As an essential ingredient in the blade element momentum theory, the tip loss effect of rotors plays an important role in the prediction of wind turbine performance. Various tip loss corrections based on the Prandtl tip loss function are analysed in the article. Comparisons with measurements and theoretical analyses show that existing tip loss correction models are inconsistent and fail to predict correctly the physical behaviour in the proximity of the tip. A new tip loss correction model is proposed that remedies the inconsistency. Comparisons between numerical and experimental data show that the new model results in much better predictions of the loading in the tip region. Copyright © 2005 John Wiley & Sons, Ltd.

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This paper presents the wind turbine airfoil development at Risø. The design method is described together with our target characteristics for wind turbine airfoils. The use of the CFD code Ellipsys2D for prediction of final target characteristics is described together with the VELUX wind tunnel testing setup. Three airfoil families were developed; Risø-A1, Risø-P and Risø-B1.The Risø-A1 airfoil family was developed for rotors of 600 kW and larger.Wind tunnel testing and field testing showed that this airfoil family is well suited for stall and active stall control. However, sensitivity to roughness was higher than expected. Field tests of a 600 kW active stall wind turbine showed an estimated reduction in blade fatigue loading of up to 15% at the same annual energy yield and at the same time reduced blade weight and blade solidity. The Risø-P airfoils were developed to replace the Risø-A1 airfoils for use on pitch controlled wind turbines.Improved design objectives should reduce the sensitivity to roughness, but measurements are not yet available.The Risø-B1 airfoil family was developed for variable speed operation with pitch control of large megawatt sized rotors. Wind tunnel testing verified the high maximum lift for these airfoils, and the airfoils were found to be very insensitive to leading edge roughness. Performance with vortex generators and Gurney flaps in combination was found to be attractive for the blade root part. Field testing of a 1·5 MW rotor is in progress.

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 present work contains a deformable trailing edge fl ap controller integrated in a numerically simulated modern, variablespeed, pitch-regulated megawatt (MW)-size wind turbine. The aeroservoelastic multi-body code HAWC2 acts as a component in the control loop design. At the core of the proposed controller, all unsteady loads are divided by frequency content. Blade pitching and generator moment react to low-frequency excitations, whereas fl aps deal with high-frequency excitations. The present work should be regarded as an investigation into the fatigue load reduction potential when applying trailing edge fl aps on a wind turbine blade rather than a conclusive control design with traditional issues like stability and robustness fully investigated. Recent works have shown that the fatigue load reduction by use of trailing edge fl aps may be greater than for traditional pitch control methods. By enabling the trailing edge to move independently and quickly along the spanwise position of the blade, local small fl utuations in the aerodynamic forces can be alleviated by deformation of the airfoil fl ap. Strain gauges are used as input for the fl ap controller, and the effect of placing strain gauges at various radial positions on the blade is investigated. An optimization routine minimizes blade root fatigue loads. Calculations are based on the 5 MW reference wind turbine part of the UpWind project primarily with a mean turbulent wind speed close to rated power. A fatigue load reduction of 25% in the blade root moment was obtained for a continuous 6.3 m long fl ap. 194Flaps and wind turbine controllers using strain gauge sensors P. B. Andersen et al. Wind Energ. 2010; 13:193-206 196 Flaps and wind turbine controllers using strain gauge sensors P. B. Andersen et al.