Flow Instability around a 2d Airfoil Induced by Acoustic Disturbances at Low Reynolds Numbers

In this paper, a numerical study is presented to investigate a boundary-layer instability on the suction side of a NACA0006 airfoil, induced by aerodynamic sound scattered from a trailingedge. By incorporating a linear stability analysis into the direct simulations with a compressible Navier-Stokes code using high-order accurate schemes, the unsteady characteristics are estimated upon resolving the acoustic effects of trailing-edge noise generation. The onset condition of an acoustic feedback loop is explored by altering the Mach number, compared with the frequency characteristics of the boundary-layer instability. While increasing the Mach number prompts the excitation of the feedback loop via the amplification of acoustic disturbances, the instability of the boundary layer seems to be suppressed at a higher Mach number; therefore, the onset condition should be determined between these two contradictory features, at a given Mach number. Also the phase requirement between acoustic and hydrodynamic waves is closely examined to attain a feedback loop. It is strongly suggested that this phasing consistency should explain the presence of discrete resonance modes. In the present result, however, the mechanism to determine the discrete frequencies is still an open question for future research.