Non-Singular Charged Black Hole Solution for Non-Linear Source

A non-singular exact black hole solution in General Relativity is presented. The source is a non-linear electromagnetic field, which reduces to the Maxwell theory for weak field. The solution corresponds to a charged black hole with |q| \leq 2s_c m \approx 0.6 m, having metric, curvature invariants, and electric field bounded everywhere.Comment: 3 pages, RevTe

Atividade ovariana pós-parto em ovelhas Santa Inês sob diferentes manejos de amamentação na região semiárida do Nordeste.

Objetivou-se investigar o efeito de diferentes manejos de amamentação sobre a atividade ovariana pós-parto em ovelhas da raça Santa Inês por meio das concentrações circulantes de progesterona. Foram utilizadas 18 fêmeas com idade entre 22 e 72 meses. Após o parto, os cordeiros ficaram com as ovelhas até o 14º diae, a partir do 15º dia foram distribuídas em dois grupos: amamentação contínua (n=9) e controlada (n=9). As coletas de sangue para a dosagem de progesterona aconteceram a cada sete dias a partir do parto até o primeiro estro e, em seguida, a cada quatro dias até o segundo estro. Houve atividade ovulatória silenciosa antes do primeiro estro pós-parto em 85,70% (5/9) e 25% (3/9) das fêmeas em amamentação contínua e controlada, respectivamente. As ovelhas com amamentação contínua apresentaram o estro mais tardiamente (37,71±3,34 dias) do que as com mamada controlada (26,12±2,4 dias). As concentrações plasmáticas de progesterona entre o parto e o primeiro estro foram 2,81±1,58ng/mL vs 1,45±0,59ng/mL e ao estro 0,25±0,34ng/mL vs 0,32±0,18ng/mL para amamentação contínua e controlada, respectivamente. Os níveis médios de progesterona entre o primeiro e o segundo estro pós-parto foram 1,53±0,54 ng/mL e 2,50±1,09ng/mL, para amamentação contínua e controlada, respectivamente, estes estros ocorreram em 77,7% vs 100% e 66.6% vs 100% das fêmeas, na mesma ordem. Conclui-se que a amamentação tem influência sobre a atividade Ovarian activity in Santa Inês sheep under different types of suckling in semi-arid area of Brazil ovariana pós-parto em ovelhas Santa Inês e ciclos curtos e silenciosos são necessários para uma atividade cíclica normal no pós-parto. Ovarian activity in Santa Inês sheep under different types of suckling in semi-arid area of Brazil. SUMMARY - Aiming to investigate the suckling effect on ovarian activity in postpartum Santa Inês ewes through progesterone levels, it was used 18 females aging between 22 and 72 months. After parturition, the lambs stayed with the dams until the 14th day and from the 15th day they were divided in two treatments: continuous and controlled suckling. The blood collection for progesterone happened each seven days from parturition until the first estrous, and, thereafter, each 4 days until the second estrous. There was a silence ovulatory activity before the first postpartum estrous in 85.70% and 25% of the animals submitted to continuos and controlled suckling, while the occurrence of first estrous in the first group was delayed (37.71±3.34 days) in relation to controlled suckling group (26.12±2.4 days). The progesterone average levels between the parturition and the first estrous postpartum were 2.81±1.58ng/mL versus 1.45±0.59ng/mL, and on estrous were 0.25±0,34ng/mL versus 0,32±0,18ng/mL, for females under continuos and controlled suckling, respectively. The progesterone average levels between the first and the second estrous postpartum were 1.53±0.54ng/mL and 2.50±1.09ng/mL, for females under continuos and controlled suckling, espectively, happened these estrous in 77.7% versus 100% and 66.6% versus 100% for females, in the same order. It is concluded that suckling stimuli has influence on the postpartum ovarian activity in Santa Inês ewes and short and silent cycles are necessaries to normal postpartum ovarian cyclic

Regular Black Hole in General Relativity Coupled to Nonlinear Electrodynamics

The first regular exact black hole solution in General Relativity is presented. The source is a nonlinear electrodynamic field satisfying the weak energy condition, which in the limit of weak field becomes the Maxwell field. The solution corresponds to a charged black hole with |q| \leq 2 s_c m \approx 0.6 m, having the metric, the curvature invariants, and the electric field regular everywhere.Comment: 5 pages, RevTex, 6 figure

