Abstract
Turbulence structure in the wake behind a full-scale horizontal-axis wind turbine under the influence of real-time atmospheric inflow conditions has been investigated using actuator-line-model based large-eddy-simulations. Precursor atmospheric boundary layer (ABL) simulations have been performed to obtain mean and turbulence states of the atmosphere under stable stratification subjected to two different cooling rates. Wind turbine simulations have revealed that, in addition to wind shear and ABL turbulence, height-varying wind angle and low-level jets are ABL metrics that influence the structure of the turbine wake. Increasing stability results in shallower boundary layers with stronger wind shear, steeper vertical wind angle gradients, lower turbulence, and suppressed vertical motions. A turbulent mixing layer forms downstream of the wind turbines, the strength and size of which decreases with increasing stability. Height dependent wind angle and turbulence are the ABL metrics influencing the lateral wake expansion. Further, ABL metrics strongly impact the evolution of tip and root vortices formed behind the rotor. Two factors play an important role in wake meandering: tip vortex merging due to the mutual inductance form of instability and the corresponding instability of the turbulent mixing layer.
Highlights
Wind turbines (WT) are influenced strongly by the atmosphere’s natural variability, including thermal stratification and wind conditions
The conclusions from these simulations is that low-level jets exist in stable atmospheric boundary layer (ABL), with increasing surface temperature the height of LLJ increases for a given surface cooling rate
Wind angle, mean temperature gradient, Reynolds stresses, and buoyancy flux, T ' w ' are the ABL metrics that are strongly dependent on atmospheric stability for stable ABL, which plays an important role in dictating the near-wake dynamics of WT
Summary
Wind turbines (WT) are influenced strongly by the atmosphere’s natural variability, including thermal stratification and wind conditions. This study is promising as it is one of the first to rigorously demonstrate the influence of large-scale mean forcings, including LLJs, on the turbine wake structure It revealed that, in addition to shear-generated turbulence from the tip and root vortices, other factors produce turbulence, including interactions between root and tip vortex and vortex merging. Kansas night-time conditions during the month of October based on CASES-99 field experiment with surface temperature in range of (275 K–287 K) [7]; (b) wind turbine locations at sweet-water, Texas with surface temperature in range of (288 K–293 K) [32]; (c) arctic stratus base experiments (256 K–269 K) [23] The conclusions from these simulations is that low-level jets exist in stable ABL, with increasing surface temperature the height of LLJ increases for a given surface cooling rate. We observed that with higher surface temperatures, the time to reach equilibrium state increases; care has to be taken to monitor surface heat-flux, surface buoyancy-flux and LLJ height for the simulation to reach a statistical equilibrium state
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