Stress, stress-asymmetry and contact moments in granular matter

The physical nature of contact moments within granular assemblies is reviewed and a new approach is developed for the homogenisation of stress within these materials. This approach revolves around capturing the effects of contact moments through the concept of contact eccentricity. By this method it is possible to calculate an expression for bulk stress that is both symmetric for material in equilibrium and fully consistent with the usual definition of bulk stress as an ensemble average of material stress over a representative volume element. The technique is demonstrated in a simple two dimensional example, as well as a larger scale discrete element modelling simulation of a steady state direct shear experiment

The prediction of permeability with the aid of computer simulations

The particular area of focus for this study is the use of theory to predict the permeability of a material through the use of the Ergun equation and the Kozeny-Carman equation along with computer simulations. The Ergun equation is well known for estimating permeability, and the Kozeny-Carman equation has also beet used to a lesser extent. Existing literature extensively covers the use of these equations with homogeneous materials containing mono-size particles. In this study, ate alternative way is sought to characterize mixtures that is based on the structure of the porosity, or void size, rather than the traditional method of using mean particle diameter. In doing so, this allows the Ergun and Kozeny-Carman equations to be rewritten to provide for an expansion in the type of mixtures that they can be applied to. Results are presented in this article on the application of these equations to mixtures including mono-size particles, then modified to include binary and distributed mixtures

The effects of particle dynamics on the calculation of bulk stress in granular media

Expressions for bulk stress within a granular material in a dynamic setting are reviewed and explicitly derived for assemblies of three dimensional arbitrary shaped particles. By employing classical continuum and rigid body mechanics, the mean stress tensor for a single particle is separated into three distinct components; the familiar Love-Webber formula describing the direct effect of contacts, a component due to the net unbalanced moment arising from contact and a symmetric term due to the centripetal acceleration of material within the particle. A case is made that the latter term be ignored without exception when determining bulk stress within an assembly of particles. In the absence of this centripetal term an important observation is made regarding the nature of the symmetry in the stress tensor for certain types of particles; in the case of particles with cubic symmetry, the effects of dynamics on the bulk stress in an assembly is captured by an entirely skew-symmetric tensor. In this situation, it is recognised that the symmetric part of the Love-Webber formula is all that is required for defining the mean stress tensor within an assembly - regardless of the dynamics of the system

Improving permeability prediction for fibrous materials through a numerical investigation into pore size and pore connectivity

The application of the traditional Kozeny–Carman equation to irregular particle shapes such as those found in fibrous materials results in a poor estimation of the permeability due to two factors; inadequate description of the void size and no accounting of the pore connectivity. A shape factor is used to attempt to describe the void size but fails for this type of particle as it can only be found empirically rather than from its definition. The use of the traditional Kozeny–Carman equation also includes a constant value for the tortuosity of a material, which represents the connectivity of the voids, whereas the tortuosity should be a variable that changes depending on the shape of the particle and the degree of packing. These two factors are investigated in this study via numerical simulations to obtain a greater understanding of the role of void space and void connectivity in the prediction of permeability