INTERFEROMETRIC INVESTIGATIONS OF COMPRESSIBLE DYNAMIC STALL OVER A TRANSIENTLY PITCHING AIRFOIL

The compressible dynamic stall flowfield over a NACA 0012 airfoil tran siently pitching from 0 to 60 deg at a constant rate under compressibl e flow conditions has been studied using real-time interferometry. A q uantitative description of the overall flowfield, including the finer details of dynamic stall vortex formation, growth, and the concomitant changes in the airfoil pressure distribution, has been provided by an alyzing the interferograms. For Mach numbers above 0.4, small multiple shocks appear near the leading edge and are present through the initi al stages of dynamic stall. Dynamic stall was found to occur coinciden tally with the bursting of the separation bubble over the airfoil. Com pressibility was found to confine the dynamic stall vortical structure closer to the airfoil surface. The measurements show that the peak su ction pressure coefficient drops with increasing freestream Mach numbe r, and also it lags the steady now values at any given angle of attack . As the dynamic stall vortex is shed, an anti-clockwise vortex is ind uced near the trailing edge, which actively interacts with the post-st all flow.