Abstract
The fluid-acoustic interactions in a flow over a cavity in a turbulent boundary layer are investigated by directly solving the compressible Navier-Stokes equations. 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 computations for backward-facing step flows with an artificial acoustic source clarify that convective disturbances in the shear layer are induced by the acoustic waves and the disturbances grow into the large-scale vortices due to the Kelvin-Helmholtz instability.
