Fluid Mechanics for Chemical Engineering

Chapter 18

Notions on Granular Materials

The mechanics of granular materials, which were once of interest only to specialists in geomechanics, have considerably developed, and its applications have widened to include other fields in physics. As far as processes are concerned, it has wide applications in the agri-food industry for understanding mechanics inside silos or in logistics for the transport of granular materials. A fluid dynamicist can no longer stay detached from the mechanics of granular materials. The notion of angle of repose, as well as of powder flowability, is frequently employed by engineers.

This chapter has quite modest ambitions. We limit ourselves to the introduction of a few fundamental notions about granular materials, in order to facilitate access to this field of mechanics, and to setting out what distinguishes a granular material from a fluid medium. These differences will be illustrated by two important applications:

1.The pressure distribution in a granular material at static equilibrium within the gravity field, which differs from the one that establishes itself in a fluid. The case of tanks/silos will be discussed.

2. The flow of a granular material, for example in an hourglass, which differs from that of a fluid. The reasons for this will be explained.

Flows of cohesive granular materials will not be discussed, setting aside the notion of powder flowability whose introduction requires to be complemented by drawing on examples from continuum mechanics. ...