An Eulerian-Lagrangian model framework for cohesive sediment transport in turbulent flows

An Eulerian-Lagrangian framework has been developed to study the aggregation dynamics of cohesive sediments in turbulent flows. Fine sediment primary particles are modeled using the discrete element method which tracks the motions of each individual particle. An adhesion-contact model is used to simulate inter-particle collisions. The DLVO theory, which models aggregation, is also implemented to bring the particles closer together. To account for long-range hydrodynamic interactions between particles, the Pairwise Interaction Extended Point-particle (PIEP) model is applied, in which these interactions are pre-computed and effects of neighboring particles are linearly superposed. The PIEP model is shown to achieve DNS-like accuracy without the need to resolve the flow around individual particles. The model framework has been applied to sedimentation of cohesive flocs in isotropic turbulent flows at different turbulent shear rates and sediment volume fractions. Initially identical adhesive elastic primary particles are dispersed in the computational domain. The floc size spectrum evolves with time and reaches a quasi-steady state when the growth and break-up rates are balanced. Results show that, as expected, the size spectrum strongly depends on the shear rate and the volume fraction, and that floc size is limited by the Kolmogorov length scale. In addition, it is shown that long-range hydrodynamic interactions tend to enhance the collision frequency and promote floc growth leading to larger settling velocities, thereby underpinning the important role of such interactions in aggregation dynamics.