The granular hopper flow rate depends on the filling height when hopper is submerged in water.
This effect is further studied with an experimental setup consisting on cylindrical flat bottomed hoppers with various diameters and orifices. The grains are spherical polydisperse glass beads. The flow rate is measured with an electric scale connected to a computer.
The submerged hopper results show a counterintuitive increase in flow rate as the filling height decreases. For small orifice sizes the hopper clogs with similar characteristics than with a dry case. The flow of grains near the wall changes from creep like behavior to mass flow as the hopper diameter decreases. Similar effect is seen in granular flows deep in riverbeds. In a dry case the increase in flow rate is also observed but in much smaller scale merely just before the hopper runs out of grains.
The hypothesis for increasing flow rate and clogging is the changes in compatible stresses and force chains. The future work includes changing fluid pressure and flow rate near the orifice as well as changing the roughness of the walls.
Below is a playlist of hopper flow videos. Switch resolution to higher (4k) from the gear symbol while playing: