Aeroelastic modeling and dynamic analysis of a wind turbine rotor by considering geometric nonlinearities

Due to the increased flexibility of modern multi-megawatt wind turbine structures, more advanced analyses are needed to investigate the effects of geometric nonlinearities originating from large blade deformations under operational loads. The main objective of this paper is to study the related dynamics and the aeroelastic effects of these nonlinearities by using a multi-flexible-body aeroelastic model of an entire three-bladed wind turbine assembly instead of a more conventional single-blade model. A geometrically-exact beam formulation is employed to model the rotating blades connected to the wind turbine tower tip via hub and nacelle components; and the revolute joint constraint is applied to model the rotor rotation. The derived governing equations are analyzed by applying a reduced-order approach based on the Galerkin method. After verifying the model by comparing it with an FEM model, it is used in several aeroelastic and dynamic studies. All the performed simulations are based on the specifications of the state-of-the-art 5 MW-NREL wind turbine. Moreover, the time-invariant forms of the governing equations are derived by applying the multi-blade coordinate transformation method. The obtained equations are used to study the whirling characteristics of wind turbine rotor by different models including the model of unloaded blades as well as the linear and nonlinear aeroelastic models. The eigenfrequencies indicate the splitting characteristics according to the gyroscopic effects that originate from the whirling motion in the forward and backward directions. The simulation results reveal that single-blade models are helpful in demonstrating the overall trends of rotor aeroelasticity. However, because single-blade models overestimate the instability limits, an analysis based on a full wind turbine model is crucial. Besides, the geometric nonlinearities significantly alter the eigenfrequency results (especially for the modes dominantly incorporated into flutter instability) as well as the dynamic responses of the system, notably around the flutter speed.