  We experimentally demonstrate that the flow rate of granular material through
an aperture is controlled by the exit velocity imposed to the particles and not
by the pressure at the base, contrary to what is often assumed in previous
works. This result is achieved by studying the discharge process of a dense
packing of monosized disks through an orifice. The flow is driven by a conveyor
belt. This two-dimensional horizontal setup allows to uncouple pressure and
velocity and, therefore, to independently control the velocity at which the
disks escape the horizontal silo and the pressure in the vicinity of the
aperture. The flow rate is found to be directly proportional to the belt
velocity, independent of the amount of disks in the container and, thus,
independent of the pressure in the outlet region. In addition, this specific
experimental configuration makes it possible to get information on the system
dynamics from a single image of the disks that rest on the conveyor belt after
the discharge.
