Showing papers by "Polytechnic University of Catalonia published in 1996"

A finite point method in computational mechanics. applications to convective transport and fluid flow

Abstract: The paper presents a fully meshless procedure fo solving partial differential equations. The approach termed generically the ‘finite point method’ is based on a weighted least square interpolation of point data and point collocation for evaluating the approximation integrals. Some examples showing the accuracy of the method for solution of adjoint and non-self adjoint equations typical of convective-diffusive transport and also to the analysis of compressible fluid mechanics problem are presented.

Numerical formulation for a simulator (CODE_BRIGHT) for the coupled analysis of saline media

Abstract: Presents numerical aspects of the program CODE_BRIGHT, which is a simulator for COupled DEformation, BRIne, Gas and Heat transport problems. It solves the equations of mass and energy balance and stress equilibrium and, originally, it was developed for saline media. The governing equations also include a set of constitutive laws and equilibrium conditions. The main peculiarities of saline media are in the dissolution/precipitation phenomena, presence of brine inclusions in the solid salt and creep deformation of the solid matrix.

Modelling strong discontinuities in solid mechanics via strain softening constitutive equations. part 1: fundamentals

Abstract: The paper addresses some fundamental aspects about the use of standard constitutive equations to model strong discontinuities (cracks, shear bands, slip lines, etc.) in solid mechanics analyzes. The strong discontinuity analysis is introduced as a basic tool to derive a general framework, in which different families of constitutive equations can be considered, that allows to extract some outstanding aspects of the intended analysis. In particular, a link between continuum and discrete approaches to the strain localization phenomena is obtained. Applications to standard continuum damage and elastoplastic constitutive equations are presented. Relevant aspects to be considered in the numerical simulation of the problem (tackled in Part 2 of the work) are also presented.

Word sense disambiguation using Conceptual Density

Abstract: This paper present a method for the resolution of lexical ambiguity of nouns and its automatic evaluation over the Brown Corpus The method relies on the use of the wide-coverage noun taxonomy of WordNet and the notion of conceptual distance among concepts, captured by a Conceptual Density formula developed for this purpose This fully automatic method requires no hand coding of lexical entries, hand tagging of text nor any kind of training process The results of the experiments have been automatically evaluted against SemCor, the sense-tagged version of the Brown Corpus

Fractal multiband antenna based on the Sierpinski gasket

Abstract: Experimental and computed results show a multiband behaviour over five bands for the new fractal Sierpinski antenna. Such a behaviour is based on the self-similarity properties of the antenna's fractal shape, which might open an alternative way for designing new multiband and frequency independent antennas.

Modelling Strong Discontinuities in Solid Mechanics via Strain Softening Constitutive Equations. Part 2: Numerical Simulation

Abstract: On the basis of the strong discontinuity analysis of standard local stress–strain constitutive equations, a finite element framework for the simulation of strong discontinuities, which belongs to the family of assumed enhanced strain methods, is presented. Taking the standard linear triangle as the underlying element, an additional incompatible mode leads to the formulation of an enriched strain field which is shown to be able to appropriately capture strong discontinuities. The presented numerical simulations show that mesh size and mesh alignment dependencies can be completely removed.

A new model for {1012} twin growth in hcp metals

Abstract: Computer simulation has been used to study the interaction of a perfect, basal dislocation with a {1012} twin boundary in a hcp metal for the situation where the 1/3{1120} Burgers vector is inclined at 60° to the interface. It is found that slip is not transferred from one crystal to the other with a residual dislocation left at the interface. Instead, the matrix dislocation decomposes into interfacial defects. We show that as a result of this decomposition the matrix dislocation becomes a new source of twinning dislocations that produce twin growth when the appropriate stress is applied to the crystal. The mechanism described does not require twinning dislocations to multiply by a pole process.

