Experiments on a pitch-plunge wing undergoing limit cycle oscillation

The aeroelastic behaviour of a wing oscillating in the heave and pitch degrees of freedom is examined by means of wind tunnel experiment. The phenomena of interest are classical flutter and limit cycle oscillation. Classical flutter is normally associated with the exponential growth of the response amplitude. Linear flutter theory only predicts the critical flutter speed. Any excitation or disturbance beyond the critical speed is assumed to cause exponential growth in the response amplitude. In contrast, any limited amplitude oscillations occurring post-fultter suggest the existence of nonlinear properties in the system. Such properties can originate from the aerodynamic forces in the form of flow separation and reattachment. On the structural side, damping and stiffness can also contribute nonlinear properties. Furthermore, these nonlinearities can manifest themselves even at pre-flutter conditions, depending on the values of some governing parameter. The focus of the present work is the transformation of classical flutter into stall flutter as the equilibrium angle of attack of heaving and pitching wing is increased. The interaction of stall-related nonlinearity with structural nonlinearities is also of interest. The measured aeroelastic responses are analyzed and the bifurcation behavior of the dynamic system is characterized. Structural responses as well as flow field visualization through Particle Image Velocimetry show the origin of nonlinearity does not solely come from the manifestation of separation and the shedding of vortices, but from the structural nonlinearity which limits the response amplitude.