Whole-field velocity measurements around an axisymmetric body with a Stratford–Smith pressure recovery

Abstract

Wind tunnel experiments are performed on a body of revolution with aft-section designed to be an axisymmetric Stratford ramp (i.e. the flow over the ‘ramp’ experiences an adverse pressure gradient that causes it to be continuously on the verge of separation). Digital particle image velocimetry (DPIV) measurements over the ramp reveal a thick boundary layer that is characterized by self-similar velocity profiles with a large wake component and organized vorticity structures. The mean skin friction quickly drops to a value near zero.

The sensitivity of the boundary layer to the degree of severity of the adverse pressure gradient is investigated by testing two additional ramps; one is slightly more conservative (i.e. less steep) than the Stratford ramp while the other is slightly more radical (i.e. steeper). In comparison to the Stratford ramp, the conservative ramp is characterized by a thinner boundary layer, with velocity profiles that start attached and gradually develop a large wake component, a much more gradual drop in the skin friction, and vorticity that is concentrated very close to the wall. On the other hand, the boundary layer over the radical ramp is unsteady and separates intermittently. Measurements of the drag force on each of the three bodies confirm that the Stratford ramp experiences the least amount of drag.

Finally, additional data are gathered on the windward and leeward sides of the Stratford ramp when subjected to a small angle of attack. This case exhibits a more complex flow structure: the flow remains attached over the windward side of the ramp while separating over the leeward side.