ACTIVE TURBULENCE CONTROL FOR DRAG REDUCTION IN WALL-BOUNDED FLOWS

The objective of this study is to explore concepts for active control of turbulent boundary layers leading to skin-friction reduction using the direct numerical simulation technique. Significant drag reduction is achieved when the surface boundary condition is modified to suppres s the dynamically significant coherent structures present in the wall region. The drag reduction is accompanied by significant reduction in the intensity of the wall-layer structures and reductions in the magni tude of Reynolds shear stress throughout the flow. The apparent outwar d shift of turbulence statistics in the controlled flows indicates a d isplaced virtual origin of the boundary layer and a thickened sublayer . Time sequences of the flow fields show that there are essentially tw o drag-reduction mechanisms. Firstly, within a short time after the co ntrol is applied, drag is reduced mainly by deterring the sweep motion without modifying the primary streamwise vortices above the wall. Con sequently, the high-shear-rate regions on the wall are moved to the in terior of the channel by the control schemes. Secondly, the active con trol changes the evolution of the wall vorticity layer by stabilizing and preventing lifting of the spanwise vorticity near the wall, which may suppress a source of new streamwise vortices above the wall.