  The Beverloo scaling for the gravity flow of granular materials through
orifices has two distinct universal features. On the one hand, the flow rate is
independent of the height of the granular column. On the other hand, less
well-known yet more striking, the flow rate is fairly insensitive to the
material properties of the grains (density, Young's modulus, friction
coefficient, etc.). We show that both universal features are lost if work is
done on the system at a high rate. In contrast to viscous fluids, the flow rate
increases during discharge if a constant pressure is applied to the free
surface of a granular column. Moreover, the flow rate becomes sensitive to the
material properties. Nevertheless, a new universal feature emerges: the
dissipated power scaled by the mean pressure and the flow rate follows a master
curve for forced and unforced conditions and for all material properties
studied. We show that this feature can be explained if the granular flow in the
silo is assumed to be a quasistatic shear flow under the $\mu(I)$-rheology.