Oracles and Queries That Are Sufficient for Exact Learning

Abstract: We show that the class of all circuits is exactly learnable in randomized expected polynomial time using weak subset and weak superset queries. This is a consequence of the following result which we consider to be of independent interest: circuits are exactly learnable in randomized expected polynomial time with equivalence queries and the aid of anNP-oracle. We also show that circuits are exactly learnable in deterministic polynomial time with equivalence queries and a ?P3-oracle. The hypothesis class for the above learning algorithms is the class of circuits of larger?but polynomially related?size. Also, the algorithms can be adapted to learn the class of DNF formulas with hypothesis class consisting of depth-3 ?-?-? formulas (by the work of Angluin this is optimal in the sense that the hypothesis class cannot be reduced to DNF formulas, i.e., depth-2 ?-? formulas). We also investigate the power of anNP-oracle in the context of learning withmembershipqueries. We show that there are deterministic learning algorithms that use membership queries and anNP-oracle to learn: monotone boolean functions in time polynomial in the DNF size and CNF size of the target formula; and the class ofO(logn)-DNF?O(logn)-CNF formulas in time polynomial inn. We also show that, with anNP-oracle and membership queries, there is a randomized expected polynomial time algorithm that learns any class that is learnable from membership queries with unlimited computational power. Using similar techniques, we show the following both for membership and for equivalence queries (when the hypotheses allowed are precisely the concepts in the class); any class learnable with unbounded ?computational-power is learnable in deterministic polynomial time with a ?:p5-oracle. Furthermore, we identify the combinatorial properties that completely determine learnability in this information-theoretic sense. Finally we point out a consequence of our result in structural complexity theory showing that if everyNPset has polynomial-size circuits then the polynomial hierarchy collapses to ZPPNP.

DiP: A Parallel Program Development Environment

Abstract: This paper describes an environment whose aim is to aid in the development and tuning of message passing applications before actually running them in a real system with a large number of processors. Our objective is not to eliminate tests on real machines but to be able to focus them in a more selective way and thereby minimize their number. The environment presented in this paper consists of three closely integrated tools: an instrumented communication library, a trace driven simulator (Dimemas) and a visualization/analysis tool (Paraver).

Fractal design of multiband and low side-lobe arrays

Abstract: Most array factor design techniques are highly dependent on the operating wavelength. In this paper, a novel technique based on fractal structures is described for multiband operation. The analysis is focused in two different approaches: the fractal spatial arrangement of array elements and the fractal design of array factors. Although the patterns of fractal arrays show some interesting similarity properties at several bands, the directivity is not held constant through the bands. Nevertheless, such structures have been shown to be useful for designing low side-lobe arrays with equally weighted current elements. On the other hand, the fractal array factors presented do keep the same shape at several bands because they are designed as selfsimilar curves. The arrays that would synthesize such patterns present a characteristic power-law current distribution analogous to the spectral distribution of the bandlimited fractal Weierstrass function.

Microplane Model for Concrete. I: Stress-Strain Boundaries and Finite Strain

Abstract: The paper presents an improvement of the microplane model for concrete—a constitutive model in which the nonlinear triaxial behavior is characterized by relations between the stress and strain components on a microplane of any orientation under the constraint that the strains on the microplane are the projections of the macroscopic strain tensor. The improvement is achieved by a new concept: the stress-strain boundaries, which can never be exceeded. The advantage of this new concept is that various boundaries and the elastic behavior can be defined as a function of different variables (strain components). Thus, whereas for compression the stress-strain boundaries are defined on the microplanes separately for volumetric and deviatoric components, for tension an additional boundary is defined in terms of the total normal strains. This is necessary to achieve a realistic triaxial response at large tensile strains. For microplane shear, a friction law with cohesion is introduced. The present model is simpler than the previous microplane model. Finally, the microplane model is generalized to finite, but only moderately large, strains. Verification and calibration by test data are left to a subsequent companion paper.

Extinction and self-organized criticality in a model of large-scale evolution.

Abstract: A simple model of large-scale biological evolution is presented. This model involves an $N$-species system where interactions take place through a given connectivity matrix, which can change with time. True extinctions, with removal of less-fit species, occur followed by episodes of diversification. An order parameter may be naturally defined in the model. Through the dynamical equations, the system moves towards the critical threshold, which triggers the extinctions. The frequency distribution $N(s)$ of extinctions of size $s$ follows a power law $N(s)\ensuremath{\approx}{s}^{\ensuremath{-}\ensuremath{\alpha}}$ with $\ensuremath{\alpha}\ensuremath{\approx}2.3$, close to known palaeobiological evidence.

