The response of airfoils to periodic disturbances - The unsteady Kutta condition

Two cases of unsteady flowfields over a NACA 0012 airfoil at an angle of attack are examined. The first is the classical pitching motion about the airfoil's quarter chord. The second is the flow over a fixed airfoil immersed in the wake of the pitching airfoil. Large reduced frequencies are examined. Measurements were obtained in a water tunnel by laser Doppler velocimetry. Ensemble-averaged velocity measurements were obtained in the vicinity of the trailing edges of both the pitching and the fixed airfoil. The results indicate that the classical unsteady Kutta condition is clearly not valid. An extension of this condition earlier proposed by Giesing and Maskell is examined and some evidence is provided for its support.