Decompaction waves in falling granular flows

We study the falling of a 2D column of granular material placed initially into a container formed by two lateral vertical walls with a positive friction coefficient by using molecular dynamics simulations. The time evolution of the mean velocity shows an intermediary region where the grains velocity is relatively constant. Follows a decompaction of the granular flow propagating from the bottom to the top of the column with a velocity of approximately 450 cm/s. Just before the flow decompaction an analysis of the forces exhibits a peculiar behavior which can be assimilated to stick-slip. Finally, after the passage of the decompaction wave low and high density regions coexist both descending with relatively the same velocity and the density exhibits a power law spectrum in 1/f ^a , with a≈ 1.51. Received: 9 May 1999