Browsing by Author "Carvalho, A."
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- Assessing parametric uncertainty on fibre reinforced composite laminatesPublication . Carvalho, A.; Silva, T. A. N.; Loja, AméliaWhen modeling a composite structure it is important to take into account its greater exposition to parametric variability, when compared to other types of materials more traditionally used. The possibility to tailor composite materials according to specific requisites is simultaneously a source of additional variability, which origin may be associated to material and to geometrical characteristics. Regardless the origin of this variability, they will produce its effect in the structure response, thus it is very important to anticipate them and to quantify them as much as possible. With the present work, it is intended to assess and quantify the influence of geometrical parameters variabilities on the composite structural response. Behind this characterization, linear static analyses were performed, considering that the layers’ thicknesses and fibre orientation angles will be affected by uncertainty. A set of simulations and numerical results are presented and discussed.
- Developments on finite element methods for medical image supported diagnosticsPublication . Almeida, Ana; Barbosa, J. I.; Carvalho, A.; Loja, M. A. R.; Portal, R.; Rodrigues, J. A.; Vieira, LinaVariational image-processing models offer high-quality processing capabilities for imaging. They have been widely developed and used in the last two decades, enriching the fields of mathematics as well as information science. Mathematically, several tools are needed: energy optimization, regularization, partial differential equations, level set functions, and numerical algorithms. For this work we consider a second-order variational model for solving medical image problems. The aim is to obtain as far as possible fine features of the initial image and identify medical pathologies. The approach consists of constructing a regularized functional and to locally analyse the obtained solution. Some parameters selection is performed at the discrete level in the framework of the finite element method. We present several numerical simulations to test the efficiency of the proposed approach.
- Finite element techniques for medical image processingPublication . Almeida, A.; Barbosa, Joaquim; Carvalho, A.; Loja, Amélia; Portal, Ricardo; Rodrigues, José Alberto; Vieira, L.We consider some second-order variational model for solving medical image problems. The aim is to obtain as far as possible fine features of the initial image and identify medical pathologies. The approach consists of constructing a family of regularized functionals and to select locally and adaptively the regularization parameters. The parameters selection is performed at the discrete level in the framework of the finite element method. We present several numerical simulations to test the efficiency of the proposed approach.
- Molecular dynamics in tilted bilayer smectic phases: a proton nuclear magnetic resonance relaxometry studyPublication . Carvalho, A.; Sebastião, P. J.; Ribeiro, A. C.; Nguyen, H. T.; Vilfan, M.A proton nuclear magnetic resonance (NMR) relaxation study of molecular dynamics in the liquid crystal 4-octylphenyl 2-chloro-4-(4-cyanobenzoyloxy)benzoate (DB8Cl) is presented. DB8Cl molecules possess a strong polar terminal group and form, in addition to the nematic phase, three different smectic phases: bilayer smectic A, bilayer smectic C, and anticliniclike smectic C phase. The proton spin-lattice relaxation times were measured in all mesophases over a broad frequency range of six decades by applying conventional and fast field-cycling NMR techniques. The parameters obtained in the analysis of the experimental data give quantitative information on molecular motions, particularly for the tilted smectic phases of DB8Cl. In contrast to former conjectures, we found that the low-frequency relaxation in the bilayer smectic C phases results from director fluctuations about the layer normal, which occur without distortion of the layers, and from layer undulations, similar to those in the smectic A phase. In the low-temperature bilayer smectic C phase, a considerable slowing-down of molecular translational diffusion is observed. It confirms indirectly the anticlinic character of this mesophase. Measurements of angular dependence of the relaxation times at 60 MHz support the conclusions obtained from the frequency dispersion data.
- Pisando arte e matemática em LisboaPublication . Carvalho, A.; Santos, C. P.; Silva, J. N.; Teixeira, R. C.Motifs in one or two directions can be classified mathematically by the types of symmetries they possess, and that classification gives rise to seven frieze patterns and to seventeen wallpaper patterns. Rosettes are other type of patterns in which the repetition of the design occurs about a single point, within a limited region of the plane. Rosettes are either dihedral or cyclic, depending on the presence or absence of mirror symmetries. Many Portuguese pavements are beautiful artistic works: all the seven friezes, cyclic rosettes, dihedral rosettes and twelve of the seventeen types of wallpapers were detected in Lisbon. In this paper, we exemplify some of these artistic works, highlighting, in particular, the work in Rossio dos Olivais, carried out by Fernando Conduto. In 2014, the Ludus Association and the University of Lisbon published the Baralho de Simetrias - Calcadas de Lisboa, a deck of cards to disseminate this subject to the largest possible number of people. We also discuss that initiative.
- A recursive process related to a partisan variation of WythoffPublication . Carvalho, A.; Santos, C. P.; Dias, C. L.; Coelho, F.; Neto, J. P.; Vinagre, S.Wythoff Queens is a classical combinatorial game related to very interesting mathematical results. An amazing one is the fact that the P-positions are given by (⌊├ φn⌋┤┤,├ ├ ⌊φ┤^2 n⌋) and (⌊├ φ^2 n⌋┤┤,├ ├ ⌊φ┤n⌋) where φ=(1+√5)/2. In this paper, we analyze a different version where one player (Left) plays with a chess bishop and the other (Right) plays with a chess knight. The new game (call it Chessfights) lacks a Beatty sequence structure in the P-positions as in Wythoff Queens. However, it is possible to formulate and prove some general results of a general recursive law which is a particular case of a Partizan Subtraction game.