GALCIT Colloquium

Flowing granular materials arise everywhere around us, in industry from pharmaceutical processes to bulk good transport lines, and in nature from snow avalanches to captivating dune fields.

At the particle-scale, collisions between grains create a fascinating network of so-called force chains, which are responsible for the inhomogeneous distribution of stresses in a granular medium. In this talk, we discover stress distributions in 2D granular avalanches, visualized with bespoke, superior-quality birefringent photoelastic particles. This technique gives us for the first time access to the full velocity, density and stress fields inside of a dynamic avalanche, and allows us to experimentally validate granular rheological models.

In contrast, at the system-scale, large desert dunes are composed of the same particles, but the effect of particle size may disappear entirely when analysing entire dunes. We present a unique, recirculating, laboratory experiment in which we create and trace aqueous dunes over long times. We examine the interaction between two dunes of different sizes, and present a phase-space diagram with interaction outcomes. Furthermore, we explore the feedback mechanism between a bedform and the flow providing the forcing, and identify a repulsion mechanism that ensures that bedforms do not coarsen without limit.