Published: Feb. 10, 2023

Nathalie Vriend, The Paul M. Rady Department of Mechanical Engineering, University of Colorado Boulder

Granular Flows: Photoelastic avalanches & Bedform dynamics

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, and processes occur over long time-scales. In the lab, by using appropriately scaled miniature subaqueous dunes, we can investigate the key physical processes in a matter of hours. We examine pairwise interaction between two dunes, and present a phase-space diagram with interaction outcomes. We perform a dynamical system analysis to explore the interaction between flow and dune, and derive both fast and slow dynamics. Lastly, we address the engineering challenge of dune-obstacle interaction.