# Investigating the Settling Dynamics of Cohesive Silt Particles With Particle-Resolving Simulations

## TL;DR

This study uses CFD-DEM simulations to explore how cohesive forces and particle polydispersity influence the settling behavior of silt particles, providing new insights into bed structure and collision dynamics.

## Contribution

First numerical investigation of how Bond number and polydispersity separately affect cohesive silt settling using CFD-DEM.

## Key findings

- Structural densities of silt beds depend on cohesion and polydispersity.
- Cohesive behavior arises from particle cohesion and polydispersity effects.
- Binary collision theory overestimates collision frequency at high solid volume fractions.

## Abstract

The settling of cohesive sediment is ubiquitous in aquatic environments. 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. 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 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.