OALib Journal期刊

ISSN: 2333-9721




2015 ( 5 )

2014 ( 19 )

2013 ( 26 )

2012 ( 63 )


匹配条件: “Barra” ,找到相关结果约542条。
Control of Doubly Fed Induction Generator Supplied via Matrix Converter for Wind Energy Conversion System  [PDF]
Zakaria Kara, Kamel Barra
International Journal of Modern Nonlinear Theory and Application (IJMNTA) , 2014, DOI: 10.4236/ijmnta.2014.33012

The present paper deals with the modeling and control of Wind Energy Conversion System WECS based Doubly Fed Induction Generator DFIG using the slip energy recovery principle. The proposed drive system uses a Matrix Converter (MC) to transfer the slip energy of the rotor into the mains instead of using cascaded ac-dc-ac converter whilst the stator side is fixed to the grid. Operation at both sub-synchronous and super-synchronous regions is possible with the proposed drive system. The different level control strategies for maximum power point tracking and active-reactive power are discussed. Simulation results of the proposed doubly fed induction generator drive system show the good performance of the control system strategy for both transient and steadystate conditions.

The Use of Anti-Platelet and/or Anticoagulant Agents in the Prevention of Large Vessel Vasculitis-Associated Ischemic Complications: A Meta-Analysis  [PDF]
James Jeong, Lillian Barra
Open Journal of Rheumatology and Autoimmune Diseases (OJRA) , 2014, DOI: 10.4236/ojra.2014.42017
Abstract: Objective: To determine the effectiveness of antiplatelet and/or anticoagulant therapy (AP/AC) at reducing ischemic events in patients with Large Vessel Vasculitis (LVV). Methods: We performed a random effects meta-analysis of studies examining antiplatelet and/or anticoagulant therapy (AP/AC) and ischemic events in Takayasu’s Arteritis (TAK) or Giant Cell Arteritis (GCA). Severe ischemic events were defined as stroke, ischemic ocular manifestations and claudication symptoms. Any ischemic event included jaw claudication in addition to the above manifestations. Results: Seven studies met inclusion criteria: 1 TAK and 6 GCA. The majority of patients (>80%) were treated with ASA and treatment was initiated prior to diagnosis of LVV. Risk of severe and any ischemic event in patients with LVV treated with AP/AC versus no treatment was not significantly different (OR 0.570, 95% CI 0.243, 1.340 and OR 0.594, 95% CI 0.248, 1.421, respectively). For studies with follow-up data (26-76 months), AP/AC was protective for severe ischemic events (OR 0.18, 95% CI 0.04, 0.83). Findings were similar when excluding studies that did not account for potential confounders, such as cardiovascular risk factors. Conclusion: At follow-up, antiplatelet therapy significantly decreases ischemic events in patients with LVV. However, in most cases of GCA, the treatment was initiated prior to the diagnosis of vasculitis. The benefit of initiating anti-platelet therapy at the time of GCA diagnosis remains unclear.
Bienestar psicológico y orientación de rol sexual en estudiantes universitarios
Enrique Barra
Terapia Psicológica , 2010,
Abstract: Este estudio examinó la relación entre el bienestar psicológico, las dimensiones de masculinidad y feminidad y las categorías de orientación de rol sexual (masculino, femenino, andrógino, indiferenciado). Los participantes fueron 346 estudiantes de ambos sexos de la Universidad de Concepción (Chile) con edades entre 18 y 31 a os, quienes respondieron instrumentos referentes a bienestar psicológico y dimensiones de rol sexual. Los resultados mostraron que el bienestar psicológico presentaba mayor relación con la dimensión de masculinidad que con la de feminidad, y que los participantes categorizados como indiferenciados presentaron un nivel significativamente menor de bienestar psicológico que el resto de las orientaciones de rol sexual, siendo los individuos andróginos los que informaron el mayor nivel de bienestar. Se proponen diversos factores para explicar los resultados obtenidos.
Fluctuations induce transitions in frustrated sparse networks
Adriano Barra
Physics , 2009,
Abstract: We analyze, by means of statistical mechanics, a sparse network with random competitive interactions among dichotomic variables pasted on the nodes, namely a Viana-Bray model. The model is described by an infinite series of order parameters (the multi-overlaps) and has two tunable degrees of freedom: the noise level and the connectivity (the averaged number of links). We show that there are no multiple transition lines, one for every order parameter, as a naive approach would suggest, but just one corresponding to ergodicity breaking. We explain this scenario within a novel and simple mathematical technique via a driving mechanism such that, as the first order parameter (the two replica overlap) becomes different from zero due to a real second order phase transition (with properly associated diverging rescaled fluctuations), it enforces all the other multi-overlaps toward positive values thanks to the strong correlations which develop among themselves and the two replica overlap at the critical line.
The mean field Ising model trough interpolating techniques
Adriano Barra
Physics , 2007, DOI: 10.1007/s10955-008-9567-2
Abstract: Aim of this work is not trying to explore a macroscopic behavior of some recent model in statistical mechanics but showing how some recent techniques developed within the framework of spin glasses do work on simpler model, focusing on the method and not on the analyzed system. To fulfil our will the candidate model turns out to be the paradigmatic mean field Ising model. The model is introduced and investigated with the interpolation techniques. We show the existence of the thermodynamic limit, bounds for the free energy density, the explicit expression for the free energy with its suitable expansion via the order parameter, the self-consistency relation, the phase transition, the critical behavior and the self-averaging properties. At the end a bridge to a Parisi-like theory is tried and discussed.
Irreducible free energy expansion and overlaps locking in mean field spin glasses
Adriano Barra
Physics , 2006, DOI: 10.1007/s10955-005-9006-6
Abstract: We introduce a diagrammatic formulation for a cavity field expansion around the critical temperature. This approach allows us to obtain a theory for the overlap's fluctuations and, in particular, the linear part of the Ghirlanda-Guerra relationships (GG) (often called Aizenman-Contucci polynomials (AC)) in a very simple way. We show moreover how these constraints are "superimposed" by the symmetry of the model with respect to the restriction required by thermodynamic stability. Within this framework it is possible to expand the free energy in terms of these irreducible overlaps fluctuations and in a form that simply put in evidence how the complexity of the solution is related to the complexity of the entropy.
The thermodynamic cost of driving quantum systems by their boundaries
Felipe Barra
Physics , 2015, DOI: 10.1038/srep14873
Abstract: The laws of thermodynamics put limits to the efficiencies of thermal machines. Analogues of these laws are now established for quantum engines weakly and passively coupled to the environment providing a framework to find improvements to their performance. Systems whose interaction with the environment is actively controlled do not fall in that framework. Here we consider systems actively and locally coupled to the environment, evolving with a so-called boundary-driven Lindblad equation. Starting from a unitary description of the system plus the environment we simultaneously obtain the Lindblad equation and the appropriate expressions for heat, work and entropy-production of the system extending the framework for the analysis of new, and some already proposed, quantum heat engines. We illustrate our findings in spin 1/2 chains and explain why an XX chain coupled in this way to a single heat bath relaxes to thermodynamic-equilibrium while and XY chain does not. Additionally, we show that an XX chain coupled to a left and a right heat baths behaves as a quantum engine, a heater or refrigerator depending on the parameters, with efficiencies bounded by Carnot efficiencies.
A Geometrical Way to Sum Powers by Means of Tetrahedrons and Eulerian Numbers
Mario Barra
Mathematics , 2011,
Abstract: We geometrically prove that in a d-dimensional cube with edges of length n, the number of particular d-dimensional tetrahedrons are given by Eulerian numbers. These tetrahedrons tassellate the cube, In this way the sum of the cubes are the sums of the tetrahedrons, whose calculation is trivial.
Towards an Intelligent Predictive Model for Analyzing Spatio-Temporal Satellite Image Based on Hidden Markov Chain  [PDF]
Houcine Essid, Imed Riadh Farah, Vincent Barra
Advances in Remote Sensing (ARS) , 2013, DOI: 10.4236/ars.2013.23027

