Turbulent flows in rough open-channels have complex structures near the channel-bed. The near-bed flow can cause bed erosion, channel instability, and damages to fish habitats. This paper aims to improve our understanding of the structures. Transverse square bars placed at the channel-bed form two-dimensional roughness elements. Turbulent flows over the bars are predicted using large eddy simulation (LES). The predicted flow quantities compare well with experimental data. The LES model predicts mean-flow velocity profiles that resemble those in the classic turbulent boundary layer over a flat plate and profiles that change patterns in the vicinity of roughness elements, depending on the pitch-to-roughness height ratio λ/k. The relative turbulence intensity and normalized Reynolds shear stress reach maxima of 15% and 1.2%, respectively, at λ/k = 8, compared to 9% and 0.2% at λ/k = 2. The predicted bottom boundary layers constitute a large portion of the total depth, indicating roughness effect on the flow throughout the water column. Fluid exchange between the roughness cavity and outer region occurs due to turbulence fluctuations. The fluctuations increase in intensity with increasing λ/k ratio. This ratio dictates the number of eddies in the cavity as well as their locations and shapes. It also controls turbulence stress distributions. LES can be used to explore strategies for erosion control, channel restoration, and habitat protection.

中文翻译：

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浅层明渠中粗糙单元上近床流动和湍流的大涡模拟

粗糙明渠中的湍流在通道床附近具有复杂的结构。近河床流会导致河床侵蚀、河道不稳定并破坏鱼类栖息地。本文旨在提高我们对结构的理解。放置在槽床处的横向方形钢筋形成二维粗糙度元素。使用大涡模拟 (LES) 预测杆上的湍流。预测的流量与实验数据比较好。LES 模型预测的平均流速剖面类似于平板上经典湍流边界层中的平均流速剖面，以及根据间距与粗糙度的高度比 λ/k 改变粗糙度元素附近模式的剖面。在 λ/k = 8 时，相对湍流强度和归一化雷诺剪切应力分别达到 15% 和 1.2% 的最大值，与 λ/k = 2 时的 9% 和 0.2% 相比。预测的底部边界层构成了总深度的很大一部分，表明粗糙度对整个水柱的流动有影响。由于湍流波动，在粗糙腔和外部区域之间发生流体交换。随着 λ/k 比率的增加，强度的波动也会增加。这个比率决定了腔中涡流的数量以及它们的位置和形状。它还控制湍流应力分布。LES 可用于探索侵蚀控制、渠道恢复和栖息地保护的策略。由于湍流波动，在粗糙腔和外部区域之间发生流体交换。随着 λ/k 比率的增加，强度的波动也会增加。这个比率决定了腔中涡流的数量以及它们的位置和形状。它还控制湍流应力分布。LES 可用于探索侵蚀控制、渠道恢复和栖息地保护的策略。由于湍流波动，在粗糙腔和外部区域之间发生流体交换。随着 λ/k 比率的增加，强度的波动也会增加。这个比率决定了腔中涡流的数量以及它们的位置和形状。它还控制湍流应力分布。LES 可用于探索侵蚀控制、渠道恢复和栖息地保护的策略。