Vortex induced vibrations of wind turbine blades: Influence of the tip geometry

The present investigation used numerical simulations to study the vortex induced vibrations (VIVs) of a 96 m long wind turbine blade. The results of this baseline shape were compared with four additional geometry variants featuring different tip extensions. The geometry of the tip extensions was generated through the variation of two design parameters: the dihedral angle bending the blade out of the rotor plane and the sweep angle bending the blade in the rotor plane. The applied numerical methods relied on a fluid structure interaction (FSI) approach, coupling a computational fluid dynamics solver with a multi-body structural solver. The methodology followed for locating VIV regions was based on the variation of the inclination angle. This variable was defined as the angle between the freestream velocity and the blade axis, being 0 degrees when these vectors were normal and positive when a velocity component from tip to root was introduced. For the baseline geometry, the FSI simulations predicted significant blade vibrations for inclination angles between 47.5 degrees and 60 degrees with a maximum peak-to-peak amplitude of 2.3 m. The installation of the different tip extensions on the blade geometry was found to significantly modify the inclination angles where VIV was observed. In particular, the simulations of three of the tip designs showed a shifting of several degrees for the point where the maximum vibrations were recorded. For the specific tip geometry where only the sweep angle was taken into account, a total mitigation of the VIV was observed.