Enhancement of flutter stability in wind turbines with a new type of passive damper of torsional rotation of blades

Classical flutter of wind turbine blades is defined as an aeroelastic instability where a torsional blade mode couples to a flapwise bending mode to result in a mutual rapid growth of the amplitude of the flapwise and torsional motions beyond certain rotational speed of the rotor. Till date, flutter instability of wind turbine rotor has not been considered to be a serious problem. However, with the advent of larger turbines fitted with relatively softer blades, classical flutter may become a more important design consideration. As an economically viable alternative to increasing the torsional rigidity (which is more expensive), the use of a type of viscous damper mounted inside the wind turbine blades is proposed to promote the flutter critical rotational speed of the rotor. The flutter critical rotational speed is determined by using a time-domain aeroelastic simulation technique based on the quasi-steady aerodynamic forces and also considering turbulence intensity of incoming wind. The optimal damper location and the optimal damping constant values are investigated for the DTU 10 MW wind turbine. It is shown that at an optimal tuning of the damper, the flutter critical rotational speed is only marginally dependent on the turbulence intensity of the incoming wind field, on the mean wind velocity and on the structural damping of the flutter torsional eigenmode. The novel tuned damper device proposed in this paper may increase the flutter critical rotational speed with more than 100%.