Effect of Streamwise Travelling Waves on Coherent Vortices and Fine Structure of Wall Turbulence

In this work we analyse the effect of streamwise travelling waves wall forcing on the near-wall quasi-streamwise vortical structures. Six different amplitudes were considered at fixed frequency and wavelength that produced drag reduction margin ranging from 30-60%. The main effect of the large amplitude STW is to attenuate the spanwise vorticity at the wall. The near-wall quasi-streamwise vortices (QSVs) are shifted away from the wall with a drift of up to 10 wall units for the largest amplitude case considered here. This drift of QSVs results in their weakened signature at the wall, and hence reduced drag. The contribution of the quadrant events to the Reynolds shear stress show a significant increase in the sweep events by up to 130% for the large amplitude cases. Furthermore, the intensity of outward (Q1) and inward (Q3) interaction events show an increase of about 80% up to the edge of the buffer layer, implying increased incoherence of the near-wall flow field.