221 research outputs found
Epitaxial Frustration in Deposited Packings of Rigid Disks and Spheres
We use numerical simulation to investigate and analyze the way that rigid
disks and spheres arrange themselves when compressed next to incommensurate
substrates. For disks, a movable set is pressed into a jammed state against an
ordered fixed line of larger disks, where the diameter ratio of movable to
fixed disks is 0.8. The corresponding diameter ratio for the sphere simulations
is 0.7, where the fixed substrate has the structure of a (001) plane of a
face-centered cubic array. Results obtained for both disks and spheres exhibit
various forms of density-reducing packing frustration next to the
incommensurate substrate, including some cases displaying disorder that extends
far from the substrate. The disk system calculations strongly suggest that the
most efficient (highest density) packings involve configurations that are
periodic in the lateral direction parallel to the substrate, with substantial
geometric disruption only occurring near the substrate. Some evidence has also
emerged suggesting that for the sphere systems a corresponding structure doubly
periodic in the lateral directions would yield the highest packing density;
however all of the sphere simulations completed thus far produced some residual
"bulk" disorder not obviously resulting from substrate mismatch. In view of the
fact that the cases studied here represent only a small subset of all that
eventually deserve attention, we end with discussion of the directions in which
first extensions of the present simulations might profitably be pursued.Comment: 28 pages, 14 figures; typos fixed; a sentence added to 4th paragraph
of sect 5 in responce to a referee's comment
The Multimode Resource Constrained Project Scheduling Problem for Repetitive Activities in Construction Projects
[EN] In construction projects, resource availability might limit the implementation of ideal schedules. Especially, when repetitive activities are involved, traditional resource¿constrained project scheduling problem (RCPSP) models fail to allocate the resource consumption in an efficient manner. Besides, actual models only provide local optimal solutions and do not incorporate activity acceleration routines. To fulfill this gap, partially, a mathematical optimization model, the multimode RCPSP for repetitive activities in construction projects, is proposed and solved to optimality; it takes into account acceleration routines under real construction scenarios using spreadsheets. The article shows a complete computational experimentation over a real construction project, considering several scenarios of resource availabilities and continuity conditions. The model allows analyzing the resources efficiency indexes comparing them to resource consumptions, continuity of activities, and objective functions that reveal that fragmented activities do not provide better resource efficiency outcomes.This research was partially supported by the FAPA program of Universidad de Los Andes, Colombia (code P14.246922.005/01). The authors would also like to thank the research group of Construction Engineering and Management (INgeco) at Universidad de los Andes.García-Nieves, J.; Ponz-Tienda, JL.; Salcedo-Bernal, A.; Pellicer Armiñana, E. (2018). The Multimode Resource Constrained Project Scheduling Problem for Repetitive Activities in Construction Projects. Computer-Aided Civil and Infrastructure Engineering. 33(8):655-671. https://doi.org/10.1111/mice.12356S65567133
A cosmological dust model with extended f(chi) gravity
Introducing a fundamental constant of nature with dimensions of acceleration
into the theory of gravity makes it possible to extend gravity in a very
consistent manner. At the non-relativistic level a MOND-like theory with a
modification in the force sector is obtained, which is the limit of a very
general metric relativistic theory of gravity. Since the mass and length scales
involved in the dynamics of the whole universe require small accelerations of
the order of Milgrom's acceleration constant a_0, it turns out that the
relativistic theory of gravity can be used to explain the expansion of the
universe. In this work it is explained how to use that relativistic theory of
gravity in such a way that the overall large-scale dynamics of the universe can
be treated in a pure metric approach without the need to introduce dark matter
and/or dark energy components.Comment: 7 pages, 1 figure. Accepted for publication in the European Physical
Journal
The hydration state of HO(aq)
The HO(aq) ion participates in myriad aqueous phase chemical processes of
biological and chemical interest. A molecularly valid description of its
hydration state, currently poorly understood, is a natural prerequisite to
modeling chemical transformations involving HO(aq). Here it is shown that
the statistical mechanical quasi-chemical theory of solutions predicts that
is the dominant inner shell coordination
structure for HO(aq) under standard conditions. Experimental observations
and other theoretical calculations are adduced to support this conclusion.
Hydration free energies of neutral combinations of simple cations with
HO(aq) are evaluated and agree well with experimental values.Comment: 10 pages, 1 figur
Structural transitions in granular packs: statistical mechanics and statistical geometry investigations
We investigate equal spheres packings generated from several experiments and
from a large number of different numerical simulations. The structural
organization of these disordered packings is studied in terms of the network of
common neighbours. This geometrical analysis reveals sharp changes in the
network's clustering occurring at the packing fractions (fraction of volume
occupied by the spheres respect to the total volume, ) corresponding to
the so called Random Loose Packing limit (RLP, ) and Random
Close Packing limit (RCP, ). At these packing fractions we
also observe abrupt changes in the fluctuations of the portion of free volume
around each sphere. We analyze such fluctuations by means of a statistical
mechanics approach and we show that these anomalies are associated to sharp
variations in a generalized thermodynamical variable which is the analogous for
these a-thermal systems to the specific heat in thermal systems.Comment: 7 pages, 6 figure
Random close packing of granular matter
We propose an interpretation of the random close packing of granular
materials as a phase transition, and discuss the possibility of experimental
verification.Comment: 6 page
Gravitational anomalies signaling the breakdown of classical gravity
Recent observations for three types of astrophysical systems severely
challenge the GR plus dark matter scenario, showing a phenomenology which is
what modified gravity theories predict. Stellar kinematics in the outskirts of
globular clusters show the appearance of MOND type dynamics on crossing the
threshold. Analysis shows a ``Tully-Fisher'' relation in these systems,
a scaling of dispersion velocities with the fourth root of their masses.
