Influence of active control on STG-based generation of streamwise vortices in near-wall turbulence

The near-wall streamwise vortices are closely related with the generation of high skin friction at the wall in turbulent flows, and in the controlled, friction-reduced turbulent flows, streamwise vortices are greatly attenuated. In present study, the streak transient growth (STG) mechanism of generating near-wall streamwise vortices is employed, and the opposition control is imposed during the transient growth process of the perturbation to disclose how the active control affects the generation of quasi-streamwise vortices. It is found that in the transient growth stage, when the detection plane is located near the wall (y⁺ _d = 15), the control can suppress the generation of the streamwise vorticity through weakening the near-wall vertical velocity; when the detection plane moves away from the wall (y⁺ _d = 28), the control has opposite effects. In the vortex generation stage, the control can not change the dominance of the stretching effect. The y⁺ _d = 15 control can achieve sustained overall attenuation of streamwise vortex generation via the suppression of the stretching term. The y⁺ _d = 28 control, however, generates multiple extreme points in the stretching term distribution, which causes increasing the number of streamwise vortices by splitting the primary vortex into smaller and shorter fractions. The opposition control using the signal at y⁺ _d = 28 with reduced strength has been proposed and tested in both minimal and full-scale channel flows. The effectiveness in turbulence suppression by the lessened y⁺ _d = 28 control confirms present analysis.