The Effect of Acoustic Forcing on Instabilities and Breakdown in Swept-Wing Flow Over a Backward-Facing Step

摘要

Instability interaction and breakdown were experimentally investigated in the flow over a swept backward-facing step. Acoustic forcing was used to excite the Tollmien-Schlichting (TS) instability and to acquire phase-locked results. The phase-averaged results illustrate the complex nature of the interaction between the TS and stationary crossflow instabilities. The weak stationary crossflow disturbance causes a distortion of the TS wavefront. The breakdown process is characterized by large positive and negative spikes in velocity. The positive spikes occur near the same time and location as the positive part of the TS wave. Higher-order spectral analysis was used to further investigate the nonlinear interactions between the TS instability and the traveling crossflow disturbances. The results reveal that a likely cause for the generation of the spikes corresponds to nonlinear interactions between the TS, traveling crossflow, and stationary crossflow disturbances. The spikes begin at low amplitudes of the unsteady and steady disturbances (2-4% U_e) but can achieve very large amplitudes (20-30% U_e) that initiate an early, though highly intermittent, breakdown to turbulence.