Models And Simulations Of Size Segregation In Granular Flows

Granular materials are pretty common in engineering and nature.Due to the discreteness and strong dissipation of granular materials,they exhibit multi-state behaviors,i.e.solid-like,liquid-like and gas-like mechanical states and their in-between transitions.Study of the mechanism of granular materials is one of the hot topics in academia.During the processes of natural hazards that result from the flow of granular materials,such as landslides and debris flows,segregation is always observed in granular mixtures consisting of particles differing in size.Usually large particles are transferred to the free surface of the flow while small particles sink downward.This configuration of particle size segregation would enhance the mobility and damage of the flow.Therefore it is very important to get a good understanding of the mechanism of size segregation in granular flows.However,researches on the rheology of granular flow are in some way not associated to that on the mechanism of size segregation,which leads to the fact that previous models are just suitable for a specific flow regime,and a unified segregation model that can be used to predict segregation pattern of granular flow when it turns from one flow regime to another is still in a lack.This thesis focuses on developing such a model and the following innovations are achieved:(1)Based on DEM simulations of binary size granular mixture flowing in open channels with different sidewall conditions,mixture theory and kinetic theory for size segregation of granular flow are compared,the influence of flow regime transition on the mechanism of size segregation is explained;A unified segregation model that can cover different flow regimes is developed based on the mixture theory.Research results suggest that changes on sidewall conditions lead to transitions of flow regime,as well as variations of the non-local effect inside the flow,and further change the way in which velocity gradient drives the segregation.The mixture theory of size segregation is suitable for slow dense granular flow,and the kinetic theory of size segregation is suitable for rapid granular flow,while the unified segregation model proposed in this thesis works well for both of these flow regimes.(2)Based on a non-linear elasticity model of granular materials,the incompressible assumption of granular flow and the ?(I)rheology,a unified granular constitutive relation is proposed,the reversible elastic effect,invertible large deformation and dissipation inside granular materials are described;With the help of Material Point Method,this model is used to simulate the mechanical behaviors of granular solid,granular liquid and the state in-between them successfully.(3)Based on the former two models,i.e.the unified granular constitutive model and the unified size segregation model,a numerical scheme for the size segregation of binary mixture granular flow in different flow regimes is proposed with the help of a modified Material Point Method.Based on this numerical model,coupling simulations of granular flow and segregation are achieved,flow features as well as final segregation patterns are simulated pretty well in both rapid and dense slow granular flows.