Rapid Granular Flows

A granular material is a collection of a large number of discrete solid particles. Generally, the interstices between the particles are filled with a fluid such as air or water, and thus, technically, a granular flow is a multiphase process. However, if the particles are closely packed or if they are much denser than the interstitial fluid, the particles alone-and not the fluid or the fluid-particle interactions-will play the greatest role in momentum transport within the material, in which case the interstitial fluid can be ignored in describing the flow behavior. Granular-material fl ows are generally taken to fall into this limiting category and thus may be considered dispersed single-phase rather than multiphase flows. This review addresses the fluidlike behavior of granular solids and, in particular, those flows for which the material is rapidly sheared. I outline what is currently known about rapid granular flows, discuss the various modeling techniques used to describe the motion of the bulk material, and point out many questions that remain to be answered. Depending on the local stress conditions, a granular material may behave as either an elastic solid or a fluid. When a granular material is showing its elastic-solid behavior, it can support the large loads of building foundations or form i nto hills with finite slopes. However, much of the load is supported across frictional bonds between the particles, and, as such, the system's strength is limited to the loads that those bonds can support. When enough of the bonds have been overcome, the system will fail and begin to flow. The initial failure will consist of many-particle blocks moving relative to one another along shear bands that roughly follow stress characteristics through the material. If the motion occurs slowly, particles will stay in contact and interact frictionally with their neighbors over long periods of time. The failure will continue in this