Volume 6 (2011) Issue 1 Pages 90-106
Fluid-acoustic interactions in fluid-dynamic oscillations of a flow over a two-dimensional cavity were investigated by directly solving the compressible Navier-Stokes equations. It was assumed that the upstream boundary layer is turbulent and the depth-to-length ratio of the cavity is 0.5. To clarify the effects of the freestream Mach number on the fluid-dynamic oscillations, the computations were performed for two freestream Mach numbers, i.e., 0.15 and 0.3. The oscillations occur with the Mach number of 0.3, while they do not occur with the Mach number of 0.15. For the Mach number of 0.3, phase-averaged flow fields show that large-scale vortices form in the shear layer. When the large-scale vortex collides with the downstream wall, the low-pressure region spreads along the downstream wall. As a result, a local pressure gradient induces a local downstream velocity, causing the upstream fluid to expand and an expansion wave to propagate to the outside of the cavity. To clarify the mechanism for the formation of the large-scale vortices, backward-facing step flows with an artificial acoustic source were also computed. These computations clarify the reason that oscillations do not occur in the cavity flow with Mach number of 0.15.