The flow patterns within a longitudinal gap area formed by discontinuous distributions of submerged canopy, as well as the momentum and mass exchange characteristics between the gap area and the overlying free-flow, were studied using high-resolution Large Eddy Simulation (LES). The gap area is located within the fully developed region of submerged canopy flow. The simulations considered four aspect ratios of the gap area, L/h (ranging from 1 to 4), where L and h represent the span of the gap area and the height of the canopy, respectively, and two canopy densities, ϕ = 0.08 and 0.15. Results indicate that recirculation vortices appear only at ϕ = 0.15 and penetrate the downstream canopy patches to varying extents, with the distance of invasion decreasing as L/h increases. Influenced by the recirculation vortices, the bed shear stress in the downstream part of the gap area and the initial section of the downstream canopy patch is significantly increased compared to the fully developed region of the upstream canopy patch. Within the parameter range covered, vertical penetration of the mixing layer into the gap area always occurs, consistently falling into the shear layer growth regime, with significant enhancement of turbulence within the gap area even at L/h = 1. The high momentum entrainment elevates longitudinal velocity and turbulence intensity in the upper corner regions of the downstream canopy patch’s leading edge, combined with localized increases in bed shear stress, potentially destabilizing plants at the forefront of the downstream canopy patch. Although the total momentum exchange across the interface between the gap area and the upper free-flow layer is approximately independent of L/h, larger canopy densities lead to stronger momentum transport, and turbulent transport always dominates. Mass exchange generally increases with L/h, with more efficient vertical mass exchange in ϕ = 0.15 cases at L/h = 3 and 4.

中文翻译：

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研究充分开发的浅水-水下冠层流内间隙区域的湍流和界面交换特征


使用高分辨率大涡模拟 （LES） 研究了由水下冠层不连续分布形成的纵向间隙区域内的流型，以及间隙区域与上覆自由流之间的动量和质量交换特性。间隙区域位于淹没冠层流的充分发展区域内。模拟考虑了间隙面积的四个纵横比 L/h（范围从 1 到 4），其中 L 和 h 分别代表间隙区域的跨度和冠层的高度，以及两个冠层密度，φ = 0.08 和 0.15。结果表明，再循环涡旋仅在 φ = 0.15 时出现，并不同程度地渗透到下游冠层斑块中，侵袭距离随着 L/h 的增加而减小。受再循环涡流的影响，与上游冠层斑块的充分发育区域相比，间隙区下游部分和下游冠层斑块初始部分的床层剪切应力显著增加。在所涵盖的参数范围内，混合层始终垂直渗透到间隙区域，始终落入剪切层生长状态，即使在 L/h = 1 时，间隙区域内的湍流也会显著增强。高动量夹带提高了下游冠层斑块前缘上角区域的纵向速度和湍流强度，再加上床层剪切应力的局部增加，可能会使下游冠层斑块前沿的植物不稳定。 尽管间隙区域和上部自由流动层之间界面上的总动量交换大致独立于 L/h，但较大的冠层密度会导致更强的动量传输，而湍流传输始终占主导地位。质量交换通常随着 L/h 的增加而增加，在 φ = 0.15 的情况下，当 L/h = 3 和 4 时，垂直质量交换效率更高。