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One of the main uncertainties in airfoil and rotor design is the determination of the position of transition on the blade as this can have considerable influence on the aerodynamic load sand in particular rotor power. Even for simulations on a rotor in uniform, steady inflow the determination of the position of transition is challenging. The atmospheric inflow on a real MW rotor adds a new level of complexity and uncertainty to the transition modelling and prediction.What is the impact of the atmospheric turbulence on transition compared with transition on a blade section in wind tunnel flow ? The present work is aiming at improving our insight into this subject by analysing transition data measured in the DanAero experiment in 2009 on an industrial 2MW turbine with an 80m rotor with LM38.8 m blades. Transition position is determined from analysing spectra of high frequency (50kHz) surface pressure fluctuations from 56 flush mounted microphones at a radius of 37 m. The position of transition is correlated with measured inflow angle close to the section with transition data. A comparison of the rotor transition with a similar set-up in the LM Glasfiber wind tunnel for transition detection on a 2D blade section copy of the rotor blade, shows a much earlier transition on the pressure side of the rotor blade. On the suction side the difference is much smaller due to an already early transition on the 2D blade section in the wind tunnel, probably caused by small surface irregularities or bumps. Analysing the pressure fluctuations in the laminar boundary layer close to the leading edge which are closely correlated with the turbulent inflow it can be seen that below 300 Hz the energy level on the rotor is much higher than in the tunnel. However,above 300 Hz they are comparable. It is finally shown that the transition on the rotor on the pressure side is moving forward when the spectral energy from 100-300 Hz in the laminar boundary layer increases. more...

In order to investigate structural coupling effects such as flapwise bend-twist, edgewise bend-twist, flapwise bend-edgewise bend, and extension-twist coupling for a passive control to decrease blade fatigue loads, parametric studies are performed. In order to analyze structural coupling effects on a blade a new beam element for a composite wind turbine blade is developed and implemented into HAWC2. Eigenvalue analysis is performed to validate the new beam element. An optimization approach is suggested to find an optimized blade configuration including structural couplings to achieve the maximum reduction of the blade fatigue loads. 5MW NREL reference wind turbine is used for the study. In total 11 different types of structural couplings are investigated. Among them when a flapwise bendedgewise bend, a flapwise bend-twist, an edgewise bend-twist, and an extension-twist are coupled together the maximum reduction of blade root flapwise fatigue is achieved around 12% decrease. Blade maximum tip deflection is reduced approximately 19% as well. Life time fatigue load is also decreased approximately 12% with the optimized design. more...

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 more...

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. more...

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. more...

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. 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. more...

This paper presents a method for deriving 3D airfoil characteristics for a wind turbine on basis of field measurements of generator power and blade bending moments. The objective is to adjust the airfoil characteristics so that calculated results from an aeroelastic code correspond to the measured power and loads by minimizing the discrepancy using a numerical optimization scheme. This implies several steps: 1) Establish an aeroelastic model of the wind turbine, 2) Provide 2D airfoil characteristics, 3) Provide field measurements that are sorted for undesired operational conditions and afterwards binned and 4) Apply a numerical optimization tool to derive corrected airfoil characteristics. Using this approach airfoil characteristics was derived for a 1.5MW active stall controlled wind turbine. Aeroelastic calculations using the derived airfoil characteristics showed good agreement between flap moments in five radial positions and the electrical power. The correction of the 2D airfoil characteristics showed that maximum lift was reduced for r/R=95%, maximum lift was close to 2D data for r/R=70% and 85%, and maximum lift was significantly higher and appeared at higher angles of attack for r/R=23.45% and 50%. The drag level was in general increased. NOMENCLATURE more...

Double stall causes more than one power level when stall-regulated wind turbines operate in stall. This involves significant uncertainty on power production and loads. To avoid double stall, a new leading edge was designed for the NACA 632-415 airfoil, an airfoil that is often used in the tip region of wind turbines. A numerical optimization tool incorporating XFOIL was used with a special formulation for the airfoil leading edge shape. The EllipSys2D CFD code was used to analyze the modified airfoil. In theory and in wind tunnel tests, the modified airfoil showed smooth and stable stall characteristics with no tendency to double stall. Also, both theory and wind tunnel tests showed that the overall aerodynamic characteristics were similar to NACA 632-415 except for an increase in the lift-drag ratio below maximum lift and an increase in maximum lift. The wind tunnel tests showed that dynamic stall and aerodynamic damping characteristics for the modified airfoil and the NACA 632-415 airfoil were the same. The modified airfoil with leading edge roughness in general had better characteristics compared with the NACA 632-415 airfoil. more...