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A Variable-Fidelity Modeling (VFM) method has been developed as an efficient and accurate aerodynamic data modeling strategy. In this approach, a set of CFD methods with varying degrees of fidelity and computational expense is exercised to reduce the number of expensive high-fidelity computations. Kriging-based bridge functions are constructed to match the low-and high fidelity CFD data. The method is demonstrated by constructing a global approximation model of the aerodynamic coefficients of an RAE 2822 airfoil based on sampled data. The model is adaptively refined by inserting additional samples. It is shown that the method is promising for efficiently generating accurate aerodynamic models that can be used for the rapid prediction of aerodynamic data across the flight envelope.

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A new airfoil family, called NPU-MWA (Northwestern Polytechnical University Multi-megawatt Wind-turbine A-series) airfoils, was designed to improve both aerodynamic and structural performance, with the outboard airfoils being designed at high design lift coefficient and high Reynolds number, and the inboard airfoils being designed as flat-back airfoils. This article aims to design a multi-megawatt wind turbine blade in order to demonstrate the advantages of the NPU-MWA airfoils in improving wind energy capturing and structural weight reduction. The distributions of chord length and twist angle for a 5 MW wind turbine blade are optimized by a Kriging surrogate model-based optimizer, with aerodynamic performance being evaluated by blade element-momentum theory. The Reynolds-averaged Navier-Stokes equations solver was used to validate the improvement in aerodynamic performance. Results show that compared with an existing NREL (National Renewable Energy Laboratory) 5 MW blade, the maximum power coefficient of the optimized NPU 5 MW blade is larger, and the chord lengths at all span-wise sections are dramatically smaller, resulting in a significant structural weight reduction (9%). It is shown that the NPU-MWA airfoils feature excellent aerodynamic and structural performance for the design of multi-megawatt wind turbine blades.Energies 2019, 12, 3330 2 of 24 designed by National Aeronautical Research Institute of Sweden [10]. The Northwestern Polytechnical University (NPU) of China has developed a family of NPU-WA (Northwestern Polytechnical University Wind-turbine A-series) airfoils for megawatt wind turbines, featuring a high lift-to-drag ratio at a high Reynolds number and a high lift coefficient [11][12][13][14]. The Chinese Academy of Sciences (CAS) has designed a family of CAS airfoils for medium Reynolds numbers [15]. The Chongqing University of China and Technical University of Denmark have developed the CQU-DTU-series airfoils, taking into account of the aerodynamic performance and low noise requirements [16]. Compared with the conventional aeronautic airfoils, the dedicated airfoils for wind turbine blades have a higher lift-to-drag ratio, which means that the aerodynamic force of the blades has a larger component in the tangential direction of the wind wheel and lower component in the axial direction (thrust), resulting in smaller thrust with the same tangential component of aerodynamic force, thus the total aerodynamic load is reduced.With the continuous development of wind turbine technology, the wind turbine generator system tends to become larger and larger. The blade diameter of wind turbines has increased from about 20 m in the 1980s to nearly 200 m, or even more now. Meanwhile, the capacity has also been increased, from 20 kW~60 kW to 5 MW~10 MW [17][18][19]. In recent years, the technology for applying the wind turbine-dedicated airfoil family to megawatt wind turbines (1.0 MW, 3.0 MW) has gradually matured. More and more attention has been paid to the development of new airfoil families dedicated t...