(486g) Bulk Solids Flow Theory then and Now - Pointing a Direction Towards the Future

The foundation of bulk solids theory came through a question. What magnitude of stresses might a bulk material support in a confined and an unconfined condition? This led to another question: What stress state is required to cause deformation in the mass of bulk solid? And then, if the bulk solid did deform under those stresses, how would it deform – and can this be related to the flow of a bulk solid material? The concept of a limiting stress state and its application to granular material gave birth to a new field of study where the behavior of the mass was dependent on the cohesive and frictional properties of the material. A bulk material can transition from a flowing state to a non-flowing state based on several parameters: its cohesive nature, the shape of the container, and the frictional characteristics of the confining boundary.

This paper reviews the traditional bulk solids flow theory where the cohesive properties are a function of stress, but independent of strain rate. However, not all bulk solid materials have simple relationships describing cohesive strength effects. Sometimes the cohesive nature of the bulk material depends on how fast the bulk material experiences shear. With some modification, the traditional theory can be extended to materials that are both granular and viscous in nature. The concept of flow or no-flow then becomes strain rate dependent, complicating the concept of arching and making limiting flow rates dependent on the cohesive flow properties of the bulk. This extension can help explain the behavior of cohesive viscous solids such as oil sands and biomass that demonstrate complex viscous behavior in process equipment.