Lattice Boltzmann simulation of the flocculation process of cohesive sediment due to differential settling

Abstract

The flocculation processes for differential settling of cohesive sediment were simulated via the Lattice Boltzmann method (LBM) in which the hydrodynamics and attractive van der Waals forces were considered. The simulations were performed with different initial sediment concentrations, and hydrodynamic properties of flocs formed by differential settling, such as size spectra, settling velocities, flow fields around flocs and particle motion trajectories, are discussed in detail. Results indicate that flocs containing the same primary particles formed by different sediment concentrations have similar settling velocities, which means that sediment concentrations exert few effects on flocs settling velocities themselves. Otherwise, sediment concentrations influence the bulk mean settling velocity of suspended sediment. The higher the sediment concentration, the larger the bulk mean settling velocity, which is attributed to the higher-concentration particles colliding easily and quickly forming larger flocs. Statistical analysis shows that numerical results clearly reproduce collision properties because of the differential settling at the mesoscale. These results suggest that LBM may be a promising approach to study the flocculation mechanisms of cohesive sediments.