The Influence of Three-Dimensional Topography on Turbulent Flow Structures Over Dunes in Unidirectional Flows
The Influence of Three-Dimensional Topography on Turbulent Flow Structures Over Dunes in Unidirectional Flows
复制标题
三维地形对单向流沙丘湍流结构的影响
DOI：
10.1029/2021jf006121
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Hardy R
中科院分区：
文献类型：
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作者：
Hardy R
沙丘是砂层河流中最常见的底形，其形态通常包括多种尺度的三维地形。然而，我们对沙丘上流动的理解主要基于二维模型，这种情况在自然界中很少见。在这里，我们提出了静态的，三维沙丘场的大涡模拟结果，使用二维和三维地形实现，研究床地形和湍流结构之间的相互作用。我们表明，与三维地形相比，二维底形上的流动增加了风坡上的速度，并减小了背风分离区的大小。三维底形上的流动产生的漩涡是二维底形上的两倍，并且这些漩涡比二维底形上的流动更长、更宽、更高。湍流主要由发夹形涡和Kelvin-Helmholtz不稳定性控制，这些不稳定性与沙丘顶部边缘点区域和背风坡下的床相互作用，并在长时间内产生高剪切应力。这些结果被用来提出一个新的概念模型，显示了二维和三维底形之间的差异。研究结果强调了如何的大小，形态和堆积的相干流结构到更大的流量上层建筑可能是至关重要的泥沙夹带，并可能决定事件持续时间和幅度之间的关系，驱动沉积物的脉冲在床上。这将最终导致床形形态的三维度增加。
Dunes are the most prevalent bedform present in sand‐bedded rivers and their morphology typically comprises multiple scales of three‐dimensional topography. However, our understanding of flow over dunes is predicated largely on two‐dimensional models, a condition which is rare in nature. Here, we present results of Large Eddy Simulations over a static, three‐dimensional dune field, using a two‐ and three‐ dimensional topographic realisation, to investigate the interaction between bed topography and turbulent flow structures. We show that flow over two‐dimensional bedforms increases the velocity over the stoss slope and reduces the size of the leeside separation zone as compared to 3D topography. Flow over three‐dimensional bedforms generates twice as many vortices as over two‐dimensional bedforms, and these vortices are longer, wider and taller than flow over their two‐dimensional counterparts. Turbulence is dominated by hairpin‐shaped vortices and Kelvin‐Helmholtz instabilities that interact with the bed in the brink point region of the dune crest and down the lee slope, and generate high shear stresses for long durations. These results are used to propose a new conceptual model showing the differences between flow over two‐ and three‐dimensional bedforms. The findings highlight how the size, morphology and stacking of coherent flow structures into larger flow superstructures may be critical in sediment entrainment, and may dictate the relationship between event duration and magnitude that drives sediment impulses at the bed. This will ultimately lead to an increased in the three‐dimensionality of bedform morphology.