Density waves and 1/f density fluctuations in granular flow.

We simulate the granular flow in a narrow pipe with a lattice–gas automaton model. We find that the density in the system is characterized by two features. One is that spontaneous density waves propagate through the system with well–defined shapes and velocities. The other is that density waves are so distributed to make the power spectra of density fluctuations as 1/fα noise. Three important parameters make these features observable and they are energy dissipation, average density and the rougness of the pipe walls. PACS numbers: 05.20.Dd, 47.50.+d, 47.20.-k, 46.10.+z Permanent address: Institute of Physics, Academia Sinica, Beijing, China.