  Discrete element method is conducted to investigate the bidisperse dense
granular flow having big and small grains in a two dimensional hopper. A
half-circular dynamical force arch is observed above the outlet and Beverloo's
law is verified to describe the relationship between flow rate and outlet size.
The bidisperse flow can reach the maximum flow rate when a small fraction of
big grains is added into the monodisperse flow with only small grains. The
distributions of contact force, packing fraction and grain velocity indicate
that the flow properties in the hopper are closely related to the local flow
characteristics of the key area which almost overlaps with the force arch. The
interior packing structures, i.e., ordered arrangement for monodisperse flow
and disordered arrangement for bidisperse flow, play a dominant role in the
flow pattern transition from mass flow to funnel flow. The earlier occurrence
of the transition can effectively increase the grain velocity and therefore
improve the flow rate.
