Heat-Flux Gauge Studies of Compressible Dynamic Stall

Compressible dynamic stall was studied using 148 closely spaced heat-flux gauges distributed over the surface of an oscillating, 15.24-cm chord NACA 0012 airfoil. The study has revealed the various surface flow features of compressible dynamic stall. These include the rapid movement of the transition onset point over the airfoil upper surface on the upstroke with increasing angle of attack, the signature of the convecting dynamic-stall vortex, and the relaminarization of the reattaching shear layer on the downstroke. The imprint of a leading-edge shock has been captured for the first time in the surface flow signature. Even though compressible dynamic stall can originate from several causes depending upon flow conditions, the study led to the conclusion that a deterministic precursor of dynamic-stall onset is a sharp rise in the surface shear stress in the leading-edge adverse pressure gradient region, which is a common and singular flow feature at all conditions tested