Blade element momentum (BEM) methods are still the most common methods used for predicting the aerodynamic loads during the aeroelastic design of wind turbine blades. However, their accuracy is limited by the availability of reliable aerofoil data. Owing to the 3D nature of the flow over wind turbine blades, the aerofoil characteristics will vary considerably from the 2D aerofoil characteristics, especially at the inboard sections of the blades. Detailed surface pressure measurements on the blade surfaces may be used to derive more realistic aerofoil data. However, in doing so, knowledge of the angle of attack distributions is required. This study presents a method in which a free wake vortex model is used to derive such distributions for the NREL Phase VI wind turbine under different operating conditions. The derived free wake geometry solutions are plotted together with the corresponding wake circulation distribution. These plots provide better insight into how circulation formed at the blades is eventually diffused into the wake. The free wake model is described and its numerical behaviour is examined. Copyright © 2006 John Wiley &Sons, Ltd.

Tip vortex locations have been measured in the wake of a model rotor in both axial flow and yaw using quantitative flow visualization. For each setting, the axial force coefficient has been derived, as well, from measurements. The results agree well with those previously published on the Delft University of Technology model rotor. The main interest is to determine the tip vortex pitch, wake skew angle, wake expansion, and to physically interpret the data. The results also help to validate and construct models. The tip vortex location data complement the existing skewed wake velocity data from hot-wire anemometry, making it a valuable experimental database.

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a b s t r a c tPrevious experimental work under controlled conditions on a small scale floating offshore horizontal axis wind turbine has shown an increasing amplitude of the cyclic thrust and power generation against tip speed ratio under the influence of surge motion. A numerical study is performed using an actuator disc Navier Stokes model, a Blade Element Momentum model and a Generalized Dynamic Wake model on the NREL 5 MW reference rotor in order to confirm or reject these observations on a full-scale surging rotor. The hypothesis was confirmed and the underlying reasons for the observed behaviour were studied on the basis of the near wake physics. It was found that the analysis of transient effects such as fatigue cannot be performed without an adequate aerodynamic model of the wake. Characterization of quasisteady and unsteady regimes may be useful to establish when detailed aerodynamic wake models are necessary.