Cohesive sediment forms flocs of various sizes and structures in the natural turbulent environment. Understanding flocculation is critical in accurately predicting sediment transport and biogeochemical cycles. In addition to aggregation and breakup, turbulence also reshapes flocs toward more stable structures. A 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 was used to simulate inter-particle collisions. The model framework has been applied to sedimentation of cohesive flocs in isotropic turbulent flows at different turbulent shear rates. 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 gyration radius and fractal dimension, were used to analyze floc structures at different development stage. We have identified two floc reshaping mechanisms, namely breakage-regrowth and restructuring by hydrodynamic drag. Floc surface erosion is found to be the primary breakup mechanism for strong flocs, while fragile flocs tend to split into fragments of similar sizes. Aggregation of flocs of sizes comparable to or greater than the Kolmogorov scale is modulated by turbulence at lower aggregation efficiency. Our findings highlight the limiting effects of turbulence on both floc size and structure.
About the Speaker
Xiao Yu obtained his PhD degree in Ocean Engineering at University of Delaware in 2012, and continued to work at the center for applied coastal research at UD as a postdoc from 2012 to 2015. He worked at the Institute of Marine Sciences at UNC Chapel Hill from 2015 to 2017 as a postdoc researcher and studied hydrodynamics of coral reefs. In 2017, he joined the department of Civil and Coastal engineering at University of Florida as an assistant professor. His research focuses on nearshore hydrodynamics, sediment transport and physical-ecological coupling of aquatic ecosystems.
Host: Professor Leonardo Chamorro