Jamming transition in a two-dimensional open granular pile with rolling resistance

We present a molecular dynamics study of the jamming/unjamming transition in two-dimensional granular piles with open boundaries. The grains are modeled by viscoelastic forces, Coulomb friction and resistance to rolling. Two models for the rolling resistance interaction were assessed: one considers a constant rolling friction coefficient, and the other one a strain dependent coefficient. The piles are grown on a finite size substrate and subsequently discharged through an orifice opened at the center of the substrate. Varying the orifice width and taking the final height of the pile after the discharge as the order parameter, one can devise a transition from a jammed regime (when the grain flux is always clogged by an arch) to a catastrophic regime, in which the pile is completely destroyed by an avalanche as large as the system size. A finite size analysis shows that there is a finite orifice width associated with the threshold for the unjamming transition, no matter the model used for the microscopic interactions. As expected, the value of this threshold width increases when rolling resistance is considered, and it depends on the model used for the rolling friction.Comment: 9 pages, 6 figure

Analysis of the velocity field of granular hopper flow

We report the analysis of radial characteristics of the flow of granular material through a conical hopper. The discharge is simulated for various orifice sizes and hopper opening angles. Velocity profiles are measured along two radial lines from the hopper cone vertex: along the main axis of the cone and along its wall. An approximate power law dependence on the distance from the orifice is observed for both profiles, although differences between them can be noted. In order to quantify these differences, we propose a Local Mass Flow index that is a promising tool in the direction of a more reliable classification of the flow regimes in hoppers

Spin-glass phase transition and behavior of nonlinear susceptibility in the Sherrington-Kirkpatrick model with random fields

The behavior of the nonlinear susceptibility $\chi_3$ and its relation to the spin-glass transition temperature $T_f$, in the presence of random fields, are investigated. To accomplish this task, the Sherrington-Kirkpatrick model is studied through the replica formalism, within a one-step replica-symmetry-breaking procedure. In addition, the dependence of the Almeida-Thouless eigenvalue $\lambda_{\rm AT}$ (replicon) on the random fields is analyzed. Particularly, in absence of random fields, the temperature $T_f$ can be traced by a divergence in the spin-glass susceptibility $\chi_{\rm SG}$, which presents a term inversely proportional to the replicon $\lambda_{\rm AT}$. As a result of a relation between $\chi_{\rm SG}$ and $\chi_3$, the latter also presents a divergence at $T_f$, which comes as a direct consequence of $\lambda_{\rm AT}=0$ at $T_f$. However, our results show that, in the presence of random fields, $\chi_3$ presents a rounded maximum at a temperature $T^{*}$, which does not coincide with the spin-glass transition temperature $T_f$ (i.e., $T^* > T_f$ for a given applied random field). Thus, the maximum value of $\chi_3$ at $T^*$ reflects the effects of the random fields in the paramagnetic phase, instead of the non-trivial ergodicity breaking associated with the spin-glass phase transition. It is also shown that $\chi_3$ still maintains a dependence on the replicon $\lambda_{\rm AT}$, although in a more complicated way, as compared with the case without random fields. These results are discussed in view of recent observations in the LiHo$_x$Y$_{1-x}$F$_4$ compound.Comment: accepted for publication in PR

Application of non-parametric geostatistical methods to the identification of aggregate deposits on the continental shelf

