Large Eddy Simulation Of Fluid Mud And Sediment Transport Under Waves And Currents

In river or coastal area, the sediment and fluid mud can be suspended and trans-ported under waves or current. The interaction of fluid, sediment and bed causes bed erosion or deposition and this induces lots of engineering sediment problems, i.e., coastline erosion, reservoir sedimentation, local scour around bridge and pier, back-silting of navigation channel. The investigation on sediment in river and coastal area is helpful for understanding the mechanism of sediment motions, accurately predicting the tendency of bed deformation, and thus address the various issues encountered in hydraulic engineering projects.In this paper, large eddy simulation method (LES) is adopted to model the mo-tions of fluid mud and sediment under breaking waves and current. The main contents are as follows:1、The non-static flow and sediment/mud transport model in the framework of LES is described. The coupled Level set and VOF method, CLSVOF, is used to track the evolution of interfaces between different phases. For the bottom boundary condition of sediment, the newly proposed flux-based model is implemented. This model is physically reasonable for LES simulations. To improve the efficiency of the code, parallel computing is achieved based on massage passing interface (MPI).2、The discretization problem related to the sediment settling term for suspended sediment simulations is analyzed. The criterion for appropriate discretization of this term is given and based on this, two new schemes on designed. From the simulations of sediment transport under turbulent channel flow and breaking waves we see that, the newly designed two schemes are superior to the conventional schemes.3、3D LES simulations are carried out to study the interaction of breaking wave and a layer of fluid mud at the bottom. The wave breaking process, the detailed flow field in the water and mud region, the energy transport and dissipation mechanism, and also the interface evolution between air-water and water-mud are analyzed. It shows that, the existence of fluid mud can weak the intensity of wave breaking and shorten the wave breaking period. Most of the energy dissipation is due to the work done to the air phase and the viscous dissipation of fluid mud. Through the investigation of energy transport in the studied system, we see that breaking wave not only enhances the intensity of energy transport in the water but also alters its transport mechanism (compared with nonbreaking wave). This is not the case in the mud layer however, for which only the intensity of energy transport is changed.4、Numerieal.simulations are implemented for the study of effect of wave break-ing on suspended sediment. The phenomenon of breaking wave generated sediment suspension is successfully modeled. The sediment motions in the water, the effect of turbulence and current caused by wave breaking are elucidated. We also present the mechanism of local scour effect caused by the large scale air-bubble. Three stages are summarized for sediment motions in the studied system. Finally, the scales for sediment transport and bed deformation are given.