Flow Visualization Studies of the Mach Number Effects on the Dynamic Stall of an Oscillating Airfoil

Abstract

Compressibility effects on dynamic stall of a NACA 0012 airfoil undergoing sinusoidal oscillatory motion were studied using a stroboscopic schlieren system. Schlieren pictures and some quantitative data derived from them are presented and show the influence of freestream Mach number and reduced frequency on the dynamic stall vortex. This study shows that a dynamic stall vortex always forms and convects over the airfoil upper surface at approximately 0.3 times the freestream velocity for all cases studied. The results also demonstrate that occurrence of deep stall is delayed to higher angles of attack with increased reduced frequency, even when compressibility effects are present, but increasing Mach number alone has the opposite effect.