Computation of Sound Generation and Flow/Acoustic Instabilities in the Flow Past an Open Cavity

The modes of oscillation and radiated acoustic fields of compressible flows over open cavities are investigated computationally. The compressible Navier-Stokes equations are solved directly (no turbulence model) for two dimensional open cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field. The results show a bifurcation from a shear layer mode, for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear layer mode is well characterized by Rossiter modes and these oscillations lead to intense upstream acoustic radiation dominated by a single frequency. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number nearly independent the Mach number. The vortex shedding causes the boundary layer to periodically separate upstream of the cavity. Acoustic radiation is more intense, with multiple frequencies present. The wake mode oscillation is similar to that reported by Gharib & Roshko (1987) for incompressible cavity flows with laminar upstream boundary layers