LES and Proper Orthogonal Decomposition analysis of vertical entrainment of kinetic energy in large wind farms (Invited)

Vertical entrainment of kinetic energy has been shown to be an important limiting factor in the performance of very large wind turbine arrays. Given high Reynolds numbers and domain sizes on the order of kilometers, we rely on wall-modeled Large Eddy Simulation (LES) to predict flow within large wind farm. We use Proper Orthogonal Decomposition (POD) to identify energetically important large-scale structures in the flow. The primary large-scale structures are found to be streamwise counter-rotating vortices located above the height of the wind turbines. The contribution of each flow structure to the kinetic energy entrainment is quantified. Surprisingly, fewer flow structures (POD modes) contribute to the vertical kinetic energy flux than to the kinetic energy in the flow, for which the POD analysis is optimal. While the general characteristics of the flow structures are robust, the net kinetic energy entrainment to the turbines depends on the orientation of the wind turbines in the array. The various modes' contributions to variability and intermittency is also quantified. The POD analysis is performed for aligned and staggered wind turbine arrays as well as for atmospheric flow without wind turbines. This research is supported by a NSF Graduate Fellowship and by the WINDINSPIRE project, funded through NSF-OISE 1243482.