Prof. Nico Gray

Frictional hysteresis in geophysical mass flows

Abstract: When a static granular material fails there is often frictional weakening with increasing velocity, before the friction begins to increase again as the flow speeds up further. This can be captured in depth-averaged geophysical mass flow models by making the friction a non-monotonic function of the Froude number as shown in figure 1.

This simple frictional behavior leads to the coexistence of flowing and stationary material, as well as a wide range of striking flow phenomena that are commonly observed in geophysical mass flows. A prime example is the formation of self-channelized flows with static levees (Rocha et al. 2019), such as the ones shown in the small scale analogue experiments in figure 2.

Frictional hysteresis is also responsible for the spontaneous formation of wave pulses that are separated by regions of static material. These have been termed erosion deposition waves (Edwards & Gray 2015, Edwards et al. 2017, Viroulet et al. 2019) because each individual pulse erodes static material at their leading edge and deposits material in their tails. The presence of easily erodible material on a slope dramatically changes the apparent mobility of such flows, allowing waves to propagate indefinitely on slopes that would otherwise rapidly bring the grains to rest. Waves may also grow or decay dependent on the slope inclination, the release mass and the amount of easily erodible material, and it is also possible to generate retrogressive failures that propagate upslope (Russell et al. 2019).

This talk will describe recent progress in modelling such diverse phenomena with a simple depth-averaged model that differs fundamentally from standard geophysical mass flow models. In particular, it is vital to include depth-averaged viscous terms (Gray & Edwards 2014; Baker et al. 2016; Gray 2018) to correctly predict steady-state channel widths (Rocha et al. 2019). The model is able to quantitatively capture all of the phenomena observed in small scale experiments. The challenge for the future is to see how these breakthroughs can be applied to improve our understanding of large scale flows in the field.

Biography

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.