Density waves in gravity-driven granular flow through a channel

A molecular-dynamic computer simulation is used to examine rapid granular flow in a vertical channel. A two-dimensional event driven algorithm is used with periodic boundary conditions in the flow direction and solid walls in the lateral direction. Flow in the channel leads to an inhomogeneous distribution of the particles and two distinct types of density waves are identified: An S-shaped wave and a clump. The density waves are further characterized by quantifying their temporal evolution using Fourier methods and examining local and global flow properties of the system, including velocities, mass fluxes, granular temperatures, and stresses. A parametric study is used to characterize the effect of the system parameters on the density waves. In particular we are able to show that the dynamics of large systems are often qualitatively and quantitatively different from those of small systems. Finally, the types of density waves and dominant Fourier modes observed in our work are compared to those that are predicted using a linear stability analysis of equations of motion for rapid granular flow.