Investigation Of Fluid-Sediment Particle Motion Based On CFD-DEM Coupling Simulation Method

The flow and sediment motion is the hot research topic in river and coastal engineering all the time.Basic understandings about the macroscopic motion of flow-sediment have been obtained by previous researchers through experimental measurements and theoretical analysis,but the mesoscopic mechanism of this problem remains to be explored.With the rapid development of computer and computational method in recent years,it is possible to use numerical model to investigate this problem from the mesoscale view.Based on CFD-DEM coupling method,a fluid-particle coupled model is developed using open source software OpenFOAM,LIGGGHTS and CFDEM.The problem of sediment particles settling in still water,the kinetic characteristics of fluid and particle in open channel flow and oscillatory flow are mainly discussed.The main research contents and results of this thesis are as follows.(1)By simulating settling process of a single sediment particle in still water,the computational grids sensitivity of this coupled model is analyzed and the ratio of grid size to particle diameter is recommended in the range of 2~5 for small particles.Comparing the settling velocity results of different drag force models,the Benyahia model has a good accuracy.The settling velocity results of different diameter particles are consistent with experiment and empirical formula,which verified the applicability of this model.Simulated distributions of group settling velocity along horizontal directionfor different volume concentration have good agreements with experiments.(2)The velocity profile,turbulence intensity and Reynold stress have been examined through simulations of different cases of sediment transport in open channel flow.Three typical sediment movement forms of settled particles,bedload transport and bedload-suspended load transport are presented.Bedload rates per unit width show good aggrement with experimental data and empirical formulae.With increasing flow intensity,inhomogeneity of bed sediment increases at first and then decreases.(3)The simulated results show that the velocity profile and turbulence intensity are different for fixed and movable bed under oscillatory flow.Under the action of symmetrical oscillatory flow,sediment particles move back and forth periodicly and regularly.The net bedload rate approaches zero in a whole period.The averaged bedload rates of half cycle are in good agreement with previous experimental data and empirical formulae.