Deriving aerofoil data for very large offshore wind turbine blades using CFD

Abstract

To improve the cost effectiveness of wind energy, the offshore wind turbine industry is constantly developing larger rotors. However, doing so creates several challenges, especially from an engineering perspective. In addition to increasing the structural demands, larger wind turbines are also subjected to complex aerodynamic flow characteristics. This is mainly due to the fact that the rotating blades are subjected to higher Reynolds numbers. Furthermore, stall delay phenomena are typically present. These phenomena are known to affect the lift and drag characteristics of a wind turbine and, therefore, affect the power output. Therefore, significant research efforts are taking place to correct any available two-dimensional (2D) aerofoil data, to account for these effects by using empirical models. In relation to this, experimental three-dimensional (3D) lift and drag data for similar operating conditions are not available for large wind turbines. The first objective of this project was to further develop a Computational Fluid Dynamics (CFD) model of the IEA 15 MW reference wind turbine, under different operating conditions, and to validate its performance characteristics. These performance characteristics mainly include the coefficients of thrust and power. The second and most important objective was to estimate the 3D coefficients of lift and drag, at different spanwise locations, by obtaining information regarding the loads and flow field at the rotor plane. By generating several simulations, in general, the CFD solver produced reasonable and expected results. Apart from validating the force distributions, the 3D coefficients were also compared to the limited 2D aerofoil available. In addition, the 3D coefficients obtained at different blade pitch angles were compared with each other. Considering the limited aerofoil data available, general agreement was present between the results. Towards the inboard sections of the rotor, stall delay effects were also present. Most of the discrepancies present were generally for angles of attack which are close to the stall region. That is, the numerical model pre-maturely predicted the stall angles of attack. In relation to this, the thesis also includes a detailed section which delves into possible reasons for this lack of performance at high angles of attack.