The core of the analysis is the iterative computation of the power coefficient of a given airfoil during each step of optimization. The integrated design of airfoil family and blade can start from the BEM analysis of an airfoil section at a given blade station. The integrated design of airfoil and blade The flow conditions, such as Reynolds number, angle of attack, Mach number and transition condition are written in an input script that is recognized by the Xfoil code.ģ. The code is used iteratively inside the optimization. The numerical tool Xfoil developed by Drela is applied for airfoil boundary layer calculations. To model the airfoil roughness condition, the free transition simulation is based on the e n model, where n = 9 the forced transition simulation is performed by giving the upper and lower transition points at 5% and 10% chords position, measured from the leading edge. Such a wide range of angle of attack also takes into account the off‐design condition. Numerical computations go through each angle of attack with a step of 1°. To consider different flow conditions, the design angle of attack is between 3° and 10°. According to the local flow at each blade section, the Mach number at each blade section also varies. The LN1xx, LN2xx and LN3xx airfoils are designed at Reynolds number around 1.6 × 10 6, 3 × 10 6 and 16 × 10 6, respectively, depending on the airfoil chord and radial location. This chapter is organized as follows: Section 2 introduces the 2D airfoil design method Section 3 presents the airfoil and blade integrated design method Section 4 presents results from aerodynamic and aeroacoustic simulations and Section 5 concludes the work. A 2D blade element momentum (BEM) model is introduced during the optimization loop, which iteratively computes the local maximum C p at each rotor cross‐section. The advantage is that the flow geometry over each rotor cross‐section is optimized. In the optimization process, the airfoils and blade design is integrated. The above‐mentioned LN2xx and LN3xx airfoils are designed for a 3 MW wind turbine and a 20 MW wind turbine, respectively. To develop high‐performance wind turbine blade, it is desired to have the airfoils designed for a specific blade. All of these airfoils are noise constrained with a semi‐empirical noise prediction model. These airfoils are generated with a shape perturbation function that uses the LN1xx airfoils as baseline airfoils. Based on these airfoils, the LN2xx and LN3xx airfoil families are further developed. Using this method, the DTU‐LN1xx airfoils are designed. To begin with airfoil and blade design, a profile shape can be described by the Joukowski transformation such that an airfoil shape can be represented by a series of trigonometric functions. As the most important design objective, these wind turbine airfoils meet the demand of high lift to drag ratio and some of them are designed for low‐noise emission. Since 1990s, many researchers at Technical University of Denmark (DTU) have made advance research in designing wind turbine airfoils. Later on, Björk, Timmer and Van Rooij, Dahl and Fuglsang, and Fuglsang and Bak made some significant contributions in designing wind turbine airfoils, and the designed airfoils were named with the institution's names (FFA, DU, and RISØ airfoils). Previous works on wind turbine airfoil design were aimed at high lift and high lift to drag ratio, such as the Wortmann FX 77‐W‐series airfoils and the NREL airfoils. In addition, there is a low frequency limit (<20 dB) applied for wind turbine noise received inside a nearby house. In Denmark, the noise limits for wind turbine in open area is 44 dB at a wind speed of 8 m/s and 42 dB at a wind speed of 6 m/s. The wind turbine noise regulation is restricted in Europe because it is more densely populated than many other countries outside EU. Therefore, the overall objective behind the design work is to make wind energy production more efficient, while at the same time lowering noise emission through gaining fundamental insight into the airfoil noise generation mechanisms. When it comes to social acceptance, the noise aspect becomes very important in particular for onshore turbines, such that low‐noise wind turbine design is an important competitive parameter. For using wind energy, one of the major objectives in wind turbine airfoil design is to achieve a high aerodynamic performance that ensures wind turbine blade to operate with high‐power performance. Also, as the size of wind turbine is getting larger, rotor noise has become a barrier for future development. Other types of air pollutants such as SO x, NO x and dust are becoming significant problems for some cities at densely populated areas. In addition to the energy shortage, CO 2 emission is the worldwide concern of using conventional energy sources. Wind energy is one of the most attractive energy sources compared with the coal‐based energy sources.
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