382 research outputs found
Study of the collapse of granular columns using DEM numerical simulation
Numerical simulations of the collapse and spreading of granular columns onto
an horizontal plane using the Contact Dynamics method are presented. The final
shape of the deposit seems to depend only on the aspect ratio of the
columns; these results are in good agreement with previous experimental work.
In particular, the renormalised runout distance shows a power law dependence on
the aspect ratio , which is incompatible with a simple friction model. The
dynamics of the collapse is shown to be mostly controlled by the free fall of
the column. The energy dissipation at the base of the column can be described
simply by a coefficient of restitution. Hence the energy available for the
sideways flow is proportional to the initial potential energy . The
dissipation process within the flow is well approximated by basal friction,
contrary to the behaviour of the runout distance. The mass ejected sideways is
showned to play a determining role in the spreading process. As increases,
the same fraction of initial potential energy drives more mass against
friction. This additional dissipation give a possible explanation for power-law
dependence of the runout distance on . Beyond the frictional properties of
the material, we show that the flow characteristics strongly depend on the
early dynamics of the collapse. We propose a new scaling for the runout
distance that matches the data well, is compatible with a friction model, and
provide a qualitative explanation to the column collapse phenomenology.Comment: 15 pages, 22 Figure
Friction vs Texture at the Approach of a Granular Avalanche
We perform a novel analysis of the granular texture of a granular bed close
to stability limit. Our analysis is based on a unique criterion of friction
mobilisation in a simulated two-dimensional packing. In this way, we recover
the bimodal character of granular texture, and the coexistence of weak and
strong phases in the sense of distinct contacts populations. Moreover, we show
the existence of a well-defined subset of contacts within the weak contact
network. These contacts are characterized by their important friction, and form
a highly coherent population in terms of fabric. They play an antagonistic role
with respect to force chains. We are thus able to discriminate between
incoherent contacts and coherent contacts in the weak phase, and to specify the
role that the latter plays in the destabilisation process.Comment: 4 pages, 6 figure
Continuum simulation of the discharge of the granular silo: a validation test for the mu(I)-visco-plastic flow law
Using both a continuum Navier-Stokes solver, with the mu(I)-flow-law
implemented to model the viscous behavior, and the discrete Contact Dynamics
algorithm, the discharge of granular silos is simulated in two dimensions from
the early stages of the discharge until complete release of the material. In
both cases, the Beverloo scaling is recovered. We first do not attempt
quantitative comparison, but focus on the qualitative behavior of velocity and
pressure at different locations in the flow. A good agreement is obtained in
the regions of rapid flows, while areas of slow creep are not entirely captured
by the continuum model. The pressure field shows a general good agreement. The
evolution of the free surface implies differences, however, the bulk
deformation is essentially identical in both approaches. The influence of the
parameters of the mu(I)-flow-law is systematically investigated, showing the
importance of the dependence on the inertial number I to achieve quantitative
agreement between continuum and discrete discharge. The general ability of the
continuum model to reproduce qualitatively the granular behavior is found to be
very encouraging.Comment: 12 pages, 15 figure
The granular silo as a continuum plastic flow: the hour-glass vs the clepsydra
The granular silo is one of the many interesting illustrations of the
thixotropic property of granular matter: a rapid flow develops at the outlet,
propagating upwards through a dense shear flow while material at the bottom
corners of the container remains static. For large enough outlets, the
discharge flow is continuous; however, by contrast with the clepsydra for which
the flow velocity depends on the height of fluid left in the container, the
discharge rate of granular silos is constant. Implementing a plastic rheology
in a 2D Navier-Stokes solver (following the mu(I)-rheology or a constant
friction), we simulate the continuum counterpart of the granular silo. Doing
so, we obtain a constant flow rate during the discharge and recover the
Beverloo scaling independently of the initial filling height of the silo. We
show that lowering the value of the coefficient of friction leads to a
transition toward a different behavior, similar to that of a viscous fluid, and
where the filling height becomes active in the discharge process. The pressure
field shows that large enough values of the coefficient of friction (
0.3) allow for a low-pressure cavity to form above the outlet, and can thus
explain the Beverloo scaling. In conclusion, the difference between the
discharge of a hourglass and a clepsydra seems to reside in the existence or
not of a plastic yield stress.Comment: 6 pages, 6 figure
Multiscale Analysis of the Stress State in a Granular Slope in Transition to Failure
By means of contact dynamics simulations, we analyze the stress state in a
granular bed slowly tilted towards its angle of repose. An increasingly large
number of grains are overloaded in the sense that they are found to carry a
stress ratio above the Coulomb yield threshold of the whole packing. Using this
property, we introduce a coarse-graining length scale at which all stress
ratios are below the packing yield threshold. We show that this length
increases with the slope angle and jumps to a length comparable to the depth of
the granular bed at an angle below the angle of repose. This transition
coincides with the onset of dilatation in the packing. We map this transition
into a percolation transition of the overloaded grains, and we argue that in
the presence of long-range correlations above the transition angle, the
granular slope is metastable.Comment: 11 pages, 14 Fig, submitted to PR
The Cohesive Granular Collapse as a Continuum : Parametrization Study
Although intensive research on the flow of dry granular materials has allowed for the proposition of continuum rheology and modelling, the behaviour of flowing cohesive material has attracted less attention so far. To start modelling such cohesive flows, we first focus on the configuration of a granular collapse, which is a simple benchmark test. Specifically, we compare granular-collapse experiments of cohesive grains with numerical simulations, where we test a simple rheology for the material : the so-called µ(I)-rheology, supplmented by a yield stress for cohesion. This document reports the sensitivity of our numerical simulations on the parameters of the rheology, often challenging to measure in experiments
Experimental evidence of ageing and slow restoration of the weak-contact configuration in tilted 3D granular packings
Granular packings slowly driven towards their instability threshold are
studied using a digital imaging technique as well as a nonlinear acoustic
method. The former method allows us to study grain rearrangements on the
surface during the tilting and the latter enables to selectively probe the
modifications of the weak-contact fraction in the material bulk. Gradual ageing
of both the surface activity and the weak-contact reconfigurations is observed
as a result of repeated tilt cycles up to a given angle smaller than the angle
of avalanche. For an aged configuration reached after several consecutive tilt
cycles, abrupt resumption of the on-surface activity and of the weak-contact
rearrangements occurs when the packing is subsequently inclined beyond the
previous maximal tilting angle. This behavior is compared with literature
results from numerical simulations of inclined 2D packings. It is also found
that the aged weak-contact configurations exhibit spontaneous restoration
towards the initial state if the packing remains at rest for tens of minutes.
