  Three-dimensional molecular dynamics simulations of dissipative particles (~
10^6) are carried out for studying the clustering kinetics of granular media
during cooling. The inter-connected high particle density regions are
identified, showing tube-like structures. The energy decay rates as functions
of the particle density and the restitution coefficient are obtained. It is
found that the probability density function of the particle density obeys an
exponential distribution at late stages. Both the fluctuation of density and
the mean cluster size of the particle density have power law relations against
time during the inelastic coalescing process.
