  The flow of granular material in a rotating cylinder was simulated by
molecular dynamics in two dimensions using spherical as well as nonspherical
grains. At very low but constant angular velocity we found that the flow varies
irregularly with time. The particles move stick-slip like i.e. there are
avalanches of different size at the surface of the granular material. Observing
the traces of the particles we found that there are unstable convection cells.
Our results agree with recent experiments by Rajchenbach [1] and Rolf [2].
