Modeling the process of cohesive sediment settling and flocculation based on CFD–DEM approach

The settling process of cohesive sediment particles exists universally in nature and many engineering fields. Some of them have negative impacts, e.g., leading to the storage loss of reservoir, prolonging the duration of the reclamation project. Thus, understanding and quantifying cohesive sediments settling and flocculation processes are of great importance. In the present work, a three-dimensional model is applied to investigate the cohesive settling process numerically by computational fluid dynamics–discrete element method (CFD–DEM) approach. In the model, the cohesiveness of the sediment particle is modeled by implementing the van der Waals force. The simulation results are in good agreement with the experimental results, which demonstrate the ability of the current model to study the settling and flocculation process. For cohesive particles, the influence of polydispersity on structural density is discussed in two aspects: (a) the particle mean diameter d50 and (b) the geometric standard deviation d85/d15. Furthermore, the results show that the influence of these two parameters is more profound when a large Hamaker constant is applied. We also study the relationship between the maximum value of structural density and the Hamaker constant. The structure density is found to be significantly influenced by Hamaker constant when Hamaker constant is smaller than 2.0×10-20J\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2.0\, \times \,10^{ - 20} \,{\text{J}}$$\end{document}. Besides, for the poly-dispersed system, different from non-cohesive particles, the segregation phenomenon is not obvious for poly-dispersed cohesive particles due to the flocculation phenomenon. Moreover, the excessive pore pressure in sediment bed does not dissipate instantaneously as the non-cohesive sediment bed. And with the increase of the Hamaker constant, the excessive pore pressure dissipation velocity of the sediment bed decreases. In this work, we study the flocculation phenomenon of cohesive particles. It’s found that the influence of cohesive force is more significant on small particles. Since with the reduction of the particle size, the inertial force decreases to a greater extent compared to the cohesive force, the particles’ motion is mainly determined by the cohesive force.