449 research outputs found
The Magnetized Electron Gas in terms of Hurwitz Zeta Functions
We obtain explicit expressions for thermodynamic quantities of a relativistic
degenerate free electron gas in a magnetic field in terms of Hurwitz Zeta
functions. The formulation allows for systematic expansion in all regimes.
Three energy scales appear naturally in the degenerate relativistic gas: the
Fermi energy Ef, the temperature T and an energy related to the magnetic field
or Landau level spacing, eB/Ef. We study the cold and warm scenarios, T <<
eB/Ef and eB/Ef << T, respectively. We reproduce the oscillations of the
magnetization as a function of the field in the cold regime and the dilution of
them in the warm regime.Comment: 33 pages, 6 figures, LaTeX 2e, uses epsf. v2: References added, minor
additions to content
La reforma agraria agroecológica como camino hacia la sostenibilidad: un estudio de caso en Brasil.
Resumen: El modelo agroexportador actualmente en marcha en Brasil, basado en el agronegocio y los grandes monocultivos para la producción de commodities, tiene intrínsecas limitaciones en alcanzar de manera satisfactoria las múltiples dimensiones de la sostenibilidad planteadas por la agroecología y la soberanía alimentaria. En base a un estudio de caso (un asentamiento campesino agroecológico en la región cañera de Ribeirão Preto, estado de São Paulo), argumentamos que los procesos de transición hacia la sostenibilidad en zonas dominadas por estos grandes monocultivos agroindustriales pueden ser viables a partir de un nuevo modelo de reforma agraria de base agroecológica, que impulse procesos sociales de construcción de alternativas más sostenibles en el campo. Las evidencias obtenidas en la investigación nos permiten plantear que la reforma agraria, y las políticas agroecológicas asociadas, tienen un importante papel de recuperar la agrobiodiversidad y hacer emerger ?memorias campesinas? que de otra forma estarían condenadas al olvido, abriendo las posibilidades para un proceso de recampesinización en contraposición al modelo de desarrollo hegemónico en la región. Concluimos que la perspectiva agroecológica permite una resignificación de la reforma agraria, en la medida que no la restringe a una dimensión solamente económico-productivista, rescatando su naturaleza multidimensionaly rompiendo el histórico divorcio entre la ?cuestión agraria?y la ?cuestión ambiental? en Brasil
Low magnetic Prandtl number dynamos with helical forcing
We present direct numerical simulations of dynamo action in a forced Roberts
flow. The behavior of the dynamo is followed as the mechanical Reynolds number
is increased, starting from the laminar case until a turbulent regime is
reached. The critical magnetic Reynolds for dynamo action is found, and in the
turbulent flow it is observed to be nearly independent on the magnetic Prandtl
number in the range from 0.3 to 0.1. Also the dependence of this threshold with
the amount of mechanical helicity in the flow is studied. For the different
regimes found, the configuration of the magnetic and velocity fields in the
saturated steady state are discussed.Comment: 9 pages, 14 figure
A Spectral Method for Elliptic Equations: The Neumann Problem
Let be an open, simply connected, and bounded region in
, , and assume its boundary is smooth.
Consider solving an elliptic partial differential equation over with a Neumann boundary condition. The problem is converted
to an equivalent elliptic problem over the unit ball , and then a spectral
Galerkin method is used to create a convergent sequence of multivariate
polynomials of degree that is convergent to . The
transformation from to requires a special analytical calculation
for its implementation. With sufficiently smooth problem parameters, the method
is shown to be rapidly convergent. For
and assuming is a boundary, the convergence of
to zero is faster than any power of .
Numerical examples in and show experimentally
an exponential rate of convergence.Comment: 23 pages, 11 figure
A spectral method for elliptic equations: the Dirichlet problem
An elliptic partial differential equation Lu=f with a zero Dirichlet boundary
condition is converted to an equivalent elliptic equation on the unit ball. A
spectral Galerkin method is applied to the reformulated problem, using
multivariate polynomials as the approximants. For a smooth boundary and smooth
problem parameter functions, the method is proven to converge faster than any
power of 1/n with n the degree of the approximate Galerkin solution. Examples
in two and three variables are given as numerical illustrations. Empirically,
the condition number of the associated linear system increases like O(N), with
N the order of the linear system.Comment: This is latex with the standard article style, produced using
Scientific Workplace in a portable format. The paper is 22 pages in length
with 8 figure
Avaliação econômica da implantação e manutenção de um sistema agroflorestal com cultivo diversificado.
