The role of the in-plane solidity on canopy flows
The role of the in-plane solidity on canopy flows
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面内坚固性对冠层流动的作用
DOI：
10.1017/jfm.2023.848
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发表时间：
2023
影响因子：
3.7
通讯作者：
Nicholas S
中科院分区：
文献类型：
--
作者：
Nicholas S
在由垂直于河床的细长圆柱体组成的淹没顶盖上发展的湍流明渠流动已知在很大程度上受稠度参数(以及细丝的直径和高度，以及细丝之间的平均间距)的控制。当细丝足够细长时，茎的高度和间距之间的比率决定了树冠内外发展的水动力状况。这一比率还建立了导致从密集冠层流态向稀疏冠层流态转变的条件(NEPF，Annu。《流体机械》，第44卷，2012年，第123-142页)。在之前的一次伴随的数值研究中，Monti等人(J.Fluid Mech，Vol.891,2020，A9)使用大涡模拟(LES)研究了树冠高度对不同区域开始的影响，而不改变树干之间的平均间距。在LES研究中，我们观察了树干高度恒定而树冠花丝数密度变化的互补情况。结果表明，对于低密度(稀疏或中等密度)的树冠，由这两种方法得到的稠度值所对应的流量非常相似。不同的是，对于较高的值(即在较密的树冠中)，和的影响开始发散，尽管共享相同的标称值。在这篇文章中，我们分析了不同的物理机制对通过改变或改变得到的密集组态起作用的物理机制。特别是，我们将重点放在最相关的长度尺度上，并使用三重分解方法对流量进行详细分析。我们发现，以高高的树干为特征的密集冠层流动的内部区域，主要是由在墙附近提供高动量的墙法线扫掠所主导的。这里，床层的不可穿透性条件使可用动量在平行于壁面的方向重新分布，从而使原本停滞不前的流动重新获得能量。不同的是，在密实的树冠中，外喷流的穿透和来自外部流动的动量传递受到平行于壁面的渗透率的减小的限制，从而导致不同的行为，包括减少树冠提供的总阻力。
Turbulent open channel flows developing above submerged canopies made of slender cylinders mounted perpendicular to the channel bed are known to be largely governed by the solidity parameter(andbeing the diameter and height of the filament, andthe average spacing between filaments). When the filaments are sufficiently slender, the ratio between the height of the stems and the spacing sets the hydrodynamic regime developing inside and outside the canopy. This ratio also establishes the conditions leading to the transition from a dense to a sparse canopy flow regime (Nepf, Annu. Rev. Fluid Mech., vol. 44, 2012, pp. 123–142). In a previous, companion numerical investigation, Monti et al.(J. Fluid Mech., vol. 891, 2020, A9) used large eddy simulation (LES) to study the influence of the canopy height on the onset of the different regimes without modifying the average spacingbetween the stems. In that LES study, we were looking at the complementary situation in which the height of the stems is constant while the filaments’ number density of the canopy is changed. It was found that for low values of(ie sparse or moderately dense canopies:), the flows sharing the value of the solidity obtained by either varyingorare very similar. Differently, for higher values of(ie in denser canopies), the effects ofandstart to diverge although sharing the same nominal value of. In this paper, we analyse the different physical mechanisms that come into play for dense configurations obtained by varying eitheror. In particular, we focus on the most relevant length scales and carry out a detailed analysis of the flows using a triple decomposition approach. We show that the inner region of dense canopy flows, characterised by tall stems, is dominated by wall-normal sweeps delivering high momentum in the wall vicinity. Here, the impenetrability condition of the bed redistributes the available momentum in the wall-parallel directions re-energising an otherwise stagnating flow. Differently, in densely packed canopies, the penetration of the outer jet and the momentum transfer from the external flow are limited by the decreasing value of the wall-parallel permeabilities leading to different behaviours, including a reduction of the total drag offered by the canopy.
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