Hydrodynamics: A Sea of Grains

A granular fluid with typical boundary conditions used in laboratory or in silico, will develop structures and inhomogeneities in space and time. When spatial and temporal gradients are small, slow fields such as density, flow velocity and granular temperature evolve accordingly to the equations of granular hydrodynamics. The main steps to derive and close those equations, starting from granular Boltzmann equation, are described in this chapter. The application of the method to common situations are discussed. The interesting and still debated problem of fluctuations is introduced, in the last part of the chapter.