Showing papers by "Instituto Superior Técnico published in 2002"
TL;DR: In this paper, the authors consider the evolution of a universe evolving from a phase dominated by nonrelativistic matter to a cosmological constant via an intermediate period where the effective equation of state is given by $p=\ensuremath{\alpha{-}A/{\ensemath{\rho}}^{\ensemblem{\alpha}},$ where A is a positive constant and $0l √ √ 1/ √ l 1/1.
Abstract: We consider the scenario emerging from the dynamics of a generalized Born-Infeld theory. The equation of state describing this system is given in terms of the energy density $\ensuremath{\rho}$ and pressure p by the relationship $p=\ensuremath{-}A/{\ensuremath{\rho}}^{\ensuremath{\alpha}},$ where A is a positive constant and $0l\ensuremath{\alpha}l~1.$ We discuss the conditions under which homogeneity arises and show that this equation of state describes the evolution of a universe evolving from a phase dominated by nonrelativistic matter to a phase dominated by a cosmological constant via an intermediate period where the effective equation of state is given by $p=\ensuremath{\alpha}\ensuremath{\rho}.$
1,885 citations
TL;DR: In this paper, the progress in wave energy conversion in Europe during the past ten years is reviewed and current activities and initiatives in the wave energy sector at National and Union level are described.
Abstract: The progress in wave energy conversion in Europe during the past ten years is reviewed and current activities and initiatives in the wave energy sector at National and Union level are described. Other important activities worldwide are summarized. The technical and econ
908 citations
10 Dec 2002
TL;DR: Results on both synthetic and real data show the ability of the K-means method to identify arbitrary shaped clusters in multidimensional data.
Abstract: We explore the idea of evidence accumulation for combining the results of multiple clusterings. Initially, n d-dimensional data is decomposed into a large number of compact clusters; the K-means algorithm performs this decomposition, with several clusterings obtained by N random initializations of the K-means. Taking the co-occurrences of pairs of patterns in the same cluster as votes for their association, the data partitions are mapped into a co-association matrix of patterns. This n/spl times/n matrix represents a new similarity measure between patterns. The final clusters are obtained by applying a MST-based clustering algorithm on this matrix. Results on both synthetic and real data show the ability of the method to identify arbitrary shaped clusters in multidimensional data.
534 citations
TL;DR: While the density and solid-liquid phase transition properties are similar for both series, the new RTILs present a considerably lower viscosity and an increased ability to dissolve HgCl(2) and LaCl(3) (up to 16 times higher).
Abstract: A new series [CnOm mim][X] of imidazolium cation-based room temperature ionic liquids (RTILs), with ether and alcohol functional groups on the alkyl side-chain has been prepared. Some physical properties of these RTILs were measured, namely solubility in common solvents, viscosity and density. The solubility of LiCl, HgCl2 and LaCl3 in room temperature ionic liquids was also determined. The features of the solid–liquid phase transition were analysed, namely the glass transition temperature and the heat capacity jump associated with the transition from the non-equilibrium glass to the metastable supercooled liquid. These properties were compared with those reported for the 1-n-alkyl-3-methylimidazolium [Cn mim][X] series. While the density and solid–liquid phase transition properties are similar for both series, the new RTILs present a considerably lower viscosity and an increased ability to dissolve HgCl2 and LaCl3 (up to 16 times higher).
504 citations
TL;DR: The Alpha Magnetic Spectrometer (AMS) was flown on the space shuttle Discovery during flight STS-91 (June 1998) in a 51.7° orbit at altitudes between 320 and 390 km as discussed by the authors.
423 citations
10 Dec 2002
TL;DR: This paper presents an evaluation of the QoS enhancements to the IEEE 802.11e standard, currently under specification, both the enhanced distributed coordination function (EDCF) and hybrid coordinationfunction (HCF) modes of medium access control (MAC) operation are analysed and compared with legacy distributed coordination functions (DCF) and point coordination function(PCF).
