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Dense granular clusters often behave like macro-particles. We address thisinteresting phenomenon in a model system of inelastically colliding hard disksinside a circular box, driven by a thermal wall at zero gravity. Moleculardynamics simulations show a close-packed cluster of an almost circular shape,weakly fluctuating in space and isolated from the driving wall by a low-densitygas. The density profile of the system agrees very well with the azimuthallysymmetric solution of granular hydrostatic equations employing constitutiverelations by Grossman et al., whereas the widely used Enskog-type constitutiverelations show poor accuracy. We find that fluctuations of the center of massof the system are Gaussian. This suggests an effective Langevin description interms of a macro-particle, confined by a harmonic potential and driven by adelta-correlated noise. Surprisingly, the fluctuations persist when increasingthe number of particles in the system.

Close-packed granular clusters: hydrostatics and persistent Gaussian fluctuations