The Impact of Nonequilibrium Flow on the Structure of Turbulence Over River Dunes
The Impact of Nonequilibrium Flow on the Structure of Turbulence Over River Dunes
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DOI：
10.1029/2017wr021377
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发表时间：
2018-09
影响因子：
5.4
通讯作者：
C. Unsworth;D. Parsons;R. Hardy;A. Reesink;J. Best;P. Ashworth;G. Keevil
中科院分区：
文献类型：
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作者：
C. Unsworth;D. Parsons;R. Hardy;A. Reesink;J. Best;P. Ashworth;G. Keevil
大多数过去的实验研究的流动在河流沙丘集中在条件下，匹配半经验的流深标度律，但这样的平衡条件是有限的价值，因为它们很少发生在自然渠道。本文量化的平均和湍流的结构在固定的2-D实验室沙丘在一系列的非平衡条件下的沙丘流制度。使用二维粒子成像测速仪对12种条件下的流场进行了量化，包括对于固定沙丘的大小来说太深、太浅、太快或太慢的流动。结果表明，主要偏离的平均流量和湍流结构的模式相比，沙丘下形成的平衡流条件。流动再附着长度与沙丘顶部的平均深度平均流向速度与剪切速度之比（U <$c/uc*）呈线性关系，这为流动再附着长度提供了一种新的预测措施。沙丘顶部的深度平均垂直速度（V ² c）与U ² c呈抛物线关系，在U ² c ~0.60 m/s处达到峰值，这与沙丘纵横比与移动的河床条件下存在的运输阶段的关系相匹配。紊流尾流的空间位置随流深和流速而变化，较低的U ′ c和较大的流深使尾流向自由表面上升。由于湍流与自由表面的相互作用减少，较深的流动可能在沙丘顶部显示较少的流动会聚，从而导致运输阶段减少。
Most past experimental investigations of flow over river dunes have focused on conditions that match semiempirical flow‐depth scaling laws, yet such equilibrium conditions are of limited value because they rarely occur in natural channels. This paper quantifies the structure of mean and turbulent flow over fixed 2‐D laboratory dunes across a range of nonequilibrium conditions within the dune flow regime. The flow field was quantified using 2‐D particle imaging velocimetry for 12 conditions, including flows that are too deep, too shallow, too fast, or too slow for the size of the fixed dunes. The results demonstrate major departures in the patterns of the mean flow and structure of turbulence when compared to dunes formed under equilibrium flow conditions. The length of flow reattachment scales linearly with the ratio of mean depth‐averaged streamwise velocity to shear velocity at the dune crest ( U¯c/uc* ), which provides a new predictive measure for flow reattachment length. Depth‐averaged vertical velocities at the dune crest ( V¯c ) show a parabolic relationship with U¯c , peaking at U¯c ~0.60 m/s, which matches the relationship of dune aspect ratio with transport stage present in mobile bed conditions. The spatial location of the turbulent wake was found to vary with flow depth and velocity, with lower U¯c and greater flow depths causing the wake to rise toward the free surface. Deeper flows are likely to show less flow convergence over the crests of dunes due to reduced interaction of turbulence with the free surface, resulting in a reduction of transport stage.