Abstract: This paper presents an evaluation of the QoS enhancements to the IEEE 802.11 standard, named IEEE 802.11e, currently under specification. Both the enhanced distributed coordination function (EDCF) and hybrid coordination function (HCF) modes of medium access control (MAC) operation are analysed and compared with legacy distributed coordination function (DCF) and point coordination function (PCF). Performance evaluation is attained through computer simulation of a scenario of 802.11 b/e access to an IP core network through an access point (AP) in an infrastructure WLAN.
398 citations
TL;DR: A hierarchical computational procedure for optimizing material distribution as well as the local material properties of mechanical elements, leading to single scale microstructures, which may be restricted in various ways, based on design and manufacturing criteria.
Abstract: This paper describes a hierarchical computational procedure for optimizing material distribution as well as the local material properties of mechanical elements. The local properties are designed using a topology design approach, leading to single scale microstructures, which may be restricted in various ways, based on design and manufacturing criteria. Implementation issues are also discussed and computational results illustrate the nature of the procedure.
367 citations
TL;DR: The aim of this paper is to define energy indicators used in the assessment of energy systems which meet the sustainability criterion, and takes into consideration energy resources, environment capacity, social indicators and economic indicators.
358 citations
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01 Jan 2002TL;DR: In this paper, Lyapunov stability theory of differential equations and nonuniform hyperbolic theory of local manifold theory have been studied in the context of smooth hyperbolc measures.
Abstract: Introduction Lyapunov stability theory of differential equations Elements of nonuniform hyperbolic theory Examples of nonuniformly hyperbolic systems Local manifold theory Ergodic properties of smooth hyperbolc measures Bibliography Index
346 citations
TL;DR: In this paper, the notion of connection on a Lie algebroid has been extended in order to study singular geometric structures, namely, the singular geometric structure of singular geometric objects.
302 citations
TL;DR: It is concluded that shortest-widest paths can neither be computed with a generalized Dijkstra's algorithm nor can packets be routed hop-by-hop over those paths.
Abstract: Prompted by the advent of quality-of-service routing in the Internet, we investigate the properties that path weight functions must have so that hop-by-hop routing is possible and optimal paths can be computed with a generalization of E.W. Dijkstra's algorithm (see Numer. Math., vol.1, p.269-71, 1959). We define an algebra of weights which contains a binary operation, for the composition of link weights into path weights, and an order relation. Isotonicity is the key property of the algebra. It states that the order relation between the weights of any two paths is preserved if both of them are either prefixed or appended by a common, third, path. We show that isotonicity is both necessary and sufficient for a generalized Dijkstra's algorithm to yield optimal paths. Likewise, isotonicity is also both necessary and sufficient for hop-by-hop routing. However, without strict isotonicity, hop-by-hop routing based on optimal paths may produce routing loops. They are prevented if every node computes what we call lexicographic-optimal paths. These paths can be computed with an enhanced Dijkstra's algorithm that has the same complexity as the standard one. Our findings are extended to multipath routing as well. As special cases of the general approach, we conclude that shortest-widest paths can neither be computed with a generalized Dijkstra's algorithm nor can packets be routed hop-by-hop over those paths. In addition, loop-free hop-by-hop routing over widest and widest-shortest paths requires each node to compute lexicographic-optimal paths, in general.
TL;DR: In this article, the growth rate for the purely transverse Weibel instability is determined from relativistic kinetic theory using a waterbag distribution function in the momenta perpendicular to the main propagation direction of the beam.
Abstract: The growth rate for the purely transverse Weibel instability is determined from relativistic kinetic theory using a waterbag distribution function in the momenta perpendicular to the main propagation direction of the beam. A parametric study is presented for conditions relevant to the fast ignitor. It is shown that for expected parameters the purely transverse Weibel instability will be significantly suppressed or even eliminated due to the transverse energy spread or emittance.
TL;DR: It is shown in the Joint European Torus that NTMs can be readily destabilized by long-period sawteeth, such as obtained by sawtooth stabilization from ion-cyclotron heating or current drive, which has important implications for burning plasma scenarios, as alpha particles strongly stabilize the saweeth.
