  We report two-dimensional simulations of circular and polygonal grains
passing through an aperture at the bottom of a silo. The mass flow rate for
regular polygons is lower than for disks as observed by other authors. We show
that both the exit velocity of the grains and the packing fraction are lower
for polygons, which leads to the reduced flow rate. We point out the importance
of the criteria used to define when two objects of different shape are
considered to be of the same size. Depending on this criteria, the mass flow
rate may vary significantly for some polygons. Moreover, the particle flow rate
is non-trivially related to a combination of mass flow rate, particle shape and
particle size. For some polygons, the particle flow rate may be lower or higher
than that of the corresponding disks depending on the size comparison criteria.
