Peter Fuglsang

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

Site-Specific Design Optimization of 1.5–2.0 MW Wind Turbines

A method is presented for site-specific design of wind turbines where cost of energy is minimized. A numerical optimization algorithm was used together with an aeroelastic load prediction code and a cost model. The wind climate was modeled in detail including simulated turbulence. Response time series were calculated for relevant load cases, and lifetime equivalent fatigue loads were derived. For the fatigue loads, an intelligent sensitivity analysis was used to reduce computational costs. Extreme loads were derived from statistical response calculations of the Davenport type. A comparison of a 1.5 MW stall regulated wind turbine in normal onshore flat terrain and in an offshore wind farm showed a potential increase in energy production of 28% for the offshore wind farm, but also significant increases in most fatigue loads and in cost of energy. Overall design variables were optimized for both sites. Compared to an onshore optimization, the offshore optimization increased swept area and rated power whereas hub height was reduced. Cost of energy from manufacture and installation for the offshore site was reduced by 10.6% to 4.6 O. This reduction makes offshore wind power competitive compared with todays onshore wind turbines. The presented study was made for one wind turbine concept only, and many of the involved sub models were based on simplified assumptions. Thus there is a need for further studies of these models.

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

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.

Modification of the NACA 632-415 leading edge for better aerodynamic performance

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 which 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. The modified airfoil showed in theory and in wind tunnel tests smooth and stable stall characteristics and no tendency to double stall. Also, in theory and in wind tunnel tests 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.

A METHOD FOR DERIVING 3D AIRFOIL CHARACTERISTICS FOR A WIND TURBINE

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