  We numerically study the flow of dense granular materials between parallel
plates driven by an external force. The granular materials form a jammed
solid-like state when the external force is below a critical force, while they
flow like fluids above the critical force. The transition is characterized by
the mass flux. The critical force depends on the average packing fraction and
the distance between the plates. The scaling laws for the critical force and
the mass flux are predicted theoretically based on a continuum model. They are
numerically verified.
