Large Eddy Simulations and Theoretical Analysis of Wind Turbine Aerodynamics Using an Actuator Line Model

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Johns Hopkins University
The actuator line model (ALM) is a widely used tool to represent the wind turbine blades when performing numerical simulations of flow over wind turbines and wind farms. The ALM is used to represent wind turbine blades without the need to resolve the full geometry of the blades. In this work, the ALM is implemented in a large-eddy simulation (LES) research code and compared with 3 other numerical codes in the wind energy community. Excellent agreement is observed in the implementation of the ALM amongst all the codes. Comparisons against experimental measurements are also performed. From the experimental comparisons, it is found that in the case of a small scale wind tunnel experiment, a nacelle and tower models are required to match the near wake. After the comparisons with other codes and experiments, a new theoretical approach is used to improve the ALM. In the ALM, the parameter $\epsilon$ establishes the width over which body forces are distributed. An optimal $\epsilon_{\rm opt}$ is found based on two dimensional aerodynamics. This optimal body force is then tested in a three dimensional simulation of a wind turbine under uniform inflow. The optimal body force is able to resolve the tip vortex and the tip losses are well reproduced. Then, a new theoretical framework is developed to predict the behavior near the tip in the ALM. This framework can be used to predict the tip losses for a given body force width $\epsilon$ and to correct them based on the optimal $\epsilon_{\rm opt}$. This improved ALM has ability to better predict the quantities along the blades such as lift and drag forces, and thus can provide better power predictions from the rotor.
computational fluid dynamics, wind energy, large eddy simulations, actuator line model