1,032 research outputs found
Polygons vs. clumps of discs: a numerical study of the influence of grain shape on the mechanical behaviour of granular materials
We performed a series of numerical vertical compression tests on assemblies
of 2D granular material using a Discrete Element code and studied the results
with regard to the grain shape. The samples consist of 5,000 grains made from
either 3 overlapping discs (clumps - grains with concavities) or six-edged
polygons (convex grains). These two grain type have similar external envelopes,
which is a function of a geometrical parameter .
In this paper, the numerical procedure applied is briefly presented followed
by the description of the granular model used. Observations and mechanical
analysis of dense and loose granular assemblies under isotropic loading are
made. The mechanical response of our numerical granular samples is studied in
the framework of the classical vertical compression test with constant lateral
stress (biaxial test). The comparison of macroscopic responses of dense and
loose samples with various grain shapes shows that when is considered
a concavity parameter, it is therefore a relevant variable for increasing
mechanical performances of dense samples. When is considered an
envelope deviation from perfect sphericity, it can control mechanical
performances for large strains. Finally, we present some remarks concerning the
kinematics of the deformed samples: while some polygon samples subjected to a
vertical compression present large damage zones (any polygon shape), dense
samples made of clumps always exhibit thin reflecting shear bands. This paper
was written as part of a CEGEO research project www.granuloscience.comComment: This version of the paper doesn't include figures. Visit the journal
web site to download the final version of the paper with the figure
Jamming transition in a two-dimensional open granular pile with rolling resistance
We present a molecular dynamics study of the jamming/unjamming transition in
two-dimensional granular piles with open boundaries. The grains are modeled by
viscoelastic forces, Coulomb friction and resistance to rolling. Two models for
the rolling resistance interaction were assessed: one considers a constant
rolling friction coefficient, and the other one a strain dependent coefficient.
The piles are grown on a finite size substrate and subsequently discharged
through an orifice opened at the center of the substrate. Varying the orifice
width and taking the final height of the pile after the discharge as the order
parameter, one can devise a transition from a jammed regime (when the grain
flux is always clogged by an arch) to a catastrophic regime, in which the pile
is completely destroyed by an avalanche as large as the system size. A finite
size analysis shows that there is a finite orifice width associated with the
threshold for the unjamming transition, no matter the model used for the
microscopic interactions. As expected, the value of this threshold width
increases when rolling resistance is considered, and it depends on the model
used for the rolling friction.Comment: 9 pages, 6 figure
Scale separation in granular packings: stress plateaus and fluctuations
It is demonstrated, by numerical simulations of a 2D assembly of polydisperse
disks, that there exists a range (plateau) of coarse graining scales for which
the stress tensor field in a granular solid is nearly resolution independent,
thereby enabling an `objective' definition of this field. Expectedly, it is not
the mere size of the the system but the (related) magnitudes of the gradients
that determine the widths of the plateaus. Ensemble averaging (even over
`small' ensembles) extends the widths of the plateaus to sub-particle scales.
The fluctuations within the ensemble are studied as well. Both the response to
homogeneous forcing and to an external compressive localized load (and gravity)
are studied. Implications to small solid systems and constitutive relations are
briefly discussed.Comment: 4 pages, 4 figures, RevTeX 4, Minor corrections to match the
published versio
Relaxation kinetics in two-dimensional structures
We have studied the approach to equilibrium of islands and pores in two
dimensions. The two-regime scenario observed when islands evolve according to a
set of particular rules, namely relaxation by steps at low temperature and
smooth at high temperature, is generalized to a wide class of kinetic models
and the two kinds of structures. Scaling laws for equilibration times are
analytically derived and confirmed by kinetic Monte Carlo simulations.Comment: 6 pages, 7 figures, 1 tabl
A knowledge-based view of the extending enterprise for enhancing a collaborative innovation advantage
In animal societies as well as in human crowds, many observed collective
behaviours result from self-organized processes based on local interactions
among individuals. However, models of crowd dynamics are still lacking a
systematic individual-level experimental verification, and the local mechanisms
underlying the formation of collective patterns are not yet known in detail. We
have conducted a set of well-controlled experiments with pedestrians performing
simple avoidance tasks in order to determine the laws ruling their behaviour
during interactions. The analysis of the large trajectory dataset was used to
compute a behavioural map that describes the average change of the direction
and speed of a pedestrian for various interaction distances and angles. The
experimental results reveal features of the decision process when pedestrians
choose the side on which they evade, and show a side preference that is
amplified by mutual interactions. The predictions of a binary interaction model
based on the above findings were then compared to bidirectional flows of people
recorded in a crowded street. Simulations generate two asymmetric lanes with
opposite directions of motion, in quantitative agreement with our empirical
observations. The knowledge of pedestrian behavioural laws is an important step
ahead in the understanding of the underlying dynamics of crowd behaviour and
allows for reliable predictions of collective pedestrian movements under
natural conditions
Interest of a systematic screening of comorbidities in chronic inflammatory rheumatisms
