Abstract
In this paper the experiments of Fickie (1989) on the density of a granular material flowing through a wedge-shaped hopper are discussed with respect to their impact on the theoretical modelling of the flow from a hopper or bin under the influence of gravity. It is concluded that the degree of dilation found in these experiments is most significant. Furthermore, the experiments suggest that the material is less dense at the outlet than might be expected from the current theory. This in turn suggests that no discontinuity in the stress and velocity fields is needed to model the flow. The dilation is capable of reducing the flow so much that the challenge to the theoretician is no longer to find ways of increasing the dissipation of energy in their models but to decrease it.