  The density relaxation phenomenon is modeled using both Monte Carlo and
dissipative MD simulations to investigate the effects of regular taps applied
to a vessel having a planar floor filled with monodisperse spheres. Results
suggest the existence of a critical tap intensity that depends on the mass
overburden, which produces a maximum bulk solids fraction. We find that the
mechanism responsible for the relaxation phenomenon is evolving quasi-ordered
packing structure propagating upwards from the plane floor.
