Short-Time Critical Dynamics of Damage Spreading in the Two-Dimensional Ising Model

The short-time critical dynamics of propagation of damage in the Ising ferromagnet in two dimensions is studied by means of Monte Carlo simulations. Starting with equilibrium configurations at $T= \infty$ and magnetization $M=0$, an initial damage is created by flipping a small amount of spins in one of the two replicas studied. In this way, the initial damage is proportional to the initial magnetization $M_0$ in one of the configurations upon quenching the system at $T_C$, the Onsager critical temperature of the ferromagnetic-paramagnetic transition. It is found that, at short times, the damage increases with an exponent $\theta_D=1.915(3)$, which is much larger than the exponent $\theta=0.197$ characteristic of the initial increase of the magnetization $M(t)$. Also, an epidemic study was performed. It is found that the average distance from the origin of the epidemic ($\langle R^2(t)\rangle$) grows with an exponent $z^* \approx \eta \approx 1.9$, which is the same, within error bars, as the exponent $\theta_D$. However, the survival probability of the epidemics reaches a plateau so that $\delta=0$. On the other hand, by quenching the system to lower temperatures one observes the critical spreading of the damage at $T_{D}\simeq 0.51 T_C$, where all the measured observables exhibit power laws with exponents $\theta_D = 1.026(3)$, $\delta = 0.133(1)$, and $z^*=1.74(3)$.Comment: 11 pages, 9 figures (included). Phys. Rev. E (2010), in press

Adição de metionina protegida da degrabilidade ruminal em rações para cordeiros alimentados com dois níveis de ptoteína não degradável no rúmen: degrabilidade ruminal da matéria seca1.

bitstream/CPATC/19776/1/f_14_2007.pd

Numerical simulation of partially premixed combustion using a flame surface density approach

Partially premixed combustion is characterized by a variable equivalence ratio of the mixture in space and time, and where there are both lean and rich mixture zones. Thus the reaction evolves along with a turbulent mixture process, which modifies the composition of reactants and products. In this situation a so-called triple flame could be encountered, in which a rich and a lean premixed flame front as well as a diffusion flame are present. The diffusion flame develops behind the premixed flame front due to turbulent mixing in the hot combustion products. This kind of combustion could be found in Direct Injection Spark Ignition (DISI) engines when they are operated in the stratified charge mode. The model considered in this work assumes a simplified one-step irreversible chemical reaction in which fuel and oxidant react together in stoichiometric proportions giving products with the composition corresponding to a complete combustion. A transport equation is solved for the oxidant and fuel, from which the amount of products and the combustion progress are computed, while the turbulence is modeled with RANS (Reynolds-Average Navier-Stokes). The reaction rate is assumed in the model as proportional to the product of the Flame Surface Density (FSD) by the local laminar flame speed. Aside from the state and composition of the mixture, the local laminar flame speed is afected by the turbulent mixing process. This mixing process is taken into account by means of the classical β-PDF (Probability Density Function), which is a function of the mixture fraction and its variance. A transport equation is solved for both, the mixture fraction and its variance, and the FSD is computed through a transport equation where several models are available for the source terms. The model is implemented in the open-source toolkit OpenFOAM®. Computational results are obtained for partially premixed combustions inside constant-volume vessels with several initial configurations, which are compared with numerical results available in the literature.Publicado en: Mecánica Computacional vol. XXXV, no. 16.Facultad de Ingenierí

Numerical simulation of partially premixed combustion using a flame surface density approach

