Skin-Friction Drag Reduction in a Turbulent Channel Flow based on Wall Shear-Free Control

Abstract

Active flow control of wall-bounded turbulent flow for skin-friction drag reduction (DR) has been received great attention in recent years due to large economical and ecological interest. In the present study, direct numerical simulations (DNSs) of turbulent channel flows are utilized to explore an active flow control concept using streamwise shear-free control (SSFC) at the wall. The control is only applied to half of the entire wall comprised of spanwise-alternating longitudinal regions of no-control and control surfaces for simplicity, and the simulations are systematically performed with changing the spanwise periodicity (P/h) of the control surface. In addition, an amplitude parameter (A) imposing the strength of the actuating streamwise velocity at the wall is introduced to enhance the skin-friction DR. Significant DR is observed with increasing the two parameters with accompanying reduction of the Reynolds stresses and vorticity fluctuations, although further increase of the parameters amplifies the turbulence activity in the near-wall region. In order to study the direct relationship between turbulent vortical structures and DR under the control, temporal evolution with initial eddies extracted by the conditional averages for Reynolds-stress-maximizing Q2 events are examined. It is shown that the generation of new vortices is dramatically inhibited with increasing the parameters throughout the flow, and thus fewer vortices are induced under the control. But, when P/h is sufficiently large, the autogeneration process for new vortex is not suppressed over the no-control surface in the near-wall region, thus resulting in increase of the second- and fourth-quadrant Reynolds shear stresses. Although strong amplitude A intensifies near-wall flow properties of turbulence, the increase of the turbulence activity is attributed to generation of counter-clockwise near-wall vortices by the increased vortex transport.