Tamás Börzsönyi Group leader |
Ellák Somfai Scientific Advisor |
Sára Lévay PostDoc (on leave) |
Bo Fan PostDoc |
Alumni: 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.

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) |