Influence of Yawed Wind Flow on the Blade Forces/Bending Moments and Blade Elastic Torsion for an Axial-Flow Wind Turbine

Effects of yawed incoming flow on wind turbine blades forces and root bending moments (RBMs) are not fully understood. To advance our current understanding, numerical studies of a small-scale three-bladed horizontal axis wind turbine at TSR = 6.7 with yaw angles of zero and 45° have been carried out to examine the variations of blade and rotor loading due to the yawed incoming flow. An approach combining Large Eddy Simulation (LES) with Actuator Line Modelling (ALM) has been employed in the present study. The predicted phase-averaged blade forces reveal that the blade tangential force, in-plane RBM and power coefficient are much more sensitive to the upstream streamwise velocity variations and are much more strongly affected than the blade axial force, out-of-plane RBM and thrust coefficient. It also shows that for yawed incoming flows the blade axial force to the blade tangential force ratio fluctuates significantly during one rotor revolution, resulting in large variations of the blade elastic torsion and that the total blade force (magnitude and direction) undergoes a non-linear change in the circumferential and radial directions, which will likely lead to the reduction in the turbine operational life significantly, especially for long lightweight blades of large size wind turbines.