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Active control for drag reduction in turbulent channel flow: the opposition control schemes revisited

The opposition control schemes first proposed by Choi et al (1994 J. Fluid Mech. 262 75) employing wall-normal ( v ) and spanwise ( w ) velocity are revisited in the present study by performing direct numerical simulation to turbulent channel flow at R e τ = 180 . Special attention is paid to the combined control, in which the wall-normal and spanwise velocities are imposed at the wall just instantaneously opposite to those at a small distance to the wall. In comparison to the v- and w-controls, combined-control could achieve the best drag reduction rate and control efficiency, with the greatest suppression of turbulence intensities. The influence of control on the statistical properties of vortices is scrutinized. By control, the numbers of vortices with every circulation and radius apparently decrease at the same normal location near the wall, while the vortex radius scaled by the actual wall-friction velocity almost remains the same. The streamwise vortices and the induced Reynolds shear stress undergo the greatest suppression by combined control. It is shown that combined control achieves a better efficacy, attributed to the co-work of the mechanisms of the v- and w-controls. At a higher Reynolds number R e τ = 1000 , combined control is also more effective than v- and w-controls. The better suppression effect on the outer large scales is the primary reason for the larger drag reduction rate in combined control.