On wind turbine loading induced by non-uniform approaching flow at high Reynolds numbers

Influences of non-uniform incoming flow on the wind turbines blades forces and root bending moments (RBMs) are not fully understood. To advance our current understanding, numerical studies of a three-bladed horizontal axis wind turbine with cylinders placed in front of it to produce non-uniform flow approaching the turbine with different non-uniformity levels have been carried out to examine the variations of blade and rotor loading due to the non-uniform incoming flow. The phase-averaged predicted 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 non-uniform 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. This study also shows different behaviors of the blade forces along the blade span under non-uniform upstream flows in terms of the amplitudes and standard deviations of their oscillations. For the blade tangential force, λ and σ increase monotonously along the blade span up to near the blade tip, whereas those of the blade axial force increase up to approximately 0.6 blade span and show an opposite trend behind that.