Granular group:

Tamás Börzsönyi
Group leader

Ellák Somfai
Scientific Advisor

Sára Lévay
(on leave)

  Bo Fan

Balázs Szabó  (MSc 2010, PhD 2015, PostDoc 2015-2017, TB, now at Mediso Kft)
Katalin Gillemot (PostDoc 2014-2016, TB, now MC Fellow at the University of Vienna)
Gábor Törös (MSc 2011, TB, now at GE Hungary)
  Zsolt Kovács (BSc 2011, MSc 2012, TB, now at Semilab)
  Gábor Bíró (BSc 2013, TB, now at Wigner RMI)
Béla Csengeri (BSc 2014, TB)
Bence Szabó (BSc 2017, TB)
Dávid Kálmán (MSc 2017, ES)
Viktor Nagy (BSc 2018, TB)
Dániel Nagy (BSc 2015, MSc 2017, ES)

Aims and techniques:

We study the flow properties of granular materials in various geometries taking benefit of high speed digital imaging, X-Ray Computed Tomography or MRI.

Latest results:

Shear induced orientational ordering of asymmetric elongated particles was investigated. Corn grains and pegs with one end sharpened were studied using x-ray computed tomography during quasistatic shearing and for silo flow. We have shown that asymmetries can be detected in the orientational distributions of the particles, which are related to the modulated rotation of the particles during shear flow.  J. Stat. Mech. 2023 113201    (download pdf)

The flow of elliptical particles out of a 2-dimensional silo when extracted with a conveyor belt was analyzed. The conveyor belt —placed directly below the silo outlet — reduces the flow rate, increases the size of the stagnant zone, and it has a very strong influence on the relative velocity fluctuations as they strongly increase everywhere in the silo with decreasing belt speed. In other words, instead of slower but smooth flow, flow reduction by belt leads to intermittent flow. Interestingly, we show that the intermittency correlates with a strong reduction of the orientational order of the particles at the orifice region.  Phys. Rev. E 108, 044902 (2023)     (download pdf)

The force on an obstacle exerted by a flowing granular material was investigated in a silo. For frictional hard glass beads, the force on the obstacle was practically flow-rate independent. In contrast, for nearly frictionless soft hydrogel spheres the drag force increases with flow rate. The dependence of the total force on the obstacle diameter is qualitatively different for the two types of material: the force grows quadratically with the obstacle diameter in the soft, low-friction material, while it grows much weaker, nearly linearly with the obstacle diameter, in the bed of glass spheres Phys. Rev. E 108, L062901 (2023)    (download pdf)

Numerical simulations were performed for studying the discharge of elongated grains from a silo with rotating bottom. The introduction of a slight transverse shear reduces the flow rate by up to 70% compared with stationary bottom, but the flow rate shows a modest increase by further increasing the external shear. Main findings of our previous experimental studies are recovered.  Phys. Rev. E 106, 034904 (2022)    (download pdf)

The time evolution of silo discharge was investigated for different granular materials made of spherical or elongated grains in laboratory experiments and with discrete element model (DEM) calculations. For spherical grains, we confirmed the widely known typical behavior with constant discharge rate. For elongated particles we find a peculiar flow rate increase before the end of the discharge process for a certain range of the normalized orifice diameter D/d* (see Figure). New J. Phys. 24,103036 (2022)     (download pdf)

Laboratory experiments have been carried out to study the discharge of elongated grains from a silo with a rotating bottom plate. In the continuous flow regime the rotation of the silo base leads to a strong — more than 50% — decrease of the discharge rate, which otherwise depends only weakly on the rotation rate.   Phys. Rev. E 103, 062905 (2021)    (download pdf)

We analyzed the statistical properties of granular packings to test Edwards approach based on equiprobable jammed states. Identical spheres packed in a nearly two-dimensional geometrical confinement were studied in experiments and numerical simulations. When tapped, the system evolves toward a ground state, but due to incompatible domain structures it gets trapped. Analytical calculations reproduce relatively well our simulation results, which allows us to test Edwards theory on a coupled system of two subsystems with different properties. We find that the joint system can only be described by the Edwards theory if considered as a single system due to the constraints in the stresses. The results show counterintuitive effects as in the coupled system the change in the order parameter is opposite to what is expected from the change in the compactivity. Phys. Rev. E  103, 042901 (2021)     (download pdf)

