Drag Reduction by Controlled Base Flow Separation.

Abstract

A missile shaped body, with and without the stepped afterbodies, was examined for measuring reduction in pressure drag at a hypersonic Mach number of 5.75 in a shock tunnel at 0 degree angle of incidence. Two types of stepped afterbodies, one with five steps of nonuniform heights and lengths, and other with three steps of nonuniform heights and lengths, were used, and a single-component accelerometer balance was used to measure the drag force on the body. It was observed that in the case of a stepped afterbody, if the step height is kept approximately equal to the boundary layer thickness ahead of it, then the separated flow at each step would reattach at the horizontal face of the step, giving rise to to a controlled flow separation and thereby generating a weak separation bubble at the step. It was also observed that the flow separation reduced skin friction on each step thereby reducing viscous drag on the afterbody.