Scales and kinetics of granular flows

When a granular material experiences strong forcing, as may be the case, e. g., for coal or gravel flowing down a chute or snow (or rocks) avalanching down a mountain slope, the individual grains interact by nearly instantaneo us collisions, much like in the classical model of a gas. The dissipative n ature of the particle collisions renders this analogy incomplete and is the source of a number of phenomena which are peculiar to "granular gases," su ch as clustering and collapse. In addition, the inelasticity of the collisi ons is the reason that granular gases, unlike atomic ones, lack temporal an d spatial scale separation, a fact manifested by macroscopic mean free path s, scale dependent stresses, "macroscopic measurability" of "microscopic fl uctuations" and observability of the effects of the Burnett and super-Burne tt "corrections." The latter features may also exist in atomic fluids but t hey are observable there only under extreme conditions. Clustering, collaps e and a kinetic theory for rapid flows of dilute granular systems, includin g a derivation of boundary conditions, are described alongside the mesoscop ic properties of these systems with emphasis on the effects, theoretical co nclusions and restrictions imposed by the lack of scale separation. (C) 199 9 American Institute of Physics. [S1054-1500(99)02603-8].