Partially premixed combustion is characterized by a variable equivalence ratio of the mixture in space and time, and where there are both lean and rich mixture zones. Thus the reaction evolves along with a turbulent mixture process, which modifies the composition of reactants and products. In this situation a so-called triple flame could be encountered, in which a rich and a lean premixed flame front as well as a diffusion flame are present. The diffusion flame develops behind the premixed flame front due to turbulent mixing in the hot combustion products. This kind of combustion could be found in Direct Injection Spark Ignition (DISI) engines when they are operated in the stratified charge mode. The model considered in this work assumes a simplified one-step irreversible chemical reaction in which fuel and oxidant react together in stoichiometric proportions giving products with the composition corresponding to a complete combustion. A transport equation is solved for the oxidant and fuel, from which the amount of products and the combustion progress are computed, while the turbulence is modeled with RANS (Reynolds-Average Navier-Stokes). The reaction rate is assumed in the model as proportional to the product of the Flame Surface Density (FSD) by the local laminar flame speed. Aside from the state and composition of the mixture, the local laminar flame speed is afected by the turbulent mixing process. This mixing process is taken into account by means of the classical β-PDF (Probability Density Function), which is a function of the mixture fraction and its variance. A transport equation is solved for both, the mixture fraction and its variance, and the FSD is computed through a transport equation where several models are available for the source terms. The model is implemented in the open-source toolkit OpenFOAM®. Computational results are obtained for partially premixed combustions inside constant-volume vessels with several initial configurations, which are compared with numerical results available in the literature.Publicado en: Mecánica Computacional vol. XXXV, no. 16.Facultad de Ingenierí

Numerical simulation of partially premixed combustion using a flame surface density approach

Partially premixed combustion is characterized by a variable equivalence ratio of the mixture in space and time, and where there are both lean and rich mixture zones. Thus the reaction evolves along with a turbulent mixture process, which modifies the composition of reactants and products. In this situation a so-called triple flame could be encountered, in which a rich and a lean premixed flame front as well as a diffusion flame are present. The diffusion flame develops behind the premixed flame front due to turbulent mixing in the hot combustion products. This kind of combustion could be found in Direct Injection Spark Ignition (DISI) engines when they are operated in the stratified charge mode. The model considered in this work assumes a simplified one-step irreversible chemical reaction in which fuel and oxidant react together in stoichiometric proportions giving products with the composition corresponding to a complete combustion. A transport equation is solved for the oxidant and fuel, from which the amount of products and the combustion progress are computed, while the turbulence is modeled with RANS (Reynolds-Average Navier-Stokes). The reaction rate is assumed in the model as proportional to the product of the Flame Surface Density (FSD) by the local laminar flame speed. Aside from the state and composition of the mixture, the local laminar flame speed is afected by the turbulent mixing process. This mixing process is taken into account by means of the classical β-PDF (Probability Density Function), which is a function of the mixture fraction and its variance. A transport equation is solved for both, the mixture fraction and its variance, and the FSD is computed through a transport equation where several models are available for the source terms. The model is implemented in the open-source toolkit OpenFOAM®. Computational results are obtained for partially premixed combustions inside constant-volume vessels with several initial configurations, which are compared with numerical results available in the literature.Publicado en: Mecánica Computacional vol. XXXV, no. 16.Facultad de Ingenierí

Clustering of Intermediate Luminosity X-ray selected AGN at z~3

We present the first clustering results of X-ray selected AGN at z~3. Using Chandra X-ray imaging and UVR optical colors from MUSYC photometry in the ECDF-S field, we selected a sample of 58 z~3 AGN candidates. From the optical data we also selected 1385 LBG at 2.8<z< 3.8 with R<25.5. We performed auto-correlation and cross-correlation analyses, and here we present results for the clustering amplitudes and dark matter halo masses of each sample. For the LBG we find a correlation length of r_0,LBG = 6.7 +/- 0.5 Mpc, implying a bias value of 3.5 +/- 0.3 and dark matter (DM) halo masses of log(Mmin/Msun) = 11.8 +/- 0.1. The AGN-LBG cross-correlation yields r_0,AGN-LBG = 8.7 +/- 1.9 Mpc, implying for AGN at 2.8<z<3.8 a bias value of 5.5 +/- 2.0 and DM halo masses of log(Mmin/Msun) = 12.6 +0.5/-0.8. Evolution of dark matter halos in the Lambda CDM cosmology implies that today these z~3 AGN are found in high mass galaxies with a typical luminosity of 7+4/-2 L*.Comment: Accepted for publication in ApJ Letters. 4 pages, 4 figures (1 in color

