Elizabeth D. Liss

1) The Influence of Clusters on the Stress in a Sheared Granular Material

Although the transport of bulk solids materials is an integral part of many industrial processes, many fundamental questions about the basic flow behavior of granular materials remain unanswered. Microstructure formation has been shown to exist in several types of granular flow due to the inelastic nature of collisions between particles of a granular material and due to the presence of confining walls. In this work particle dynamic simulations are carried out to examine the effect of cluster formation on the stress tensor in a rapidly sheared granular material. The simulation used for this study is an event driven algorithm with Lees-Edwards boundary conditions. Although most previous work has focused on a system of a particular size or a fixed number of particles, it has been determined that the degree of clustering depends on the system size. It was found that the long time average stresses and granular temperature initially increased with the system size and then approached limiting values as the size of the system is further increased. It is also observed that the distribution of stresses initially broadened with system size and then approached limiting distributions as the size of the system is further increased. Combining the values of the stresses obtained from our simulations with a previous theory for the clustering length scale, it has been determined that once clusters are fully formed in a system, the stresses no longer increase. These results suggest that under certain circumstances a small system can capture the overall behavior of a much larger system, and by knowing the variation of stress with system size it is possible to determine whether or not the effect of clustering is significant for a particular application. In addition, we conclude that it is important to identify relevant microstructures in granular flows and simulate a large enough system so that the effects related to these microstructures are not missed.

2) Microstructure Formation during Gravity-Driven Flows in Vertical Channels

In addition to the questions that remain regarding the effects of microstructure formation on granular flows, the effect of a driving force, such as gravity, is not fully understood. In this work a particle dynamic computer simulation is used to examine gravity flow of two-dimensional disks in a channel and to investigate how microstructure affects the flow properties of this system. The simulation used for this study is an event driven algorithm with periodic boundary conditions in the flow direction and flat walls in the lateral direction. During rapid gravity-driven flow of granular material in a channel, three distinct forms of microstructure (a plug, a wavy flow and a clump) have been identified. The parameters of the system, which have been shown to affect structure, include average solids fraction, coefficient of restitution, particle size, the size of the periodic cell, and the distance between confining walls. Simulations of large systems for long times have been performed due to the fact that the dynamics of large systems are often quantitatively different from those of small systems and the type of structures which form are dependent on system size. Microstructure has been characterized by examining local and averaged steady flow properties of the system, such as velocity, mass flux, granular temperature and stresses. Also, the types of microstructure observed by our simulations have been compared to those that are predicted using a linear stability analysis of equations of motion of rapid granular flow and good agreement is found.

3) Segregation during Gravity Flow through Vertical Pipes

The final product required by many industries, for example the pharmaceutical industry, is an exact mixture of bulk solid materials that have a variety of different physical properties. Often, much time and energy is expended to ensure that the product is well mixed. However, products must often be moved during the final phases of processing and the transport of the powder can lead to segregation. Previous researchers have observed that transport can both enhance as well as diminish segregation of powders, with different physical properties such as size, shape or density. In this work, the physical mechanisms that can lead to segregation during gravity-driven flow of powders is investigated. An experimental investigation of size segregation is being conducted which examines the effect of a vertical drop (i.e. sedimentation) on segregation in vertical ducts (or pipes) for a variety of different powders. A quantitative framework for understanding such segregation is being developed by carrying out experiments for different physical and process parameters such as particle size ratio, average particle size, amount of material dropped, pipe diameter and drop height. The sampling of the sedimented powder is accomplished through a collection unit that allows the amount of material with a specific property to be examined as a function of height of material in the collection unit. For cohesionless materials we have observed a general trend whereby the mass fraction of the fines increases with height, but in two distinct regions. In the first region (close to the bottom of the collection unit) the fraction of fines increases gradually while in the second region there is a relatively large increase in the fraction of fines with height. The experimental effort is complemented by the development of a model based on continuum mechanics and particle dynamic simulations to describe segregation of polydisperse materials. Through this work we have been able to show that for the conditions we have examined, drag effects govern the segregation phenomenon.