Vortex development behind a finite porous obstruction in a channel
Vortex development behind a finite porous obstruction in a channel
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DOI：
10.1017/jfm.2011.479
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发表时间：
2011-12
影响因子：
3.7
通讯作者：
Lijun Zong;H. Nepf
中科院分区：
文献类型：
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作者：
Lijun Zong;H. Nepf
摘要本实验研究描述了一个由圆柱体组成的二维多孔障碍物后的湍流尾迹。圆柱体从河床延伸到水面，模仿一片新兴的植被。测试了三个贴片直径（$D$）和七个固体体积分数（$\Phi $）。由于水流可以穿过斑块，因此在直接下游有一个稳定的、非零的流向速度区域${U}_{1} $，称为定常尾迹。对于这里考虑的斑块直径和固体体积分数，${U}_{1} $仅是$\Phi $的函数。稳定尾迹（${L}_{1} $）的长度随着$\Phi $的减小而增大，可以通过平面剪切层的增长来预测。von-Kármán涡旋街的形成被推迟到稳定尾流结束。有两个横向速度波动升高的区域（${v}_{\mathit{rms}} $）：在补丁的正后方，与单个圆柱体的尾迹湍流有关；在距离斑块${L}_{1} $处，与大规模尾流振荡的形成有关。在斑块尺度涡街形成后，沿尾迹中心线的速度才开始增加，并在距离${L}_{2} $上接近自由流速度。整个尾流的无因次长度$({L}_{1} + {L}_{2})/ D$随补片孔隙度增大而增大。
Abstract This experimental study describes the turbulent wake behind a two-dimensional porous obstruction, consisting of a circular array of cylinders. The cylinders extend from the channel bed through the water surface, mimicking a patch of emergent vegetation. Three patch diameters ($D$) and seven solid volume fractions ($\Phi $) are tested. Because flow can pass through the patch, directly downstream there is a region of steady, non-zero, streamwise velocity, ${U}_{1} $, called the steady wake. For the patch diameters and solid volume fractions considered here, ${U}_{1} $ is a function of $\Phi $ only. The length of the steady wake (${L}_{1} $) increases as $\Phi $ decreases and can be predicted from the growth of a plane shear layer. The formation of the von-Kármán vortex street is delayed until the end of the steady wake. There are two regions of elevated transverse velocity fluctuation (${v}_{\mathit{rms}} $): directly behind the patch, associated with the wake turbulence of individual cylinders; and at the distance ${L}_{1} $ from the patch, associated with the formation of large-scale wake oscillation. Velocity along the centreline of the wake starts to increase only after the patch-scale vortex street is formed, and it approaches the free-stream velocity over a distance ${L}_{2} $. The dimensionless length of the entire wake, $({L}_{1} + {L}_{2} )/ D$, increases with patch porosity.