Low-level jets (LLJs) are the wind maxima in the lowest 50 to 1000 m of atmospheric boundary layers. Due to their significant influence on the power production of wind farms, it is crucial to understand the interaction between LLJs and wind farms. In the presence of a LLJ, there are positive and negative shear regions in the velocity profile. The positive shear regions of LLJs are continuously turbulent, while the negative shear regions have limited turbulence. We present large eddy simulations of wind farms in which the LLJ is above, below, or in the middle of the turbine rotor swept area. We find that the wakes recover relatively quickly when the LLJ is above the turbines. This is due to the high turbulence below the LLJ and the downward vertical entrainment created by the momentum deficit due to the wind farm power production. This harvests the jet's energy and aids wake recovery. However, when the LLJ is below the turbine rotor swept area, the wake recovery is very slow due to the low atmospheric turbulence above the LLJ. The energy budget analysis reveals that the entrainment fluxes are maximum and minimum when the LLJ is above and in the middle of the turbine rotor swept area, respectively. Surprisingly, we find that the negative shear creates a significant entrainment flux upward when the LLJ is below the turbine rotor swept area. This facilitates energy extraction from the jet, which is beneficial for the performance of downwind turbines.

中文翻译：

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低空射流高度对风电场性能的影响

低空急流（LLJs）是大气边界层最低50至1000 m处的最大风。由于它们对风电场的电力生产产生重大影响，因此了解LLJ和风电场之间的相互作用至关重要。在存在LLJ的情况下，速度剖面中存在正剪切区域和负剪切区域。LLJs的正剪切区是连续湍流，而负剪切区的湍流是有限的。我们介绍了风电场的大型涡流模拟，其中LLJ位于涡轮转子扫掠区域的上方，下方或中间。我们发现，当LLJ位于涡轮机上方时，尾流恢复得相对较快。这是由于LLJ下方的湍流高以及风电场发电产生的动量不足所造成的向下垂直夹带。这样可以收集喷气机的能量并帮助尾流恢复。但是，当LLJ低于涡轮转子扫掠区域时，由于LLJ上方的低大气湍流，尾流恢复非常缓慢。能量预算分析表明，当LLJ分别位于涡轮转子扫掠区域的上方和中间时，夹带通量分别为最大值和最小值。出乎意料的是，我们发现当LLJ低于涡轮转子扫掠面积时，负剪力会产生向上的显着夹带通量。这有利于从射流中提取能量，这对于顺风涡轮的性能是有利的。能量预算分析表明，当LLJ分别位于涡轮转子扫掠区域的上方和中间时，夹带通量分别为最大值和最小值。出乎意料的是，我们发现当LLJ低于涡轮转子扫掠面积时，负剪切力会产生明显的向上夹带通量。这有利于从射流中提取能量，这对于顺风涡轮的性能是有利的。能量预算分析表明，当LLJ分别位于涡轮转子扫掠区域的上方和中间时，夹带通量分别为最大值和最小值。出乎意料的是，我们发现当LLJ低于涡轮转子扫掠面积时，负剪切力会产生明显的向上夹带通量。这有利于从射流中提取能量，这对于顺风涡轮的性能是有利的。