  When rough grains in standard packing conditions are discharged from a silo,
a conical depression with a single slope is formed at the surface. We observed
that the increase of the volume fraction generates a more complex depression
characterized by two angles of discharge: a lower angle close to the one
measured for standard packing and a considerably larger upper angle. The change
in slope appears at the boundary between a densely packed stagnant region at
the periphery and the central flowing channel formed over the aperture. Since
the material in the latter zone is always fluidized, the flow rate is
unaffected by the initial packing of the bed. On the other hand, the contrast
between both angles is markedly smaller when smooth particles of the same size
and density are used, which reveals that high volume fraction and friction must
combine to produce the observed geometry. Our results show that the surface
profile helps to identify by simple visual inspection the packing conditions of
a granular bed, and this can be useful to prevent undesirable collapses during
silo discharge in industry.
