Abstract
We investigate the fluid-acoustic interactions in fluid-dynamic oscillations of a flow over a two-dimensional cavity by directly solving the compressible Navier-Stokes equations. 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, we perform computations for the two freestream Mach numbers of 0.15 and 0.3. The oscillations occur with the Mach number of 0.3, while the oscillations 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, the local pressure gradient induces a local downstream velocity, causing the upstream fluid to expand. Finally, an expansion wave propagates to the outside of the cavity. In order to clarify the mechanism for the formation of the large-scale vortices, we also perform the computations of backward-facing step flows with an artificial acoustic source. As a result, it is shown that the large-scale vortices originate from the convective disturbances, which are generated by the acoustic waves. The disturbances grow due to the Kelvin-Helmholtz instability, similar to the growth of the disturbances in a laminar cavity flow. Also, these computations of the backward-facing step flows clarify the reason why oscillations do not occur in the cavity flow for the Mach number of 0.15.
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