Fluid-acoustic interactions in self-sustained oscillations in turbulent cavity flows. I. Fluid-dynamic oscillations

The fluid-acoustic interactions in a flow over a two-dimensional rectangular cavity are investigated by directly solving the compressible Navier–Stokes equations. The upstream boundary layer is turbulent. The depth-to-length ratio of the cavity is 0.5. Phase-averaged flow fields reveal the mechanism for the acoustic radiation. Large-scale vortices form in the shear layer that separates from the upstream edge of the cavity. When a large-scale vortex collides with the downstream wall, the low-pressure fluid in the vortex spreads along the downstream wall. As a result, a local downward velocity is induced by the local pressure gradient, causing the upstream fluid to expand. Finally, an expansion wave propagates to the outside of the cavity. The large-scale vortices originate from the convective disturbances that develop in the shear layer. The disturbances grow due to the Kelvin–Helmholtz instability, similar to the growth of those in a laminar cavity flow. To clarify the mechanism for the generation of the initial convective disturbances, computations for backward-facing step flows with an artificial acoustic source are also performed. As the artificial acoustic waves become more intense, the initial convective disturbances in the shear layer become more intense while the spatial growth rate of these disturbances does not change. This means that the initial convective disturbances in the shear layer are induced by the acoustic waves.