We have performed fully resolved simulations of turbulent flows over various submerged rigid canopies covering the wall of an open channel. All the numerical predictions have been obtained considering the same nominal bulk Reynolds number (i.e. $Re_{b}=U_{b}H/\unicode[STIX]{x1D708}=6000$ , $H$ being the channel depth and $U_{b}$ the bulk velocity). The computations directly tackle the region occupied by the canopy by imposing the zero-velocity condition on every single stem, while the outer flow is dealt with a highly resolved large-eddy simulation. Four canopy configurations have been considered. All of them share the same in-plane solid fraction while the canopy to channel height ratios have been selected to be $h/H=(0.05,0.1,0.25,0.4)$ . The lowest and the highest values lead to flow conditions approaching the two asymptotic states that in the literature are usually termed the sparse and dense regimes (see Nepf ( Annu. Rev. Fluid Mech. , vol. 44, 2012, pp. 123–142)). The other two $h/H$ selected ratios are representative of transitional regimes, a generic category that incorporates all the non-asymptotic states. While the interaction of a turbulent flow with a filamentous canopy in the two asymptotic conditions is relatively well understood, not much is known on the transitional flows and on the physical mechanisms that are responsible for the variations of flow regimes when the canopy solidity is changed. The effects of the latter on the flow developing in the intra-canopy region, on the outer flow and on their mutual interactions have been numerically explored and are reported in this work. By systematically varying the canopy height, we have unravelled the main character of the different regimes that are generated by the interplay between the outer flow structures, the emerging instabilities driven by the canopy drag and the interstitial flow between the canopy stems. The key role played by the relative positions of the inflection points of the mean velocity profile and the location of the virtual wall origin (as seen from the outer flow) is put forward and used to define a new condition to infer the canopy flow regime when the solidity is changed. Finally, the presence and the effects of an instability occurring close to the bed, nearby the interior inflectional point of the mean velocity profile is highlighted together with its consequences on the flow structure within the canopy region. To the best of our knowledge, this is the first time that the emergence of close-to-the-bed coherent structures induced by the inner inflection point is reported in the literature.

中文翻译：

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关于冠层流中不同制度的起源：数值研究

我们已经对覆盖明渠壁的各种水下刚性檐篷上的湍流进行了完全解析的模拟。考虑到相同的标称体积雷诺数（即 $Re_{b}=U_{b}H/\unicode[STIX]{x1D708}=6000$ ，$H$ 是通道深度，$U_ {b}$ 体积速度）。通过在每个茎上施加零速度条件，计算直接处理冠层占据的区域，而外流则由高分辨率大涡模拟处理。已经考虑了四种顶篷配置。它们都共享相同的平面内固体分数，而冠层与通道的高度比已选择为 $h/H=(0.05,0.1,0.25,0.4)$ 。最低和最高值导致接近两种渐近状态的流动条件，这在文献中通常称为稀疏和密集状态（参见 Nepf（Annu. Rev. Fluid Mech. , vol. 44, 2012, pp. 123–142 ))。其他两个 $h/H$ 选择的比率代表过渡状态，一个包含所有非渐近状态的通用类别。虽然在两种渐近条件下湍流与丝状冠层的相互作用相对较好，但对过渡流和导致冠层固体变化时流态变化的物理机制知之甚少。后者对冠层内区域流动发展的影响，对外部流动及其相互相互作用进行了数值探索，并在这项工作中进行了报告。通过系统地改变冠层高度，我们揭示了由外部流动结构之间的相互作用、由冠层阻力驱动的新出现的不稳定性和冠层茎之间的间隙流动所产生的不同状态的主要特征。提出了平均速度剖面的拐点的相对位置和虚拟壁原点（从外流看）的位置所起的关键作用，并用于定义一个新的条件来推断冠层流态，当坚固性发生了变化。最后，靠近床的不稳定的存在和影响，平均速度剖面的内部拐点附近被突出显示，以及它对冠层区域内流动结构的影响。据我们所知，这是第一次在文献中报道由内拐点引起的近床相干结构​​的出现。