Langevin equation approach to granular flows in narrow pipes
  The flow of granular material through a rough narrow pipe is described by the Langevin equation formalism. The stochastic force is caused by irregular interaction between the wall and the granular particles. In correspondence with experimental observations we find clogging and density waves in the flowing material. 
Tino Riethmueller, Lutz Schimansky-Geier, Dirk Rosenkranz, Thorsten   Poeschel