Abstract: The onset of a neoclassical tearing mode (NTM) depends on the existence of a large enough seed island. It is shown in the Joint European Torus that NTMs can be readily destabilized by long-period sawteeth, such as obtained by sawtooth stabilization from ion-cyclotron heating or current drive. This has important implications for burning plasma scenarios, as alpha particles strongly stabilize the sawteeth. It is also shown that, by adding heating and current drive just outside the inversion radius, sawteeth are destabilized, resulting in shorter sawtooth periods and larger beta values being obtained without NTMs.
TL;DR: In this article, the characteristics and applications of optical glass ceramics are reviewed, with particular emphasis on rare-earth-doped transparent glass-ceramics for photonics, including search for new transparent glass ceramic compositions and the development of suitable methods to process such materials into functional devices.
TL;DR: In this paper, a detailed analytic and numerical study of baryogenesis through leptogenesis is performed in the framework of the standard model of electroweak interactions extended by the addition of three right-handed neutrinos, leading to the seesaw mechanism.
TL;DR: In this article, an extensive review of the application of pressure-driven membrane separation processes in the treatment of seafood processing effluents and recovery of proteins therein is presented, and the results show that UF reduces the organic load from the fish meal wastewaters and allows the recovery of valuable raw materials comprising proteins.
TL;DR: A specific multicriteria socio-technical approach to facilitating bid evaluation processes is presented and several issues that warrant its use are discussed, including practical aspects of structuring criteria and creating a computer-based additive value model in direct interaction with Evaluation Committees responsible for bid evaluation.
Abstract: A speci2c multicriteria socio-technical approach to facilitating bid evaluation processes is presented and several issues that warrant its use are discussed. Some real-world interventions in international public call for tenders illustrate practical aspects of structuring criteria and creating a computer-based additive value model in direct interaction with Evaluation Committees responsible for bid evaluation, supported by the MACBETH approach. ? 2002 Elsevier Science Ltd. All rights reserved.
TL;DR: In this paper, a stochastic model was applied to devise an optimal algorithm for the rotational speed control of an oscillating-water-column (OWC) wave power plant equipped with a Wells turbine and to evaluate the average power output of the plant.
TL;DR: The peculiarities of molybdenum chemistry are illustrated here and how they have become part of certain enzymes in life and on chemical superiority in carrying out the essential tasks.
TL;DR: The large reduction of the effective binding constant from pH 2 to 7 suggests that electrostatic interactions are a major contribution to the binding of TSPP to the aforementioned proteins.
TL;DR: In this paper, a stochastic method has been developed to evaluate the average performance of an oscillating water column wave energy device equipped with an assumedly linear Wells turbine, represented by a set of sea states, characterized by their power spectra, the free-surface elevation being a Gaussian random variable in each sea state.
TL;DR: In this article, the performance of hardfaced material was evaluated by microstructure, chemical analysis and microhardness measurements, and results showed that for operating conditions that do not require elevated temperature, laser hardfacing shows the best results, due to the refine solidification structure produced.
TL;DR: In this paper, strong discontinuities embedded in finite elements are used to model discrete cracking in quasi-brittle materials and a numerical algorithm is introduced to enforce the continuity of the crack path.
TL;DR: In this article, the average bubble size in a dual turbine stirred tank is investigated and compared with data from the literature, obtained under many different conditions, including different types and numbers of stirrers, different media and measuring systems.
TL;DR: The study of the folding of free Cp and Ind, combined with the (eta(5/3)-Cp')-M bond analysis, shows that the observed difference is not the result of an intrinsic characteristic of the indenyl ligand, such as the traditionally accepted aromaticity gain in the benzene ring formed in eta(3)-Ind complexes.
