Granular Pressure and the Thickness of a Layer Jamming on a Rough Incline

Dense granular media have a compaction between the random loose and random close packings. For these dense media the concept of a granular pressure depending on compaction is not unanimously accepted because they are often in a "frozen" state which prevents them to explore all their possible microstates, a necessary condition for defining a pressure and a compressibility unambiguously. While periodic tapping or cyclic fluidization have already being used for that exploration, we here suggest that a succession of flowing states with velocities slowly decreasing down to zero can also be used for that purpose. And we propose to deduce the pressure in \emph{dense and flowing} granular media from experiments measuring the thickness of the granular layer that remains on a rough incline just after the flow has stopped.Comment: 10 pages, 2 figure

Rheophysics of dense granular materials : Discrete simulation of plane shear flows

We study the steady plane shear flow of a dense assembly of frictional, inelastic disks using discrete simulation and prescribing the pressure and the shear rate. We show that, in the limit of rigid grains, the shear state is determined by a single dimensionless number, called inertial number I, which describes the ratio of inertial to pressure forces. Small values of I correspond to the quasi-static regime of soil mechanics, while large values of I correspond to the collisional regime of the kinetic theory. Those shear states are homogeneous, and become intermittent in the quasi-static regime. When I increases in the intermediate regime, we measure an approximately linear decrease of the solid fraction from the maximum packing value, and an approximately linear increase of the effective friction coefficient from the static internal friction value. From those dilatancy and friction laws, we deduce the constitutive law for dense granular flows, with a plastic Coulomb term and a viscous Bagnold term. We also show that the relative velocity fluctuations follow a scaling law as a function of I. The mechanical characteristics of the grains (restitution, friction and elasticity) have a very small influence in this intermediate regime. Then, we explain how the friction law is related to the angular distribution of contact forces, and why the local frictional forces have a small contribution to the macroscopic friction. At the end, as an example of heterogeneous stress distribution, we describe the shear localization when gravity is added.Comment: 24 pages, 19 figure

Eficiência relativa na avaliação de instituições de pesquisa agropecuária.

Apresenta-se um modelo que serve ao proposito de avaliacao das relacoes de producao em instituicoes de pesquisa agropecuaria. A nocao basica que permite avaliacoes comparativas entre unidades produtoras de pesquisa e a de medida radial de eficiencia relativa de producao. Esta medida de eficiencia, aplicada neste estudo aos centros de pesquisa da EMBRAPA, e geral e pode ser estendida a outros contextos. A abordagem que se utiliza na definicao do conceito de eficiencia e a do estudo de fronteiras economicas via envoltoria de dados - modelo DEA.Na publicação: Eliseu Alves

Produtividade e eficiência relativa de produção em sistemas de produção de pesquisa agropecuária.

Coleção Eliseu Alves: Biblioteca da Embrapa Sede. Na publicação: Eliseu Alves