Overbank flows in rivers, also termed compound open‐channel flows, are rarely uniform in the streamwise direction. Flow nonuniformity involves transverse currents directed from the river main channel (MC) to adjacent floodplains (FPs) or vice versa. Depending on their direction and magnitude, the transverse currents can laterally displace the planform shear layer that forms at the interface between MC and FP. They can also modify the cross‐sectional distribution of the helical secondary currents (SCs). This paper reports a numerical study that focuses on assessing the ability of a hybrid RANS (Reynolds Averaged Navier‐Stokes) ‐ LES (Large Eddy Simulation) approach, namely, the Scale Adaptive Simulation (SAS), to capture the evolving structure of both depth‐uniform and nonuniform flows in a straight compound open‐channel. The simulated flows have recently been studied in an 18 m long and 3 m wide laboratory flume by Proust and Nikora (2020, https://doi.org/10.1017/jfm.2019.973). The originality of the study lies in the modeling of flows over the entire flume length and in the estimate of the length‐scales of the large Kelvin‐Helmholtz‐type coherent structures (KHCSs). The SAS approach can accurately predict the longitudinal evolution of: (i) the streamwise time‐averaged flow, including the planform shear layer width, (ii) the SC patterns, and (iii) the turbulence statistics. The emergence and development of the KHCSs are qualitatively retrieved and are confirmed to be controlled by dimensionless velocity shear irrespective of flow depth. The simulations also show that SC patterns depend on the magnitude and direction of the transverse currents and also on the development of KHCSs.

中文翻译：

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复合明渠中的非均匀流：混合RANS‐LES方法的评估

河流中的过岸流量，也称为复合明渠流量，在河流方向上很少均匀。流量不均匀性涉及从河流主河道（MC）到相邻洪泛区（FPs）的横向流，反之亦然。根据它们的方向和大小，横向电流可以使在MC和FP之间的界面处形成的平面剪切层横向移位。他们还可以修改螺旋次级电流（SC）的横截面分布。本文报告了一项数值研究，其重点在于评估混合RANS（雷诺平均Navier-Stokes）-LES（大涡模拟）方法（即尺度自适应模拟（SAS））捕获两个深度变化结构的能力。在直的复合明渠中均匀流动和非均匀流动。Proust和Nikora（2020，https://doi.org/10.1017/jfm.2019.973）最近在18 m长和3 m宽的实验室水槽中研究了模拟流量。该研究的独创性在于整个水槽长度上的流场建模以及大型Kelvin-Helmholtz型相干结构（KHCS）的长度尺度估计。SAS方法可以准确地预测以下方面的纵向演变：（i）沿流的时间平均流量，包括平面剪切层的宽度；（ii）SC模式；以及（iii）湍流统计。定性地检索了KHCS的出现和发展，并证实了其不受流度的影响而受到无量纲速度剪切的控制。