Numerical Simulations of the Aeroelastic Behavior of Large Horizontal-Axis Wind Turbines: the Drivetrain Case

Abstract

In this work an aeroelastic model that describes the interaction between aerodynamics and drivetrain dynamics of a large horizontal–axis wind turbine is presented. Traditional designs for wind turbines are based on the output of specific aeroelastic simulation codes. The output of these codes gives the loads acting on the wind turbine components caused by external forces such as the wind, the electricity grid and (for offshore applications) sea waves. Since the focus in the traditional codes lies mainly on the rotor loads and the dynamic behavior of the overall wind turbine, the model of the drive train in the wind turbine is reduced to only a few degrees of freedom. This means that, for the design of the drive train, the simulated load time series need to be further processed to applied loads on the individual components, such as gears and bearings. Furthermore, the limitation of the model implies that vibrations of these internal drive train components are not taken into account and, as a consequence, dynamic loads on these components cannot be simulated. In this effort an aerodynamical model based on the non–linear and unsteady vortex–lattice method is used to compute the aerodynamic loads and their evolution in the space and the time domains, considering multiple aerodynamic interactions among blades, wakes, hub, nacelle, support tower, ground and land–surface boundary layer. All these in combination affect substantially the total efficiency of the turbine. In addition, a flexible multibody model for the drivetrain is developed as a way to include directly the high speed shaft’s (which connects the gear box and generator) flexibility. For the inter–model combination, a strong interaction scheme based on the fourth order Hamming predictor–corrector method is used. The models and the interaction scheme are implemented in a computational tool; using this tool, the behavior of the turbine in the starting initial regime is investigated considering different laws of brake releasing. The capability to simulate these phenomena is a novel aspect in the present effort.