Impact of Yaw–Induced Unsteady Aerodynamics on BEM Prediction Accuracy: CFD Analysis Based on the NREL Phase VI Wind Turbine

Abstract. Accurately predicting the aerodynamic loads on wind turbine blades under yawed inflow remains a major challenge due to the complexity of three–dimensional unsteady flow phenomena. This study combines high-fidelity computational fluid dynamics (CFD) simulations and NREL Phase VI experimental data with a newly proposed normalized absolute error metric to evaluate the prediction accuracy of the modified blade element momentum (BEM) method under yawed and non-yawed inflow conditions, thereby quantitatively assessing the differences between different yaw angles, inflow velocities, and spanwise blade positions. In addition, the CFD results are employed to analyze the potential flow mechanisms responsible for the deterioration of BEM prediction accuracy. The results show that while the BEM method maintains high accuracy under non–yawed attached flow conditions, its performance deteriorates significantly under flow separation and yawed inflow. At a yaw angle of 30 and an inflow velocity of 15 m/s, the force coefficient (C_n) prediction error at the blade root increases to 48.6 %, exceeding the non–yawed case by more than 20 %. Flow field analyses reveal that yawed inflow intensifies vortex interactions on the leeward side and induces strong spanwise vortex bands driven by Coriolis forces, causing stall regions to propagate from blade tip to root. These phenomena lead to severe local aerodynamic load fluctuations that are not captured by conventional BEM formulations based on steady–state assumptions. This study quantitatively demonstrates the degradation of BEM prediction accuracy under yawed conditions and systematically reveals the direct impact of stall vortex evolution on aerodynamic load variations. These findings provide physical insights for the development of next–generation aerodynamic models incorporating three–dimensional unsteady flow corrections.