Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Harry L. Swinney. Vita. Includes bibliographical references. Also available from UMI.

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A study of the constitutive equations of the flow of cohesionless granular materials at large rates of deformation and low stress levels is presented. The interstitial fluid effects are assumed negligible and the particles approximately incompressible. / Dimensional arguments and experimental observations suggest that two different regimes of flow may occur. At low rates of deformation, the Coulomb friction gives rise to stresses that are rate independent (of the plastic type). At large rates of deformation, momentum transfer arises from particles collisions and the resulting stresses become rate dependent (of the viscous type). A continuum model that includes both types of behaviour is proposed. The equilibrium part of the stress tensor satisfies the Mohr-Coulomb yield criterion and a non-coaxial flow rule. The viscous part is assumed to have a form corresponding to an isotropic Reiner-Rivlin fluid. / Experiments of simple shear flow flow in an annular shear cell were performed on several types of dry granular materials. The purpose of the tests was to obtain the information needed for the continuum description of the material behaviour. At lower solids concentrations and high shear rates where the inter-particle collisions are dominant, both shear and normal stresses were proportional to the square of the shear rate. At higher concentrations and lower shear rates, Coulomb friction between particles became increasingly important and the stresses were proportional to the shear rate raised to a power less than two. All tests showed a strong dependence upon solids concentration.

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Thesis (PhD (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2004. / Granular flow occurs in a broad spectrum of industrial applications that range from separation and mixing in the pharmaceutical industry, to grinding and crushing, blasting, stockpile construction, flow in and from hoppers, silos, bins, and conveyer belts, agriculture, mining and earthmoving. Two totally different approaches of modelling granular flow are the Discrete Element Method (DEM) and continuum methods such as Finite Element Methods (FEM). Continuum methods can be divided into nonpolar or classic continuum methods and polar continuum methods. Large displacements are usually present during granular flow which, without remeshing, cannot be solved with standard finite element methods due to severe mesh distortion. The Particle-in-Cell (PIC) method, which is a so-called meshless method, eliminates this problem since all the state variables are traced by material points moving through a fixed mesh. The main goal of this research was to model the flow of noncohesive granular material in front of flat bulldozer blades and into excavator buckets using a continuum method. A PIC code was developed to model these processes under plane strain conditions. A contact model was used to model Coulomb friction between the material and the bucket/blade. Analytical solutions, published numerical and experimental results were used to validate the contact model and to demonstrate the code’s ability to model large displacements and deformations. The ability of both DEM and PIC to predict the forces acting on the blade and bucket and the material flow patterns were demonstrated. Shear bands that develop during the flow of material were investigated. As part of the PIC analyses, a comparison between classic continuum and polar continuum (Cosserat) results were made. This includes mesh size and orientation dependency, flow patterns and the forces acting on the blade and the bucket. It is concluded that the interaction of buckets and blades with granular materials can successfully be modelled with PIC. In the cases conducted here, the nonpolar continuum was more accurate than the polar continuum, but the polar continuum results were less dependent on the mesh size. The next step would be to apply this technology to solve industrial problems.

Granular materials are abundant in the natural and industrial environment. Typical granular materials are collections of inert, passive particles in which the constituent grains of the material are macroscopic; thus they fill space, are athermal, and interact through only local contact forces. This definition can be broadened to include non-inert particles as well-active particles-in which the grains of an active granular material possess an internal energy source which drives motion. Active granular materials are found in many areas of the biological world, from cattle stampedes and pedestrian traffic flow, to the subterranean world of ant colonies and their collective motion within the nest. We study the rheology and dynamics of inert granular material, and an active granular system of collections of fire-ants, which together we call matter with granularity. In both of these systems we observe bifurcations in the force and flow dynamics which results from confinement effects of the effectively rigid granular materials. In inert granular systems, the onset of flow among particles that are closely packed together causes them to dilate as particles must separate away from each other to accommodate flow. Dilation is a property unique to matter with granularity and other complex fluids in which particles interact locally and occupy space. We explore how dilation influences the inert granular system in situations of local and global forcing: drag of an immersed intruder and avalanche flow respectively. We next study collections of fire ants which also interact with each other locally through contact forces and exclude volume. We study the construction of, and locomotion within subterranean tunnels by groups of fire ants. We find that the traffic dynamics of ants within confined tunnels are significantly affected by tunnel diameter. Reducing tunnel diameter increases the formation of traffic jams due to the inability of ants to pass each other easily. However, we show that jamming within tunnels may have beneficial effects on subterranean locomotion. Individual ants jam there bodies against the walls of vertical tunnels to resist falling. From physics studies of fire ant mobility in confined spaces, we show that subterranean tunnel size has a significant effect on the stability and mobility of ants within these environments.

This thesis is an experimental study of the fluidization of binary mixture in particulate flows. A fluidized bed with two distributors was built with water being used as carrying fluid. Three types of solid particles of nylon, glass and aluminum of the same size and different densities are used in the experiments. The wall effect on a single particle fluidization, the fluidization of binary mixture of large density difference (nylon and aluminum of density ratio of 0.42), and the fluidization of binary mixture of close density (glass and aluminum with density ratio of 0.91) were investigated. Also, the effect of distributors on mono-disperse and bi-disperse particle fluidization was investigated. Results show that the presence of narrow walls reduces the minimum fluidization velocity for a single particle by as much as nearly 40%. Also, in the case of binary mixture of close density particles, uniform mixing was easily achieved and no segregation was observed, but in the case of large density difference particles, there exists significant segregation and separation. At high velocity, the uniform distributor behaves like a transport bed. To achieve a full bed in the single jet, it requires 1.5 times velocity of the uniform distributor. This behavior determines their application in the industries.