Abstract: The eta(5)-to-eta(3) coordination shift of cyclopentadienyl (Cp=C(5)H(5)(-)) and indenyl (Ind=C(9)H(7)(-)) ligands in molybdenocene complexes, [(eta(5)-Cp')(eta(5)-Cp)Mo(CO)(2)](2+) (Cp'=Cp or Ind), driven by a two-electron reduction of those species, was studied and compared by means of molecular orbital calculations (B3LYP HF/DFT hybrid functional, DZP basis sets). The results obtained, in terms of optimized geometries, relative energies, and bond analysis parameters, compare well with the experimental data, and verify the well-known indenyl effect, that is, a significantly more facile eta(5)-to-eta(3) rearrangement for the indenyl ligand when compared to cyclopentadienyl. However, the study of the folding of free Cp and Ind, combined with the (eta(5/3)-Cp')-M bond analysis, shows that the observed difference is not the result of an intrinsic characteristic of the indenyl ligand, such as the traditionally accepted aromaticity gain in the benzene ring formed in eta(3)-Ind complexes. Instead, it is directly related to the Cp'-M bond strength. While the difference in the energy required to fold the two free ligands is negligible (< or =1 kcal mol(-1) for folding angles up to 20 degrees), the (eta(5)-Cp)-M bond is stronger than that of (eta(5)-Ind)-M; however, the opposite situation is found for the eta(3) coordination mode. The net result, for Cp'=Ind, is a destabilization of the eta(5) complexes and a stabilization of the eta(3) intermediates or transition states yielding smaller activation energies and faster reaction rates for processes in which that is the rate-determining step.
TL;DR: In this paper, a higher-dimensional version of multifractal analysis for several classes of hyperbolic dynamical systems is presented, where the level sets of pointwise dimensions, local entropies and Lyapunov exponents can be simultaneously approximated by the entropy of ergodic measures.
TL;DR: In this paper, the authors used a ladder-type spacers with inter-filament distances/channel height ratios (L f ) of 1.9, 3.8, and 5.7 to visualize the flow in a nanofiltration/reverse osmosis (RO) spiral wound module.
TL;DR: This review describes the use of the naturally occurring polysaccharide carrageenan as a support for the immobilisation of biocatalysts.
Abstract: Immobilisation of both enzymes and whole-cell systems is of major importance in the improvement of the stability, activity and reusability of these biocatalysts. This review describes the use of the naturally occurring polysaccharide carrageenan as a support for the immobilisation of biocatalysts. Carrageenan is a food-grade and biocompatible support material extracted from red seaweeds. Before focusing on the use of carrageenan as an immobilisation support, an overview is given of the present uses of biocatalysts in industrial processes. The basic concepts of enzyme and whole-cell immobilisation are discussed, as well as the background of carrageenan as a biopolymer. Several examples of enzymes and whole-cell systems immobilised in carrageenan are discussed. A list of the most relevant patents in this field is presented as well as a list of enzymes and cell systems immobilised in carrageenan as described in the literature.
10 Dec 2002
TL;DR: In this article, a nonlinear adaptive controller is proposed that steers an AUV so as to track a sequence of points consisting of desired positions (x, y) in an inertial reference frame, followed by vehicle positioning at the final target point.
Abstract: This paper addresses the problem of dynamic positioning and way-point tracking of underactuated autonomous underwater vehicles (AUVs) in the presence of constant unknown ocean currents and parametric model uncertainty. A nonlinear adaptive controller is proposed that steers an AUV so as to track a sequence of points consisting of desired positions (x, y) in an inertial reference frame, followed by vehicle positioning at the final target point. The controller is first derived at the kinematic level assuming that the ocean current disturbance is known. An exponential observer is then designed and convergence of the resulting closed loop system trajectories is analyzed. Finally, integrator backstepping and Lyapunov based techniques are used to extend the kinematic controller to the dynamic case and to deal with model parameter uncertainty. Simulation results are presented and discussed.
TL;DR: In this paper, the authors constructed concrete supersymmetric grand unified theories based on the Pati-Salam gauge group SU(4)c×SU(2)L×SU (2)R which naturally lead to a moderate violation of theasymptotic Yukawa unification and thus can allow an acceptable b-quark mass even with universal boundary conditions.