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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 fluc...

The fundamental mechanisms of a hitherto unstudied approach to control the crossflow-induced transition in a three-dimensional boundary layer employing unsteady control vortices are investigated by means of direct numerical simulations. Using a spanwise row of blowing/suction or volume-force actuators, subcritical travelling crossflow vortex modes are excited to impose a stabilizing (upstream) flow deformation (UFD). Volume forcing mimics the effects of alternating current plasma actuators driven by a low-frequency sinusoidal signal. In this case the axes of the actuators are aligned with the wave crests of the desired travelling mode to maximize receptivity and abate the influence of other unwanted, misaligned modes. The resulting travelling crossflow vortices generate a beneficial mean-flow distortion reducing the amplification rate of naturally occurring steady or unsteady crossflow modes without invoking significant secondary instabilities. It is found that the stabilizing effect achieved by travelling control modes is somewhat weaker than that achieved by the steady modes in the classical UFD method. However, the energy requirements for unsteady-UFD plasma actuators would be significantly lower than for steady UFD because the approach makes full use of the inherent unsteadiness of the plasma-induced volume force with alternating-current-driven actuators. Also, the input control amplitude can be lower since unsteady crossflow vortex modes grow stronger in the flow.

It has been shown recently by direct numerical simulations that plasma actuators can be used to delay laminar-turbulent transition caused by steady crossflow vortices (CFVs) in three-dimensional boundary layers on swept aerodynamic surfaces. In the current work the applicability of such actuators to control transition caused by traveling CFVs is explored by two techniques. In the first technique, named upstream flow deformation, the actuators are used to excite steady CFV control modes. The resulting narrow spaced control CFVs induce a beneficial mean-flow distortion and weaken the primary crossflow instability, yielding delayed transition. In the second technique, the direct attenuation of nonlinear traveling CFVs, the actuators are positioned more downstream, where the traveling CFVs have already established. The localized unsteady forcing against the direction of the crossflow is then aimed at attenuating the amplitude of the traveling CFVs by directly tackling the three-dimensional nonlinear disturbance state. With both techniques transition can be delayed, however with a significantly higher efficiency for the method of upstream flow deformation.