Systematic description of the effect of grain elongation in silo discharge
Résumé
By means of 2D Contact Dynamics simulations, we investigate the flow properties of
granular materials composed of elongated grains within a silo of varying orifice size. The
grains have a rounded-cap rectangular shape described by their aspect ratio parameter
varying from 1 for a disk to 7 for a thin or long grain. In order to isolate the effects of grain
shape, both the mass and surface of the grains are kept constant as well as the total mass
of the system and the condition of the discharge. By simulating a large number of
avalanches, the flow rates “Q”, the velocity profiles and the packing fraction profiles close
to the orifice are studied for increasing grain aspect ratio and for various characteristic
dimensionless numbers such the orifice size normalized by the grain size, or the system
size normalized either by the orifice size or by the grain size. A counterintuitive finding of
this work is that the effect of grain elongation depends on the size of the orifice. Indeed,
we find that Q is independent of grain elongation for small (normalized) orifice size. In
contrast, Q increases with grain elongation for intermediate (normalized) orifice size, and
it may saturate or continue to increase at larger grain aspect ratio. Based on the
methodology proposed by Janda et al. (2012) to deduce the flow rate from the packing
fraction and velocity profiles close to the orifice, we show that the nonlinear variation of Q
with grain elongation results from compensation mechanisms between the velocity and
the packing fraction measured at the center of the orifice.
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