Assessing the Importance of Geometric Nonlinear Effects in the Prediction of Wind Turbine Blade Loads

As the size of commercial wind turbines increases, new blade designs become more flexible in order to comply with the requirement for reduced weights. In normal operation conditions, flexible blades undergo large bending deflections, which exceed 10% of their radius, while significant torsion angles toward the tip of the blade are obtained, which potentially affect performance and stability. In the present paper, the effects on the loads of a wind turbine from structural nonlinearities induced by large deflections of the blades are assessed, based on simulations carried out for the NREL 5MW wind turbine. Two nonlinear beam models, a second order (2nd order) model and a multibody model that both account for geometric nonlinear structural effects, are compared to a first order beam (1st order) model. Deflections and loads produced by finite element method based aero-elastic simulations using these three models show that the bending-torsion coupling is the main nonlinear effect that drives differences on loads. The main effect on fatigue loads is the over 100% increase of the torsion moment, having obvious implications on the design of the pitch bearings. In addition, nonlinearity leads to a clear shift in the frequencies of the second edgewise modes.