ONE-DEGREE-OF-FREEDOM ROTATIONAL GALLOPING UNDER STRONG WIND CONDITIONS

We consider the non-linear dynamics of a mechanical oscillator with on e degree of rotational freedom in a uniform wind field. A quasi-steady approach is used to model the aerodynamic forces. For weak wind the a erodynamic forces can be assumed to be a perturbation to the mechanica l system (weak forcing approach). For high wind speeds a strong forcin g approach is developed, where the system is described as a weakly per turbed Hamiltonian system, by including in the Hamiltonian the leading -order aerodynamic terms, which are of conservative nature. The conser vative system is characterized by a non-linear centre and two saddles connected by a heteroclinic loop. The perturbed system is analysed for limit cycles using Pontryagins's method for perturbed Hamiltonian sys tems. Dynamic tests were performed on an experimental oscillator set-u p in a wind-tunnel, in which limit cycle amplitudes were recorded and compared to the quasi-steady analysis. Tests at increased wind speed r evealed the predicted behaviour of increased limit cycle amplitude, am plitude-dependent oscillation frequency and dynamic divergence above a critical wind speed. (C) 1997 Elsevier Science Ltd.