Vector lattice model for stresses in granular materials

A vector lattice model for stresses in granular materials is proposed. A two dimensional pile built by pouring from a point is constructed numerically according to this model. Remarkably, the pile violates the Mohr Coulomb stability criterion for granular matter, probably because of the inherent anisotropy of such poured piles. The numerical results are also compared to the earlier continuum FPA model and the (scalar) lattice $q$-model

Models of stress fluctuations in granular media

We investigate in detail two models describing how stresses propagate and fluctuate in granular media. The first one is a scalar model where only the vertical component of the stress tensor is considered. In the continuum limit, this model is equivalent to a diffusion equation (where the r\^ole of time is played by the vertical coordinate) plus a randomly varying convection term. We calculate the response and correlation function of this model, and discuss several properties, in particular related to the stress distribution function. We then turn to the tensorial model, where the basic starting point is a wave equation which, in the absence of disorder, leads to a ray-like propagation of stress. In the presence of disorder, the rays acquire a diffusive width and the angle of propagation is shifted. A striking feature is that the response function becomes negative, which suggests that the contact network is mechanically unstable to very weak perturbations. The stress correlation function reveals characteristic features related to the ray-like propagation, which are absent in the scalar description. Our analytical calculations are confirmed and extended by a numerical analysis of the stochastic wave equation.Comment: 32 pages, latex, 18 figures and 6 diagram

Response of a Hexagonal Granular Packing under a Localized External Force: Exact Results

We study the response of a two-dimensional hexagonal packing of massless, rigid, frictionless spherical grains due to a vertically downward point force on a single grain at the top layer. We use a statistical approach, where each mechanically stable configuration of contact forces is equally likely. We show that this problem is equivalent to a correlated $q$-model. We find that the response is double-peaked, where the two peaks, sharp and single-grain diameter wide, lie on the two downward lattice directions emanating from the point of the application of the external force. For systems of finite size, the magnitude of these peaks decreases towards the bottom of the packing, while progressively a broader, central maximum appears between the peaks. The response behaviour displays a remarkable scaling behaviour with system size $N$: while the response in the bulk of the packing scales as $\frac{1}{N}$, on the boundary it is independent of $N$, so that in the thermodynamic limit only the peaks on the lattice directions persist. This qualitative behaviour is extremely robust, as demonstrated by our simulation results with different boundary conditions. We have obtained expressions of the response and higher correlations for any system size in terms of integers corresponding to an underlying discrete structure.Comment: Accepted for publication in JStat; 33 pages, 10 figures; Section 2.2 reorganized and rewritten; Details about the simulation procedure added in Sec.3.1. ; A new section, summarizing the final results and the calculation procedure adde

Stresses in silos: Comparison between theoretical models and new experiments

We present precise and reproducible mean pressure measurements at the bottom of a cylindrical granular column. If a constant overload is added, the pressure is linear in overload and nonmonotonic in the column height. The results are {\em quantitatively} consistent with a local, linear relation between stress components, as was recently proposed by some of us. They contradict the simplest classical (Janssen) approximation, and may pose a rather severe test of competing models.Comment: 4 pages, 2 figures, final version to appear in Phys. Rev. Let

Force Distribution in a Granular Medium

We report on systematic measurements of the distribution of normal forces exerted by granular material under uniaxial compression onto the interior surfaces of a confining vessel. Our experiments on three-dimensional, random packings of monodisperse glass beads show that this distribution is nearly uniform for forces below the mean force and decays exponentially for forces greater than the mean. The shape of the distribution and the value of the exponential decay constant are unaffected by changes in the system preparation history or in the boundary conditions. An empirical functional form for the distribution is proposed that provides an excellent fit over the whole force range measured and is also consistent with recent computer simulation data.Comment: 6 pages. For more information, see http://mrsec.uchicago.edu/granula

From fire suppression to fire management:Advances and resistances to changes in fire policy in the savannas of Brazil and Venezuela

International audienc

Force distributions in 3D granular assemblies: Effects of packing order and inter-particle friction

We present a systematic investigation of the distribution of normal forces at the boundaries of static packings of spheres. A new method for the efficient construction of large hexagonal-close-packed crystals is introduced and used to study the effect of spatial ordering on the distribution of forces. Under uniaxial compression we find that the form for the probability distribution of normal forces between particles does not depend strongly on crystallinity or inter-particle friction. In all cases the distribution decays exponentially at large forces and shows a plateau or possibly a small peak near the average force but does not tend to zero at small forces.Comment: 9 pages including 8 figure