Abstract
Understanding the effect of thermal stratification on wind turbine wakes in complex terrain is essential to optimize wind farm design. The effect of a three-dimensional hill on the performance of a downwind turbine is studied by performing large eddy simulations for different atmospheric conditions. The distance between the hill and the turbine is six times the turbine diameter, and the hill height is equal to the hub height. It is shown that the hill wake reduces the power production of the downstream turbine by 35% for the convective boundary layer case under consideration. However, surprisingly, the wind turbine power production is increased by about 24% for the stable boundary layer case considered here. This phenomenon results from the entrainment of kinetic energy from the low-level jet due to the increased mixing induced by the hill wake. This effect strongly depends on the Coriolis force-induced wind veer. The increased turbulence intensity by the hill results in a strong increase in the forces experienced by the blades, which suggest the turbine is experiencing much higher unsteady turbulence loading. It is shown that the increase in the power fluctuations may not fully reflect the increase in the force fluctuations on the blades.
Highlights
As more and more wind turbines will be installed in complex terrains, it is crucial to study the effect of atmospheric stability on the performance of wind turbines in hilly terrain [1e3]
This speedup is caused by the interaction between the wind veer, the low level jet, and the vortex shedding caused by the hill
The speedup is a result of vortex shedding created by the hill due to which the right side of the hill wake ðy =h > 0Þ, which has been deflected by the wind veer, entrains high-velocity wind from the low level jet
Summary
As more and more wind turbines will be installed in complex terrains, it is crucial to study the effect of atmospheric stability on the performance of wind turbines in hilly terrain [1e3]. Howard et al [13] performed wind tunnel measurements on the effect of a threedimensional hill on the performance of a model wind turbine for different thermal stratifications. They showed that the upstream hill has a negative effect on the power production of a turbine located downstream of the hill. Englberger & Do€rnbrack [16] used LES to study the wake development from a wind turbine located on a hilltop for different thermal stratification. Field experiments allow one to measure wind turbine performance in realistic conditions, but it can be challenging to isolate various physical effects. Where C ,D denotes filtering at a scale of 2D, Àu2* is the wall sub-grid scale shear stress, k 1⁄4 0:4 is the von Karman constant, z is the vertical distance from the wall, z0 is the roughness height, q* is the wall sub-grid scale heat flux, qs is the potential temperature at the wall surface, and jm and jh are, respectively, the stability corrections for the momentum and heat fluxes [21,44],
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