Drag reduction in wall-bounded turbulence via a transverse travelling wave

Computational experiments based on direct numerical simulation of wall-bounded flow reveal that turbulence production can be suppressed by the action of a transverse travelling wave. Flow visualizations show that the near-wall flow structure is altered substantially, compared to other turbulence control techniques, leading to a large amount of shear stress reduction (i.e., more than 30%). The travelling wave can be induced by a spanwise force that is confined within the viscous sublayer, it has its maximum at the wall, and decays exponentially away from it. We demonstrate the robustness of this approach, and its application in salt water using arrays of electro-magnetic tiles that can produce the required travelling wave excitation. We also study corresponding results from spanwise oscillations using a similar force, which also leads to large drag reduction. Although the turbulence statistics for the two approaches are similar, the near-wall structures appear to be different: in the spanwise oscillatory excitation there is a clear presence of wall-streaks whereas in the travelling wave excitation these streaks have disappeared. From the fundamental point of view, the new finding of this work is that appropriate enhancement of the streamwise vortices leads to weakening of the streak intensity, as measured by the normal vorticity component, and correspondingly substantial suppression of turbulence production. From the practical point of view, our findings provide guidance for designing different surface-based actuation techniques including piezoelectric materials, shape memory alloys, and electro-magnetic tiles.