Control of roughness-induced transition under the influence of inflow disturbance

Abstract

Control of roughness-induced transition influenced by two- or three-dimensional inflow disturbances using spanwise-uniform wall blowing is investigated from the perspective of unstable modal growth with direct numerical simulations. Visualization of flow structures for two-dimensional disturbances indicates that the counter-rotating vortex pair in the wake bends the high-shear layer, which is related to both symmetric and antisymmetric unstable modes. The interaction of these structures is found to be responsible for the initial growth of the symmetric mode, which dominates the transition process. Both upstream and downstream position wall blowing are effective for transition control. Upstream position blowing works by lifting up the boundary layer and diminishing the wake structures, resulting in inhibition of the symmetric mode. Downstream position blowing has an even better effect, attributable to reconfiguration of the streamwise vorticity field in the wake and further distortion of the high-shear layer. When the inflow disturbance is a three-dimensional wave, the antisymmetric mode becomes dominant in the near-wake region, resulting in more rapid initial growth of the symmetric mode. As oblique waves have higher growth rates in a compressible boundary layer, the background disturbance is obviously stronger. Therefore, transition occurs earlier than for two-dimensional disturbances. When upstream position blowing is introduced, the spanwise shear related to the antisymmetric mode is reduced, and the background disturbance is suppressed as the disturbed boundary layer is pushed away from the roughness. Downstream position blowing can still inhibit the symmetric mode, but the antisymmetric mode is barely influenced, since this blowing is less effective in suppressing the counter-rotating vortex pair.