H2020-MSCA-IF-2017

This EU funded project is titled "A theoretical, experimental and numerical study of the formation of coarse dry granular fronts and
spontaneously self-channelizing levees in debris flows", which aims to spur the development of a novel and combined study of geophysical flows.

Research

This project looks at (1) the mechanism of the formation of dry granular fronts in debris flows and (2) their feedback on the dynamics of debris flows. The MSCA Research Fellow conducting this research is particularly interested in the development of a depth-averaged model to account for the formation of dry granular fronts and validation of this proposed
model by comparing numerical results with experimental measurements.

These topics are of interest to fluid dynamicists, civil engineers, geophysicists, and geologists working on the practical mitigation of natural hazards. The formation of large amplitude dry bouldery flow fronts is a spectacular and counterintuitive effect, that is directly responsible for the increased destructive power of debris flows. These poorly understood phenomena are often observed in the field (e.g. Pierson (1986)), large-scale experiments (e.g. Iverson et al. (2010) and Johnson et al. (2012)), and small-scale experiments (e.g. Haas et al. (2015) and Lanzoni et al. (2017)). Similar mechanisms have also been observed in the suspension flows, snow avalanches, and pyroclastic flows. The traditional geological perspective asserts that the coarse grains are segregated to the top of the flow and they are then preferentially sheared forwards and recirculated by particle-size segregation to develop a dry front. Nevertheless, recent laboratory experiments of mono-disperse flows of grains and water, e.g. Davies (1990) and Taylor-Noonan (2020), also exhibit such phenomena, which motivated us to
explore new formation mechanisms in this project.

News & Events

Xiannan has participated in the 7th International Conference on Debris-Flow Hazards Mitigation held June 10 - 13, 2019 in Golden, Colorado, USA on the campus of Colorado School of Mines;
Xiannan has participated in the 3rd IMA Conference on Dense Granular Flows in Cambridge in 2019 and gave a talk on modeling the grain and water mixture flows by coupling mixture theory and dilatancy law;
Xiannan has presented the work titled "Formation of dry granular fronts and watery tails in debris flows" in EGU General Assembly 2021.

Latest Publications

Investigation of influence of an obstacle on granular flows by virtue of a depth-integrated theory

Understanding granular flows past an obstacle is very important to most possibly avoid damage to human properties and infrastructures. The present paper investigates the influence of an obstacle on dry and fluid-saturated granular flows to gain insights into the physics behind them. To this end, we extend the existing depth-integrated theory by considering additional effects from the pore fluid pressure and the granular dilatancy. We revisit a large-scale experiment to validate the extended theory. The good agreement between numerical results and experimental data reveals that granular dilatancy plays a crucial role in mobility and peak depth. Furthermore, we investigate the influence of obstacles on the dynamics of dry granular flows by comparing numerical results with experimental data. It is shown that shock waves, dead zones, and vacuum (grain-free zone), well observed in the experiments, can be captured. Additionally, a fluid-saturated granular flow past the same obstacle is numerically simulated to interpret the role of the interstitial fluid, especially the pore fluid pressure, in the fluid-granular mixture causing distinct dynamic behaviors from those of a dry granular flow. It is also found that the granular dilatancy has a significant influence on the pore fluid pressure which can mitigate the granular friction. This is consistent with many experimental observations. Additionally, it is demonstrated that the pore fluid pressure is prone to elevate in front of a cuboid dam (but not in front of a forward-facing tetrahedral wedge), which in turn mitigates the granular friction. The findings are helpful to understand complex behaviors encountered in geophysical flows and industrial processes.

The article appeared in the European Journal of Mechanics - B/Fluids in 2020

DOI: https://doi.org/10.1016/j.euromechflu.2020.06.014