Kinetic study of the setting reaction of a calcium phosphate bone cement

Abstract: The setting reaction of a calcium phosphate bone cement consisting of a mixture of 63.2 wt % alpha-tertiary calcium phosphate (TCP)[alpha-Ca3(PO4)2], 27.7 wt % dicalcium phosphate (DCP) (CaHPO4), and 9.1 wt % of precipitated hydroxyapatite [(PHA) used as seed material] was investigated. The cement samples were prepared at a liquid-to-powder ratio of: L/P = 0.30 ml/g. Bi-distilled water was used as liquid solution. After mixing the powder and liquid, some samples were molded and aged in Ringer's solution at 37 degrees C. At fixed time intervals they were unmolded and then immediately frozen in liquid nitrogen at a temperature of TN = -196 degrees C, lyofilized, and examined by X-ray diffraction as powder samples. The compressive strength versus time was also measured in setting samples of this calcium phosphate bone cement. The crystal entanglement morphology was examined by scanning electron microscopy. The results showed that: 1) alpha-TCP reacted to a calcium-deficient hydroxyapatite (CDHA), Ca9(HPO4)(PO4)5O H, whereas DCP did not react significantly; 2) the reaction was nearly finished within 32 h, during which both the reaction percentage and the compressive strength increased versus time, with a strong correlation between them; and 3) the calcium phosphate bone cement showed in general a structure of groups of interconnected large plates distributed among agglomerations of small crystal plates arranged in very dense packings.

A rate-dependent isotropic damage model for the seismic analysis of concrete dams

Abstract: In this paper a rate-dependent isotropic damage model developed for the numerical analysis of concrete dams subjected to seismic excitation is presented. The model is shown to incorporate two features essential for seismic analysis: stiffness degradation and stiffness recovery upon load reversals and strain-rate sensitivity. The issue of mesh objectivity is addressed using the concept of the ‘characteristic length’ of the fracture zone, to show that both the softening modulus and the fluidity parameter must depend on it to provide consistent results as the computational mesh is refined. Some aspects of the numerical implementation of the model are also treated, to show that the model can be easily incorporated in any standard non-linear finite element code. The application of the proposed model to the seismic analysis of a large gravity concrete dam shows that the structural response may vary significantly in terms of the development of damage. The inclusion of rate sensitivity is able to reproduce the experimental observation that the tensile peak strength of concrete can be increased up to 50 percent for the range of strain rates that appear in a structural safety analysis of a dam subjected to severe seismic actions.

Perturbation of the Sierpinski antenna to allocate operating bands

Abstract: A scheme for modifying the spacing between the bands of the Sierpinski antenna is introduced. Experimental results of two novel designs of fractal antennas suggest that the fractal structure can be perturbed to enable the log-period to be changed while still maintaining the multiband behaviour of the antenna.

Crystallization behavior of polypropylene filled with surface‐modified talc

Abstract: Talc-filled polypropylene (PP) composites were prepared by extrusion in a wide composition range (0–40 wt %). To improve the affinity relation between talc and the PP matrix, we modified the talc surface with silane coupling agents. Differential scanning calorimetry investigations on test samples, prepared by injection moulding, revealed that the talc content and its surface modification had a pronounced effect on the crystallization behavior of the filled PP composites. The experimental results indicate that a talc concentration of 2 wt % strongly affects the nonisothermal crystallization process of the PP, especially when talc is silane treated Isothermal crystallization experiments on samples with minimum amounts of talc (2 wt %) revealed an improved nucleation activity with silane-treated talc. © 1996 John Wiley & Sons, Inc.

Damage Scenarios Simulation for Seismic Risk Assessment in Urban Zones

Abstract: A methodology for simulating seismic damage of unreinforced masonry buildings for seismic risk assessment of urban areas is presented in this paper. The methodology is based on the Italian vulnerab...

Solvent Effects in Chloroform Solution: Parametrization of the MST/SCRF Continuum Model

Abstract: The suitability of the self-consistent reaction field (SCRF) strategy for the study of solutes in chloroform solution has been examined. The SCRF method developed by Miertus, Scrocco and Tomasi (MST) has been parametrized at both ab initio 6-31G* and semiempirical AM1, MNDO, and PM3 levels. The reliability of the MST/SCRF model has been assessed from the comparison with results derived from classical Monte Carlo−free energy perturbation simulations and from mixed Monte Carlo−quantum mechanical/molecular mechanical computations, as well as with experimental data. The parametrized MST/SCRF method estimates the free energy of solvation with a root-mean-square deviation of 0.4 kcal/mol from the experimental value for the molecules studied. Further confidence in the optimized method stems from its ability to reproduce the tautomeric change of 2- and 4-pyridone upon transfer from gas phase to chloroform and the partition coefficient for compounds not considered in the parametrization. The results indicate the s...

Adaptive estimation of QRS complex wave features of ECG signal by the Hermite model.

