Spatial dimensions of bedforms relative to the flow depth are of great interest for both engineers and geoscientists, and continue to be an active area of research. These morphological features are of significant consequence for critical hydrodynamic parameters, which in turn has an impact on sediment and solute transport through the river system. In this study, we present results from three-dimensional large eddy simulation of flows over such bedforms in a straight channel. Rigid three-dimensional pool–riffle structures occupying the full span of the channel were considered as the macro-bedforms. The presence of pool–riffle affects the spatial heterogeneity of the flow parameters such as lateral flow concentration. We observed that the lateral flow concentration increases in convective deceleration (CDF) zone with peak values at the central plane in channels with narrower span. The peak values shift from the central plane towards sidewall in channels with larger width. Flow recovery is found to be faster in narrower channels in the conducted simulations. High-magnitude counter-rotating vortices (HCRVs) in CDF and pool transport the momentum of the flow towards the centre of the channel. Larger channel width ensures persistent presence of HCRV in CDF and pool. Longer riffle length in the streamwise direction is observed to further alter the lateral flow concentration compared to the shorter riffles. Instantaneous bed shear stress mapping reveals the presence of coherent high- and low-velocity streaks on riffles. The coherence of these structures is lost in CDF and pool accompanied with an increase in turbulence represented by an increase in turbulent kinetic energy and Reynolds stress.

中文翻译：

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通道宽度，床形长度和湍流：实验室规模水池－浅滩序列上流动动力学的数值研究

相对于流动深度而言，床形的空间尺寸对于工程师和地球科学家都非常感兴趣，并且仍然是研究的活跃领域。这些形态特征对于关键的水动力参数具有重要意义，这反过来又影响了沉积物和溶质通过河流系统的运输。在这项研究中，我们提出了三维流场的模拟，通过大涡模拟流在这种床形上的直通道。占据通道整个跨度的刚性三维池格结构被认为是宏床型。池格的存在会影响流量参数的空间异质性，例如侧向流量浓度。我们观察到，在对流减速（CDF）区域中，侧向流浓度在跨度较窄的通道中的中心平面处达到峰值。在具有较大宽度的通道中，峰值从中心平面向侧壁移动。在进行的仿真中，发现在较窄的通道中流量恢复更快。CDF和池中的高幅反向旋转涡流（HCRV）将流的动量向通道中心传输。较大的通道宽度可确保HCRV在CDF和池中持续存在。与较短的浅滩相比，观察到沿流向的浅滩长度更长，从而进一步改变了侧向流的浓度。瞬时河床切应力图显示了浅滩上存在连贯的高速和低速条纹。