Theory and computation of directional nematic phase ordering

A computational study of morphological instabilities of a two-dimensional nematic front under directional growth was performed using a Landau-de Gennes type quadrupolar tensor order parameter model for the first-order isotropic/nematic transition of 5CB (pentyl-cyanobiphenyl). A previously derived energy balance, taking anisotropy into account, was utilized to account for latent heat and an imposed morphological gradient in the time-dependent model. Simulations were performed using an initially homeotropic isotropic/nematic interface. Thermal instabilities in both the linear and non-linear regimes were observed and compared to past experimental and theoretical observations. A sharp-interface model for the study of linear morphological instabilities, taking into account additional complexity resulting from liquid crystalline order, was derived. Results from the sharp-interface model were compared to those from full two-dimensional simulation identifying the specific limitations of simplified sharp-interface models for this liquid crystal system. In the nonlinear regime, secondary instabilities were observed to result in the formation of defects, interfacial heterogeneities, and bulk texture dynamics.Comment: first revisio

Casimir force between integrable and chaotic pistons

We have computed numerically the Casimir force between two identical pistons inside a very long cylinder, considering different shapes for the pistons. The pistons can be considered as quantum billiards, whose spectrum determines the vacuum force. The smooth part of the spectrum fixes the force at short distances, and depends only on geometric quantities like the area or perimeter of the piston. However, correcting terms to the force, coming from the oscillating part of the spectrum which is related to the classical dynamics of the billiard, are qualitatively different for classically integrable or chaotic systems. We have performed a detailed numerical analysis of the corresponding Casimir force for pistons with regular and chaotic classical dynamics. For a family of stadium billiards, we have found that the correcting part of the Casimir force presents a sudden change in the transition from regular to chaotic geometries.Comment: 13 pages, 10 figure

A numerical study of the development of bulk scale-free structures upon growth of self-affine aggregates

During the last decade, self-affine geometrical properties of many growing aggregates, originated in a wide variety of processes, have been well characterized. However, little progress has been achieved in the search of a unified description of the underlying dynamics. Extensive numerical evidence has been given showing that the bulk of aggregates formed upon ballistic aggregation and random deposition with surface relaxation processes can be broken down into a set of infinite scale invariant structures called "trees". These two types of aggregates have been selected because it has been established that they belong to different universality classes: those of Kardar-Parisi-Zhang and Edward-Wilkinson, respectively. Exponents describing the spatial and temporal scale invariance of the trees can be related to the classical exponents describing the self-affine nature of the growing interface. Furthermore, those exponents allows us to distinguish either the compact or non-compact nature of the growing trees. Therefore, the measurement of the statistic of the process of growing trees may become a useful experimental technique for the evaluation of the self-affine properties of some aggregates.Comment: 19 pages, 5 figures, accepted for publication in Phys.Rev.

Ischaemic Stroke in a Child of a Vegan Mother

O acidente vascular cerebral é uma doença rara na infância (2,7 por 100 000 crianças por ano). A hiperhomocisteinemia é um fator de risco independente para a aterosclerose prematura, sendo habitualmente secundário a mutações na enzima metiltetrahidrofolato redutase ou défice de vitamina B12, na idade pediátrica. Descreve-se o caso clínico de um lactente de 10 meses, observado no serviço de urgência por queda da própria altura, com diminuição dos movimentos espontâneos do hemicorpo esquerdo e com postura flexora do membro superior esquerdo. A tomografia computorizada realizada na admissão mostrava imagens de hipodensidade nos corpos estriados bilaterais, e a ressonância magnética lesões isquémicas nas áreas das artérias lentículo-estriadas externas. Analiticamente apresentava aumento da homocisteína sérica (25,2 µmol/L) e défice de vitamina B12 (< 150 pg/mL). A mãe apresentava igualmente défice de vitamina B12, associado a dieta vegana. Foi feita suplementação com vitamina B12, iniciada a diversificação alimentar com introdução da carne, verificando-se normalização dos níveis de homocisteína sérica. Após seis meses de cumprimento do plano terapêutico de medicina física e de reabilitação não eram objetiváveis assimetrias sequelares. A dieta vegana pode associar-se a défices nutricionais que obrigam a uma vigilância nutricional por um profissional experiente, nos casos em que esta seja a opção materna durante a gravidez, sobretudo se prolongada no período de aleitamento materno, e/ou na criança ou adolescente.info:eu-repo/semantics/publishedVersio