Abstract: The most characteristic wave set in ECG signals is the QRS complex. Automatic procedures to classify the QRS are very useful in the diagnosis of cardiac dysfunctions. Early detection and classification of QRS changes are important in real-time monitoring. ECG data compression is also important for storage and data transmission. An Adaptive Hermite Model Estimation System (AHMES) is presented for on-line beat-to-beat estimation of the features that describe the QRS complex with the Hermite model. The AHMES is based on the multiple-input adaptive linear combiner, using as inputs the succession of the QRS complexes and the Hermite functions, where a procedure has been incorporated to adaptively estimate a width related parameter b. The system allows an efficient real-time parameter extraction for classification and data compression. The performance of the AHMES is compared with that of direct feature estimation, studying the improvement in signal-to-noise ratio. In addition, the effect of misalignment at the QRS mark is shown to become a neglecting low-pass effect. The results allow the conditions in which the AHMES improves the direct estimate to be established. The application is shown, for subsequent classification, of the AHMES in extracting the QRS features of an ECG signal with the bigeminy phenomena. Another application is highlighted that helps wide ectopic beats detection using the width parameter b.

Microplane Model for Concrete: II: Data Delocalization and Verification

Abstract: The new microplane model developed in the preceding companion paper is calibrated and verified by comparison with test data. A new approximate method is proposed for data delocalization, i.e., decontamination of laboratory test data afflicted by localization of strain-softening damage and size effect. This method, applicable more generally to any type of constitutive model, is based on the series-coupling model and on the size-effect law proposed by Bazant. An effective and simplified method of material parameter identification, exploiting affinity transformations of stress-strain curves, is also given. Only five parameters need to be adjusted if a complete set of uniaxial, biaxial, and triaxial test data is available, and two of them can be determined separately in advance from the volumetric compression curve. If the data are limited, fewer parameters need to be adjusted. The parameters are formulated in such a manner that two of them represent scaling by affinity transformation. Normally only these two parameters need to be adjusted, which can be done by simple closed-form formulas. The new model allows good fit of all the basic types of uniaxial, biaxial, and triaxial test data for concrete.

Extension of MST/SCRF method to organic solvents: Ab initio and semiempirical parametrization for neutral solutes in CCl4

Abstract: The self-consistent reaction field (SCRF) method proposed by Miertus, Scrocco, and Tomasi (MST) was extended to solutions of neutral solutes in CCl4. A detailed parametrization of the solute/solvent interface and of the “hardness” atomic parameters determining the van der Waals interactions was performed from comparison with experimental data and Monte Carlo simulations. The parametrization was carried out at both ab initio (6-31G*) and semiempirical (MNDO, AM1, PM3) levels. The MST/SCRF optimized versions provide accurate estimates of the free energy of solvation in CCl4 for the series of molecules studied. Furthermore, a precise description of the solvent effect on different chemical processes in CCl4 solution supports the reliability of the parametrization. © 1996 by John Wiley & Sons, Inc.

On the computational efficiency and implementation of block-iterative algorithms for nonlinear coupled Problems

Abstract: Outlines a general methodology for the solution of the system of algebraic equations arising from the discretization of the field equations governing coupled problems. Considers that this discrete problem is obtained from the finite element discretization in space and the finite difference discretization in time. Aims to preserve software modularity, to be able to use existing single field codes to solve more complex problems, and to exploit computer resources optimally, emulating parallel processing. To this end, deals with two well‐known coupled problems of computational mechanics – the fluid‐structure interaction problem and thermally‐driven flows of incompressible fluids. Demonstrates the possibility of coupling the block‐iterative loop with the nonlinearity of the problems through numerical experiments which suggest that even a mild nonlinearity drives the convergence rate of the complete iterative scheme, at least for the two problems considered here. Discusses the implementation of this alternative to the direct coupled solution, stating advantages and disadvantages. Explains also the need for online synchronized communication between the different codes used as is the description of the master code which will control the overall algorithm.

General matching of two spherically symmetric spacetimes.

Abstract: In this paper we show some very general results concerning the general matching of any two spherically symmetric spacetimes through a timelike hypersurface. We present a set of necessary conditions for this case which are very simple to use. As an important result, these conditions allow us to ascertain, by mere inspection of the conformal diagrams, which matchings are feasible in principle and which are not allowed. We shall illustrate these results by applying them to the general matching of Vaidya's radiating metric and the general flat Robertson-Walker spacetime with a linear equation of state, where all possible models are obtained. These particular models are relevant on their own as they describe interesting physical situations; some of them had not been considered hitherto.