Numerical Simulation and Characterisation of the Packing of Granular Materials
Creators: name: Guises, Romain
Publisher: name: Imperial College London
Year: 2008

description: The scientific problems related to granular matter are ubiquitous. It is currently an active area of research for physicists and earth scientists, with a wide range of applications within the industrial community. Simple analogue experiments exhibit behaviour that is neither predicted nor described by any current theory. The work presented here consists of modelling granular media using a two-dimensional combined Finite-Discrete Element Method (FEM-DEM). While computationally expensive, as well as modelling accurately the dynamic interactions between independent and arbitrarily shaped grains, this method allows for a complete description of the stress state within individual grains during their transient motion. After a detailed description of FEM-DEM principles, this computational approach is used to investigate the packing of elliptical particles. The work is aimed at understanding the influence of the particle shape (the ellipse aspect ratio) on the emergent properties of the granular matrix such as the particle coordination number and the packing density. The diff erences in microstructure of the resultant packing are analysed using pair correlation functions, particle orientations and pore size distributions. A comparison between frictional and frictionless systems is carried out. It shows great diff erences not only in the calculated porosity and coordination number, but also in terms of structural arrangement and stress distribution. The results suggest that the particle's shape a ffects the structural order of the particle assemblage, which itself controls the stress distribution between the pseudo-static grains. The study then focuses on describing the stress patterns or \force chains" naturally generated in a frictional system. An algorithm based on the analysis of the contact force network is proposed and applied to various packs in order to identify the force chains. A statistical analysis of the force chains looking at their orientation, length and proportion of the particles that support the loads is then performed. It is observed that force chains propagate less efficiently and more heterogeneously through granular systems made of elliptical particles than through systems of discs and it is proposed that structural diff erences due to the particle shape lead to a signifi cant reduction in the length of the stress path that propagates across connected particles. Finally, the e ffect of compression on the granular packing, the emergent properties and the contact force distribution is examined. Results show that the force network evolves towards a more randomly distributed system (from an exponential to a Gaussian distribution), and it confi rms the observations made from simulations using discs. To conclude, the combined finite-discrete element method applied to the study of granular systems provides an attractive modelling strategy to improve the knowledge of granular matter. This is due to the wide range of static and dynamic problems that can be treated with a rigorous physical basis. The applicability of the method was demonstrated through to a variety of problems that involve di fferent physical processes modelled with the FEM-DEM (internal deformations, fracture, and complex geometry). With the rapid extension of the practical limits of computational models, this work emphasizes the opportunity to move towards a modern generation of computer software to understand the complexity of the phenomena associated with discontinua.