Effects of continuously changing inlet wind direction on near-to-far wake characteristics behind wind turbines over flat terrain

The wake characteristics of a utility-scale wind turbine under realistic atmospheric boundary layer conditions are affected by the continuously changing wind direction arriving at the wind turbine. In the present study, the effects of continuous changes in the incoming wind direction were studied for a wind turbine on flat terrain, with the objective of understanding the wake characteristics of the wind turbine. Thus, understanding the effects of continuously changing incoming wind direction on the wake characteristics of wind turbines over flat terrain is important in the design of wind farm layouts, including in the design of offshore wind power plants. For this purpose, a computational fluid dynamics (CFD) approach using large-eddy simulations (LES) was adopted in the present study. An in-house LES-solver based on the actuator line (AL) aerodynamics technique was constructed in order to successfully capture the wake structure behind the wind turbine. First, experimental investigations on both a blade-only wind turbine scale model and a full 3D wind turbine scale model (isolated wind turbine) were conducted for a fixed inlet wind condition, the latter including the nacelle and the tower. Through a detailed comparison of the wind tunnel experimental and numerical results, the prediction accuracy of the in-house LES-solver was verified and validated for fixed inlet wind conditions. On the basis of the validation results obtained, and using the full 3D wind turbine scale model, the effects of the continuously changing inlet wind conditions on the wake characteristics in the near- and far-wake regions were numerically investigated. In addition, the effects of the wind turbine nacelle and tower on the wake characteristics were also investigated. The numerical results show that the most significant impact of the effects of the continuously changing wind direction was the rapid recovery of the mean velocity deficits in the wind turbine wake region. Further, at the x = 10D position (D is the rotor diameter) downstream of the wind turbine, the non-dimensional streamwise mean velocity was 0.93, which nearly matches the approaching flow speed, under an optimal tip speed ratio of 4.0, compared to the fixed wind direction scenario.