Coupled-Mode Flutter of Wind Turbines and its Suppression Using Torsional Viscous Damper

The trend towards lighter and more flexible blades may lead to aeroelastic instability of wind turbines under certain circumstances, resulting in rapid destructive failure or limit-cycle oscillations of the structural components. For pitch-regulated wind turbines, classical flutter is believed to become an increasingly important design consideration as blades become longer and slender. Flutter is an aeroelastic instability phenomenon where a torsional blade mode couples to a flap-wise bending mode, leading to a mutual rapid growth of the amplitude of the flap-wise and torsional motions. In this paper, a detailed finite element (FE) model for the wind turbine system is developed, with consistent modeling of the coupling between aerodynamic, elastic and inertial loads which have significant influence on the coupled-mode (flap-wise and torsional) vibration and stability. Further, elastic coupling between blade vibrations with tower and drivetrain motions are also considered, making this model capable for coupled-mode flutter analysis of a complete wind turbine system. The parameters of the model have been calibrated to the DTU 10MW wind turbine, and the critical flutter speed of the rotor is shown to be about 1.6 times its nominal rotational speed. A novel torsional viscous damper is then proposed to suppress torsional blade vibration and to enhance flutter stability of wind turbines.