Physical and numerical modelling of segregation in sediment transport

When a population of solid particles is transported by a carrying fluid, a physical separation of the particles into size classes is observed in the flow, a phenomenon referred to as size segregation or size sorting. In mountains where sediment transport is particularly intense, steep slopes carry huge amounts of coarse particles of very different sizes, which migrate differentially along the vertical, longitudinal or lateral axis of the flow. This behaviour leads to very complex and varied morphologies of bed surface and subsurface such as armouring for instance (see Fig.1a), and can drastically modify the fluvial geomorphological equilibrium. The complexity of the phenomenon is largely responsible for our limited ability to predict sediment flux and river morphology. Size segregation is particularly important when it comes to sediment transport, and can have major consequences for public safety, water resources and environmental sustainability. More generally, size segregation is a main feature of geophysical flows such as debris flows, pyroclastic flows or snow avalanches, but it also has huge implications for industrial applications in which a perfect homogeneous mixture is often desired: size segregation can alter the resistance properties of concrete due to the migration of aggregates; in the pharmaceutical industry, the non‐uniformity of powder mixtures due to segregation can lead to non‐acceptable quality of tablets and capsules.

The present study focuses on vertical size segregation. An overlooked reason for our limited ability to predict segregation is that we have no general understanding of the effect of grain/grain interactions. Both fluid/grain and grain/grain interactions should be considered, to model segregation patterns.

For large size ratios, the small particles can percolate spontaneously by gravity, without external forcing, into the bed of larger particles. For the smaller size ratios, spontaneous percolation is not possible without deformation of the bed, which can be achieved by shearing or vibrating. This dynamic segregation also called kinetic sieving results in a net downward flux of small particles and an upward flux of large grains, leading to an inversely graded bed (Fig.1b,c).

Focusing on granular interactions, the overall objective of this PhD project is to investigate experimentally and numerically vertical size segregation.

The main originalities of the project include:

• Setting up the Refractive Index Matching Method (RIM) to better measure two‐phase flow variables;
• Concentrating on granular interactions which have been overlooked by both the fluid mechanical engineering and fluvial geomorphology communities;
• Investigating two important scientific bottlenecks: (1) the asymmetric behaviour (small particles segregating faster in regions of many large particles and large particles segregating slower in regions of many small particles); (2) the existence or not of an optimum size ratio for segregation.