The topology of turbulent coherent structures is known to regulate the transport of energy, mass and momentum in the atmospheric boundary layer (ABL). While previous research has primarily focused on characterizing the structure of turbulence in stationary ABL flows, real-world scenarios frequently deviate from stationarity, giving rise to nuanced and poorly understood changes in the turbulence geometry and associated transport mechanisms. This study sheds light on this problem by examining topological changes in ABL turbulence induced by non-stationarity and their effects on momentum transport. Results from a large-eddy simulation of pulsatile open channel flow over an array of surface-mounted cuboids are examined. The analysis reveals that the flow pulsation triggers a phase-dependent shear rate, and the ejection-sweep pattern varies with the shear rate during the pulsatile cycle. From a turbulence structure perspective, it is attributed to the changes in the geometry of hairpin vortices. An increase (decrease) in the shear rate intensifies (relaxes) these structures, leading to an increase (decrease) in the frequency of ejections and an amplification (reduction) of their percentage contribution to the total momentum flux. Furthermore, the size of the hairpin packets undergoes variations, which depend on the geometry of the constituting hairpin vortices, yet the packet inclination preserves its orientation throughout the pulsatile cycle. These observations reinforce the important role non-stationarity holds in shaping the structure of ABL turbulence and the momentum transport mechanisms it governs.

中文翻译：

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城市冠层非定常流湍流结构

众所周知，湍流相干结构的拓扑结构可以调节大气边界层（ABL）中的能量、质量和动量的传输。虽然之前的研究主要集中在表征静止 ABL 流中的湍流结构，但现实世界的场景经常偏离静止状态，从而导致湍流几何形状和相关传输机制发生微妙且难以理解的变化。这项研究通过检查非平稳性引起的 ABL 湍流的拓扑变化及其对动量传递的影响，揭示了这个问题。检查了一系列表面安装的长方体上脉动明渠流的大涡模拟结果。分析表明，流动脉动触发了相位相关的剪切速率，并且喷射-扫掠模式在脉动循环期间随着剪切速率而变化。从湍流结构的角度来看，这归因于发夹涡几何形状的变化。剪切速率的增加（减少）会增强（松弛）这些结构，从而导致喷射频率的增加（减少）以及它们对总动量通量的贡献百分比的放大（减少）。此外，发夹形包的尺寸会发生变化，这取决于构成发夹形涡流的几何形状，但发夹形包的倾斜度在整个脉动周期中保持其方向。这些观察结果强化了非平稳性在塑造 ABL 湍流结构及其控制的动量传递机制中的重要作用。