Pattern formation and shocks in granular gases
  Granular media such as sand and sugar are ubiquitous in nature and industry but are less well understood than fluids or solids. We consider the behavior of rapid granular flows where the transfer of momenta by collisions dominates. The physics is quite different for the opposite limit of static or slowly moving grains (e.g., sand piles). To gain understanding of granular flows we consider two problems that have been investigated with experiments, particle simulations and hydrodynamic theory: vertically oscillating granular layers and flow past an obstacle. Oscillating granular layers spontaneously form spatial patterns when the container acceleration amplitude exceeds a critical value, about 2.5 times the gravitational acceleration. Simulations with hard spheres that conserve linear momentum and dissipate energy in collisions are in qualitative accord with some but not all aspects of the observed patterns. It is necessary to include friction and angular momentum conservation in the simulations to achieve quantitative accord with observations. [Abridged] 
Harry L. Swinney and E.C. Rericha