A comparative study of rigid-lid and level-set methods for LES of open-channel flows: morphodynamics

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.