Close - packed granular clusters : hydrostatics and persistent Gaussian fluctuations

Dense granular clusters often behave like macroparticles. We address this interesting phenomenon in a model system of inelastically colliding hard disks inside a circular box, driven by a thermal wall at zero gravity. Molecular dynamics simulations show a close-packed cluster of an almost circular shape, weakly fluctuating in space and isolated from the driving wall by a low-density gas. The density profile of the system agrees very well with the azimuthally symmetric solution of granular hydrostatic equations employing constitutive relations by Grossman et al, whereas the widely used Enskog-type constitutive relations show poor accuracy. We find that fluctuations of the center of mass of the system are Gaussian. This suggests an effective Langevin description in terms of a macro-particle, confined by a harmonic potential and driven by a delta-correlated noise. Surprisingly, the This research was supported by the German-Israel Foundation for Scientific Research and Development (Grant I-795-166.10/2003) and by MEyC and FEDER (project FIS2005-00791). Baruch Meerson Racah Institute of Physics Hebrew University of Jerusalem Jerusalem 91904 Israel E-mail: [email protected] Manuel Dı́ez-Minguito Institute “Carlos I” for Theoretical and Computational Physics University of Granada Spain E-mail: [email protected] Thomas Schwager Charité Augustenburger Platz 1 13353 Berlin Germany E-mail: [email protected] Thorsten Pöschel Charité Augustenburger Platz 1 13353 Berlin Germany E-mail: [email protected] fluctuations persist when increasing the number of particles in the system.