Investigating the settling dynamics of cohesive silt particles with particle-resolving simulations

Abstract

The settling of cohesive sediment is ubiquitous in aquatic environments, and the study of the settling process is important for both engineering and environmental reasons. In the settling process, the silt particles show behaviors that are different from non-cohesive particles due to the influence of inter-particle cohesive force. For instance, the flocs formed in the settling process of cohesive silt can loosen the packing, and thus the structural densities of cohesive silt beds are much smaller than that of non-cohesive sand beds. While it is a consensus that cohesive behaviors depend on the characteristics of sediment particles (e.g., Bond number, particle size distribution), little is known about the exact influence of these characteristics on the cohesive behaviors. In addition, since the cohesive behaviors of the silt are caused by the inter-particle cohesive forces, the motions of and the contacts among silt particles should be resolved to study these cohesive behaviors in the settling process. However, studies of the cohesive behaviors of silt particles in the settling process based on particle-resolving approach are still lacking. In the present work, three-dimensional settling process is investigated numerically by using CFD-DEM (Computational Fluid Dynamics-Discrete Element Method). The inter-particle collision force, the van der Waals force, and the fluid-particle interaction forces are considered. The numerical model is used to simulate the hindered settling process of silt based on the experimental setup in the literature. The results obtained in the simulations, including the structural densities of the beds, the characteristic lines, and the particle terminal velocity, are in good agreement with the experimental observations in the literature. To the authors' knowledge, this is the first time that the influences of non-dimensional Bond number and particle polydispersity on the structural densities of silt beds have been investigated separately. The results demonstrate that the cohesive behavior of silt in the settling process is attributed to both the cohesion among silt particles themselves and the particle polydispersity. To guide to the macro-scale modeling of cohesive silt sedimentation, the collision frequency functions obtained in the numerical simulations are also presented based on the micromechanics of particles. The results obtained by using CFD-DEM indicate that the binary collision theory over-estimated the particle collision frequency in the flocculation process at high solid volume fraction.

Settling of cohesive sediment: particle-resolved simulations

“…It represents the ratio of the cohesive force maximum for particles of diameter D 50 to the characteristic gravitational force scale of the problem. A similar characteristic number was used by Sun et al (2018) in the framework of a point-particle approach. A complete derivation for the origin of Co within the present numerical framework is provided in appendix B.…”

“…Numerical investigations have attempted to tackle this issue by employing point-particle approaches in conjunction with a hard-sphere model to account for particle-particle interactions (e.g. Ho & Sommerfeld 2002;Kosinski & Hoffmann 2010;Breuer & Almohammed 2015;Sun et al 2018). The hard-sphere model resolves collisions instantaneously by changing the particle velocity according to a restitution coefficient for inelastic collisions.…”

Consolidation of freshly deposited cohesive and noncohesive sediment: Particle-resolved simulations

“…It represents the ratio of the maximum cohesive force for particles of diameter D 50 to the characteristic gravitational force scale of the problem [62]. We can transfer the DLVO-theory (7) to our simpler model (9) by choosing the following parameters: (i) A H = 1 · 10 −20 J, which reflects silica materials in water according to [64], (ii) R eff = RpRq Rp+Rq = Rp 2 = 5µm for monodisperse silt particles of grain size D p = 20µm, (iii) ζ 0 = 4.7nm; (iv) we determine κ using the approximation for the monovalent salt sodium chloride given by [65] as κ = 0.304·10 −9 m −1 |z| √ C salt in meters, where z is the valency of the salt and C salt is the salt concentration in mol/liter.…”

NUFEB: A Massively Parallel Simulator for Individual-based Modelling of Microbial Communities

“…The EPS adhesive force F a,i is a pair-wise interaction, which is modelled as a van 175 der Waals force [25]:…”

“…The drag force F d,i is the fluid-particle interaction force due to fluid flow, with 180 direction opposite the microbe motion in a fluid. It is formulated as [25]:…”