Abstract: This paper presents a flow visualization study of the flow structure arounda constant-rate pitching airfoil using hydrogen bubble technique. It is shown that there exist three types of separated flow structure for different pitching rate. The time evolution of the dymamic stall vortex, shear layer vortices and trailing edge vortex is visualized, and their mutual interaction is studied. The flow visualization results, combining with theoretical analysis in literature, qualitatively explains the mechanism of ...