  This paper describes methods to measure flow rate and jamming onset of
granules discharged through a flat-bottom cylindrical hopper with a circular
orifice. The intrinsic jamming onset for ideal particles (spherical,
monodisperse, smooth) is experimentally measured by two independent methods,
with good agreement. For non-ideal particles, the normalized jamming onset
increases with elongated granule shape, broadened size distribution and
increased friction as measured by the drained angle of repose. An empirical
model of the jamming onset is introduced to quantify these effects over the
range of materials investigated. The jamming onset can be used as a measure of
differentiation between relatively free-flowing granules.
