  We investigate the coupling between interstitial medium and granular
particles by studying the hopper flow of dry and submerged system
experimentally and numerically. In accordance with earlier studies, we find,
that the dry hopper empties at a constant rate. However, in the submerged
system we observe the surging of the flow rate. We model both systems using the
discrete element method, which we couple with computational fluid dynamics in
the case of a submerged hopper. We are able to match the simulations and the
experiments with good accuracy. To do that, we fit the particle-particle
contact friction for each system separately, finding that submerging the hopper
changes the particle-particle contact friction from $\mu_{vacuum}=0.15$ to
$\mu_{sub}=0.13$, while all the other simulation parameters remain the same.
Furthermore, our experiments find a particle size dependence to the flow rate,
which is comprehended based on arguments on the terminal velocity and drag.
These results jointly allow us to conclude that at the large particle limit,
the interstitial medium does not matter, in contrast to small particles. The
particle size limit, where this occurs depends on the viscosity of the
interstitial fluid.
