  The flow rate of water through an orifice at the bottom of a container
depends on the hydrostatic pressure whereas for a dry granular material is
nearly constant. But what happens during the simultaneous discharge of grains
and liquid from a silo? By measuring the flow rate as a function of time, we
found that: (i) different regimes appear, going from constant flow rate
dominated by the effective fluid viscosity to a hydrostatic-like discharge
depending on the aperture and grain size, (ii) the mixed material is always
discharged faster than dry grains but slower than liquid, (iii) for the
mixture, the liquid level drops faster than the grains level but they are
always linearly proportional to one another, and (iv) a sudden growth in the
flow rate happens during the transition from a biphasic to a single phase
discharge. These results are associated to the competition between the decrease
of hydrostatic pressure above the granular bed and the hydrodynamic resistance.
A model combining the Kozeny-Carman, Bernoulli and mass conservation equations
is proposed and the numerical results are in good agreement with experiments.
