Abstract Summary
Modern wind turbines towers have a circular cross-section (commonly tubular and made of steel), which is needed to have the same behaviour in all wind directions. This entails that it has similar natural frequencies in the wind direction and in the perpendicular direction, the difference being due to the offset of the rotor blades with respect to the vertical axis of the tower, since they are at the end of the nacelle. Thus, there is the possibility to have an internal resonance between along-wind and across-wind oscillations, that can interact when nonlinearities are taken into account at the structural level, a fact that is a consequence of the slenderness of the towers. Thus, it is possible that the wind force acting on the tower produces an oscillation in the perpendicular direction, similarly to what happens for vortex induced vibration, but due to a completely different reason. This investigation is aimed at investigating this phenomenon by means of a simplified nonlinear model, by obtaining an analytical solution allowing to have a full understanding and a rigorous treatment of the problem. As a by-product, this gives the possibility to control the phenomenon by properly modifying the parameters of the structure. This work represents a first attempt to describe the internal resonance problem in wind turbine towers, and will be developed in the future by considering more sophisticated mechanical models, that however will require numerical simulations and thus will not be very performant for parametric analyses.
