4. TITLE AND SUBTITLE 5. FUNDING NUMBERS Schlieren Studies of Compressibility Effects on Dynamic Stall of Transiently Pitching Airfoils

The view, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy, or decision, unless so designated by other documentation. 12a. DISTRIBUTION/AVAILABILITY STATEMENT 2b. DISTRIBUTION CODE Approved for public release; distribution unlimited. LECUEOCTs19.19 13. ABSTRACT (Maximum 200 words) W Compressibility effects on the flowfield of an airfoil executing rapid transient pitching motion from 0-60 deg over a wide range of Mach numbers and pitching rates were studied using a stroboscopic schlieren flow visualization technique. The studies have led to the first direct experimental documentation of multiple shocks on the airfoil upper surface flow for certain conditions. Also, at low Mach numbers, additional coherent vortical structures were found to be present along with the dynamic stall vortex, whereas at higher Mach numbers the flow was dominated by a single vortex. The V' delineating Mach number for significant compressibility effects was 0.3 and the dynamic stall process was accelerated by increasing the Mach number above that value