All Positions

Research
Physics

Physics of dense granular media

DC-34
AMU and USYD
Marseille (FR) and Sydney (AU)

Host organizations

Projects

Option 1

Mixing in Granular materials

Mixing granular materials such as sand, ores, powders, and cereals is key to many industrial processes. These include glass production as well as food and mineral processing. It is typically achieved by large, energy-intensive and costly machines which design requires a precise understanding of the physical mechanisms controlling mixing in granular matter. However, these mechanisms remain elusive, which makes predicting the behaviour of mixing machines difficult.

To address this problem, this PhD will seek to identify robust, physically based models describing the mixing of granular materials. In this aim, laboratory experiments and numerical simulations will be used to perform granular flows and reveal how the mixing behaviour is influenced by parameters such as grain size, rate of shear. Experiments will use a “stadium shear device”, purposely built to shear granular materials and track individual grain trajectories. These will be complemented by numerical simulations based on a discrete element method, which will allow to explore a wider range of grain and flow properties.

The experimental part will be primarily conducted in Marseille under the supervision of B. Metzger. The numerical part will be done in Sydney under the supervision of P. Rognon. The project will benefit from a collaboration with P. Jop from CNRS/Saint Gobain who will provide us with scientific and industrial inputs.

Option 2

Flowing behaviour of granular materials

Flow of granular materials are common in nature (e.g. landslides, snow avalanches and earthquakes) and in industrial processes (e.g. mineral and food processing, concrete mixing, glass production). Accurately predicting these flows is vital to mitigate natural hazards and to design effective industrial processes. Recent strides in research have revealed how the behaviour of granular flows differs from that of common fluids like water, and identified physical laws to capture this behaviour. These account for the specificities of the grains, be they sand, snow flakes or powders. However, the behaviour of granular flows near fixed boundaries remains elusive, which undermine predictions of the overall flow dynamics in all practical settings.

To address this problem, this PhD will seek to identify robust, physically based models describing the flowing behaviour of granular materials near fixed boundaries. In this aim, laboratory experiments and numerical simulations will be used to perform granular flows and reveal how their dynamics is influenced by the proximity of a boundary. Experiments will use a “stadium shear device”, purposely built to shear granular materials and track individual grain trajectories. These will be complemented by numerical simulations based on a discrete element method, which will allow to explore a wider range of grain and flow properties.

The experimental part will be primarily conducted in Marseille under the supervision of B. Metzger. The project will benefit from a collaboration with P. Jop from CNRS/Saint Gobain who will provide us with scientific and industrial inputs.

Option 3

Drag force in dilatant suspensions

Dilatant fluids are very concentrated particulate suspensions which must dilate to be able to flow. Dilation of the granular network under shear induces a pore-pressure feedback having dramatic consequences on the flow response, particularly in transient situations. Such coupling occurs in macrosocopic frictional suspensions initially prepared above their critical volume fraction (e.g. in dense sediments), or in concentrated shear-thickening suspensions above a critical applied stress. It has been shown to control the initiation of granular avalanches and to lead to a spectacular hardening of granular beds when submitted to an impact.

These experiments will then be extended to the case of shear-thickening suspensions for which the effect of dilation and associated pore-pressure feedback has never been studied, yet we expect it plays a key role in their transient behaviour. Major progresses have recently been done in the comprehension of shear-thickening, now understood as a frictional transition occurring above an onset stress required to overcome repulsive forces between particles. This framework shows that shear-thickening suspensions have two rheological branches with distinct critical jamming fractions; Whenever the suspension is prepared at a volume fraction lying between these two critical jamming fractions and sheared above the onset stress, dilation effects should become dominant as the micro-metric particle size in shear thickening suspension imposes strong Darcy flow resistance, which in turn must results in strong pore pressure feedback effects. The ultimate goal will be to describe how such dilatancy effects set the drag force on objects moving in dilatant fluids — a key question to elucidate the behaviour of impact hardening materials.

The experimental part will be primarily conducted in Marseille under the supervision of B. Metzger. The numerical part will be done in Sydney under the supervision P. Rognon. The project will benefit from a collaboration with P. Boustingorry from Chryso/Saint-Gobain who will provide us with scientific and industrial inputs.

Research Areas

Physics, Granular Matter, Soft Matter, Mixing, Rheology