Conquistas e desafios do agronegócio brasileiro.

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Propagation of Partially Coherent Light in non-Hermitian Lattices

Band theory for partially coherent light is introduced by using the formalism of second-order classical coherence theory under paraxial approximation. It is demonstrated that the cross-spectral density function, describing correlations between pairs of points in the field, can have bands and gaps and form a correlation band structure. The propagation of a partially coherent beam in non-Hermitian periodic structures is considered to elucidate the interplay between the degree of coherence and the gain/loss present in the lattice. We apply the formalism to study partially coherent Bloch oscillations in lattices having parity-time symmetry and demonstrate that the oscillations can be sustained in such media but they are strongly dependent upon the spatial correlations of the beam. A transition between breathing and oscillating modes is shown to be induced by the degree of spatial coherence

Análise de preços agrícolas.

Recentemente, muitos analistas vêm se dedicando a examinar o crescimento dos preços de commodities. A Tabela 1 mostra que tem havido aumentos de preços expressivos para commodities agrícolas. Entre 2004 e 2007, o preço do trigo aumentou 48 %, o do milho 33 % e o do arroz 23 %. A soja ? que já estava com preços elevados em 2004 ? subiu mais 4 %. Os óleos vegetais, a soja e a palma, também mostraram aumentos importantes: 23 % e 50 %, respectivamente, entre 2004 e 2007. O açúcar atingiu o preço mais elevado em 2006, mas continua com seu preço 20 % acima do valor alcançado em 2004.Na publicação: Eliseu Alves

Commodities agrícolas e preço do petróleo.

Este artigo tem por objetivo documentar estatisticamente a relação entre preços internacionais de produtos agrícolas e o preço internacional do petróleo. Inicialmente caracteriza-se a relação entre o preço do petróleo e o preço da ureia e, em seguida, como o preço desta e das commodities escolhidas para o estudo se comportam. Os resultados mostram que a tendência no preço desses produtos é influenciada de maneira significativa pela tendência no preço do petróleo. As elasticidades estimadas estão entre 0,47 e 0,61, indicando que entre 50% e 60% das variações no preço do petróleo são transmitidas para os produtos. Ademais, choques transitórios no preço do petróleo têm impactos fortes e duradouros sobre os preços desses produtos.Na publicação: Eliseu Alves

Solos do campo experimental da Embrapa Milho e Sorgo: suas características e classificação no novo Sistema Brasileiro.

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Nuno Portas and the Spanish influence on the definition of housing policies in Portugal in the period of democratic transition

Taking the housing crisis and the SAAL program as a central interest point of architects and sociologists in the aftermath of the Portuguese revolution, this chapter tracks the influence of Spanish architecture in Portugal and the relations of Portuguese and Spanish architects, signaling the main role of Nuno Portas. It begins by introducing the background of the architecture exchange between Portugal and Spain, since the 1960’s, through the diffusion and interchange activities of Nuno Portas (section 2). It continues to discuss the role of architects on urban change during the revolutionary process from the viewpoint Joan Antonio Solans experiences and writings (Section 3). Then it takes on the social movements debate with Manuel Castells reflections and writings about the new housing policies and experiences (Section 4). Finally, a short reflection on the interchange of ideas and experiences between Portugal and Spain in presented in the conclusion (Section 5).info:eu-repo/semantics/submittedVersio

Thermal Convection of an Ellis Fluid Saturating a Porous Layer with Constant Heat Flux Boundary Conditions

The present work analyzes the thermal instability of mixed convection in a horizontal porous channel that is saturated by a shear-thinning fluid following Ellis’ rheology. The fluid layer is heated from below by a constant heat flux and cooled from above by the same heat flux. The instability of such a system is investigated by means of a small-disturbances analysis and the resulting eigenvalue problem is solved numerically by means of a shooting method. It is demonstrated that the most unstable modes on the instability threshold are those with infinite wavelength and an analytical expression for such conditions is derived from an asymptotic analysis. Results show that the non-Newtonian character of the fluid has a destabilizing role

Dissipation instability of Couette-like adiabatic flows in a plane channel

The mixed convection flow in a plane channel with adiabatic boundaries is examined. The boundaries have an externally prescribed relative velocity defining a Couette-like setup for the flow. A stationary flow regime is maintained with a constant velocity difference between the boundaries, considered as thermally insulated. The effect of viscous dissipation induces a heat source in the flow domain and, hence, a temperature gradient. The nonuniform temperature distribution causes, in turn, a buoyancy force and a combined forced and free flow regime. Dual mixed convection flows occur for a given velocity difference. Their structure is analysed where, in general, only one branch of the dual flows is compatible with the Oberbeck-Boussinesq approximation, for realistic values of the Gebhart number. A linear stability analysis of the basic stationary flows with viscous dissipation is carried out. The stability eigenvalue problem is solved numerically, leading to the determination of the neutral stability curves and the critical values of the P\'eclet number, for different Gebhart numbers. An analytical asymptotic solution in the special case of perturbations with infinite wavelength is also developed.Comment: 24 pages, 11 figure

Unstable Convection in a Vertical Double–Layer Porous Slab

A convective stability analysis of the flow in a vertical fluid-saturated porous slab made of two layers with different thermophysical properties is presented. The external boundaries are isothermal with one of them impermeable while the other is open to an external fluid reservoir. This study is a development of previous investigations on the onset of thermal instability in a vertical heterogeneous porous slab where the heterogeneity may be either continuous or piecewise as determined by a multilayer structure. The aim of this paper is investigating whether a two-layer structure of the porous slab may lead to the onset of cellular convection patterns. The linear stability analysis is carried out under the assumption that one porous layer has a thermal conductivity much higher than the other layer. This assumption may be justified for the model of a heat transfer enhancement system involving a saturated metal foam. A flow model for the natural convection based on Darcy’s momentum transfer in a porous medium is adopted. The buoyancy-induced basic flow state is evaluated analytically. Small-amplitude two-dimensional perturbations of the basic state are introduced, thus leading to a linear set of governing equations for the disturbances. A normal mode analysis allows one to formulate the stability eigenvalue problem. The numerical solution of the stability eigenvalue problem provides the onset conditions for the thermal instability. Moreover, the results evidence that the permeability ratio of the two layers is a key parameter for the critical conditions of the instability