Along with the upscaling tendency, lighter and so more flexible wind turbine blades are introduced for reducing material and manufacturing costs. The flexible blade deforms under aerodynamic loads and in turn affects the flow field, arising the aeroelastic problems. In this paper, the impacts of blade flexibility on the wind turbine loads, power production, and pitch actions are discussed. An advanced aeroelastic model is developed for the study. A free wake vortex lattice model instead of the traditionally used blade element momentum (BEM) method is used to calculate the aerodynamic loads, and a geometrically exact beam theory is adopted to compute the blade structural dynamics. The flap, lead-lag bending, and torsion degrees-of-freedom (DOFs) are all included and nonlinear effects due to large deflections are considered. The National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine is analyzed. It is found that the blade torsion deformations are significantly affected by both the aerodynamic torsion moment and the sectional aerodynamic center offset with respect to the blade elastic axis. Simulation results further show that the largest bending deflection of the blade occurs at the rated wind speed, while the torsion deformation in toward-feather direction continuously increases along with the above-rated wind speed. A significant reduction of the rotor power is observed especially at large wind speed when considering the blade flexibility, which is proved mainly due to the blade torsion deformations instead of the pure-bending deflections. Lower pitch angle settings are found required to maintain the constant rotor power at above-rated wind speeds.
Modeling wind turbine blades by geometrically-exact beam and shell elements: a comparative approach.