Floc Size Distribution of Cohesive Sediment in Isotropic Turbulence

A two-phase Eulerian-Lagrangian framework was implemented to study the flocculation dynamics in isotropic turbulence flows, especially the physical origin of different equilibrium floc size distributions. The primary particles of sediment were modeled as sticky soft spheres using the discrete element method, which tracks the motions of individual particles. The particle adhesive contact mechanics was simulated by the Johnson-Kendall-Roberts (JKR) model with the rolling friction. The equilibrium floc size distributions were investigated by varying physicochemical properties of primary particles, such as the diameter (Dp), the normalized Youngs modulus (E), damping coefficient () and the normalized surface energy density (). The fractal dimension is used to examine the floc structure. The model result shows that when aggregation is the dominant mechanism of flocs construction at equilibrium, a lognormal distribution of floc size develops. However, when breakup and aggregation mechanisms play an equally important role for flocs with size close to Kolmogorov length, the floc size distribution is better represented by the Weibull distribution with a shape factor between 2.5 and 3. For the cases that follow Weibull distribution, we identified flocs are built in a hierarchy way. Microflocs have a similar fractal dimension and are the building blocks for macroflocs, whose fractal dimension decreases with the floc size.