We present a collision model for phase-resolved Direct Numerical Simulations of sediment transport that couple the fluid and particles by the Immersed Boundary Method. Typically, a contact model for these types of simulations comprises a lubrication force for particles in close proximity to another solid object, a normal contact force to prevent particles from overlapping, and a tangential contact force to account for friction. Our model extends the work of previous authors to improve upon the time integration scheme to obtain consistent results for particle-wall collisions. Furthermore, we account for polydisperse spherical particles and introduce new criteria to account for enduring contact, which occurs in many sediment transport situations. This is done without using arbitrary values for physically-defined parameters and by maintaining the full momentum balance of a particle in enduring contact. We validate our model against several test cases for binary particle-wall collisions as well as the collective motion of a sediment bed sheared by a viscous flow, yielding satisfactory agreement with experimental data by various authors.

We analyze the consolidation of freshly deposited cohesive and non-cohesive sediment by means of particle-resolved direct Navier-Stokes simulations based on the Immersed Boundary Method. The computational model is parameterized by material properties and does not involve any arbitrary calibrations. We obtain the stress balance of the fluid-particle mixture from first principles and link it to the classical effective stress concept. The detailed datasets obtained from our simulations allow us to evaluate all terms of the derived stress balance. We compare the settling of cohesive sediment to its non-cohesive counterpart, which corresponds to the settling of the individual primary particles. The simulation results yield a complete parameterization of the Gibson equation, which has been the method of choice to analyze self-weight consolidation. arXiv:1907.03700v1 [cond-mat.soft]

We employ direct numerical simulations of the three-dimensional Navier-Stokes equations, based on a continuum formulation for the sediment concentration, to investigate the physics of turbidity currents in complex situations, such as when they interact with seafloor topography, submarine engineering infrastructure and stratified ambients. In order to obtain a more accurate representation of the dynamics of erosion and resuspension, we have furthermore developed a grain-resolving simulation approach for representing the flow in the high-concentration region near and within the sediment bed. In these simulations, the Navier-Stokes flow around each particle and within the pore spaces of the sediment bed is resolved by means of an immersed boundary method, with the particle-particle interactions being taken into account via a detailed collision model.

Abstract

One of the most important aspects in hydraulic engineering is to describe flows over mobile porous media in a continuum sense to derive models for sediment transport. This remains a challenging task due to the complex coupling of the particle and the fluid phase. Computational Fluid Dynamics can provide the data needed to understand the coupling of the two phases. To this end, we carry out grain-resolving Direct Numerical Simulations of multiphase flow. The particle phase is introduced by the Immersed Boundary Method and the particle-particle interaction is described by a sophisticated Discrete Element Method. We derive the stress budgets of the fluid and the particle phases separately through a rigorous analysis of the governing equations using the Double Averaging Methodology and the Coarse-Graining Method. As a next step, we perform a simple simulation of a heavy particle exposed to a Poiseuille flow rolling along a wall to understand the physical implications of the fluid-particle coupling. All terms of the stress balances can be computed in a straightforward manner allowing to close the budgets for the two phases separately. However, we encounter problems when attempting to combine the fluid-resolved local stresses with the coarse-grained particle stresses into a single balance for the fluid-particle mixture.