Showing papers by "Instituto Superior Técnico published in 1994"

The fractional Fourier transform and time-frequency representations

Abstract: The functional Fourier transform (FRFT), which is a generalization of the classical Fourier transform, was introduced a number of years ago in the mathematics literature but appears to have remained largely unknown to the signal processing community, to which it may, however, be potentially useful. The FRFT depends on a parameter /spl alpha/ and can be interpreted as a rotation by an angle /spl alpha/ in the time-frequency plane. An FRFT with /spl alpha/=/spl pi//2 corresponds to the classical Fourier transform, and an FRFT with /spl alpha/=0 corresponds to the identity operator. On the other hand, the angles of successively performed FRFTs simply add up, as do the angles of successive rotations. The FRFT of a signal can also be interpreted as a decomposition of the signal in terms of chirps. The authors briefly introduce the FRFT and a number of its properties and then present some new results: the interpretation as a rotation in the time-frequency plane, and the FRFT's relationships with time-frequency representations such as the Wigner distribution, the ambiguity function, the short-time Fourier transform and the spectrogram. These relationships have a very simple and natural form and support the FRFT's interpretation as a rotation operator. Examples of FRFTs of some simple signals are given. An example of the application of the FRFT is also given. >

Stress and strain in liquid and solid foams

Abstract: Both liquid and solid foams, together with analogous cellular materials, have distinctive mechanical or rheological properties which find many applications. This review concentrates on the search for a basic understanding of the underlying mechanisms, in terms of specific structural models. Computer simulations play an increasing role and are beginning to be applied to three-dimensional models.

An axiomatic definition of holonomy

Abstract: A group of loops is associated to every smooth pointed manifold M using a strong homotopy relation. It is shown that the holonomy of a connection on a principal G-bundle may be presented as a group morphism and that every such morphism satisfying a natural smoothness condition is the holonomy of some unique connection up to isomorphism.

Kinetic studies of the urease-catalyzed hydrolysis of urea in a buffer-free system

Abstract: The kinetics of urea hydrolysis catalyzed by urease, mainly in the absence of buffers by use of the self-buffer effect of the products, was investigated. The effect of pH, temperature, and concentration of enzyme, substrate, product, salt ions, and buffers on the kinetic behavior of urease was examined. A kinetic model of a modified Michaelis-Menten form, incorporating substrate and product inhibition, pH dependence, and temperature effect, was developed to describe the reaction rate. Experimental data indicated that urease in a buffer-free solution was less susceptible to the inhibition of substrate product. The Michaelis constant keeps almost constant with the variation of pH and temperature, and increases with the addition of buffers and salts. The data also suggested that the noncompetitive pattern of the product inhibition, which is not significantly affected by temperature, increases gently with increasing pH. A Monod form rate expression was proposed to analyze the pH effect on the maximum rate. The proposed kinetic model was also examined by the long-time experiments in which pH, substrate, and product concentration varied obviously during the reaction course.

Production planning and scheduling using a fuzzy decision system

Abstract: In this paper short-range planning and scheduling problems are addressed using a nonclassical approach supported by fuzzy theory. The proposed methodology uses a hierarchical structure which includes three decision levels (higher, middle, lower), each responsible for a different production problem with a different time scale. The methodology approaches the tasks associated with each level using a heuristic formulation and solves the short-range planning and scheduling problems with a nonstationary policy. The higher decision level determines safety stock levels used to compensate for future resource failures. At the middle level, loading rates are computed. This is accomplished through a fuzzy controller that tends to minimize the error between the cumulative production and the cumulative demand while keeping the work in process below acceptable values. Finally, the lower level controls the flow of parts among the resources, using a modified version of the Yager's fuzzy decision method. This method has the ability to use several criteria to generate a decision. Simulation results reveal that the proposed system exhibits good performance, in terms of a high production percentage and a low WIP, under resource failures and demand variations. >

Smart sensor interface with A/D conversion and programmable calibration

Abstract: In this paper we present a new architecture for a smart sensor interface. It is based on an oversampled A/D converter associated with a small ROM containing calibration coefficients. The nonlinear function desired is obtained by piecewise linear interpolation between the values stored in the ROM, without any additional circuits. This solution has the advantage of high programming flexibility, long-term stability, and low area consumption. Moreover, it is suitable for co-integration with sensors because of its minimum analog content. A prototype was integrated in a CMOS 1.2-/spl mu/m technology. Simulation and experimental results are reported together with a detailed theoretical analysis and some design guidelines. >

Completely integrable bi-Hamiltonian systems

Abstract: We study the geometry of completely integrable bi-Hamiltonian systems and, in particular, the existence of a bi-Hamiltonian structure for a completely integrable Hamiltonian system. We show that under some natural hypothesis, such a structure exists in a neighborhood of an invariant torus if, and only if, the graph of the Hamiltonian function is a hypersurface of translation, relative to the affine structure determined by the action variables. This generalizes a result of Brouzet for dimension four.

Molecular cloning and sequence analysis of the gene of the molybdenum-containing aldehyde oxido-reductase of Desulfovibrio gigas. The deduced amino acid sequence shows similarity to xanthine dehydrogenase.

Abstract: In this report, we describe the isolation of a 4020-bp genomic PstI fragment of Desulfovibrio gigas harboring the aldehyde oxido-reductase gene. The aldehyde oxido-reductase gene spans 2718 bp of genomic DNA and codes for a protein with 906 residues. The protein sequence shows an average 52% (+/- 1.5%) similarity to xanthine dehydrogenase from different organisms. The codon usage of the aldehyde oxidoreductase is almost identical to a calculated codon usage of the Desulfovibrio bacteria.

Black holes of a general two-dimensional dilaton gravity theory.

