Crossflow transition control by upstream flow deformation using plasma actuators

Abstract

Control of laminar-turbulent transition in a swept-wing-type boundary-layer flow, subject to primary crossflow instability, is investigated using direct numerical simulations. In our previous works, we explored a direct base-flow stabilization aimed at a spanwise homogenous flow manipulation or a direct crossflow-vortex manipulation by plasma actuators. In this paper, the technique of upstream flow deformation (UFD) is applied, needing by far the least energy input. The actuators, modeled by local volume forcing, are set to excite amplified steady crossflow vortex (CFV) control modes with a higher spanwise wavenumber than the most amplified modes. The resulting nonlinear control CFVs are spaced narrower than the naturally occurring vortices and are less unstable with respect to secondary instability. They generate a beneficial mean-flow distortion attenuating the primary crossflow instability, and thus a delay of the transition to turbulence. Unlike roughness elements for UFD, the employed dielectric barrier discharge plasma actuators allow to set the force direction: Forcing against the crossflow has a direct, fundamental stabilizing effect due to a reduction of the mean crossflow, whereas forcing in the crossflow direction locally invokes the opposite due to a local increase of the mean crossflow. The differences between these settings, also with respect to forcing in streamwise direction, are discussed in detail, and it is shown that a significant transition delay can be achieved indeed with both, however with a differing efficiency and robustness. Additionally, a comparison to a set-up with an excitation of the control modes by synthetic blowing and suction is performed to clarify the role of the direct effect on the base flow.