More than 1 400 samples were used in the assessment of the sand and gravel potential of the continental shelf north of Espinho (northern Portugal). This study deals primarily with the determination of the probabilities of the occurrence of some textural (gravel, sand, silt and clay contents) and chemical (carbonate content) parameters and of some textural groups, according to Nickless' (1973) classification. Sediment samples were collected between 1986 and 1989, during several cruises sponsored by the Portuguese Hydrographic Institute. For the purpose of the present study, a non-parametric geostatistical methodology was used on the whole set of samples, encompassing the following steps: a) a preliminary variography analysis based on the raw data, to determine structural features, such as trends, anisotropies and nested models; b) binary codification of the initial data, in ‘less than' or ‘equal to' specific cut-offs determined with economic criteria; c) calculation and modelling of variograms of this indicator; and d) application of indicator kriging to obtain probability (risk) maps. The results obtained by the application of the methodology to the two data groups (sediment texture and Nickless classification) were compared in order to characterise the aggregate deposits which could be economically exploited in the near future. This study should help decision-makers to define an exploitation strategy, to be implemented in the near future. However, this preliminary approach requires further studies, namely by using cross-variography procedures and conditional probabilities.Más de 1 400 muestras sedimentarias fueron utilizadas en la evaluación del potencial en gravas y arenas que se pueden explotar para áridos naturales y de trituración para la construcción y obras públicas. El presente estudio se atiene a la determinación de la probabilidad de ocurrencia de algunos parámetros texturales (contenido en gravas, en arena y en fango) y químicos (contenido en carbonatos) y de determinados grupos texturales, según la clasificación de Nickless (1973). Las muestras fueron recogidas entre 1986 y 1989 en el transcurso de diversas campañas oceanográficas realizadas por el Instituto Hidrográfico de la Marina Portuguesa. Se ha realizado un estudio de geoestadística no paramétrica incluyendo las siguientes etapas: a) variografía preliminar de los datos de base para identificar características estructurales (por ejemplo, tendencias, anisotropías y estructuras imbricadas); b) se codificaron los datos iniciales en formato binario, según valores inferiores a o iguales a un patamar específico determinado por un criterio económico; c) se calcularon y modelaron variogramas de esta indicatriz; y d) la indicatriz fue krijeada para obtener mapas de probabilidad o riesgo. La referida metodología fue aplicada sobre los dos tipos de datos (granulometría sedimentaria y clasificación de Nickless). Se compararon los resultados obtenidos para caracterizar los depósitos de gravas y arenas que pueden ser explotados en un futuro próximo. La presente aplicación pretende ayudar a los organismos decisorios a definir una futura estrategia de explotación. Sin embargo, éste es solamente un estudio preliminar. Se necesitan estudios más profundos, utilizando, por ejemplo, procedimientos de variografía cruzada y probabilidades condicionales.Instituto Español de Oceanografí

Quantum critical point in the spin glass-antiferromagnetism competition in Kondo-lattice systems

A theory is proposed to describe the competition among antiferromagnetism (AF), spin glass (SG) and Kondo effect. The model describes two Kondo sublattices with an intrasite Kondo interaction strength $J_{K}$ and an interlattice quantum Ising interaction in the presence of a transverse field $\Gamma$. The interlattice coupling is a random Gaussian distributed variable (with average $-2J_0/N$ and variance $32 J^{2}/N$) while the $\Gamma$ field is introduced as a quantum mechanism to produce spin flipping. The path integral formalism is used to study this fermionic problem where the spin operators are represented by bilinear combinations of Grassmann fields. The disorder is treated within the framework of the replica trick. The free energy and the order parameters of the problem are obtained by using the static ansatz and by choosing both $J_0/J$ and $\Gamma/J \approx (J_k/J)^2$ to allow, as previously, a better comparison with the experimental findings. The results indicate the presence of a SG solution at low $J_K/J$ and for temperature $T<T_{f}$ ($T_{f}$ is the freezing temperature). When $J_K/J$ is increased, a mixed phase AF+SG appears, then an AF solution and finally a Kondo state is obtained for high values of $J_{K}/J$. Moreover, the behaviors of the freezing and Neel temperatures are also affected by the relationship between $J_{K}$ and the transverse field $\Gamma$. The first one presents a slight decrease while the second one decreases towards a Quantum Critical Point (QCP). The obtained phase diagram has the same sequence as the experimental one for $Ce_{2}Au_{1-x}Co_{x}Si_{3}$, if $J_{K}$ is assumed to increase with $x$, and in addition, it also shows a qualitative agreement concerning the behavior of the freezing and the Neel temperatures.Comment: 11 pages, 3 figures, accepted for publication in J. Phys.

Busca e análise de notícias agrícolas sobre cana-de-açúcar.

Pretende-se fazer uma análise textual através do uso de mineração de texto, pelo método descrito por Moura et al. (2009), com o intuito de obter tópicos específicos, numa base de textos que consiste em notícias agrícolas sobre cana-de-açúcar. A base de textos é buscada por meio de um robô de busca em fontes previamente escolhidas (Jornal Cana: http://www.jornalcana.com.br). Após extração das notícias, é feito o tratamento dos textos e obtidas taxonomias de tópicos, isto é, são identificados assuntos e categorias cobertos pelo texto. Com os resultados da análise, pretende-se posteriormente localizar no espaço e no tempo os tópicos extraídos, para possibilitar uma inferência de fatores sócio-econômicos que possam influenciar na produção e safra da cana-de-açúcar

Utilização de infravermelho próximo para determinação das propriedades da madeira.

Organizado por Patricia Póvoa de Mattos, Celso Garcia Auer, Rejane Stumpf Sberze, Katia Regina Pichelli e Paulo César Botosso