ON ACTIVE CONTROL OF SEPARATION FROM BLUFF BODIES(Keynote Lecture)

Abstract

Active control of flow separation from a bluff object embodies the flow of curved jets and wakes both steady and unsteady and their effect on the forces experienced by the object. The efficacy of separation control by steady blowing, suction, or by periodic excitation is being extensively reinvestigated. The efficacy of steady blowing was measured by the increase in lift or the reduction in drag relative to the momentum added to the flow, while the efficacy of suction was assessed by the mass flow removed from the flow field. Recent experiments suggest that a combination of momentum and mass coefficients represents a better characterization of both methods of control. In the absence of mass flux, periodic excitation makes separation and circulation control much more effective than steady blowing provided the momentum input is small, but its advantage is smaller with respect to suction. When the momentum input is large blowing becomes advantageous because most of the input is recovered as thrust. Various experiments combining blowing suction and periodic excitation will be described.