The interface between fluid-like and solid-like behaviour in two-dimensional granular flows

The effective phase change from fluid behaviour to solid behaviour, that too often occurs in granular flow and brings with it such unwelcome events as funnel flows in hoppers and clogging of other material handling devices, is studied using a discrete particle computer simulation of a Couette flow with gravity. This simulation exhibits the full range of granular flow behaviour, from a stagnant solid-like material, through a quasi-static transition zone, to a rapid granular flow. The most important result is that the first movement in the material just above the static bed occurs in a quasi-static mode at a fixed value of the stress ratio τxy/τyy. Thus, it appears that the primary transition from solid to fluid behaviour is a yield-like phenomenon and can be described by a Mohr-Coulomb-type failure criterion.