Numerical simulation to the effect of rotation on blade boundary layer of horizontal axial wind turbine

Abstract

Two-dimensional blade element/momentum theory (BEMT) is often used in designing and calculating the performance of the blades of the wind turbine. However, in stalled conditions, the wind turbine rotor power output is under-predicted. This phenomenon, in which there are differences between the measured performances and predications based on 2D aerofoil characteristics in stalled condition, is so-called stall-delay. The reason for the stall delay has been the cause of much discussion, but a convincing physical process has not yet been established. What is agreed is that, for whatever reason, the adverse pressure gradient experienced by the flow passing over the downwind surface of the blade is reduced by the blade's rotation. The adverse pressure gradient slows down the flow as it approaches the trailing edge of the blade after the velocity peak reaches close the leading edge. In the boundary layer viscosity also slows down the flow and the combination of the two effects, and if sufficiently large, can bring the boundary layer flow to a standstill (relative to the blade surface) or even cause a reversal of flow direction. When flow reversal takes place, the flow separates from the blade surface and stall occurs, giving rise to loss of lift and a dramatic increase in pressure drag. This paper is aimed at describing the effect of rotation on the blade boundary layer of a wind turbine by solving the 3D- and 2D-NS equations. An NREL Phase VI test turbine is used as the numerical model. The grid is generated in ANSYS ICEM 12.0. Both Hex and Tetra mesh are used to increase the accuracy with small-scale computations. Commercial code FLUENT and the MRF method were chosen to solve the fluid fields around 3D wind turbine blade and 2D airfoil. We found that, compared with 2D airfoil, the stall on 3D blade is postponed due to the rotation and the separation point is delayed with the increase of rotation speed or decrease of the blade spanwise position.