Discrete element modeling of granular flows

A two-dimensional discrete element model for dynamical simulation of granular material behavior has been developed. The contact force is viscoelastic with the friction limit on the tangential force component. The model has been applied to three granular flow problems: (1) simple shear flow of disks at high solids concentrations. An emphasis is placed on the transition between rapid and quasi-static granular flows. A semi-empirical power law relationship between the stresses and the shear rate has been obtained. The transition is characterized by a gradual decrease in the power of the shear rate from two for the rapid granular flow towards zero for the quasi-static behavior. Distinct mechanisms that govern transitional and quasi-static regimes are observed and discussed. (2) uniform gravity-driven flow of disks in a channel. A theoretical solution for the uniform gravity-driven channel flow has been obtained utilizing the kinetic theory formulations and adding the fluid drag effects. The results compare favorably with the results of discrete element simulations and the experimental data. (3) transport and jam of surface river ice. A steady, non-uniform (decelerating) river flow in a straight channel is considered. Nonuniform ice flows have been simulated using the newly developed "upstream feeding scheme". Two examples are presented: in the wider channel (width/diameter = 8), a moving surface jam formed; while in the narrower channel (width/diameter = 4), a static surface jam formed. This static jam was initiated by the formation of a stable arch spanning the channel.;The results of this study can be used to aid the future development of a complete mathematical model for surface ice transport processes.