Arching in three-dimensional clogging

Abstract

Abstract. Arching in dry granular material is a long established concept, however it remains still an open question how three-dimensional orifices clog. We investigate by means of numerical simulations and experimental data how the outflow creates a blocked configuration of particles. We define the concave surface of the clogged dome by two independent methods (geometric and density based). The average shape of the cupola for spheres is almost a hemisphere but individual samples have large holes in the structure indicating a blocked state composed of two-dimensional force chains rather than three-dimensional objects. The force chain structure justifies this assumption. For long particles the clogged configurations display large variations, and in certain cases the empty region reaches a height of 5 hole diameters. These structures involve vertical walls consisting of horizontally placed stable stacking of particles. MotivationDischarge of silos and the flow of granular material through orifices are important questions of everyday life and technological processes. Several experimental and numerical studies were carried out to understand the processes of arch formation, force chains and arch breaking [1][2][3][4]. From the technological point of view one is interested in the prevention of silo clogging which blocks the outflow. The jamming probability as the function of the diameter of the orifice (D) and the aspect ratio (Q) of the granulate is also an unsolved question [5][6][7].The accurate analysis of the microscopic details of the clogged configuration in three-dimensional (3D) hoppers is a challenge. In 2D it is straightforward to define the blocking arch by finding a continuous chain of particles from one side of the orifice to the other. Here, one can visualize the force network and the structure of the arch experimentally by using photoelastic particles [3,4]. With the help of discrete element (DEM) simulations one can quantitatively compare the morphology of clogging arches with experimental data, and accurately map the force distribution inside the silo [1,2]. In the present work we identify the blocking structure in three-dimensional hoppers with DEM simulations focusing on particle configurations and forces and compare our finding to our measurements using X-ray computed tomography. We compare the case of spherical particles with elongated grains. e-mail: torok@phy.bme.hu Methods Discrete element simulationDiscrete Element Method (DEM) simulations were performed to study three-dimensional clogging. We used a modified version [8] of the molecular dynamics code LAMMPS [9] to generate a cylindrical hopper with an orifice of given size on the bottom wall. We used elongated particles with given aspect ratios (Q = 2, 3.3, 5) and spheres (Q = 1). Elongated particles were created by gluing spheres together with overlaps between them. We created polydisperse samples with the variation of particle diameters by ±20%, the length of particles by ±10% and the overlap of the spheres in elongated particles between 30...

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“…These transitions also occur when particles are driven through confined geometries and clog [Zuriguel, 2014;Thomas and Durian, 2015] or disordered landscapes, where obstacles inhibit movement by pinning particles that would otherwise be mobile [Péter et al, 2018;Reichhardt and Olson Reichhardt, 2017]. Granular materials fall within the jamming paradigm [Majmudar et al, 2007] and clogging behavior is observed in dense flows of elongated grains [Börzsönyi et al, 2016;Török et al, 2017]. Logjams have been explicitly citepd as macroscopic manifestations of granular phenomenology [Börzsönyi and Stannarius, 2013] and invoked as an inspiration in the assembly of nanomaterials [Whang et al, 2003;Yang, 2003].…”

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“…Few experiments of non-spherical shapes [19,[26][27][28] demonstrated that many features of the discharge can be compared to those of spheres when an effective particle radius is introduced. Examples of numerical simulations of silo discharge of non-spherical particles with multisphere-DEM include rods [29] and mixtures of spherical and rodlike grains [30].…”

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