Non-Gaussian Velocity Distribution Function in a Vibrating Granular Bed
  The simulation of granular particles in a quasi two-dimensional container under the vertical vibration as an experimental accessible model for granular gases is performed. The velocity distribution function obeys an exponential-like function during the vibration and deviates from the exponential function in free-cooling states. It is confirmed that this exponential-like distribution function is produced by Coulomb's friction force. A Langevin equation with Coulomb's friction is proposed to describe the motion of such the system. 
Atsushi Kawarada and Hisao Hayakawa