Abstract
A numerical study of atmospheric turbulence effects on wind-turbine wakes is presented. Large-eddy simulations of neutrally-stratified atmospheric boundary layer flows through stand-alone wind turbines were performed over homogeneous flat surfaces with four different aerodynamic roughness lengths. Emphasis is placed on the structure and characteristics of turbine wakes in the cases where the incident flows to the turbine have the same mean velocity at the hub height but different mean wind shears and turbulence intensity levels. The simulation results show that the different turbulence intensity levels of the incoming flow lead to considerable influence on the spatial distribution of the mean velocity deficit, turbulence intensity, and turbulent shear stress in the wake region. In particular, when the turbulence intensity level of the incoming flow is higher, the turbine-induced wake (velocity deficit) recovers faster, and the locations of the maximum turbulence intensity and turbulent stress are closer to the turbine. A detailed analysis of the turbulence kinetic energy budget in the wakes reveals also an important effect of the incoming flow turbulence level on the magnitude and spatial distribution of the shear production and transport terms.
Highlights
Wind turbines operate in the atmospheric boundary-layer (ABL), where they are exposed to different wind speeds, wind shears and turbulence levels, depending on the land/sea surface characteristics, Energies 2012, 5 synoptic forcings and thermal stability conditions
We present results from large-eddy simulations of the wake flows behind wind turbines operating in neutral ABLs over flat surfaces with different roughness lengths (z0 = 0.5, 0.05, 0.005, and 0.00005 m)
A series of numerical experiments were carried out to study the effect of atmospheric turbulence on stand-alone wind-turbine wakes
Summary
Wind turbines operate in the atmospheric boundary-layer (ABL), where they are exposed to different wind speeds, wind shears and turbulence levels, depending on the land/sea surface characteristics, Energies 2012, 5 synoptic forcings and thermal stability conditions. Evidence shows that the turbulence level in the incoming flow affects the rate at which this velocity deficit decreases with downwind distance from the turbine (i.e., the wake recovery rate) Both simulations [9,10,11] and experimental studies [6,12,13,14] have shown that stand-alone turbine wakes recover faster in conditions of higher turbulence intensity levels in the incoming flows. This effect is found to affect the performance (power output) of the downwind turbines in large wind farms.
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