We studied the outflow dynamics and clogging phenomena of mixtures of soft, elastic low-friction spherical grains and hard frictional spheres of similar size in a quasi-two-dimensional (2D) silo with narrow orifice at the bottom. We show that the addition of small amounts, even as low as 5%, of hard grains to an ensemble of soft, low-friction grains already has significant consequences. The mixtures allow a direct comparison of the probabilities of the different types of particles to clog the orifice. We analyze these probabilities for the hard, frictional and the soft, slippery grains on the basis of their participation in the blocking arches, and compare outflow velocities and durations of non-permanent clogs for different compositions of the mixtures. Experimental results are compared with numerical simulations. The latter strongly suggest a significant influence of the inter-species particle friction.  Soft Matter 17, 4282 (2021)    (download pdf)

We performed laboratory experiments and numerical simulations of silo discharge with traditional (frictional hard) granular materials and grains with reduced surface friction and hardness. We show, that particle stiffness has a strong effect on the qualitative features of flow rate. For deformable grains lowering the friction coefficient leads to a gradual change in the discharge curve: the flow rate becomes filling height dependent, it decreases during the discharge process. For hard grains the flow rate is much less sensitive to the value of the friction coefficient. New J. Phys. 23, 023001 (2021)     (download pdf)

Silo discharge was studied numerically for the case of a silo with a rotating bottom plate. We show, that in accordance with recent experimental findings, for intermediate orifice diameter the discharge rate shows a non-monotonic behaviour on the rotation frequency of the bottom plate. Phys. Rev. E  102, 042902 (2020)     (download pdf)

We studied the discharge of low friction soft (hydrogel) particles from a 2D silo with narrow orifice experimentally. The outflow velocity and the clogging probability both depend on the filling height, which is different from the case of hard grains where these quantities are fill height independent. The reason is the fill height dependence of the pressure at the orifice. Decreasing orifice size leads to increasing fluctuations of the flow. Decreasing orifice size leads to increasing intermittency of the flow.For an orifice size smaller than 2 particle diameters the outflow can stop completely, but in contrast to clogs formed by rigid particles, these congestions may dissolve spontaneously. This is connected to slow reorganizations of the grains due to the viscoelastic character of the particles. Restarting the flow leads to delayed particle movement at heigher locations above orifice (seee space-time plot).  Soft Matter 16, 8013 (2020)    (download pdf)

The rheology of frictional spherocylinders was investigated in shear flows. We explored how the effective friction changes with the inertial number and the particle aspect ratio L/D.  For frictional grains the effective friction increases with the elongation of the particles, while for low friction particles a non-monotonic behavior is observed. New J. Phys. 22, 073008 (2020)     (download pdf)

We used ultrafast X-ray tomography (1000 fps) to study discharge of a granular material from a cylindrical container. In this procedure 2 horizontal slices (separated by 11 mm) can be recorded continuously. Space time plots were used to reconstruct velocity profiles and packing structures for low friction soft grains and frictional hard particles. New J. Phys.  21, 113054 (2019)     (download pdf)

We studied the packing and flow profiles of low friction soft grains using X‐ray computed tomography during discharge of a 3D bin. For hard frictional grains (left image), flow is concentrated in the middle part of the bin (red region) with stagnant zones near the wall, for low friction soft grains we find grain motion everywhere in the bin (right image). Granular Matter 21, 56 (2019)    (download pdf)

The rheology of elongated frictional particles has been investigated in inclined plane flows. In this geometry we can easier test the systems response at higher inertial numbers (compared to simple shear). Our discrete element simulations and laboratory experiments revealed that density and friction are well-defined functions of the effective inertial number.  The effective friction clearly increases with grain elongation. Phys. Rev. Fluids  3, 074301 (2018)     (download pdf)