# Friction controls submerged granular flows

## TL;DR

This study explores how the presence of a fluid affects granular flow in a hopper, revealing that fluid coupling causes flow surging and that particle size and fluid viscosity influence the flow behavior.

## Contribution

It provides a combined experimental and numerical analysis of submerged versus dry granular hopper flows, highlighting the impact of fluid on particle contact friction and flow dynamics.

## Key findings

- Submerging reduces particle contact friction from 0.15 to 0.13.
- Flow rate depends on particle size, influenced by terminal velocity and drag.
- Interstitial fluid effects diminish at large particle sizes, depending on fluid viscosity.

## Abstract

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.