Suppression of viscous fluid fingering: a piecewise constant-injection process

The injection of a fluid into another of larger viscosity in a Hele-Shaw cell usually results in the formation of highly branched patterns. Despite the richness of these structures, in many practical situations such convoluted shapes are quite undesirable. In this letter we propose an efficient and easily reproducible way to restrain these instabilities based on a simple piecewise constant pumping protocol. It results in a reduction in the size of the viscous fingers by one order of magnitude.Comment: Published in Phys. Rev.

Exact Renormalization of Massless QED2

We perform the exact renormalization of two-dimensional massless gauge theories. Using these exact results we discuss the cluster property and confinement in both the anomalous and chiral Schwinger models.Comment: 14 pages, no figures, introduction and conclusions modifie

The relationship between ENSO and Paraná River flow

International audienceSeveral studies indicate that there is a relationship between the climatic variability in the South American continent and alterations of the position and intensity of the heat sources in the equatorial region. The El Niño phenomenon can influence the precipitation over some regions of South America such as the Brazilian Northeast, Amazonia, South of Brazil and Uruguay. Over 80% of Brazil's energy comes from hydropower, and decisions concerning future availability and pricing require forecasts of river flow, ideally several months in advance. In this work the relationship between the Paraná River flow and the ENSO (El Niño/Southern Oscillation) mode is investigated and statistical forecasts of river flow are tested. An evaluation of the relationship between the Pacific sea surface temperature and the Paraná River flow indicates an ENSO pattern over the equatorial Pacific. The time series of the ENSO mode obtained by applying principal components analysis on the sea surface temperature (SST) were used as predictors for the Paraná River flow forecast. Improvement in the model forecast skill is also obtained by considering the lagged river flow time series as a predictor

CVD of CrO2: towards a lower temperature deposition process

We report on the synthesis of highly oriented a-axis CrO2 films onto (0001) sapphire by atmospheric pressure CVD from CrO3 precursor, at growth temperatures down to 330 degree Celsius, i.e. close to 70 degrees lower than in published data for the same chemical system. The films keep the high quality magnetic behaviour as those deposited at higher temperature, which can be looked as a promising result in view of their use with thermally sensitive materials, e.g. narrow band gap semiconductors.Comment: 13 pages, 4 figure

Near surface mounted CFRP laminates for shear strengthening of concrete beams

The authors wish to acknowledge the supports provided by the S&P, BeTTor MBT Portugal, Secil (Unibetão), and the colaboration of Cemacom.A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminate strips into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets; and two beams strengthened with NSM precured laminate strips of CFRP, one of them with laminates positioned at 90º and the other with laminates positioned at 45º in relation to the beam axis. Influences of the laminate strip inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminate strips at 45º was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. This technique was also faster and easier to apply than the externally bonded one. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.The study reported in this paper forms a part of the research program “CUTINSHEAR - Performance assessment of an innovative structural FRP strengthening technique using an integrated system based on optical fiber sensors” supported by FCT, POCTI/ECM/59033/2004

NSM CFRP laminates for the shear strengthening of T section RC beams

A new strengthening technique for concrete structures failing in shear was developed based on introducing laminate strips of carbon fiber reinforced polymer (CFRP) into slits made on the concrete cover of the lateral faces of the beams to be strengthened. This strengthening technique is designated Near Surface Mounted (NSM) and the laminate strips are fixed to concrete by epoxy adhesive. Exploratory tests with rectangular cross-section RC beams, conducted by the authors of the present work have shown that, in comparison to the externally bonded CFRP (EBR) technique, the NSM is easier and faster to apply, can mobilize higher percentage of the CFRP tensile strength, is more efficacy in terms of increasing the beams load carrying capacity, and assures higher resistance to vandalism acts. To assess the efficiency of NSM as a shear strengthening technique for real-case-situations, series of T cross section RC beams of 2.45 m span length were tested (Fig. 1). The influence of the percentage and inclination of the CFRP laminates on the increase of the beam shear resistance is analyzed. In the present work, this experimental program is described and the main results are presented and analysed. Fig. 2 represents the relationship between the shear resistance provided by the CFRP arrangements and the CFRP percentage for the three shear strengthening configurations analyzed. The obtained results indicate that: •Independently of the CFRP percentage and inclination of the laminates, NSM technique provided a significant contribution for the shear resistance of T section RC beams; •The beams with the highest CFRP percentage assured a maximum load 97%, 99% and 93% of the maximum load of 6S-R reference beam for the CFRP configurations of laminates at 90º, 60º and 45º, respectively (Fig. 3). The highest CFRP percentages had been designed to provide a maximum load similar to the one of 6S-R reference beam, with a reinforcing system composed of six steel stirrups; •After shear crack formation, the load carrying capacity of the beams shear strengthened with the highest CFRP percentage was significantly larger than the load carrying capacity of the 6S-R reference beam. This means that these CFRP shear strengthening arrangements contributed significantly for the increase of the beam stiffness after shear crack formation.The authors of the present work wish to acknowledge the support provided by the “Empreiteiros Casais”, S&P, degussa Portugal, and Secil (Unibetão, Braga). The study reported in this paper forms a part of the research program “CUTINSHEAR - Performance assessment of an innovative structural FRP strengthening technique using an integrated system based on optical fiber sensors” supported by FCT, POCTI/ECM/59033/2004

Reforço ao corte de vigas T de betão armado usando a técnica NSM com laminados de CFRP

Relatório técnico 05-DEC/E-2