Granular Hydrodynamics

Sand flowing through the constriction of an hourglass or jumping on a vibrating plate is fluidized in the sense that it moves analogously to a fluid. Dense flows of grains driven by gravity down inclines occur in nature and in industrial processes. Natural examples include rock avalanches and landslides. Applications are found in the chemical, pharmaceutical and petroleum industry. Grain flow can be modeled as a fluid-mechanical phenomenon. However, granular fluids teach us about an astounding complexity that emerges from simple, macroscopic particles. For example, starting from an homogenous fluidized system, structures evolve and a dilute granular fluid co-exists with much denser solid-like clusters. Another example is the so-called Brazil nut effect, whereby larger and heavier particles placed into an agitated granular bed rise to the top. We present an outlook of the hydrodynamic description of granular materials. Our purpose is to outline a theory of grain flow which is based upon the description of continuous matter fields derived from the kinetic theory for dense gases, as is usually encountered in fluid dynamics.