Local fluctuations in a fluidized granular medium

Molecular dynamics simulations have been carried out on a two-dimensional model granular material made up of inelastically colliding discs subjected to gravity. The granular system is fluidized by a constant energy input from a vibrating base, and a stationary non-equilibrium state is achieved. Density and granular temperature profiles are determined, while the former goes through a maximum at intermediate altitudes, the latter exhibits a minimum at a higher altitude, beyond which the granular temperature is found to increase. Local density fluctuations are characterized by the time-displaced density autocorrelation function. Longitudinal compression (sound) waves propagating in the horizontal direction are found to be strongly damped, while they are overdamped in the vertical direction at the longest accessible wavelength.