1,638 research outputs found
Spherical collapse of a heat conducting fluid in higher dimensions without horizon
We consider a scenario where the interior spacetime,described by a heat
conducting fluid sphere is matched to a Vaidya metric in higher
dimensions.Interestingly we get a class of solutions, where following heat
radiation the boundary surface collapses without the appearance of an event
horizon at any stage and this happens with reasonable properties of matter
field.The non-occurrence of a horizon is due to the fact that the rate of mass
loss exactly counterbalanced by the fall of boundary radius.Evidently this
poses a counter example to the so-called cosmic censorship hypothesis.Two
explicit examples of this class of solutions are also given and it is observed
that the rate of collapse is delayed with the introduction of extra
dimensions.The work extends to higher dimensions our previous investigation in
4D.Comment: 6 page
Potential bio-activity of whey fermented extract as sanitizer of organic grown lettuce
Short CommunicationConsumption of vegetables is increasing due to demand for healthy products in peoples' diets. To reduce
microbial contamination and maintain freshness, industrial processes in Portugal rely on minimally
processing of vegetables with hypochlorite as sanitizer. Formation of toxic chlorine derivatives has raised
concern restrictions to its use and alternatives with whey permeate as a disinfection agent has been
attempted. The aim of this work was to evaluate the bio potential of fermented cheese whey, for use on
disinfection of minimally processed lettuce organically grown.
Assays were made with whey obtained from inoculated milk during cheese processing, fermented for
120 h at 37 C, after which, among other carbohydrates, lactic acid was measured by HPLC, giving average
yields of 18 g L 1.
The sanitizing effect of whey, undiluted, 75 and 50% solutions, was compared with 110 ppm sodium
hypochlorite, after rinsing. Aerobic Microorganisms (AM), Psychrotrophic Microorganisms (PM) and
Enterobacteriaceae (ENT), were used as indicators for hygiene quality. For a level of significance of
P < 0.05, the hygiene quality standards of lettuce samples, were better using 75% whey solution (AM 6.62,
PM 7.48 cfu g 1), than using sodium hypochlorite (AM 7.48, PM 8.15 cfu g 1), for the 7 days of shelf life
studied. Evaluation of Enterobacteriaceae showed significant differences after 3 days, betweenwater (ENT
4.98 cfu g 1) sodium hypochlorite (ENT 4.81 cfu g 1) and 75% solution of whey (ENT 4.63 cfu g 1).
Considering the actual limitations imposed to chlorine sanitation, these results point a good alternative
to the food industry, especially for organic fresh vegetables, which are chemical free brandsinfo:eu-repo/semantics/publishedVersio
The second and third Sonine coefficients of a freely cooling granular gas revisited
In its simplest statistical-mechanical description, a granular fluid can be
modeled as composed of smooth inelastic hard spheres (with a constant
coefficient of normal restitution ) whose velocity distribution
function obeys the Enskog-Boltzmann equation. The basic state of a granular
fluid is the homogeneous cooling state, characterized by a homogeneous,
isotropic, and stationary distribution of scaled velocities, .
The behavior of in the domain of thermal velocities ()
can be characterized by the two first non-trivial coefficients ( and
) of an expansion in Sonine polynomials. The main goals of this paper are
to review some of the previous efforts made to estimate (and measure in
computer simulations) the -dependence of and , to report new
computer simulations results of and for two-dimensional systems,
and to investigate the possibility of proposing theoretical estimates of
and with an optimal compromise between simplicity and accuracy.Comment: 12 pages, 5 figures; v2: minor change
Laser cooling of a trapped two-component Fermi gas
The collective Raman cooling of a trapped two-component Fermi gas is
analyzed. We develop the quantum master equation that describes the collisions
and the laser cooling, in the festina lente regime, where the heating due to
photon reabsorption can be neglected. The numerical results based on Monte
Carlo simulations show, that three-dimensional temperatures of the order of
0.008 T_F can be achieved. We analyze the heating related to the background
losses, and conclude that our laser-cooling scheme can maintain the temperature
of the gas without significant additional losses. Finally we derive an analytic
expression for the temperature of a trapped Fermi gas heated by background
collisions, that agrees very well with the data obtained from the numerical
simulation.Comment: 5 pages, 3 figure
Evaluación de reservas corporales de grasa de vacas lecheras a través de un sistema de análisis de imagen
publishedTomo I . Sección: Sistemas Ganaderos-Economía y Gestión. Sesión: Producción de leche. Ponencia nº 3
Relación entre la condición corporal y medidas de grasa y de músculo obtenidas por ultrasonografía en tiempo real con vacas de raza Barrosã
publishedTomo I . Sección: Sistemas Ganaderos-Economía y Gestión. Sesión: Vacuno carne II. Ponencia nº 1
Reionization: Characteristic Scales, Topology and Observability
Recently the numerical simulations of the process of reionization of the
universe at z>6 have made a qualitative leap forward, reaching sufficient sizes
and dynamic range to determine the characteristic scales of this process. This
allowed making the first realistic predictions for a variety of observational
signatures. We discuss recent results from large-scale radiative transfer and
structure formation simulations on the observability of high-redshift Ly-alpha
sources. We also briefly discuss the dependence of the characteristic scales
and topology of the ionized and neutral patches on the reionization parameters.Comment: 4 pages, 5 figures (4 in color), to appear in Astronomy and Space
Science special issue "Space Astronomy: The UV window to the Universe",
proceedings of 1st NUVA Conference ``Space Astronomy: The UV window to the
Universe'' in El Escorial (Spain
Critical temperature for the two-dimensional attractive Hubbard Model
The critical temperature for the attractive Hubbard model on a square lattice
is determined from the analysis of two independent quantities, the helicity
modulus, , and the pairing correlation function, . These
quantities have been calculated through Quantum Monte Carlo simulations for
lattices up to , and for several densities, in the
intermediate-coupling regime. Imposing the universal-jump condition for an
accurately calculated , together with thorough finite-size scaling
analyses (in the spirit of the phenomenological renormalization group) of
, suggests that is considerably higher than hitherto assumed.Comment: 5 pages, 6 figures. Accepted for publication in Phys. Rev.
Variational and DMRG studies of the Frustrated Antiferromagnetic Heisenberg S=1 Quantum Spin Chain
We study a frustrated antiferromagnetic isotropic Heisenberg chain
using a variational ansatz and the DMRG. At , there is a
disorder point of the second kind, marking the onset of incommensurate
correlations in the chain. At there is a Lifshitz point,
at which the excitation spectrum develops a doubly degenerate structure. These
points are the quantum remnants of the transition from antiferromagnetic to
spiral order in the classical frustrated chain. At there
is a first order phase transition from an AKLT phase to a next-nearest neighbor
generalization of the AKLT model. At the transition, the string order parameter
shows a discontinuous jump of 0.085 to 0; the correlation length and the gap
are both finite at the transition. The problem of edge states in open
frustrated chains is discussed at length.Comment: 37 pages, 14 figures, submitted to Phys.Rev.
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