Discrete Simulations of Fluid-Driven Transport of Naturally Shaped Sediment Particles

Abstract

The particles in natural bedload transport processes are usually aspherical and span a range of shapes and sizes, which is challenging to be represented in numerical simulations. We assemble existing numerical methods to simulate the transport of natural gravel (NG). Starting with computerized tomographic scans of natural grains, our method approximates the shapes of these grains by “gluing” spheres (SP) of different sizes together with overlaps. The conglomerated SP move using a Discrete Element Method which is coupled with a Lattice Boltzmann Method fluid solver, forming the first complete workflow from particle shape measurement to high-resolution simulations with hundreds of distinct shapes. The simulations are quantitatively benchmarked by flume experiments. Beyond the flume, in a more generalized wide wall-free geometry, the numerical tool is used to further test a recently proposed modified sediment transport relation, which takes particle shape effects into account, including the competition between hydrodynamic drag and material friction. Unlike a physical experiment, our simulations allow us to vary the hydrodynamic drag coefficient of the NG independently of the material friction. The results support the modified sediment transport relation. The simulations also provide insights into particle-level kinematics, such as particle orientations. Though particles below the bed surface prefer to orient with their shortest axes perpendicular to the bed surface, with a decaying tendency with an increasing height above the bed surface, the orientational preferences in transport processes are much weaker than those in settling processes. NG rotates relatively freely during bedload transport.