So far we have confined our interest to undriven rapid granular flow, and we now will give a short overview of the different methods used in this field. Kinetic theory and hydrodynamic description Kinetic theory in the context of rapid flow in granular media is almost exclusively connected with the hard-sphere model, which uses the idea of freely moving particles which suffer instantaneous inelastic binary collisions. Classical gases Since the hard-sphere model has been a very useful reference

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... ystem for our understanding of classical liquids [Ha86], many results and techniques are available and not surprisingly the model has become very popular also in the context of granular media. As far as static correlations are concerned, an analytical expression for the pair correlation is available [Th63, We63, VeLe82] . This provides a good first approximation for particles interacting via smooth potential functions. The hard-sphere model is even more important for the dynamics, because it allows for approximate analytical solutions based on the Boltzmann equation and its generalization by Enskog to account for a finite particle diameter and pair correlations at contact [LePe69, KoLe64, FuMa75, BeEr79] . The model has the additional advantage that it is particularly well-suited for numerical simulations [AlGa70] and in fact many of