The dynamics of granular materials – towards grasping the fundamentals

The manufacture of a wide range of materials in particulate form currently has to rely upon a design basis that is not at all strong theoretically. The most developed methods exist in the design of storage vessels for particles but many weaknesses remain. In those areas of processing concerned with the development of product properties, the knowledge of the dynamics of granular motion is more rudimentary. However, we are now in a position where progress of a fundamental character elucidating the mechanisms operating in engineering systems can now be undertaken. This is illustrated by recent studies aimed at understanding how the axial motion of particles occurs in mixers which the particles are stirred mechanically by blades. Representation is possible in terms of an axial diffusion coefficient, for instance. A more physical approach reveals a cellular structure to the flow with transfers between cells limiting behaviour. The axial limits of these cells correspond to the plane of rotation of ploughshare blades or of the mechanical supports for long flat blades.