Abstract: A general dilaton gravity theory in 1+1 spacetime dimensions, with a cosmological constant $\ensuremath{\lambda}$ and a new dimensionless parameter $\ensuremath{\omega}$, contains as special cases the constant curvature theory of Teitelboim and Jackiw, the theory equivalent to vacuum planar general relativity, the first-order string theory, and a two-dimensional purely geometrical theory. The equations of this general two-dimensional theory admit several different black holes with various types of singularities. The singularities can be spacelike, timelike, or null, and there are even cases without singularities. Evaluation of the ADM mass, as a charge density integral, is possible in some situations by carefully subtracting the black hole solution from the corresponding linear dilation at infinity.

Influence of impingement edge geometry on cavity flow oscillations

Abstract: and back portion of the airfoil is shown in Figs. 2b and 2c, respectively. The smooth transition of the grid from one type to another and the decency of the grid at sharp corners and between close surfaces are clearly shown, which indicate excellent compatibility and communication between the two grid strategies as controlled by a common background grid. The grid was generated using a Silicon Graphics IRIS 4D/210 VOX workstation. The code's performance is about 360 triangles/s on that workstation and 2350 triangles/s on a Cray Y-MP. The total turnaround time required for generating a grid around a complex geometry, including the setup time, is about 1 h. To examine the fidelity of the grids generated with the present method, a turbulent viscous-flow computation was performed on the grid shown in Fig. 2. The flow solution was obtained with an available node-based, upwind flow solver8 using the BaldwinEarth turbulence model at a Mach number of 0.2, an angle of attack of 16.2 deg, and a Reynolds number of 9 X 106. Comparisons for surface pressure distributions and velocity profiles are shown in Figs. 3a and 3b, respectively. Excellent agreement with experimental data confirms the viability of the generated grid. Concluding Remarks A new method of unstructured viscous grid generation has been introduced. The approach is conceptually simple but powerful, and capable of producing high-quality viscous unstructured grids for complex configurations with ease. Being based on a totally unstructured grid strategy, the method is fully automatic and flexible and, thus, alleviates the difficulties stemming from the structural limitations of the existing structured or semi-unstruct ured techniques. Because of an efficient grid-marching strategy and a simple front-detection algorithm, the method is highly efficient and operational on small workstations. The method is also self-sufficient for insertion of grid points in the boundary layer and beyond. The method has been applied to two-dimensional problems with satisfactory results. The basic elements of the technique, however, have been primarily designed for its subsequent extension for generating three-dimensional highly stretched tetrahedral grids which is currently in progress. The full benefit of the method will be realized for generation of three-dimensi onal viscous grids where the complexity of the problem becomes excessive.

Design, construction and hydrodynamic testing of the AUV MARIUS

Abstract: MARIUS (Marine Utility Vehicle System) is an autonomous underwater vehicle for environmental surveying in coastal waters. This paper describes the design, construction and hydrodynamic testing of the vehicle and analyzes its expected performance in terms of mission duration and range.

The In-vivo Activation of Saccharomyces-cerevisiae Plasma-membrane H+-atpase By Ethanol Depends On the Expression of the Pma1 Gene, But Not of the Pma2 Gene

Abstract: The expression of the PMA1 and PMA2 genes during Saccharomyces cerevisiae growth in medium with glucose plus increasing concentrations of ethanol was monitored by using PMA1-lacZ and PMA2-lacZ fusions and Northern blot hybridizations of total RNA probed with PMA1 gene. The presence of sub-lethal concentrations of ethanol enhanced the expression of PMA2 whereas it reduced the expression of PMA1. The inhibition of PMA1 expression by ethanol corresponded to a decrease in the content of plasma membrane ATPase as quantified by immunoassays. Although an apparent correspondence could exist between the increase of plasma membrane ATPase activity and the level of PMA2 expression, the maximal level of PMA2 expression remained about 200 times lower than PMA1. On the other hand, ethanol activated the plasmamembrane H+-ATPase activity from a strain expressing only the PMA1 ATPase but did not activate that from a strain expressing only the PMA2 ATPase. These results provide evidence that in the presence of ethanol it is the PMA1 ATPase which is activated, probably by a post-translational mechanism and that the PMA2 ATPase is not involved.

Continuous lipolysis in a reversed micellar membrane bioreactor

Abstract: The enzymatic hydrolysis of olive oil using Chromobacterium viscosum lipase B encapsulated in reversed micelles of AOT in isooctane was carried out in a continous reversed micellar membrane bioreactor. A tubular ceramic membrane installed in an ultrafiltration module was used to retain the lipase and separate the products from the reaction media. Water filled micelles were supplemented to the reactor together with the substrate/solvent solution to compensate for the permeation of reversed micelles. The influence of substrate concentration, residence time and water content in the productivity and conversion of the system were investigated. A linear relationship between productivity and conversion degree was found for each substrate concentration tested. Operational stability of the bioreactor was tested in a long term operation confirming the high stability of this catalytic system.

Numerical modelling of suspended sediment transport in tidal estuaries: A comparison between the Tagus (Portugal) and the Scheldt (Belgium-the Netherlands)

Abstract: A numerical model to simulate the transport of suspended sediment in tidal estuaries is presented. The model is applied to the two large European estuaries the Tagus (Portugal) and the Scheldt (Belgium-The Netherlands). Calculated suspended sediment concentrations compare favourably with observations in the Tagus (r=0.84) and in the Scheldt (r=0.73). The parametrization scheme indicates that the bottom content of fine sediment is correlated with depth in the Tagus; but a different relationship is used in the Scheldt. Because of tidal range differences, average suspended sediment concentrations are lower in the Tagus (80 mg l−1) than in the Scheldt (130 mg l−1), but a larger relative variation between spring and neap tide concentrations may occur in the Tagus.