Granular Gas: Vibrating Walls, Two-Peak Distribution and Hydrodynamics
  Vibrating walls, used to maintain the temperature in a granular gas, modify the system strongly. Most conspicuously, the usual one-peak velocity distribution splits into two, asymmetrically positioned. A surgical repair of the usual hydrodynamic description is presented that provides an account for, and an understanding of, the situation. 
Yanpei Chen, Meiying Hou, Yimin Jiang, Mario Liu
cond-mat.soft