A Study on the Capability of Backward Swept Blades to Mitigate Loads of Wind Turbines in Shear Flow

Abstract

The vertical wind shear is one of the major sources of fatigue loads on the blades of a horizontal axis wind turbine. Traditionally, the active individual pitch control system is used to alleviate the cyclic load fluctuations, which requires sensors and actuators. In this paper, a passive load control technique by using backward swept blades is explored of its potential capability of mitigating load variations in shear wind. An advanced aeroelastic model based on free wake vortex lattice model (FWVLM) and geometrically exact beam theory (GEBT) is developed for the study. Both the aerodynamic and structural solvers are able to account for the three-dimensional (3D) shape effects of the undeformed and deformed swept blades. The NREL 5-MW reference wind turbine is analyzed by adding backward sweep to the baseline blade. Comparisons are made between the baseline and swept blades for the time-varying root moments and rotor moments under sheared inflow. Different amounts of tip sweep are discussed. An effective reduction in the amplitude of the flapwise root moment variations is obtained, while the edgewise root moment is less influenced. Reduced mean values of the rotor yaw and tilt moments are also achieved, at the cost of increased blade root torsion moment.