BackgroundPatients with chronic inflammatory rheumatisms (CIR) have a greater risk of cardiovascular events, infections, lung diseases and osteoporosis. European League against Rheumatisms (EULAR) recommends annual evaluation of the cardiovascular risks.MethodsA program of comorbidity screening was set up in a daily clinic of our Rheumatology department and includes:– rheumatism evaluation;– cardiovascular evaluation; clinical examination, blood tests, modified systematic coronary risk evaluation (mSCORE) calculation, vessel ultrasound and echocardiography;– lung evaluation; self-questionnaires and spirometry;– osteoporosis; bone mineral density and FRAX calculation;– check-up of vaccinal status and the recommended neoplasic screenings.ResultsNinety-two patients already benefited from this systematic screening with 83% (n=76) of rheumatoid arthritis, 11% (n=10) of spondyloarthritis, 3% (n=2) of psoriatic arthritis and 4% (n=4) of other diseases. The mean rheumatism duration was 14±9 years, the mean age was 59±11 years and 64% were women. Hypertension was diagnosed in 8.7% (n=8) of the patients; dyslipidemia in 9.8% (n=9); diabetes in 6.5% (n=6) of the patients. The echocardiography showed significant abnormalities (valvular and hypokinesia) in 9% (n=8) of the patients, a significant supra-aortic vessel stenosis was found in 4.5% (n=4) of the population and an abdominal aortic aneuvrysm was diagnosed in 5.7% (n=5). Among 92 patients, 18.4% (n=14) were estimated at high risk of lethal cardiovascular event with a mSCORE≥5 and 27.5% (n=25) patients were sent to a cardiologist to pursue further cardiovascular investigations. Among these, 8 had a myocardial scintigraphy and all were normal. Moreover, 32.6% (n=30) of the patients were estimated at risk of chronic obstructive pulmonary disease or sleep apnea syndrome and were recommended to consult pneumologist. An anti-osteoporosis drug was introduced in 12% (n=11) of the patients. The update of the vaccinations and the neoplasic screenings were prescribed for respectively 52.7% (n=48) and 35.2% (n=32) of the patients.DiscussionA daily hospitalization for comorbidity screening seems worthy with significant abnormalities discovered in 36.2% of the patients. Further investigations were recommended in 50% of the patients. Patient’ satisfaction and the effective impact of the proposed or prescribed measures are under evaluation
The contamination of the surface of Vesta by impacts and the delivery of the dark material
The Dawn spacecraft observed the presence of dark material, which in turn
proved to be associated with OH and H-rich material, on the surface of Vesta.
The source of this dark material has been identified with the low albedo
asteroids, but it is still a matter of debate whether the delivery of the dark
material is associated with a few large impact events, to micrometeorites or to
the continuous, secular flux of impactors on Vesta. The continuous flux
scenario predicts that a significant fraction of the exogenous material
accreted by Vesta should be due to non-dark impactors likely analogous to
ordinary chondrites, which instead represent only a minor contaminant in the
HED meteorites. We explored the continuous flux scenario and its implications
for the composition of the vestan regolith, taking advantage of the data from
the Dawn mission and the HED meteorites. We used our model to show that the
stochastic events scenario and the micrometeoritic flux scenario are natural
consequences of the continuous flux scenario. We then used the model to
estimate the amounts of dark and hydroxylate materials delivered on Vesta since
the LHB and we showed how our results match well with the values estimated by
the Dawn mission. We used our model to assess the amount of Fe and siderophile
elements that the continuous flux of impactors would mix in the vestan
regolith: concerning the siderophile elements, we focused our attention on the
role of Ni. The results are in agreement with the data available on the Fe and
Ni content of the HED meteorites and can be used as a reference frame in future
studies of the data from the Dawn mission and of the HED meteorites. Our model
cannot yet provide an answer to the fate of the missing non-carbonaceous
contaminants, but we discuss possible reasons for this discrepancy.Comment: 31 pages, 7 figures, 4 tables. Accepted for publication on the
journal ICARUS, "Dark and Bright Materials on Vesta" special issu
Determination of the basic timescale in kinetic Monte Carlo simulations by comparison with cyclic-voltammetry experiments
While kinetic Monte Carlo simulations can provide long-time simulations of
the dynamics of physical and chemical systems, it is not yet possible in
general to identify the inverse Monte Carlo attempt frequency with a physical
timescale. Here we demonstrate such an identification by comparing simulations
with experimental data. Using a dynamic lattice-gas model for the
electrosorption of Br on Ag(100), we measure the scan-rate dependence of the
separation between positive-and negative-going peaks in cyclic-voltammetry (CV)
and compare simulated and experimental peak separations. By adjusting the Monte
Carlo attempt frequency, good agreement between simulated and experimental peak
separations is achieved. It is also found that the uniqueness of such a
determination is dependent on the relative values of the adsorption/desorption
and diffusion free-energy barriers.Comment: Accepted for publication in Surface Science Letters,8 pages, 4
figure
Internal states of model isotropic granular packings. I. Assembling process, geometry and contact networks
This is the first paper of a series of three, reporting on numerical
simulation studies of geometric and mechanical properties of static assemblies
of spherical beads under an isotropic pressure. Frictionless systems assemble
in the unique random close packing (RCP) state in the low pressure limit if the
compression process is fast enough, slower processes inducing traces of
crystallization, and exhibit specific properties directly related to
isostaticity of the force-carrying structure. The different structures of
frictional packings assembled by various methods cannot be classified by the
sole density. While lubricated systems approach RCP densities and coordination
number z^*~=6 on the backbone in the rigid limit, an idealized "vibration"
procedure results in equally dense configurations with z^*~=4.5. Near neighbor
correlations on various scales are computed and compared to available
laboratory data, although z^* values remain experimentally inaccessible. Low
coordination packings have many rattlers (more than 10% of the grains carry no
force), which should be accounted for on studying position correlations, and a
small proportion of harmless "floppy modes" associated with divalent grains.
Frictional packings, however slowly assembled under low pressure, retain a
finite level of force indeterminacy, except in the limit of infinite friction.Comment: 29 pages. Published in Physical Review
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