Floc reshaping by turbulence

Abstract:

An Eulerian-Lagrangian framework has been implemented to study the flocculation dynamics of cohesive sediments in isotropic 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 is implemented to model sediment cohesion. The model framework has been applied to sedimentation of cohesive flocs in isotropic turbulent flows at different turbulent shear rates. Initially identical adhesive elastic primary particles were dispersed in the computational domain. Equilibrium floc size and structure distributions develop when the balance between aggregation and breakup is achieved. Peak around Kolmogorov length scale was identified in the floc size distribution. Geometric measures, including Corey shape factor and sphericity, were used to analyze floc structures at different development stage. Floc structure distribution at equilibrium stage is narrow compared to that at initial stage for flocs comparable to the Kolmogorov length scale, suggesting a stable preferred structure of floc for a given floc size at equilibrium. Fragmentation by turbulence and regrowth of flocs lead to more stable compact floc structures.