Active Control of Skin Friction and Separation in Certain Class of Wall-bounded Flows

Abstract

Experimental investigations of large coherent structures in turbulent shear flows bypassed the boundary layer in the presence of a strong, adverse-pressure-gradient and the wall-jet. Both are wall bounded flows having one characteristic in common: their mean velocity profile is inviscidly unstable to two dimensional perturbations. Thus the identification of the large coherent structures with the predominant instability modes might be extended to this class of flows and could be quantitatively analyzed. The apparent similarity between the wall-jet and a combination of a free jet and a boundary layer is explored was and the relevance of the solid surface to the evolution of the large coherent structures was assessed. From this point of view, the boundary layer in a strong adverse pressure gradient, is regarded as a wake evolving in the vicinity of a solid surface. In both flows the significance- of the outer region is accentuated while the no-slip conditions at the solid surface are maintained.

The response of these flows to external, two dimensional excitation is currently being investigated in an attempt to further the understanding of the interactions between the inner and the outer structures in a turbulent boundary layer. While doing so, two technologically important effects were discovered. The mean flow in the wall-wake remained attached in spite of the strong adverse pressure gradient which caused separation in the absence of the excitation while the skin friction in the wall-jet was reduced as a consequence of the excitation.