Turbulent flows over dense filament canopies
Turbulent flows over dense filament canopies
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密集丝冠上的湍流
DOI：
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发表时间：
2019
影响因子：
3.7
通讯作者：
R. García
中科院分区：
文献类型：
--
作者：
Akshath Sharma;R. García
用直接数值模拟的方法研究了由小尺寸刚性细丝组成的密集冠层上的湍流流动。研究了冠层单元的高度和间距对水流的影响。流动由元素相干色散流动和非相干流动组成，非相干流动包括背景湍流和由密集冠层上方典型的Kelvin-Helmholtz类混合层不稳定引起的流动的贡献。对于目前的冠层，间隔约为3-50美元，背景湍流基本上不能在冠层内穿透。由于这些元素都是高的，高度与间距之比为$h/S，冠层的粗糙度由它们的间距决定，延伸到树冠顶端上方约2-3s$。观测到的色散速度波动也主要取决于间距，并且在冠层深处很小，在那里开尔文-亥姆霍兹类不稳定的足迹占主导地位。这种不稳定性是由冠层阻力决定的，它决定了平均速度分布的形状，从而决定了冠层尖端附近的剪切长度。对于这里考虑的高大树冠，这个阻力由单元间距和宽度决定，也就是树冠的平面布局。混合长度决定了不稳定的长度尺度，本质上是它在冠层尖端上方和下方的高度之和。前者以墙单位计大致相同，后者与所有考虑的雨篷的S元呈线性关系。对于很小的元素间距，如$S^{+}\小于10$，元素阻碍了涨落，抑制了不稳定性。在目前冠层的S范围内，障碍随间距的增大而减小，类开尔文-亥姆霍兹滚柱的特征增强。然而，对于较稀疏的冠层，由于空间均匀平均流的假设将被打破，预计不稳定的加剧将停止。对于目前密集的位形，冠层深度对不稳定的发展也有影响。对于较浅的树冠，$h/S\sim 1$，深度的缺乏阻碍了开尔文-亥姆霍兹式的滚柱。对于较深的树冠，$h/S 6$，滚子看不到底壁，树冠高度对流动的影响是饱和的。通过线性分析，可以捕捉到冠层参数对不稳定性的一些影响。
Turbulent flows over dense canopies consisting of rigid filaments of small size are investigated using direct numerical simulations. The effect of the height and spacing of the canopy elements on the flow is studied. The flow is composed of an element-coherent, dispersive flow and an incoherent flow, which includes contributions from the background turbulence and from the flow arising from the Kelvin–Helmholtz-like, mixing-layer instability typically reported over dense canopies. For the present canopies, with spacings $s^{+}\approx 3{-}50$, the background turbulence is essentially precluded from penetrating within the canopy. As the elements are ‘tall’, with height-to-spacing ratios $h/s\gtrsim 1$, the roughness sublayer of the canopy is determined by their spacing, extending to $y\approx 2{-}3s$ above the canopy tips. The dispersive velocity fluctuations are observed to also depend mainly on the spacing, and are small deep within the canopy, where the footprint of the Kelvin–Helmholtz-like instability dominates. The instability is governed by the canopy drag, which sets the shape of the mean velocity profile, and thus the shear length near the canopy tips. For the tall canopies considered here, this drag is governed by the element spacing and width, that is, the planar layout of the canopy. The mixing length, which determines the length scale of the instability, is essentially the sum of its height above and below the canopy tips. The former remains roughly the same in wall units and the latter is linear with $s$ for all the canopies considered. For very small element spacings, $s^{+}\lesssim 10$, the elements obstruct the fluctuations and the instability is inhibited. Within the range of $s^{+}$ of the present canopies, the obstruction decreases with increasing spacing and the signature of the Kelvin–Helmholtz-like rollers intensifies. For sparser canopies, however, the intensification of the instabilities can be expected to cease as the assumption of a spatially homogeneous mean flow would break down. For the present, dense configurations, the canopy depth also has an influence on the development of the instability. For shallow canopies, $h/s\sim 1$, the lack of depth blocks the Kelvin–Helmholtz-like rollers. For deep canopies, $h/s\gtrsim 6$, the rollers do not perceive the bottom wall and the effect of the canopy height on the flow saturates. Some of the effects of the canopy parameters on the instability can be captured by linear analysis.
影响因子：
3.7
作者：
Abderrahaman-Elena N
通讯作者：
Abderrahaman-Elena N