On the relevance of collision modeling for interface-resolving simulations of sediment transport in open channel flow

This paper deals with the interface-resolving simulation of particle transport by a turbulent flow over a rough bed. It aims at clarifying the importance of the type of collision model employed for the computed particle transport and the resulting fluid motion. For this purpose, a collision model based on a repulsive potential often used in the literature and a more complex collision model, the Adaptive Collision Model [Kempe & Fröhlich, J. Fluid Mech. 709 (2012) 445–489] are applied in turbulent open channel flow with bed-load sediment transport. In a first step, the Adaptive Collision Model is validated for multiple simultaneous collisions. This is done using simple test cases where the fluid surrounding the particles is neglected, as well as the sedimentation of multiple particles towards a bed of fixed particles in a viscous fluid. Numerical experiments on sediment transport are undertaken with two different prototypical setups, a single mobile particle traveling over a fixed rough bed and a cloud of mobile particles. The results show significant differences in the statistical quantities of the fluid and the disperse phase for different collision models. Comparison with experimental observations indicate significant improvement of the results with the use of the more sophisticated collision model, which takes all governing physical and numerical effects into account. Beyond the modeling issue the paper presents relevant physical information in the transport of a single particle over a rough bed by means of numerous statistical data. The same is done for collective particle transport in the regime of small sediment supply.