François Guillard

Lift forces in granular media

The paper presents an experimental and numerical study of the forces experienced by a cylinder moving horizontally in a granular medium under gravity. Despite the symmetry of the object, a strong lift force is measured. Whereas the drag force increases linearly with depth, the lift force is shown to saturate at depths much greater than the cylinder diameter, and to scale like the buoyancy with a large amplification factor of order 20. The origin of this high lift force is discussed based on the stress distribution measured in discrete numerical simulations. The lift force comes from the gravitational pressure gradient, which breaks the up/down symmetry and strongly modifies the flow around the obstacle compared to the case without pressure gradient.

Dynamic X-ray radiography reveals particle size and shape orientation fields during granular flow

When granular materials flow, the constituent particles segregate by size and align by shape. The impacts of these changes in fabric on the flow itself are not well understood, and thus novel non-invasive means are needed to observe the interior of the material. Here, we propose a new experimental technique using dynamic X-ray radiography to make such measurements possible. The technique is based on Fourier transformation to extract spatiotemporal fields of internal particle size and shape orientation distributions during flow, in addition to complementary measurements of velocity fields through image correlation. We show X-ray radiography captures the bulk flow properties, in contrast to optical methods which typically measure flow within boundary layers, as these are adjacent to any walls. Our results reveal the rich dynamic alignment of particles with respect to streamlines in the bulk during silo discharge, the understanding of which is critical to preventing destructive instabilities and undesirable clogging. The ideas developed in this paper are directly applicable to many other open questions in granular and soft matter systems, such as the evolution of size and shape distributions in foams and biological materials.

Tracking time with ricequakes in partially soaked brittle porous media

Timing your breakfast? Soak cereal in milk, apply pressure, and get recurring collapses whose sound resembles a slowing metronome. When brittle porous media interact with chemically active fluids, they may suddenly crumble. This has reportedly triggered the collapse of rockfill dams, sinkholes, and ice shelves. To study this problem, we use a surrogate experiment for the effect of fluid on rocks and ice involving a column of puffed rice partially soaked in a reservoir of liquid under constant pressure. We disclose localized crushing collapse in the unsaturated region that produces incremental global compaction and loud audible beats. These “ricequakes” repeat perpetually during the experiments and propagate upward through the material. The delay time between consecutive quakes grows linearly with time and is accompanied by creep motion. All those new observations can be explained using a simple chemomechanical model of capillary-driven crushing steps progressing through the micropores.

Numerical Investigation on the Failure Behavior of Brittle Granular Chain under Impact

In brittle granular materials, the fragmentation waves have received far less attention due to their complexity despite of their significant role in mineral processes, earthquake hazards control, etc. In this research, the Material Point Method (MPM) is used to analyze how fragmentation waves propagate in a 3-dimensional 10 brittle beads chain with a rate-dependent elasto-damage model. The simulations show that generally, the second bead will become the most severely damaged one, followed by the third bead. Most failure points will appear near the contact surface between the brittle spheres and extend to interior conically. An interesting phenomenon is that with a lower damage threshold or fracture energy, despite of the increase of total damage in the whole chain, less damage is developed in some beads after a period of time. This is mainly because more damage in the beginning dissipates excessive stress wave energy to the extent such that the reflected wave will not be able to cause more damage in the local system.