Computational modeling of open-channel flows is usually carried out using the rigid-lid (RL) assumption to prescribe the water surface. Such an approach deems more appropriate for sub-critical flow regimes with Froude number ≪ 1. When the Froude number is locally high, e.g. flows around hydraulic structures, the free-surface fluctuations affect the induced hydrodynamics. Indeed, prior investigations on the effect of the RL assumption on large-eddy simulation (LES) of open-channel flows around bridge abutments revealed that such an assumption may influence both the first and second order turbulence statistics. In this work, we investigate the effect of the RL assumption on bed-morphodynamics calculations. The fully coupled flow and bed morphodynamics virtual flow simulator (VFS-Geophysics) model is herein employed to numerically investigate the sediment transport around a laboratory-scale model of abutment. Numerical simulations using a mass-conserving level-set (LS) method and a RL assumption are performed to evaluate the influence of free-surface elevation on the computed bed-morphodynamics at a Reynolds number of 7.9 × 10⁴. We also conducted an experimental study to observe the scour development around the abutment model. The instantaneous scour patterns and bed profiles computed with the LS and RL are compared with the measurements. The mean absolute errors of bed-profile predictions at or near quasi-equilibrium of bed evolution, for the LS and RL computed results, are about 4.1% and 4.7%, respectively. Despite the differences in flow field computations of the two methods, the computed bed morphology appeared relatively insensitive to the two numerical approaches. However, the computational cost of the coupled LES-LS-morphodynamics is at least one order of magnitude greater than that of the LES-RL-morphodynamics.

中文翻译：

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明渠流动LES的刚性盖和水平集方法的比较研究：形态动力学

明渠水流的计算模型通常使用刚性盖（RL）假设来规定水面。这种方法认为更适合于Froude数为1的亚临界流态。当Froude数局部较高时，例如在水力结构周围流动时，自由表面的波动会影响诱导的流体动力学。实际上，先前对RL假设对桥台周围明渠流大涡模拟（LES）的影响的调查表明，这种假设可能会影响一阶和二阶湍流统计。在这项工作中，我们调查了RL假设对床层形态动力学计算的影响。本文采用完全耦合的流动和床层形态动力学虚拟流动模拟器（VFS-Geophysics）模型，以数字方式研究沉积物在实验室规模的基台模型周围的传输。进行了使用质量守恒水平集（LS）方法和RL假设的数值模拟，以评估雷诺数为7.9×10时自由表面高程对计算的床层形态动力学的影响。⁴。我们还进行了一项实验研究，以观察基台模型周围的冲刷情况。将用LS和RL计算的瞬时冲刷模式和床剖面与测量值进行比较。对于LS和RL计算的结果，在床演化的准平衡处或附近，床剖面预测的平均绝对误差分别约为4.1％和4.7％。尽管这两种方法的流场计算有所不同，但计算的床层形态对这两种数值方法似乎相对不敏感。然而，耦合的LES-LS-形态动力学的计算成本至少比LES-RL-形态动力学的计算成本大一个数量级。