On Flow Control

On Flow Control

Israel Wygnanski The University of Arizona, Tucson, AZ, 85721, USA

Abstract

Active Flow Control (AFC) emerged from Boundary Layer Control (BLC) where the entire boundary layer concept implied that the pressure distribution over a surface is dictated by the surface’s geometry and hence the lift and the pitching moment acting on a streamlined body are determined by an inviscid flow solution. This is incorrect in the presence of a jet, a wall-jet and even more so when either the jet or the surrounding flow is three dimensional. Thus, in order that AFC to become a viable technology, some of the preconceptions associated with BLC must be discarded. In particular, the momentum coefficient used to characterize the effectiveness of AFC should be replaced by another variable that represents a conserved quantity that is independent of specific installations. Injected momentum is a vector quantity whose effect on a surface like a wing depends on its specific design, location, and orientation. This simple fact was rarely considered by companies and researchers attempting to use AFC for improving wing performance or replacing traditional control surfaces on an airplane. A successful flow control device should be small and use little energy to create a large, desirable change in the flow. It often exploits local instabilities and interacts with large vortices to achieve the desired effect, and the present seminar will provide some examples of this approach.