Granular material flows – An overview

The paper attempts to give a critical overview of the field of granular flow with attention both to the history and the underlying physics that govern the field. It starts with a discussion of the basic transport mechanisms in a granular flow. It continues with a discussion of contact mechanics – the way that individual particles see each other mechanically. It then discusses the historical limiting regimes of granular flow, the Quasistatic and the Rapid-Flow regimes. Finally, it concludes with a review of the Elastic picture of granular flow, which both unifies the Quasistatic and Rapid regimes and fills in the intervening space. It shows that the rheological behavior of granular systems changes with system scale constraints, and, in particular, that the materials behave differently under controlled-stress and controlled-concentration conditions. The Elastic model defines an entire flowmap of granular flow and thus allows one to place boundaries on where the Quasistatic and Rapid-Flow models (sometimes called kinetic theory models) are something of a red herring and cannot be applied to common granular flows.