3D MODELLING OF A WIND TURBINE USING CFD

Turbine efficiency remains a critical component of the overall economic justification for a potential wind farm. There is therefore a requirement for prediction methodologies that are capable of addressing the in-situ performance of multiple turbine installations within a specific local environment and operating in a range of conditions. The work presented here is the first stage in a programme of work that aims to develop a practical engineering methodology for the CFD-based assessment of multiple turbine installations. In this first stage a CFD benchmarking exercise was performed using the wind turbine design in the Unsteady Aerodynamics Experiment (UAE) conducted by the US National Renewable Energy Laboratory. Initially blade sections were analysed in 2D and the results used to construct and validate a 3D CFD model of the turbine. The 3D results were used to develop estimates for actuator disk induction factors. Finally these factors were used to modify the classical actuator disk treatment of wind turbines. The results from the modified actuator disk model were in good agreement both with CFD and experiment. 1.0 Maximum Ideal Efficiency The ideal, frictionless efficiency of a wind turbine was predicted by A. Betz in 1920 using a simple one-dimensional model. The rotor is represented by an "actuator disk" that creates a pressure discontinuity of area A and local velocity V (1, 2). The control volume of the model is defined by a stream tube whose fluid passes through the rotor disk. The wind at the inlet to the model has an approach velocity V0 over an area A0, and a slower downstream velocity V3 over a larger area A3 at the outlet. A simple schematic of the model is given in Figure 1.