Passive and active control of near-wall turbulence

Wall skin friction reduction is the primary focus of this paper. Turbulent boundary layer structure and near-wall turbulence structure are considered in particular along with how passive and active modifications to the structure can lead to reductions in drag. Direct numerical simulation of low Reynolds number turbulent flow over streamwise aligned micro-grooved striations or riblets, particularly V-groove riblets and the application of a near-wall viscous model to the flow over more complicated riblet shapes (V and U groove, thin element, compound and spanwise non-continuous V-groove) are assessed. This assessment provides a qualitative basis for identifying features that may limit riblets to skin friction reduction of about 10%. This leads to consideration of three dimensional riblets or "humplets", use of oscillations (both fluid and structural), selective interference with existing turbulence and selective introduction of new scales of turbulence with the aim always towards reducing skin friction. (C) 1998 Elsevier Science Ltd.