INVESTIGATION OF FLOW AT LEADING AND TRAILING EDGES OF PITCHING-UP AIRFOIL

The dynamic stall process of an NACA 0012 airfoil undergoing a constan t-rate pitching-up motion is studied experimentally in a water towing tank facility. This study focuses on the detailed measurement of the u nsteady separated flow in the vicinity of the leading and trailing edg es of the airfoil. The measurements are carried out using the particle image velocimetry technique. This technique provides the two-dimensio nal velocity and associated vorticity fields, at various instants in t ime, in the midspan of the airfoil. Near the leading edge, large vorti cal structures emerge as a consequence of van Dommelen and Shen type s eparation and a local vorticity accumulation. The interaction of these vortices with the reversing boundary-layer vorticity initiates a seco ndary flow separation and the formation of a secondary vortex. The mut ual induction of this counter-rotating vortex pair eventually leads to the ejection process of the dynamic stall vortex from the leading-edg e region, It is found that the trailing edge flowfield only plays a se condary role on the dynamic stall process.