Nico Gray


Granular materials composed of particles with differing grain sizes, densities, shapes, or surface properties may experience unexpected segregation during flow. This review focuses on kinetic sieving and squeeze expulsion, whose combined effect produces the dominant gravity-driven segregation mechanism in dense sheared flows. Shallow granular avalanches that form at the surface of more complex industrial flows such as heaps, silos, and rotating drums provide ideal conditions for particles to separate, with large particles rising to the surface and small particles percolating down to the base. When this is combined with erosion and deposition, amazing patterns can form in the underlying substrate. Gravity-driven segregation and velocity shear induce differential lateral transport, which may be thought of as a secondary segregation mechanism. This allows larger particles to accumulate at flow fronts, and if they are more frictional than the fine grains, they can feedback on the bulk flow, causing flow fingering, levee formation, and longer runout of geophysical mass flows.
Reference: Particle Segregation in Dense Granular Flows, John Mark Nicholas Timm Gray, Annual Review of Fluid Mechanics 2018 50:1, 407-433

Nico Gray is Professor of Applied Mathematics at The University of Manchester and is an expert on granular avalanches and the particle segregation that occurs within them. He holds a BSc in Mathematics from Manchester, a PhD on “sea ice dynamics” from the University of Cambridge and a Habilitation in “continuum mechanics and geophysical dynamics” from the Technical University of Darmstadt. A key feature of Nico’s research is that he performs small scale experiments that provide a strong motivation for his theoretical and computational work. Over recent years he has also collaborated extensively with geologists working on hazardous geophysical flows, such as debris-flows, rockfalls and pyroclastic flows. This has included field work, as well as novel large-scale experiments at the United States Geological Survey (USGS) debris-flow flume in Oregon. Nico holds a prestigious Royal Society Wolfson Research Merit Award as well as EPSRC Established Career Fellowship. Both these awards are focussed at applying the significant theoretical breakthroughs that he has made in understanding the rheology of granular flows and how they segregate to important industrial unit operations, such as chute flows, silos, conveyor belts and rotating drums.