Christian Bak

Potential load reduction using airfoils with variable trailing edge geometry

This paper presents an investigation of the potential for reduction of fluctuating loads on wind turbine blades with the use of flaplike deflectable trailing edges. More specifically, the aeroelastic response of an elastically mounted airfoil section with a deflectable trailing edge is investigated. This is done by coupling a model for the aerodynamic forces on a deforming airfoil with a linear spring/damper model for the elastic deformation of a rigid airfoil to which the forces associated with the deflection of the trailing edge are added. The analysis showed that when the airfoil experienced a wind step from 10 to 12 m/s the standard deviation of the normal force could be reduced by up to 85% when the flap was controlled by the reading of the airfoil flapwise position and velocity, while reductions of up to 95% could be obtained when the flap was controlled by the reading of the angle of attack. When the airfoil experienced a turbulent wind field, the standard deviation of the normal force could be reduced by 81% for control based on measured angle of attack. The maximum reduction using a combination of flapwise position and velocity was 75%. The maximum deflection of the trailing edge geometry was, in all the considered cases, small enough to justify the use of a potential flow code for calculation of the aerodynamic forces. Calculations showed that the effect of a time lag in the actuators and sensors may drastically reduce the efficiency of the control algorithm. Likewise, the effect of a low maximum actuation velocity reduces the efficiency of the control algorithm. The analysis of the two-dimensional (2D) aeroservoelastic system shown in this paper indicates that the potential of using trailing edge flaps for reduction of fluctuating loads is significant.

Wind Tunnel Test on Wind Turbine Airfoil with Adaptive Trailing Edge Geometry

,A wind tunnel test of the wind turbine airfoil Ris oe-B1-18 airfoil equipped with an Adaptive Trailing Edge Geometry (ATEG) was carried out. The ATEG was made by piezo electric actuators attached to the trailing edge of a non-deformable airfoil and controlled by an amplifier. The airfoil was tested at Re = 1.66x10 6 . Steady state and dynamic tests were carried out with prescribed deflections of the ATEG. The steady state tests showed that deflecting the ATEG towards the pressure side (posi tive β) translated the lift curve to higher lift values and deflecting the ATEG towards the suc tion side (negative β β β β) translated the lift curve to lower lift values. Furthermore, cd was almost unaffected by the ATEG actuation. Testing the airfoil for a step change of the ATEG f rom β=-3.0 to +1.8 showed that the obtainable Δcl was 0.10 to 0.13 in the linear part of the lift cu rve. Modeling the step response with an indicial function formulation showed that t he time constant in the step change and in sinusoidal deflections in dimensionless terms was T0* =0.6. Testing the ability of the ATEG to cancel out the load variations for an airfoil in si nusoidal pitch motion showed that it was possible to reduce the amplitude with around 80% from Δ Δ Δ Δcl=0.148 to Δcl=0.032.

Design and Verification of the Risø-B1 Airfoil Family for Wind Turbines

This paper presents the design and experimental verification of the Riso-B1 airfoil family for MW-s ize wind turbines with variable speed and pitch control . Seven airfoils were designed with thickness-to-chor d ratios between 15% and 53% to cover the entire span of a wind turbine blade. The airfoils were designed to have high maximum lift coefficient to allow a slender flexible blade while maintaining high aerodynamic efficiency. The design was carried out with a Riso inhouse multi disciplinary optimization tool. Wind tu nnel testing was done for Riso-B1-18 and Riso-B1-24 in t he VELUX wind tunnel, Denmark, at a Reynolds number of 1.6 ×10 6 . For both airfoils the predicted target characteristics were met. Results for Riso-B1-18 showed a maximum lift coefficient of 1.64. A standa rd case of zigzag tape leading edge roughness caused a drop in maximum lift of only 3.7%. Cases of more severe roughness caused reductions in maximum lift between 12% and 27%. Results for the Riso-B1-24 airfoil showed a maximum lift coefficient of 1.62. The standard case leading edge roughness caused a drop in maximum lift of 7.4%. Vortex generators and Gurney flaps in combination could increase maximum lift up to 2.2 (32%). NOMENCLATURE

A Detailed investigation of the Blade Element Momentum (BEM) model based on analytical and numerical results and proposal for modifications of the BEM model

The paper presents the results of a comprehensive investigation of the BEM model based on detailed results from actuator disc simulations as well as analytical derivations. The objectives of this work has been to investigate the deficiencies in the BEM model, which is the most common engineering model for computation of the aerodynamic loads on wind turbine rotors and used widely within the industry. An additional objective has been to derive modifications to the BEM model in order to improve the accuracy of the model. Our comparisons of numerical results from the actuator disc simulations with BEM results have shown two areas of deviations. On the inner part of the rotor the BEM model overestimates the induction due to neglecting the pressure term from wake rotation. On the outer part of the rotor the tendency is opposite with an underestimation of the induction by the BEM model which seems to be an effect from expansion of the stream tubes. Two simple correction models to the BEM model were derived to account for these deviations and the results of the modified BEM model correlate very well with actuator disc results for different load distributions.

Trailing Edge Noise Model Validation and Application to Airfoil Optimization

The aim of this article is twofold. First, an existing trailing edge noise model is validated by comparing with airfoil surface pressure fluctuations and far field sound pressure levels measured in three different experiments. The agreement is satisfactory in one case but poor in two other cases. Nevertheless, the model reproduces the main tendencies observed in the measurements with respect to varying flow conditions. Second, the model is implemented into an airfoil design code that is originally used for aerodynamic optimization. An existing wind turbine airfoil is optimized in order to reduce its noise emission, trying at the same time to preserve some of its aerodynamic and geometric characteristics. The new designs are characterized by less cambered airfoils and flatter suction sides. The resulting noise reductions seem to be mainly achieved by a reduction in the turbulent kinetic energy across the boundary layer near the trailing edge and to a lesser extent by a smaller boundary layer displacement thickness.

