Vertically-hinged wing as a noise-driven Duffing oscillator

The interaction between solid structure and fluid flow often results in complex behaviors. Here, we report a novel experimental finding of flow-induced oscillation with the bi-stability of a simple hinged wing. The wing is vertically hinged about a fixed shaft in a wind tunnel, such that it can rotate freely under external forcing. By shifting the shaft position away from the leading edge of the wing, it is found that the system experiences a supercritical bifurcation from a mono-stationary regime to a sequence of bi-stable regimes with distinct oscillation dynamics. The switching events between the two meta-stable states could be either periodic, chaotic or extremely rare, depending on both the shaft position and the wind speed. Detailed theoretical analysis reveals that the bifurcation is determined by the balance between the lift and pitching moments, and the bi-stable dynamics can be modeled by a modified noise-driven Duffing oscillator. This study suggests that a simple solid structure (not limited to the wing investigated here) freely hinged in a turbulent flow can be used as a canonical model for studying bi-stability systems that are ubiquitous in nature and engineering.