Mode analysis of high-speed flow over open cavity

The flow mechanisms of the M219 cavity with an aspect ratio of L*/W*/D*=5:1:1 were investigated using the Improved Delayed Detached Eddy Simulation at Mach numbers of 0.85, 1.5, 2.0, and 4.0. The results demonstrate significant suppression of flow instabilities, including shear layer breakdown and flow-acoustic resonance at higher Mach numbers, accompanied by a reduction in the Overall Sound Pressure Level on the cavity surface. Furthermore, a transition in Rossiter mode behavior is observed. Quantitative analysis of the local Power Spectral Density reveals that a new low-frequency mode dominates the spectral energy distribution in high-speed flows (Ma ≥ 2.5). This mode is further identified via Dynamic Mode Decomposition (DMD), with three-dimensional DMD-based mode shapes confirming that the low-frequency mode is characterized by spanwise-oscillating structures driven by large-scale vortices within the cavity. In contrast, the higher-order Rossiter modes are associated with streamwise-oscillating discrete vortices in the shear layer.