Nowadays remote sensing is an important technique for observing Earth surface applied to different areas such as, land use, urban planning, remote monitoring, real time deformation of the soil that can be associated with earthquakes or landslides, the variations in thickness of the glaciers, the measurement of volume changes in the case of volcanic eruptions, deforestation, etc. To follow the evolution of these phenomena and to predict their future states, many approaches have been proposed. However, these approaches do not respond completely to the specialists who process yet more commonly the data extracted from the images in their studies to predict the future. In this paper, we propose an innovative methodology based on hidden Markov models (HMM). Our approach exploits temporal series of satellite images in order to predict spatio-temporal phenomena. It uses HMM for representing and making prediction concerning any objects in a satellite image. The first step builds a set of feature vectors gathering the available information. The next step uses a Baum-Welch learning algorithm on these vectors for detecting state changes. Finally, the system interprets these changes to make predictions. The performance of our approach is evaluated by tests of space-time interpretation of events conducted over two study sites, using different time series of SPOT images and application to the change in vegetation with LANDSAT images.

Visualizadores de imagens médicas gratuitos: é possível trabalhar apenas com eles?
Barra, Filipe Ramos;Barra, Renato Ramos;Barra Sobrinho, Alaor;
Radiologia Brasileira , 2010, DOI: 10.1590/S0100-39842010000500010
Abstract: objective: to search in the internet for freeware medical image viewers capable of running as a pacs (picture archiving and communication system) client, and to evaluate their main functions as well as the feasibility of their use in personal computers. materials and methods: the google search engine and specialized sites were utilized in the search for freeware softwares for windows. the authors have found about 70 and among them 11 were able to run as pacs clients. six were selected for analysis: clearcanvas workstation, kpacs, onis, synedra view personal, mito and tudor dicomviewer. sixteen functions selected according to the authors' needs were evaluated. results: among the six applications, two presented 10 of the 16 functions, and one of them presented only two. three perform mpr (multiplanar reconstruction), one performs mip (maximum intensity projection), two perform vr (volume rendering), two can run as a pacs server, two can create cds, one performs images fusion, three allow the use of multiple monitors and only one is not compatible with windows 7. conclusion: although several freeware applications are available, no one of them is complete. it is up to the users to analyze and select the software that best suits their needs. however, onis, synedra and clearcanvas stand out because of their own peculiarities. the use of freeware image viewers is entirely feasible in the radiologists' daily routine.

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