10,961 research outputs found
Towards a Landau-Ginzburg-type Theory for Granular Fluids
In this paper we show how, under certain restrictions, the hydrodynamic
equations for the freely evolving granular fluid fit within the framework of
the time dependent Landau-Ginzburg (LG) models for critical and unstable fluids
(e.g. spinodal decomposition). The granular fluid, which is usually modeled as
a fluid of inelastic hard spheres (IHS), exhibits two instabilities: the
spontaneous formation of vortices and of high density clusters. We suppress the
clustering instability by imposing constraints on the system sizes, in order to
illustrate how LG-equations can be derived for the order parameter, being the
rate of deformation or shear rate tensor, which controls the formation of
vortex patterns. From the shape of the energy functional we obtain the
stationary patterns in the flow field. Quantitative predictions of this theory
for the stationary states agree well with molecular dynamics simulations of a
fluid of inelastic hard disks.Comment: 19 pages, LaTeX, 8 figure
Dynamics of deviations from the Gaussian state in a freely cooling homogeneous system of smooth inelastic particles
The time dependence of deviations from the Gaussian state in a freely cooling
homogeneous system of smooth inelastically colliding spheres is investigated by
kinetic theory. We determine the full time dependence of the coefficients of an
expansion around the Gaussian state in Generalized Laguerre polynomials.
Approximating this system of equations to sixth order, we find that the
asymptotic state, where the mean energy T follows Haff's law with time
independent cooling rate, is reached within a few collisions per particle.
Two-dimensional molecular dynamics simulations confirm our results and show
exponential behavior in the high-energy tails.Comment: 11 pages, 13 eps figures, to be published in Granular Matte
On the dependence of the avalanche angle on the granular layer thickness
A layer of sand of thickness h flows down a rough surface if the inclination
is larger than some threshold value theta which decreases with h. A tentative
microscopic model for the dependence of theta with h is proposed for rigid
frictional grains, based on the following hypothesis: (i) a horizontal layer of
sand has some coordination z larger than a critical value z_c where mechanical
stability is lost (ii) as the tilt angle is increased, the configurations
visited present a growing proportion $_s of sliding contacts. Instability with
respect to flow occurs when z-z_s=z_c. This criterion leads to a prediction for
theta(h) in good agreement with empirical observations.Comment: 6 pages, 2 figure
UM Soluçar de Vida! Cantos Ecoando Com Projetos Sociais de Barra do Riacho
RESUMO
Um soluçar de vida: cantos ecoando com os projetos sociais de Barra do Riacho é um dos estudos do Núcleo de Pesquisa Contornos de Cidades: movimentos e composições (PPGPSIUFES). Esta pesquisa toma como terreno analítico os movimentos sociais de uma comunidade de periferia de Aracruz/ES. Um bairro que pode ser entendido como bolsão de pobreza, convergência de efeitos e contingências de uma região altamente industrializada e portuária com fortes tradições pesqueiras e indígenas. A pesquisa trabalhou na perspectiva metodológica aberta pela história oral, ouvindo a voz de moradores do bairro, fazendo das narrativas uma produção humana e da narração uma atitude. Um recurso político diante do contar uma história, diante da vida, de seus impasses e conflituosidades. Constrói, a partir desse eixo, narrativas de histórias de vida com moradores que se encontram com os projetos sociais, formalizados ou não por ONGs no bairro. A aposta e a experimentação das construções narrativas, com base nas lutas sociais e sonhos dos moradores, inserem-se na panorâmica acadêmica que contextualiza projetos sociais como uma ação no cerne da confluência perversa que caracterizaria o Terceiro Setor. Assim, ideias como cidadania, participação popular, ação e movimento social ganham contornos fundamentados agora por uma política do tempo e pelo entendimento de uma realidade social que é inventiva, plural e cheia de paradoxos, necessariamente como a condição humana
Domain wall description of superconductivity
In the present work we shall address the issue of electrical conductivity in
superconductors in the perspective of superconducting domain wall solutions in
the realm of field theory. We take our set up made out of a dynamical complex
scalar field coupled to gauge field to be responsible for superconductivity and
an extra scalar real field that plays the role of superconducting domain walls.
The temperature of the system is interpreted through the fact that the soliton
following accelerating orbits is a Rindler observer experiencing a thermal
bath.Comment: 9 pages, 5 figures, Latex. Version to appear in PL
On the rigidity of a hard sphere glass near random close packing
We study theoretically and numerically the microscopic cause of the
mechanical stability of hard sphere glasses near their maximum packing. We show
that, after coarse-graining over time, the hard sphere interaction can be
described by an effective potential which is exactly logarithmic at the random
close packing . This allows to define normal modes, and to apply recent
results valid for elastic networks: mechanical stability is a non-local
property of the packing geometry, and is characterized by some length scale
which diverges at [1, 2]. We compute the scaling of the bulk and
shear moduli near , and speculate on the possible implications of these
results for the glass transition.Comment: 7 pages, 4 figures. Figure 4 had a wrong unit in abscissa, which was
correcte
Dissipative collapse of the adiabatic piston
An adiabatic piston, separating two granular gases prepared in the same
macroscopic state, is found to eventually collapse to one of the sides. This
new instability is explained by a simple macroscopic theory which is
furthermore in qualitative agreement with hard disk molecular dynamics.Comment: 7 pages, 5 figure
- …