Effects of Low-Frequency Noise in Crossflow Transition Control

Abstract

Direct numerical simulations were performed to investigate crossflow transition control by the three-dimensional steady upstream flow deformation technique under the influence of low-frequency fluctuations. In wind-tunnel or flight experiments with discrete roughness elements, the fluctuations most likely arise from the interaction of the elements with background turbulence or base-flow fluctuations. If plasma actuators are applied instead, they directly enhance—unintended by discharge randomness—the low-frequency fluctuation level of the flow, degrading the control effectiveness that is based on inducing steady control vortices. To elucidate the fundamental mechanisms responsible for the promoted transition due to low-frequency fluctuations, the present work focused on localized volume forcing, modeling two actuator configurations, that is, plasma virtual roughness elements and streamwise vortex generators. The noise is introduced by modulating the steady volume force using a broadband low-frequency signal. Even when the steady control mode clearly dominates over low-frequency traveling disturbances, transition can still be significantly accelerated by the known secondarily amplified type III modes. Compared with the much localized active or passive roughness elements investigated in previous crossflow experiments, elongated vortex generators spreading over a larger streamwise distance diminish the receptivity of misaligned traveling modes and thus offer a better control performance.