  Very recently, To et al.~have experimentally explored granular flow in a
cylindrical silo, with a bottom wall that rotates horizontally with respect to
the lateral wall \cite{Kiwing2019}. Here, we numerically reproduce their
experimental findings, in particular, the peculiar behavior of the mass flow
rate $Q$ as a function of the frequency of rotation $f$. Namely, we find that
for small outlet diameters $D$ the flow rate increased with $f$, while for
larger $D$ a non-monotonic behavior is confirmed. Furthermore, using a
coarse-graining technique, we compute the macroscopic density, momentum, and
the stress tensor fields. These results show conclusively that changes in the
discharge process are directly related to changes in the flow pattern from
funnel flow to mass flow. Moreover, by decomposing the mass flux (linear
momentum field) at the orifice into two main factors: macroscopic velocity and
density fields, we obtain that the non-monotonic behavior of the linear
momentum is caused by density changes rather than by changes in the macroscopic
velocity. In addition, by analyzing the spatial distribution of the kinetic
stress, we find that for small orifices increasing rotational shear enhances
the mean kinetic pressure $\langle p^k \rangle$ and the system dilatancy. This
reduces the stability of the arches, and, consequently, the volumetric flow
rate increases monotonically. For large orifices, however, we detected that
$\langle p^k \rangle$ changes non-monotonically, which might explain the
non-monotonic behavior of $Q$ when varying the rotational shear.
