Drag-reducing flow structure modification generated by spanwise traveling surface waves*

One primary obstacle for exploiting the full potential of flow control methods is that the physical causes leading to drag reduction are still not fully understood. Therefore, the present study analyzes the flow dynamics of the drag-reducing property of spanwise traveling transversal surface waves at two friction Reynolds numbers Re_τ ≈ 390 & 1,500. The approach involves an inner-outer interaction analysis based on the 2D Noise-Assisted Multivariate Empirical Mode Decomposition, which extracts common scales across different variates, and a study of the effects of the superimposed secondary flow field.

The study reveals that the actuation introduces near-wall large-scale ejections that increase the bottom-up communication and weaken downwards oriented high-speed fluid damping the impact of the outer layer on the near-wall dynamics. In addition, specific velocity gradient combinations of the secondary flow field deform the quasi-streamwise vortices into an elliptical shape yielding vortex disintegration. The reduced number of near-wall vortices attenuates the wall-normal momentum exchange and is accompanied by less intense and widened streaks, which, in turn, yields a reduced wall-shear stress and adds up to the overall friction drag reduction.