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

Abstract

The Rossiter modes of an open cavity were studied using bi-global linear analysis and nonlinear numerical simulations. In the study, 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 bi-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 had a very destabilizing effect at low Mach. At moderate Mach the instability either saturated with Mach or had an irregular dependence on Mach. Analysis of the Rossiter mode eigenfunctions indicated that the acoustic feedback scaled to Ma 3 , and explained the strong destabilizing effect of compressibility at low Mach. The irregular dependence on Mach 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. Twodimensional simulations were performed to evaluate how much of the nonlinear regime could be captured by the linear stability results. The simulations showed that, as the flow becomes more unstable, an increasing more complex final stage is reached. Yet the spectra presents fairly distinct tones that are not far from 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 relatively close to critical, the spectra is well described by the mode R1 and a cascade of nonlinearly generated harmonics, with no reminiscence of the linear instability.