Coherent near-wall structures and drag reduction by spanwise forcing

TL;DR

This paper investigates how spanwise traveling waves of wall velocity influence near-wall vortical structures in turbulent flow, revealing mechanisms for both drag reduction and increase, and proposing a predictive model based on flow parameters.

Contribution

It extends previous work on uniform wall oscillation to spanwise traveling waves, analyzing their impact on vortex structures and developing a new predictive correlation for drag control.

Findings

Spanwise traveling waves significantly alter near-wall vortex distributions.

Suppression of Q2 events correlates with drag reduction.

Enhancement of Q2 and Q4 events correlates with drag increase.

Abstract

The effect of streamwise-traveling waves of spanwise wall velocity (StTW) on the quasi-streamwise vortices (QSV) populating the near-wall region of turbulent channels is studied via a conditional averaging technique applied to flow snapshots obtained via Direct Numerical Simulation. The analysis by A. Yakeno, Y. Hasegawa, N. Kasagi, "Modification of quasi-streamwise vortical structure in a drag-reduced turbulent channel flow with spanwise wall oscillation", Phys. Fluids 26, 085109 (2014), where the special case of spatially uniform wall oscillation (OW) was considered, is extended to the general case of StTW, which yield both reduction and increase of turbulent skin-friction drag. StTW are found to significantly impact the wall-normal distribution of the vortex population. The conditionally averaged velocity field around the vortices shows that the contributions of the QSV to the quadrant Reynolds shear stresses change significantly during the control cycle. On the one hand, as for OW, the suppression of Q2 events (with upwelling of low-speed fluid away from the wall) dominates the drag-reduction process. On the other, the enhancement of Q2 and also Q4 events (with downwelling of high-speed fluid toward the wall) is related to drag increase. Based on the link identified between the phase changes of the Reynolds stresses and the principal directions of the rate-of-strain tensor induced by the StTW, a predictive correlation for drag reduction by StTW is proposed which uses physically significant parameters to overcome the shortcomings of existing models.