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Investigations of vortex structures during granular flow by DEM

We present numerical investigation results on vortex-structures during quasi-static plane strain compression on cohesionless sand under constant lateral pressure using a three-dimensional discrete element method (DEM). A three-dimensional discrete model YADE developed at University of Grenoble was applied [1]-[3]. Grains were modelled by means of spheres with contact moments or clusters of combined spheres imitating irregular particle shapes. Sand was initially dense. The material behaviour was studied at the global and local level. The stress-strain and volumetric-strain curves, distribution of void ratio, resultant grain rotation and contact forces were calculated. In addition, every kind of energy was analyzed. The discrete global results of plane strain compression tests were compared with corresponding experimental data from performed by Vardoulakis (1980) at Karlsruhe University with the so-called Karlsruhe sand. Special attention was paid in calculations to vortex and anti-vortex structures within a granular shear zone whose existence is still unexplained. The major contribution of the manuscript is the estimation of the appearance frequency of vortices and anti-vortices within a granular shear zone using a method based on orientation angles of displacement fluctuation vectors of neighbouring spheres [4]. A direct link between force chains, vortex structures and void ratio changes and shear localization was established.

[1] Kozicki J., Tejchman J. and Mróz Z. Effect of grain roughness on strength, volume changes, elastic and dissipated energies during quasi-static homogeneous triaxial compression using DEM. Granular Matter 14, 4, 457-468, 2012.
[2] Kozicki, J., Tejchman, J. and Műhlhaus, H.-B. Discrete simulations of a triaxial compression test for sand by DEM, IJNAMG, doi: 10.1002/nag.2285, 2014.
[3] Nitka, M., Tejchman, J., Kozicki, J. and Leśniewska, D. DEM analysis of micro-structural events within granular shear zones under passive earth pressure conditions. Granular Matter 2014 (under review).
[4] Gould, H., Tobochnik, J. and Christian, W. Simulations in Physics. Open Source Physics Eclipse Workspace. 2011.

Author(s):

Jacek Tejchman    
Gdansk University of Technology
Poland

Jan Kozicki    
Gdansk University of Technology
Poland