(120d) DEM Studies of the Ordering and Force Transmission in Packings of Non-Spherical Particles

DEM studies
of the ordering and force transmission in packings
of non-spherical particles

So far most
studies concerning the packing of granular materials have only considered
systems composed of spherical particles, irrespective of the fact that granular
materials are commonly of non-spherical shape and show highly anisotropic characteristics
[1]. However, very recently, Hidalgo et al. [2, 3] and Lu et al. [4] reported
that in packings of 2D, elongated particles a more uniform stress distribution
than in packings of spheres can be observed. Due to the preferred horizontal
alignment of the particles, the force in the vertical direction of the bed does
not saturate with height. Instead, the vertical stress profile becomes similar
to the linear hydrostatic pressure profile in a liquid.

However, despite the observations summarized above, the
influence of the particle surface roughness on the packing characteristics of
non-spherical grains remains an open question. In this study, non-spherical
particles of different aspect ratios were modeled in a discrete element method
(DEM) framework by either gluing together of spheres or using the super-quadric
equation [5, 6]. The surface roughness of the non-spherical particles was
adjusted by the number of spheres used to construct their non-spherical shape.
On the other hand, a super-quadric equation-based particle possesses a smooth
surface. In total 4500 elongated particles of aspect ratio five were simulated.
It was found that the surface roughness of the particles crucially influenced
both the orientational ordering of the particles and the force transmission
within the bed of grains. Super-quadrics-based particles showed horizontal
alignments (Fig. 1a),
with local clusters of elongated particles forming (Fig. 1b). In addition, a
smooth particle surface favored the direct transmission of the bed weight to
the bottom of the container (Fig. 1c).
On the other hand, the horizontal alignment of grains was less pronounced in
packings of elongated particles constructed by gluing together spheres with, surprisingly,
coarser particle surfaces resulting in a more pronounced horizontal alignment.
Finally, the stresses measured at the bottom of a container filled with
particles constructed by gluing together spheres were much smaller than for the
case that the container was filled with smooth super-quadric particles.

References

[1]
Aranson, I. S., & Tsimring, L. S. (2006).
Patterns and collective behavior in granular media: Theoretical concepts. Reviews of modern physics, 78(2),
641.

[2] Hidalgo, R. C., Zuriguel, I., Maza, D., &
Pagonabarraga, I. (2009). Role of particle
shape on the stress propagation in granular packings. Physical review letters, 103(11), 118001.

[3] Hidalgo, R. C., Zuriguel, I., Maza, D., &
Pagonabarraga, I. (2010). Granular packings of
elongated faceted particles deposited under gravity. Journal of Statistical Mechanics: Theory and Experiment, 2010(06),
P06025.

[4] Lu, G., Third, J. R., & M¨¹ller, C. R. (2012). Packing and discharging of
non-spherical particles in hoppers. In: Proceedings
of the 7^th International
Conference for Conveying and Handling of Particulate Solids, Friedrichshafen, Germany, 10-13 September 2012.

[5] Third,
J. R., Scott, D. M., Scott, S. A., & M¨¹ller, C. R. (2010). Tangential
velocity profiles of granular material within horizontal rotating cylinders
modelled using the DEM. Granular
Matter, 12(6), 587-595.

[6] Lu,
G., Third, J. R., & M¨¹ller, C. R. (2012). Critical assessment of two
approaches for evaluating contacts between super-quadric shaped particles in
DEM simulations. Chemical
Engineering Science, 78, 226-235.

Fig. 1  DEM simulations of granular packings of
elongated particles with different surface roughnesses in a 2D, flat bottom
container: (a) Particle orientation distribution, (b) radial distribution
function ( is given as , where  is the number of
particles in the differential area  at a distance  relative to the
central particle and  is the average
number of particles per unit area in the packing region analyzed) and (c)
transmission of contact forces within the beds of grains.