Secondly, an anomaly has been found at the unexpected scales of wide binaries
in the solar neighbourhood. Binary orbital velocities cease to fall along
Keplerian expectations, and settle at a constant value, exactly on crossing the
threshold. Finally, the inferred infall velocity of the bullet cluster
is inconsistent with the standard cosmological scenario, where much smaller
limit encounter velocities appear. This stems from the escape velocity limit
present in standard gravity; the ``bullet'' should not hit the ``target'' at
more than the escape velocity of the joint system, as it very clearly did.
These results are consistent with extended gravity, but would require rather
contrived explanations under GR, each. Thus, observations now put us in a
situation where modifications to gravity at low acceleration scales cease to be
a matter of choice, to now become inevitable.Comment: 10 pages, 5 figures, Astrophysics and Space Science Proceedings 38,
4
Theory of a Human Ecology of Communication: empirical evidence of the Internet consumption ecosystem in Ecuador
Since 2010 we have been analyzing Internet consumption
indicators in Ecuador, characterized by the rapid growth in
relation to nearby countries. This phenomenon, that transcends
the statistical explanation, has been conducted in more than ten
research reports published by the Research Center of
Communication and Public Opinion (CICOP) of the Faculty of
Communication at the Universidad de Los Hemisferios, who
leads in the country the World Internet Project -WIP- project.
This "transgression" suggests an interdisciplinary explanation,
according to the complexity of human social ecosystem, to
understand "how" it is possible to discern political and
economic different and specific behaviors in social
communication, according to sociodemographic
characteristics, in a society like Ecuador, where the classic
paradigms of communication theory breaks. In this sense we
present the theoretical proposal entitled "Human Ecology of
Communication" from the Internet consumption research
conducted during the years 2010-2014 in Ecuador
Isotropic-nematic phase equilibria in the Onsager theory of hard rods with length polydispersity
We analyse the effect of a continuous spread of particle lengths on the phase
behavior of rodlike particles, using the Onsager theory of hard rods. Our aim
is to establish whether ``unusual'' effects such as isotropic-nematic-nematic
(I-N-N) phase separation can occur even for length distributions with a single
peak. We focus on the onset of I-N coexistence. For a log-normal distribution
we find that a finite upper cutoff on rod lengths is required to make this
problem well-posed. The cloud curve, which tracks the density at the onset of
I-N coexistence as a function of the width of the length distribution, exhibits
a kink; this demonstrates that the phase diagram must contain a three-phase
I-N-N region.
Theoretical analysis shows that in the limit of large cutoff the cloud point
density actually converges to zero, so that phase separation results at any
nonzero density; this conclusion applies to all length distributions with
fatter-than-exponentail tails. Finally we consider the case of a Schulz
distribution, with its exponential tail. Surprisingly, even here the long rods
(and hence the cutoff) can dominate the phase behaviour, and a kink in the
cloud curve and I-N-N coexistence again result. Theory establishes that there
is a nonzero threshold for the width of the length distribution above which
these long rod effects occur, and shows that the cloud and shadow curves
approach nonzero limits for large cutoff, both in good agreement with the
numerical results.Comment: 20 pages, 13 figure
Annihilation vs. Decay: Constraining dark matter properties from a gamma-ray detection
Most proposed dark matter candidates are stable and are produced thermally in
the early Universe. However, there is also the possibility of unstable (but
long-lived) dark matter, produced thermally or otherwise. We propose a strategy
to distinguish between dark matter annihilation and/or decay in the case that a
clear signal is detected in gamma-ray observations of Milky Way dwarf
spheroidal galaxies with gamma-ray experiments. The sole measurement of the
energy spectrum of an indirect signal would render the discrimination between
these cases impossible. We show that by examining the dependence of the
intensity and energy spectrum on the angular distribution of the emission, the
origin could be identified as decay, annihilation, or both. In addition, once
the type of signal is established, we show how these measurements could help to
extract information about the dark matter properties, including mass,
annihilation cross section, lifetime, dominant annihilation and decay channels,
and the presence of substructure. Although an application of the approach
presented here would likely be feasible with current experiments only for very
optimistic dark matter scenarios, the improved sensitivity of upcoming
experiments could enable this technique to be used to study a wider range of
dark matter models.Comment: 29 pp, 8 figs; replaced to match published version (minor changes and
some new references
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