Modeling the effects of active wake mixing on wake behavior through large scale coherent structures

Abstract. The use of active wake mixing (AWM) to mitigate downstream turbine wakes has created new opportunities for reducing power losses in wind farms. However, many current analytical or semi-empirical wake models do not capture the flow instabilities which are excited through the blade pitch actuation. In this work, we develop a framework for modeling AWM which accounts for the impacts of the large-scale coherent structures and turbulence on the mean flow. The framework uses a triple-decomposition approach for the unsteady flow field, and models the mean flow and fine-scale turbulent scales with a parabolized Reynolds Averaged Navier-Stokes (RANS) system. The wave components are modeled using a simplified spatial linear stability formulation, which captures the growth and evolution of the coherent structures. Comparisons with the high fidelity Large Eddy Simulations (LES) of the turbine wakes showed that this framework was able to capture the additional wake mixing and faster wake recovery in the far wake regions for both the pulse and helix AWM strategies with minimal computational expense. In the near wake region, some differences are observed in both the RANS velocities profiles and initial growth of the large-scale structures, which may be due to some simplifying assumptions used in the model.