The effect of Coriolis force on the coherent structures in the wake of a 5MW wind turbine

This study aims to investigate the effect of Coriolis acceleration on the coherent structures in the wake of the NREL-5MW wind turbine, using the Dynamic Mode Decomposition (DMD). The Coriolis acceleration induces an altitude-dependent lateral deviation of the incoming wind direction (veer), which can substantially affect the performance of wind turbines. Large eddy precursor simulations are carried out to generate turbulent inlet velocity profiles. The presence of a veer stretches the turbine wake and influences the dynamics of the coherent structures. We decompose the flow structures to extract a limited subset of relevant flow features that optimally approximate the original data sequence, using the Sparsity-Promoting (SP) version of the DMD algorithm to rank the most relevant modes. It is found that wind veer induces coherent structures with a spanwise velocity component of the same order of the streamwise one and oblique shape. Using proper orthogonal decomposition, we analyze the contribution of the coherent structures to the mean kinetic entrainment. We found that due to wind veer, shorter wavelength mode pairs contribute mostly to wake recovery. Finally, we derive a reduced order model using approximately 70 modes pairs, which reproduces the flow structure with 5%-7% error.