Boundary conditions and the dynamics of a dissipative granular gas: slightly dense case

The effect of different possible kinds of motion of the exciting walls (cyclic, random, …) is investigated on the dynamics of a granular dissipative gas. It is shown that the real distribution of speed of the wall which interact with the balls depend strongly on the real ball speed, due to a screening effect, so that the transfer of excitation from the walls to the cloud of particles may drastically depend on the ball speed itself. This makes the excitation quite non linear, with a change of efficiency law. This may explain the observation of a biphasic cloud when the mean free path of balls is slightly smaller than the cell size L, or may predict that the particles in the cloud is merely at rest as soon as the mean free path is smaller than L/10. It is obvious that the vibrating cell cannot be considered as a thermostat.