes. In this parameter region three dimensional modes are stable or marginally unstable. Two-dimensional simulations were performed to evaluate how much of the nonlinear regime could be captured by the linear stability results. As the flow became more unstable, an increasing more complex final stage was reached. Yet the spectra present distinct tones that are close to linear predictions. However, a closer look reveals the dominant frequency in the saturated state corresponds to R1 while linear theory predicts R2 as the most unstable mode. This is associated with a nonlinear thickening of the mixing layer in the nonlinear regime. Moreover, the frequencies are much closer to Rossiter empirical predictions than the linear instability ones. Even at these conditions close to critical, the spectra are well described by the mode R1 and a cascade of nonlinearly generated harmonics, with no reminiscence of the linear instability.