Non-linear dynamics of an airfoil forced to oscillate in dynamic stall

The aeroelastic response of a NACA 0012 airfoil with freedom to move in pitch only, and forced to oscillate through dynamic stall in subsonic flow is simulated numerically using a finite difference method. The unsteady aerodynamic pitching moment is obtained via a semi-empirical dynamic stall model; comparison between the predicted pitching moment and experimental values is reasonable. It is demonstrated that the aerodynamic non-linearities associated with dynamic stall are sufficient to cause a chaotic response. Furthermore, it is suggested that there are at least two different routes to chaos depending on the ratio of the forcing frequency to the structural natural frequency. In the first case, where the forcing frequency is considerably higher than the structural natural frequency, the suggested route to chaos is via a period doubling cascade. For the second, where the frequency of the applied torque is approximately equal to the structural frequency, it is suggested that the response is intermittently chaotic. (C) 1996 Academic Press Limited