Granular front formation in free-surface flow of concentrated suspensions

TL;DR

This study investigates the formation of granular fronts in concentrated suspensions during free-surface flow, combining experiments and simulations to understand the mechanisms and control factors behind this phenomenon.

Contribution

It introduces a combined experimental and numerical approach to analyze granular front formation, highlighting the role of phase reorganization and a predictive non-dimensional number.

Findings

Granular fronts form due to phase reorganization induced by shearing.

A non-dimensional number predicts granular front formation.

Simulations confirm experimental observations.

Abstract

Granular fronts are a common yet unexplained phenomenon emerging during the gravity driven free-surface flow of concentrated suspensions. They are usually believed to be the result of fluid convection in combination with particle size segregation. However, suspensions composed of uniformly sized particles also develop a granular front. Within a large rotating drum, a stationary recirculating avalanche is generated. The flowing material is a mixture of a visco-plastic fluid obtained from a kaolin-water dispersion, with spherical ceramic particles denser than the fluid. The goal is to mimic the composition of many common granular-fluid materials, like fresh concrete or debris flow. In these materials, granular and fluid phases have the natural tendency to segregate due to particle settling. However, through the shearing caused by the rotation of the drum, a reorganization of the phases is induced, leading to the formation of a granular front. By tuning the material properties and the drum velocity, it is possible to control this phenomenon. The setting is reproduced in a numerical environment, where the fluid is solved by a Lattice-Boltzmann Method, and the particles are explicitly represented using the Discrete Element Method. The simulations confirm the findings of the experiments, and provide insight into the internal mechanisms. Comparing the time-scale of particle settling with the one of particle recirculation, a non-dimensional number is defined, and is found to be effective in predicting the formation of a granular front.