Lattice Boltzmann Simulation For Flocculation Dynamics Of Cohesive Sediment

The flocculation processes play an important role in cohesive sediment transport and some physical phenomena in estuaries and on coasts. There is few effective ways for direct numerical simulation of flocculation mechanism. Therefore, a three-dimensional numerical model of flocculation dynamics of cohesive sediment via the Lattice Boltzmann method was presented. The flocculation process and settling behavior of cohesive sediment were explored in still water and shear flow. The flocculation mechanism due to differential settling was disclosed. Some physical phenomena in the field were explained from the mesoscale view. The main results are summarized as follows:1. A fully resolved numerical model of flocculation dynamics of cohesive sediment via the Lattice Boltzmann method was developed. The simulated results of the laminar and transitional flows induced by single particle settling, agreed with those analytical solutions. It was shown that the present model could be applied to the simulation of the particles settling in the laminar and transitional flows.2. The turbulent flows induced by the large settling particle were obtained through the Lattice Boltzmann method combining with the large-eddy simulation method. The computational results were basically in accordance with the settling velocity and flow field measured by the Particle Imaging Velocimetry. It was concluded that the Lattice Boltzmann method and the large-eddy simulation could be used to simulate the turbulent flow induced by particle settling.3. The fractal mud flocs formed by using the diffusion limited cluster-cluster aggregation model resembled the mud flocs in the field. The simulated settling velocities of the fractal mud flocs via Lattice Boltzmann method agree with the calculated results of Winterwerp's settling velocity formula adequately. The settling behaviour of three-dimensional fractal mud floc in still water was disclosed from the mesoscale view.4. The processes of settling and breakup of three-dimensional fractal mud flocs under the shear flows were simulated. Under low shear conditions, flocs will not break and deposit to the bed. Under high shear conditions, the flocculated aggregates with weak strength will break up, and the single particles and small flocs after breakup could be suspended in the flow without settling into the bed. However, the flocs with great strength could fall through the shear zone near the bed and deposited to the bed though undergoing large deformation.5. The flocculation processes due to the differential settling of cohesive sediment were fully simulated via Lattice Boltzmann method. Two particles of the same density and different sizes, located in the same vertical line, could collide and aggregate. For the differential settling, flocs with similar sizes formed in different sediment concentrations have similar settling velocities, which show that the sediment concentrations exert few effects on the settling velocities of flocs themselves. Otherwise, the sediment concentrations have obvious influences on the bulk mean settling velocity of suspended sediment. The higher is the sediment concentration, the larger the bulk mean settling velocity will be, which can be attributed that the particles in the higher concentration water are easy to collide and form larger flocs quickly.6. Flocs quickly capture other single particles or smaller flocs and aggregate into larger flocs during the process of the differential settling, which will lead to the fast deposition of suspended sediment. This phenomenon is likely to occur during the slack water.