Large Eddy Simulation and Analysis of Kinetic Energy Entrainment in Large Wind Farms

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Johns Hopkins University
Large eddy simulation (LES) is used to investigate the behavior of turbine wakes in very large wind farms for which the wakes merge and the flow becomes fully developed. To do this, horizontally-periodic boundary conditions are applied in the LES to mimic the conditions associated with an infinite wind farm. The primary focus of this thesis is on entrainment of mean kinetic energy from above the turbines to the turbine rotor height due to turbulent momentum transfer. In one study, LES is used with proper orthogonal decomposition (POD) to study energetic flow structures in the fully developed atmospheric boundary layer (ABL) flow around a large wind farm. Several thousand three-dimensional velocity fields are analyzed with POD and the resulting POD modes are ordered by their contribution to kinetic energy as well as energy entrainment to the height of the wind turbines. Streamwise-constant counter-rotating rollers are found to contain the most kinetic energy and to contribute a significant amount to the vertical kinetic energy entrainment. The temporal nature of these spatial POD modes is also analyzed to provide insight into the meandering of streaks in the wind farm ABL and the advection of velocity perturbations. In another study, LES is used to investigate the effect of synthetic forcing at the turbine rotors, which is meant to mimic the essential properties of a hypothetical future design or control scheme with the intention of enhancing kinetic energy entrainment. Downward forcing of high-momentum flow is found to positively affect the entrainment and wind farm power generation.
wind energy, wind turbine, wind farm, turbulence, proper orthogonal decomposition, POD, large eddy simulation, LES, kinetic energy entrainment