Rotational effects on the blade flow of a horizontal axis wind turbine under axial and yawed inflow conditions

Unsteady aerodynamic loads and fluctuating output power of wind turbines remain challenging to predict accurately due to the incomplete understanding of blade flow mechanisms. Rotational effects can significantly impact the wind turbine aerodynamic performance. Therefore, this paper presents a careful investigation into rotational flow characteristics of the NREL Phase VI blade under axial and yawed inflow conditions based on the DDES method. Rotational effects change the flow regime from the massive 2D trailing-edge separation into the moderate 3D leading-edge separation on the NREL Phase VI blade. Flow separation is found sufficient but not necessary for the radial flow. Under the axial inflow, rotational effects can effectively suppress the flow sepa-ration and increase the nonlinear aerodynamic loads. Under the yawed inflow, rotational effects also effectively suppress the unsteady separated flow and accelerate the flow reattachment, particularly on the downwind side. Sectional lift coefficient is therefore greatly increased at the high angles of attack due to the pronounced rota-tional effects. Increasing the yaw angle and wind speed may also strengthen the rotational effects and widen the difference between the 2D airfoil flow and 3D sectional flow. These findings imply that flow regime and aero-dynamic hysteresis on the inboard blade can be substantially changed in comparison with the 2D airfoil flow due to strong rotational effects. This study might deepen the understanding of rotational effects on the near-wall flow of wind turbine blades and also improve the aerodynamic modelling.