The DAN-AERO MW Experiments

The paper describes the DAN-AERO MW experiments carried out within a collaborative, three years research project between Riso DTU and the industrial partners LM Glasfiber, Siemens Wind Power, Vestas Wind Systems and finally the utility company DONG Energy. The main objective of the project is to establish an experimental data base which can provide new insight into a number of fundamental aerodynamic and aeroacoustic issues, important for the design and operation of MW size turbines. The most important issue is the difference between airfoil characteristics measured under 2D, steady conditions in a wind tunnel and the unsteady 3D flow conditions on a rotor. The different transition characteristics might explain some of the difference between the 2D and 3D airfoil data and the experiments have been set up to provide data on this subject. The overall experimental approach has been to carry out a number of coordinated, innovative measurements on full scale MW size rotors as well as on airfoils for MW size turbines in wind tunnels. Shear and turbulence inflow characteristics were measured on a Siemens 3.6 MW turbine with a five hole pitot tube. Pressure and turbulent inflow characteristics were measured on a 2MW NM80 turbine with an 80 m rotor. One of the LM38.8 m blades on the rotor was replaced with a new LM38.8 m blade where instruments for surface pressure measurements at four radial sections were build into the blade during the blade production process. Additionally, the outmost section on the blade was further instrumented with around 60 microphones for high frequency surface pressure measurements. The surface

Observations and hypothesis of double stall

The double-stall phenomenon of aerofoil flows is characterized by at least two distinct stall levels for identical inflow conditions. In the present work a likely explanation of double stall is presented. Observations on full-scale rotors, in wind tunnel experiments and in CFD calculations could show at least two different distinct lift levels for identical inflow conditions, with sudden shifts between them. CFD calculations revealed the generation of a small, laminar separation bubble at the leading edge of the aerofoil for incidences near maximum lift. The bursting of this bubble could explain the sudden shift in lift levels. This investigation indicated that bursting will occur if the position of the free transition is only a small distance upstream from the position where forced transition would first cause leading-edge stall. Thus the investigation indicated that double stall is closely related to the actual geometry of the leading edge of the aerofoil and that it probably can be avoided in the design of new aerofoils. The investigation indicated further that double stall can be predicted from CFD calculations. Copyright

Sensitivity of Key Parameters in Aerodynamic Wind Turbine Rotor Design on Power and Energy Performance

In this paper the influence of different key parameters in aerodynamic wind turbine rotor design on the power efficiency, Cp, and energy production has been investigated. The work was divided into an analysis of 2D airfoils/blade sections and of entire rotors. In the analysis of the 2D airfoils it was seen that there was a maximum of the local Cp for airfoils with finite maximum Cl/Cd values. The local speed ratio should be between 2.4 and 3.8 for airfoils with maximum cl/cd between 50 and 200, respectively, to obtain maximum local Cp. Also, the investigation showed that Re had a significant impact on CP and especially for Re<2mio corresponding to rotors below approximately 400kW this impact was pronounced. The investigation of Cp for rotors was made with three blades and showed that with the assumption of constant maximum cl/cd along the entire blade, the design tip speed ratio changed from X=6 to X=12 for cl/cd=50 and cl/cd=200, respectively, with corresponding values of maximum cp of 0.46 and 0.525. An analysis of existing rotors re-designed with new airfoils but maintaining the absolute thickness distribution to maintain the stiffness showed that big rotors are more aerodynamic efficient than small rotors caused by higher Re. It also showed that the design tip speed ratio was very dependent on the rotor size and on the assumptions of the airfoil flow being fully turbulent (contaminated airfoil) or free transitional (clean airfoil). The investigations showed that rotors with diameter D=1.75m, should be designed for X around 5.5, whereas rotors with diameter D=126m, should be designed for Xbetween 6.5 and 8.5, depending on the airfoil performance.

Design and verification of airfoils resistant to surface contamination and turbulence intensity

This paper presents the design of high performance airfoils for incompressible ∞ow and for Reynolds numbers at 6mio with a lift performance which is resistant to surface contamination and turbulence intensity. The Ris?-C2 airfoil family is dedicated for MW-size wind turbines, which are exposed to varying in∞ow conditions and surface contamination from bugs and dust. The airfoils were designed to have high maximum lift coe‐cient, while maintaining high aerodynamic e‐ciency. Given these characteristics the airfoils were designed with maximum stifiness. The design was carried out with a quasi 3D multi disciplinary optimization tool to take into account the complete blade shape. The design of the Ris?-C2-18 airfoil was verifled in the LM Glasflber wind tunnel, Denmark and showed good agreement with predicted characteristics.

3D Navier-Stokes Simulations of a rotor designed for Maximum Aerodynamic Efficiency

The present paper describes the design of a three-bladed wind turbine rotor taking into account maximum aerodynamic efficiency only and not considering structural as well as offdesign issues. The rotor was designed assuming constant induction for most of the blade span, but near the tip region a constant load was assumed. The rotor design was obtained using an Actuator Disc model and was subsequently verified using both a free wake Lifting Line method and a full 3D Navier-Stokes solver. Excellent agreement was obtained using the three models. Global mechanical power coefficient, CP, reached a value of slightly above 0.51, while global thrust coefficient, CT, was 0.87. The local power coefficient, Cp, increased to slightly above the Betz limit on the inner part of the rotor as well as the local thrust coefficient, Ct, increased to a value above 1.1. This agrees well with the theory of de Vries which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation both Cp and Ct will increase towards the root. Towards the tip both Cp and Ct decrease due to tip corrections as well as drag.