  Granular flow out of a silo is studied experimentally and numerically. The
time evolution of the discharge rate as well as the normal force (apparent
weight) at the bottom of the container is monitored. We show, that particle
stiffness has a strong effect on the qualitative features of silo discharge.
For deformable grains with a Young's modulus of about $Y_m\approx 40$ kPa in a
silo with basal pressure of the order of 4 kPa lowering the friction
coefficient leads to a gradual change in the discharge curve: the flow rate
becomes filling height dependent, it decreases during the discharge process.
For hard grains with a Young's modulus of about $Y_m\approx 500$ MPa the flow
rate is much less sensitive to the value of the friction coefficient. Using DEM
data combined with a coarse-graining methodology allows us to compute all the
relevant macroscopic fields, namely, linear momentum, density and stress
tensors. The observed difference in the discharge in the low friction limit is
connected to a strong difference in the pressure field: while for hard grains
Janssen-screening is effective, leading to high vertical stress near the silo
wall and small pressure above the orifice region, for deformable grains the
pressure above the orifice is larger and gradually decreases during the
discharge process. We have analyzed the momentum balance in the region of the
orifice (near the location of the outlet) for the case of soft particles with
low friction coefficient, and proposed a phenomenological formulation that
predicts the linear decrease of the flow rate with decreasing filling height.
