  For the last 50 years, the flow of a granular material through an aperture
has been intensely studied in gravity-driven vertical systems (e.g. silos and
hoppers). Nevertheless, in many industrial applications, grains are
horizontally transported at constant velocity, lying on conveyor belts or
floating on the surface of flowing liquids. Unlike fluid flows, that are
controlled by the pressure, granular flow is not sensitive to the local
pressure but rather to the local velocity of the grains at the outlet. We can
also expect the flow rate to depend on the local density of the grains. Indeed,
vertical systems are packed in dense configurations by gravity but, in
contrast, in horizontal systems the density can take a large range of values,
potentially very small, which may significantly alter the flow rate. In the
present article, we study, for different initial packing fractions, the
discharge through an orifice of monodisperse grains driven at constant velocity
by a horizontal conveyor belt. We report how, during the discharge, the packing
fraction is modified by the presence of the outlet and we analyze how changes
in the packing fraction induce variations in the flow rate. We observe that
variations of packing fraction do not affect the velocity of the grains at the
outlet and, therefore, we establish that flow-rate variations are directly
related to changes in the packing fraction.
