(137c) Enhanced Flowability of Dense Bi-Disperse Granular Avalanche: The Role of Fine Grains

Dense granular flow on inclined surfaces is encountered in engineering applications as well as geophysical flows (e.g. pyroclastic flows, landslides and avalanches) [1]. Understanding the flowability of dense granular flow is paramount for both industrial optimisation and mitigation and prevention of geo-hazards. Polydispersity is an inherent attribute of both natural and manmade granular matters. Many studies of dense granular flow deals with monodisperse or slightly polydisperse granular media [2,3], despite granular polydispersity has been proved to be a critical factor in dense granular flow behaviour [4]. In this work, we study the flowability of dense bi-disperse granular avalanches using 3D Discrete Element Method (DEM). The objectives are to reveal the flowing features of the granular system and explore the physical mechanisms underlying the observed physical behaviour.

To pursue the objectives of this work, a flume model, which is composed of two main parts, namely an acceleration zone (inclined plane) and deposition zone (horizontal plane), is employed. Two kinds of spherical cohesionless particles, differing only in size (size ratio of 2.5), are used. We keep the total mass of particles constant and vary the mass proportion of large particles. The initial granular assembly is generated by randomly pouring bi-disperse particles into reservoir under gravity to preserve a well-mixed initial state. The contact forces between particles are simulated using a linear spring-dashpot model. Both sliding friction and rolling friction are considered. The coefficient of restitution for all types of collisions is kept the same by appropriately selecting the damping coefficient. The DEM simulations are conducted using the open-source code LAMMPS (Largescale Atomic/Molecular Massively Parallel Simulator) [5].

The numerical scheme is first verified and validated using inclined chute flow by comparing with theoretical results. The flowing properties including final runout, deposition geometry, the evolution of energy and microstructures are presented and compared for cases with different mass ratio of large particles. Similar flowability, or runout, is observed for monodisperse granular assemblies, which is independent of granular size. However, the bi-disperse granular avalanches present enhanced flowability and maximum flowability is observed at a critical mass proportion of large particles. Moreover, reverse grading, in which small granular are found at the bottom and at the rear parts of final deposition, is captured. Despite the rotational energy is small in comparison with translational energy, the rotation speed of small particle is greatly increased in dense bi-disperse granular flow.

To explore possible mechanisms responsible for such a feature, we first studied the jamming volume fraction of bi-disperse particles. The rheological properties of particles are greatly related to the distance to the jamming volume fraction. The variation of jamming volume fraction presents similar trend with the runout of avalanches. Moreover, inertial number is employed to quantify mobility of granular flows in simple shear tests, which directly presents the enhanced mobility in binary particles.

The interactions between large and small granular in term of sliding and rolling friction coefficient is further investigated. We found that the rolling friction between large and small particles plays a critical role in dense bi-disperse granular flow. Specifically, the rotation of small particles can efficiently lubricate the bi-disperse granular assembly to increase its flowability. Although more simulations will be needed using large polydisperse granular media, this work provides very useful insights into understanding the effects of polydispersity on granular flowability.

Reference

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[3] Zhu C, Huang Y, Sun J. Solid-like and liquid-like granular flows on inclined surfaces under vibration–Implications for earthquake-induced landslides. Computers and Geotechnics. 2020; 123:103598.

[4] Remy B, Khinast JG, Glasser BJ. Polydisperse granular flows in a bladed mixer: experiments and simulations of cohesionless spheres. Chemical Engineering Science. 2011;66(9):1811-1824.

[5] Plimpton S. Fast parallel algorithms for short-range molecular dynamics. Journal of Computational Physics. 1995;117(1):1–19.