Structural Analysis And Numerical Modelling Of Granular Flows

Granular materials are composed by discrete particles,which are often observed in geo-hazards,such as debris flows.The purpose of this study is to better understand the complex behaviors of granular materials,such as the transition between fluid-like and solid-like behaviors,and to numerically model granular materials for engineering applications.In this work,a plane shear granular flow is modelled using discrete element method(DEM)to explore the dynamical and structural characteristics of granular materials in different rheology regimes.For engineering applications,a software is developed based on the material point method(MPM)to simulate the large deformation problems,including granular flow,granular flow impacting blocks and sliding of fully saturated sand pile.The main contents are summarized as follows.(1)A plane shear granular flow model under constant confining pressure is established to explore the dynamic characteristics of granular materials in different rheology regimes,such as force chain networks,velocity profiles,and energy densities.The model is verified using the generalized unified constitutive models of granular materials.The temporal evolution of spatial distributions of mesoscale structures are analyzed by the radical tessellation.It is found that the degree of local fivefold symmetry is a special structural indicator,which is correlated with a shear band.(2)Using an elasto-plastic constitutive model,a granular flow is simulated based on MPM,where the material parameters are determined by an inverse analysis.Considering that the contact algorithm of MPM is hard to deal with the static contact and the contact involving multi bodies,MPM is coupled with block DEM to simulate the process of granular flow impacting blocks.It is found that the simulation result agrees with the experimental result,particularly the phenomenon of the rotation of upper blocks.(3)A multiscale modelling framework using MPM and DEM is developed to abandon the phenomenological constitutive models.In this framework,the macro deformation of a continuum is simulated by MPM,and each macro material point is linked with a representative volume element(RVE)consisting of granular assemblage.The deformation information obtained by MPM is applied to the RVE as boundary conditions,while the Cauchy's stress calculated by DEM is reflected to MPM for the next step.This MPM/DEM multi-scale modelling strategy facilitates effective cross-scale interpretation and understanding the behavior of a collapse of granular pile.(4)A fully coupled hydro-mechanical model using MPM is developed based on the mixture theory.Darcy's law,considering the inertial effect,is adopted to govern the motion of interstitial water,and the conservation of momentum of the mixture is used to govern the motion of the solid.The spatial discretization schemes for these equations are derived using the generalized integration material point method,and the proposed coupled MPM formulation is implemented in a three-dimensional numerical code.An experiment is designed to observe the failure of a saturated sand pile,in which the partial-saturated region is avoided by increasing the hydraulic head at the input boundary,and the kinetic energy of water is dissipated by a filtering cloth.The failure process is simulated with the MPM code.It is found that the location of the shear band in the simulation agrees with the location of the sliding surface in the experiment.