A comprehensive investigation of the Blade Element Momentum (BEM) model using detailed numerical simulations with an axis symmetric actuator disc (AD) model has been carried out. The present implementation of the BEM model is in a version where exactly the same input in the form of non-dimensional axial and tangential load coeffi cients can be used for the BEM model as for the numerical AD model. At a rotor disc loading corresponding to maximum power coeffi cient, we found close correlation between the AD and BEM model as concerns the integral value of the power coeffi cient. However, locally along the blade radius, we found considerable deviations with the general tendency, that the BEM model underestimates the power coeffi cient on the inboard part of the rotor and overestimates the coeffi cient on the outboard part. A closer investigation of the deviations showed that underestimation of the power coeffi cient on the inboard part could be ascribed to the pressure variation in the rotating wake not taken into account in the BEM model. We further found that the overestimation of the power coeffi cient on the outboard part of the rotor is due to the expansion of the fl ow causing a non-uniform induction although the loading is uniform. Based on the fi ndings we derived two small engineering sub-models to be included in the BEM model to account for the physical mechanisms causing the deviations. Finally, the infl uence of using the corrected BEM model, BEM cor on two rotor designs is presented.

The blade element momentum (BEM) method is widely used for calculating the quasi-steady aerodynamics of horizontal axis wind turbines. Recently, the BEM method has been expanded to include corrections for wake expansion and the pressure due to wake rotation (BEM cor ), and more accurate solutions can now be obtained in the blade root and tip sections. It is expected that this will lead to small changes in optimum blade designs. In this work, BEM cor has been implemented, and the spanwise load distribution has been optimized to find the highest possible power production. For comparison, optimizations have been carried out using BEM as well. Validation of BEM cor shows good agreement with the flow calculated using an advanced actuator disk method. The maximum power was found at a tip speed ratio of 7 using BEM cor , and this is lower than the optimum tip speed ratio of 8 found for BEM. The difference is primarily caused by the positive effect of wake rotation, which locally causes the efficiency to exceed the Betz limit. Wake expansion has a negative effect, which is most important at high tip speed ratios. It was further found that by using BEM cor , it is possible to obtain a 5% reduction in flap bending moment when compared with BEM. In short, BEM cor allows fast aerodynamic calculations and optimizations with a much higher degree of accuracy than the traditional BEM model.