Coherent flow structures in a depth-limited flow over a gravel surface : The role of near-bed turbulence and influence of Reynolds number
Coherent flow structures in a depth-limited flow over a gravel surface : The role of near-bed turbulence and influence of Reynolds number
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DOI：
10.1029/2007jf000970
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发表时间：
2009-03
影响因子：
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通讯作者：
R. Hardy;J. Best;S. Lane;P. Carbonneau
中科院分区：
文献类型：
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作者：
R. Hardy;J. Best;S. Lane;P. Carbonneau
[1]在砾石河床河流中，河床微地形对紊流结构的形成有重要影响。虽然现场和实验室测量表明，砾石床上的流动包含连贯的宏观湍流结构，但这些现象的起源及其与形成砾石床的单个粗糙元素的关系，在数量上还没有得到很好的理解。在这里，我们报道了一个水槽实验，在这个实验中，通过应用数字粒子成像测速仪来量化在砾石表面上的流动，该测速仪可以研究整个流场中的下游和垂直速度分量。结果表明，随着雷诺数的增加，(1)相干流结构的视觉清晰度变得更加明显，(2)结构的上游坡度增大，(3)结构的湍流强度增大。用象限分析方法对平均速度分量、湍流强度和流动结构的分析表明，这些大尺度的湍流结构源于水流与河床地形的相互作用。通过小波分析检测主要的时间长度尺度，可以通过标准的尺度定律计算与砾石粗糙度相关的平均隔离带长度。计算的分离区长度表明，尾流拍打是砾石床河流中产生大尺度相干流动结构的主要机制。因此，我们表明，砾石上的相干流动结构是由床层产生的湍流所致，而大尺度的外层结构是与尾迹拍打相关的近床区流动-地形相互作用的结果。
[1] In gravel bed rivers, the microtopography of the bed exerts a significant effect on the generation of turbulent flow structures. Although field and laboratory measurements have indicated that flows over gravel beds contain coherent macroturbulent flow structures, the origin of these phenomena, and their relationship to the ensemble of individual roughness elements forming the bed, is not quantitatively well understood. Here we report upon a flume experiment in which flow over a gravel surface is quantified through the application of digital particle imaging velocimetry, which allows study of the downstream and vertical components of velocity over the entire flow field. The results indicate that as the Reynolds number increases (1) the visual distinctiveness of the coherent flow structures becomes more defined, (2) the upstream slope of the structures increases, and (3) the turbulence intensity of the structures increases. Analysis of the mean velocity components, the turbulence intensity, and the flow structure using quadrant analysis demonstrates that these large-scale turbulent structures originate from flow interactions with the bed topography. Detection of the dominant temporal length scales through wavelet analysis enables calculation of mean separation zone lengths associated with the gravel roughness through standard scaling laws. The calculated separation zone lengths demonstrate that wake flapping is a dominant mechanism in the production of large-scale coherent flow structures in gravel bed rivers. Thus, we show that coherent flow structures over gravels owe their origin to bed-generated turbulence and that large-scale outer layer structures are the result of flow-topography interactions in the near-bed region associated with wake flapping.