Experimental assessment of an active L-shaped tab for dynamic stall control

Abstract

An experimental activity was performed on a NACA 23012 pitching airfoil to investigate the effectiveness of an active trailing edge L-shaped tab for deep dynamic stall control. The active control system, based on the use of micro pneumatic actuators, was designed to control the deployment and retraction of the tab along the oscillating cycle. In particular, the tab was designed to behave as a Gurney flap when deployed as its end prong protrudes at the airfoil trailing edge, while in retracted position the tab behaves as a trailing edge flap. The L-shaped tab design presents interesting features to be employed on rotor blades, due to an easier integration at the trailing edge with respect to a deployable Gurney flap. Wind tunnel tests were carried out considering two pitching cycles producing deep dynamic stall regime. Unsteady pressure measurements were performed at the model midspan section to obtain the phase-averaged aerodynamic loads curves. The tests results showed that the deployment of the tab during the upstroke produces a conspicuous increase of lift with respect to the clean airfoil case, corresponding to a higher level of available thrust on the retreating blade. The retraction of the tab before stall onset does not introduce a valuable effect in terms of pitching moment peak reduction with respect to clean airfoil. Moreover, the active control system produces a conspicuous reduction of the negatively-damped portions of the pitching cycles and of the negative aerodynamic damping peak that could account for stall flutter divergence.