Multi-scale Methods for Multi-component Granular Materials

In this paper we review recent progress made to understand granular chutes flow using multi-scale modeling techniques. We introduce the discrete particle method (DPM) and explain how to construct continuum fields from discrete data in a way that is consistent with the macroscopic concept of mass and momentum conservation. We present a novel advanced contact detection method that is able of dealing with multiple distinct granular components with sizes ranging over orders of magnitude. We discuss how such advanced DPM simulations can be used to obtain closure relations for continuum frameworks (the mapping between the micro-scale and macro-scale variables and functions): the micro-macro transition. This enables the development of continuum models that contain information about the micro-structure of the granular materials without the need for a priori assumptions. The micro-macro transition will be illustrated with two granular chute/avalanche flow problems. The first is a shallow granular chute flow where the main unknown in the continuum models is the macro-friction coefficient at the base. We investigate how this depends on both the properties of the flow particles and the surface over which the flow is taking place. The second problem is that of gravity-driven segregation in poly-dispersed granular chute flows. In both these problems we consider small steady-state periodic box DPM simulations to obtain the closure relations. Finally, we discuss the issue of the validity of such closure-relations for complex dynamic problems, that are a long way from the simple period box situation from which they were obtained. For simple situations the pre-computed closure relations will hold. In more complicated situations new strategies are required were macro-continuum and discrete micromodels are coupled with dynamic, two-way feedback between them.