When the packing is titled forth and back between zero and near-critical
angles, instead of ageing, the weak-contact configuration exhibits "internal
weak-contact avalanches" in the vicinity of both the near-critical and zero
angles. By contrast, the stronger-contact skeleton remains stable
Memory of the Unjamming Transition during Cyclic Tiltings of a Granular Pile
Discrete numerical simulations are performed to study the evolution of the
micro-structure and the response of a granular packing during successive
loading-unloading cycles, consisting of quasi-static rotations in the gravity
field between opposite inclination angles. We show that internal variables,
e.g., stress and fabric of the pile, exhibit hysteresis during these cycles due
to the exploration of different metastable configurations. Interestingly, the
hysteretic behaviour of the pile strongly depends on the maximal inclination of
the cycles, giving evidence of the irreversible modifications of the pile state
occurring close to the unjamming transition. More specifically, we show that
for cycles with maximal inclination larger than the repose angle, the weak
contact network carries the memory of the unjamming transition. These results
demonstrate the relevance of a two-phases description -strong and weak contact
networks- for a granular system, as soon as it has approached the unjamming
transition.Comment: 13 pages, 15 figures, soumis \`{a} Phys. Rev.
Characterization of BTBD1 and BTBD2, two similar BTB-domain-containing Kelch-like proteins that interact with Topoisomerase I
BACKGROUND: Two-hybrid screening for proteins that interact with the core domain of human topoisomerase I identified two novel proteins, BTBD1 and BTBD2, which share 80% amino acid identities. RESULTS: The interactions were confirmed by co-precipitation assays demonstrating the physical interaction of BTBD1 and BTBD2 with 100 kDa topoisomerase I from HeLa cells. Deletion mapping using two-hybrid and GST-pulldown assays demonstrated that less than the C-terminal half of BTBD1 is sufficient for binding topoisomerase I. The topoisomerase I sequences sufficient to bind BTBD2 were mapped to residues 215 to 329. BTBD2 with an epitope tag localized to cytoplasmic bodies. Using truncated versions that direct BTBD2 and TOP1 to the same cellular compartment, either the nucleus or the cytoplasm, co-localization was demonstrated in co-transfected Hela cells. The supercoil relaxation and DNA cleavage activities of topoisomerase I in vitro were affected little or none by co-incubation with BTBD2. Northern analysis revealed only a single sized mRNA for each BTBD1 and BTBD2 in all human tissues tested. Characterization of BTBD2 mRNA revealed a 255 nucleotide 90% GC-rich region predicted to encode the N-terminus. BTBD1 and BTBD2 are widely if not ubiquitously expressed in human tissues, and have two paralogs as well as putative orthologs in C. elegans and D. melanogaster. CONCLUSIONS: BTBD1 and BTBD2 belong to a small family of uncharacterized proteins that appear to be specific to animals. Epitope-tagged BTBD2 localized to cytoplasmic bodies. The characterization of BTBD1 and BTBD2 and their interaction with TOP1 is underway
BEER - The Beamline for European Materials Engineering Research at the ESS
The Beamline for European Materials Engineering Research (BEER) will be built at the European Spallation Source (ESS). The diffractometer utilizes the high brilliance of the long-pulse neutron source and offers high instrument flexibility. It includes a novel chopper technique that extracts several short pulses out of the long pulse, leading to substantial intensity gain of up to an order of magnitude compared to pulse shaping methods for materials with high crystal symmetry. This intensity gain is achieved without compromising resolution. Materials of lower crystal symmetry or multi-phase materials will be investigated by additional pulse shaping methods. The different chopper set-ups and advanced beam extracting techniques offer an extremely broad intensity/resolution range. Furthermore, BEER offers an option of simultaneous SANS or imaging measurements without compromising diffraction investigations. This flexibility opens up new possibilities for in-situ experiments studying materials processing and performance under operation conditions. To fulfil this task, advanced sample environments, dedicated to thermo-mechanical processing, are foreseen
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