Resumo: Este trabalho apresenta a análise dos custos de implantação e manutenção de um sistema agroflorestal com cultivos diversificados. Esta avaliação é uma etapa preliminar de uma análise integrada que considerará, além dos fatores socioeconômicos, a recuperação ambiental da área. São apresentados o modelo empregado no sistema, alguns resultados iniciais e os custos de implantação e manutenção. A análise dos dados mostra que houve uma concentração dos gastos na implantação e no primeiro ano deste sistema. Na implantação, o custo principal foi com a aquisição de mudas, enquanto na manutenção os custos se concentraram na mão de obra. Abstract: This paper presents an analysis of the costs of implementation and maintenance of a agroforestry system with diversified crops. This evaluation is a preliminary step in an integrated analysis that will consider also the environmental restoration of the area. The model used in the system, some initial results and the costs of implementation and maintenance are presented. The data analysis indicated that there was a concentration of spending in the implementation and first year of this system. The seedlings was the main cost in the deployment of the system, differently the costs are concentrated in manpower in themaintenance stage
An In-Depth Spectroscopic Analysis of the Blazhko Star RR Lyr. I. Characterisation of the star: abundance analysis and fundamental parameters
The knowledge of accurate stellar parameters is a keystone in several fields
of stellar astrophysics, such as asteroseismology and stellar evolution.
Although the fundamental parameters can be derived both from spectroscopy and
multicolour photometry, the results obtained are sometimes affected by
systematic uncertainties. In this paper, we present a self-consistent spectral
analysis of the pulsating star RR Lyr, which is the primary target for our
study of the Blazhko effect. We used high-resolution and high signal-to-noise
ratio spectra to carry out a consistent parameter determination and abundance
analysis for RR Lyr. We provide a detailed description of the methodology
adopted to derive the fundamental parameters and the abundances. Stellar
pulsation attains high amplitudes in RR Lyrae stars, and as a consequence the
stellar parameters vary significantly over the pulsation cycle. The abundances
of the star, however, are not expected to change. From a set of available
high-resolution spectra of RR Lyr we selected the phase of maximum radius, at
which the spectra are least disturbed by the pulsation. Using the abundances
determined at this phase as a starting point, we expect to obtain a higher
accuracy in the fundamental parameters determined at other phases. The set of
fundamental parameters obtained in this work fits the observed spectrum
accurately. Through the abundance analysis, we find clear indications for a
depth-dependent microturbulent velocity, that we quantified. We confirm the
importance of a consistent analysis of relevant spectroscopic features,
application of advanced model atmospheres, and the use of up-to-date atomic
line data for the determination of stellar parameters. These results are
crucial for further studies, e.g., detailed theoretical modelling of the
observed pulsations.Comment: 12 pages, accepted for publication in Astronomy & Astrophysic
Turbulence-induced melting of a nonequilibrium vortex crystal in a forced thin fluid film
To develop an understanding of recent experiments on the turbulence-induced
melting of a periodic array of vortices in a thin fluid film, we perform a
direct numerical simulation of the two-dimensional Navier-Stokes equations
forced such that, at low Reynolds numbers, the steady state of the film is a
square lattice of vortices. We find that, as we increase the Reynolds number,
this lattice undergoes a series of nonequilibrium phase transitions, first to a
crystal with a different reciprocal lattice and then to a sequence of crystals
that oscillate in time. Initially the temporal oscillations are periodic; this
periodic behaviour becomes more and more complicated, with increasing Reynolds
number, until the film enters a spatially disordered nonequilibrium statistical
steady that is turbulent. We study this sequence of transitions by using
fluid-dynamics measures, such as the Okubo-Weiss parameter that distinguishes
between vortical and extensional regions in the flow, ideas from nonlinear
dynamics, e.g., \Poincare maps, and theoretical methods that have been
developed to study the melting of an equilibrium crystal or the freezing of a
liquid and which lead to a natural set of order parameters for the crystalline
phases and spatial autocorrelation functions that characterise short- and
long-range order in the turbulent and crystalline phases, respectively.Comment: 31 pages, 56 figures, movie files not include
Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere
Numerical solutions of the incompressible magnetohydrodynamic (MHD) equations
are reported for the interior of a rotating, perfectly-conducting, rigid
spherical shell that is insulator-coated on the inside. A previously-reported
spectral method is used which relies on a Galerkin expansion in
Chandrasekhar-Kendall vector eigenfunctions of the curl. The new ingredient in
this set of computations is the rigid rotation of the sphere. After a few
purely hydrodynamic examples are sampled (spin down, Ekman pumping, inertial
waves), attention is focused on selective decay and the MHD dynamo problem. In
dynamo runs, prescribed mechanical forcing excites a persistent velocity field,
usually turbulent at modest Reynolds numbers, which in turn amplifies a small
seed magnetic field that is introduced. A wide variety of dynamo activity is
observed, all at unit magnetic Prandtl number. The code lacks the resolution to
probe high Reynolds numbers, but nevertheless interesting dynamo regimes turn
out to be plentiful in those parts of parameter space in which the code is
accurate. The key control parameters seem to be mechanical and magnetic
Reynolds numbers, the Rossby and Ekman numbers (which in our computations are
varied mostly by varying the rate of rotation of the sphere) and the amount of
mechanical helicity injected. Magnetic energy levels and magnetic dipole
behavior are exhibited which fluctuate strongly on a time scale of a few eddy
turnover times. These seem to stabilize as the rotation rate is increased until
the limit of the code resolution is reached.Comment: 26 pages, 17 figures, submitted to New Journal of Physic
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