  Dense granular clusters often behave like macro-particles. 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 fluctuations persist when increasing
the number of particles in the system.
