PIV investigation of the wake past an airfoil in unsteady flow

Though the fluid dynamic fields past stationary NACA airfoils have been deeply investigated and understood, open questions still remain when unsteady pitching conditions are considered. This is even of greater relevance in the aero-naval applications where such unsteadiness becomes critical in maneuvering situations. Rotors in helicopters and maneuvering rudders in ships and submarines are clear examples of such cases. Regarding the physics, the variation of the angle of attack generates a modification of the circulation around the airfoil which is likely to be compensated, in the framework of the Kutta theory, by a vortex shedding in order to have a net vorticity balance equal to zero. So when the angle of attack is varied continuously, a street of vortices has to be expected downstream the airfoil. In order to experimentally investigate this problem, the flow past a NACA-0009 airfoil, with span to chord ratio larger than four, has been tested; to simulate the continuous variation of the angle of attack, the airfoil has been put in rotation around its aerodynamic center according to a sinusoidal law. To investigate the influence of the oscillation amplitude and of the pitching frequency, velocity and vorticity fields are presented as well as Jeong's invariant on 2D velocity fields.