3,826 research outputs found
Heat flow in the postquasistatic approximation
We apply the postquasistatic approximation to study the evolution of
spherically symmetric fluid distributions undergoing dissipation in the form of
radial heat flow. For a model which corresponds to an incompressible fluid
departing from the static equilibrium, it is not possible to go far from the
initial state after the emission of a small amount of energy. Initially
collapsing distributions of matter are not permitted. Emission of energy can be
considered as a mechanism to avoid the collapse. If the distribution collapses
initially and emits one hundredth of the initial mass only the outermost layers
evolve. For a model which corresponds to a highly compressed Fermi gas, only
the outermost shell can evolve with a shorter hydrodynamic time scale.Comment: 5 pages, 5 figure
Radial periodic boundaries for axi-symmetric DEM simulations: development, implementation and validation
Equation of state and transport processes in self--similar spheres
We study the effect of transport processes (diffusion and free--streaming) on
a collapsing spherically symmetric distribution of matter in a self--similar
space--time. A very simple solution shows interesting features when it is
matched with the Vaidya exterior solution. In the mixed case (diffusion and
free--streaming), we find a barotropic equation of state in the stationary
regime. In the diffusion approximation the gravitational potential at the
surface is always constant; if we perturb the stationary state, the system is
very stable, recovering the barotropic equation of state as time progresses. In
the free--streaming case the self--similar evolution is stationary but with a
non--barotropic equation of state.Comment: 9 pages, 2 figure
Identificação de clones com atividade quitinolítica em Biblioteca Metagenômica de solo da Amazônia.
Interrelações entre variáveis associadas à precocidade, ao crescimento e ao teor de nutrientes absorvidos de mamoeiro.
bitstream/CNPAT/7830/1/pd_7.pd
Constriction size distributions of granular filters: a numerical study
The retention capability of granular filters is controlled by the narrow constrictions connecting the voids within the filter. The theoretical justification for empirical filter rules used in practice includes consideration of an idealised soil fabric in which constrictions form between co-planar combinations of spherical filter particles. This idealised fabric has not been confirmed by experimental or numerical observations of real constrictions. This paper reports the results of direct, particle-scale measurement of the constriction size distribution (CSD) within virtual samples of granular filters created using the discrete-element method (DEM). A previously proposed analytical method that predicts the full CSD using inscribed circles to estimate constriction sizes is found to poorly predict the CSD for widely graded filters due to an over-idealisation of the soil fabric. The DEM data generated are used to explore quantitatively the influence of the coefficient of uniformity, particle size distribution and relative density of the filter on the CSD. For a given relative density CSDs form a narrow band of similarly shaped curves when normalised by characteristic filter diameters. This lends support to the practical use of characteristic diameters to assess filter retention capability
Nonadiabatic charged spherical evolution in the postquasistatic approximation
We apply the postquasistatic approximation, an iterative method for the
evolution of self-gravitating spheres of matter, to study the evolution of
dissipative and electrically charged distributions in General Relativity. We
evolve nonadiabatic distributions assuming an equation of state that accounts
for the anisotropy induced by the electric charge. Dissipation is described by
streaming out or diffusion approximations. We match the interior solution, in
noncomoving coordinates, with the Vaidya-Reissner-Nordstr\"om exterior
solution. Two models are considered: i) a Schwarzschild-like shell in the
diffusion limit; ii) a Schwarzschild-like interior in the free streaming limit.
These toy models tell us something about the nature of the dissipative and
electrically charged collapse. Diffusion stabilizes the gravitational collapse
producing a spherical shell whose contraction is halted in a short
characteristic hydrodynamic time. The streaming out radiation provides a more
efficient mechanism for emission of energy, redistributing the electric charge
on the whole sphere, while the distribution collapses indefinitely with a
longer hydrodynamic time scale.Comment: 11 pages, 16 Figures. Accepted for publication in Phys Rev
Bondian frames to couple matter with radiation
A study is presented for the non linear evolution of a self gravitating
distribution of matter coupled to a massless scalar field. The characteristic
formulation for numerical relativity is used to follow the evolution by a
sequence of light cones open to the future. Bondian frames are used to endow
physical meaning to the matter variables and to the massless scalar field.
Asymptotic approaches to the origin and to infinity are achieved; at the
boundary surface interior and exterior solutions are matched guaranteeing the
Darmois--Lichnerowicz conditions. To show how the scheme works some numerical
models are discussed. We exemplify evolving scalar waves on the following fixed
backgrounds: A) an atmosphere between the boundary surface of an incompressible
mixtured fluid and infinity; B) a polytropic distribution matched to a
Schwarzschild exterior; C) a Schwarzschild- Schwarzschild spacetime. The
conservation of energy, the Newman--Penrose constant preservation and other
expected features are observed.Comment: 20 pages, 6 figures; to appear in General Relativity and Gravitatio
Grupos de desenvolvimento humano: uma proposta para a promoção da auto-análise e autogestão na Embrapa Soja.
Área de ocorrência do mexilhão dourado (Limnoperna fortunei) na bacia do Alto Paraguai, entre os anos de 1998 e 2004.
O uso de água armazenada nos grandes navios marítimos para obter maior estabilidade, ajudar na propulsão e em manobras, a chamada "água de lastro", é o principal meio de introdução de organismos marinhos em ambientes aquáticos, tanto marinhos como de água doce. Em todo o mundo são transferidas anualmente cerca de 12 bilhões de toneladas de "água de lastro", que transportam aproximadamente 4.500 espécies diferentes. No Brasil, aproximadamente 95% de todo o comércio exterior é feito por via marítima e estima-se que 40.000 navios visitem os portos brasileiros anualmente, deslastrando 40 milhões de toneladas de água por ano (Silva & Souza, 2004). Pela "água de lastro" a espécie de bivale de água doce Limnoperna fortunei (Dunker, 1857) foi introduzida no estuário do rio da Prata, em 1991, provavelmente trazido por navios da Korea e Hong Kong (Pastorino et al., 1993). E a navegação no sistema Paraguai-Paraná foi o principal vetor da introdução desta espécie na Bacia do Alto Paraguai, onde foi observada pela primeira vez em 1998, no rio Paraguai, próximo à cidade de Corumba (MS).bitstream/item/39645/1/DOC64.pd
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