Loading...
29 results
Search Results
Now showing 1 - 10 of 29
- Computação de simetrias variacionais e optimização da resistência aerodinâmica newtonianaPublication . Gouveia, Paulo D.F.Neste trabalho exploram-se alguns dos actuais recursos de computação científica no contexto da optimização estática e dinâmica. Começa-se por propor um conjunto de procedimentos computacionais algébricos que permitem automatizar todo o processo de obtenção de simetrias e leis de conservação, quer no contexto clássico do cálculo das variações, quer no contexto mais abrangente do controlo óptimo. A utilidade do package de funções desenvolvido é demonstrada com a identificação de novas leis de conservação para alguns problemas do controlo óptimo conhecidos na literatura. Estabelece-se depois uma relação entre as simetrias variacionais do controlo óptimo e as simetrias de equações diferenciais ordinárias. A partir dessa relação, deduz-se um método construtivo, alternativo aos já existentes, para obtenção de simetrias nesta segunda classe de problemas. Numa segunda parte do trabalho, investigam-se, com recurso a simulações computacionais, formas de corpos não convexos que maximizem a sua resistência aerodinâmica quando se desloquem em meios rarefeitos e, simultaneamente, exibam um ligeiro movimento rotacional. É obtido um importante resultado original para o caso bidimensional. Trata-se de uma forma geométrica que confere ao corpo uma resistência muito próxima do seu limite teórico (R=1.4965<1.5). In this thesis some of the scientific computational resources are explored in the context of static and dynamic optimization. A set of analytical computational tools is proposed in order to allow the identification, in an automatic way, of variational symmetries and conservation laws in the calculus of variations and optimal control. The usefulness of the developed routines is showed with the identification of new conservation laws to concrete optimal control problems found in the literature. A relationship between the variational symmetries of optimal control and the symmetries of ordinary differential equations is established. Based in this relationship, a constructive method is created for the purpose of getting the symmetries in this second class of problems. Finally, we investigate, by means of computational simulations, shapes of nonconvex bodies that maximize resistance to its motion on a rarefied medium, considering that bodies are moving forward and at the same time slowly rotating. An important result is obtained for the two-dimensional case which consists of a geometric shape that confers to the body a resistance very close to the supremum value (R = 1.4965 < 1.5). Some results of the thesis are available in the English language in the following references: the research reports [29, 35, 37, 79], the poster [36], the conference proceedings with referee [34] and the refereed journals [31, 32, 38, 80].
- Bodies of maximal aerodynamic resistance on the planePublication . Plakhov, Alexander; Gouveia, Paulo D.F.A two-dimensional body moves through a rarefied medium; the collisions of the medium particles with the body are absolutely elastic. The body performs both translational and slow rotational motion. It is required to select the body, from a given class of bodies, such that the average force of resistance of the medium to its motion is maximal. There are presented numerical and analytical results concerning this problem. In particular, the maximum resistance in the class of bodies contained in a convex body K is proved to be 1.5 times resistance of K. The maximum is attained on a sequence of bodies with very complicated boundary. The numerical study was made for somewhat more restricted classes of bodies. The obtained values of resistance are slightly lower, but the boundary of obtained bodies is much simpler, as compared to the analytical solutions.
- Reflection of parallel rays by a two-dimensional body of nearly maximal resistancePublication . Gouveia, Paulo D.F.This Demonstration shows a class of nonconvex bodies that maximize resistance as they move forward while slowly rotating through a rarefied medium. This class of shapes yields a resistance very close to the theoretical maximum, improving all previous results found by the authors.
- On the two-dimensional rotational body of maximal Newtonian resistancePublication . Gouveia, Paulo D.F.; Plakhov, Alexander; Torres, Delfim F.M.We investigate, by means of computer simulations, shapes of nonconvex bodies that maximize resistance to their motion through a rarefied medium, considering that the bodies are moving forward and at the same time slowly rotating. A two-dimensional geometric shape that confers to the body a resistance very close to the theoretical supremum value is obtained, which improves previous results.
- Automatic computation of conservation laws in the calculus of variations and optimal controlPublication . Gouveia, Paulo D.F.; Torres, Delfim F.M.We present analytic computational tools that permit us to identify, in an automatic way, conservation laws in optimal control. The central result we use is the famous Noether's theorem, a classical theory developed by Emmy Noether in 1918, in the context of the calculus of variations and mathematical physics, and which was extended recently to the more general context of optimal control. We show how a Computer Algebra System can be very helpful in finding the symmetries and corresponding conservation laws in optimal control theory, thus making useful in practice the theoretical results recently obtained in the literature. A Maple implementation is provided and several illustrative examples given.
- Automatic computation of conservation laws in the calculus of variations and optimal controlPublication . Gouveia, Paulo D.F.; Torres, Delfim F.M.We present analytical computational tools that permit us to identify, in an automatic way, conservation laws in optimal control. The central result we use is the famous Noether's theorem, a classical theory developed by Emmy Noether in 1918, in the context of the calculus of variations and mathematical physics, which was extended recently to the more general context of optimal control. We show how a Computer Algebra System can be very helpful in ¯nding the symmetries and corresponding conservation laws in optimal control theory, thus making useful in practice the theoretical results recently obtained in the literature. A Maple implementation is provided and several illustrative examples are given.
- Codificação de fala por modelos variáveis no tempoPublication . Gouveia, Paulo D.F.O trabalho apresentado nesta tese representa uma contribuição para a optimização da codificação da fala. Utilizam-se para o efeito modelos de codificação baseados em filtros LP (filtros de Predição Linear) de parâmetros variáveis no tempo, contrastando com os modelos fixos utilizados nos métodos convencionais. Nestes, a adaptação dos filtros de predição realiza-se simplesmente através de actualizações periódicas dos seus parâmetros, não traduzindo por isso uma evolução gradual e contínua ao longo do tempo. A técnica utilizada na implementação dos modelos variáveis tem por base a utilização de funções do tipo B-spline na representação das formas de onda dos parâmetros LP. Para o estudo da viabilidade do modelo proposto, analisou-se o desempenho de um vocoder de predição linear incluindo, quer o modelo LP de parâmetros variáveis, quer o modelo LP de parâmetros fixos convencional, por forma a possibilitar a comparação de desempenhos. Dos resultados obtidos concluímos que a codificação de fala por modelos variáveis no tempo, embora não tenha evidenciado vantagens convincentes, pode ser encarada como outra forma de codificação, competindo por isso com as metodologias já existentes. The work presented in this thesis aims at to be a contribution to speech coding. To accomplish this objective, coding models based on LP filters (Linear Predictive Filters) with time-varying parameters are used, and compared with fixed models used in conventional methods. In these models, the predictive filters adaptation is carried on simply through periodic updatings of its parameters, therefore doesn’t representing a gradual and continuous evolution in time. The technique used in varying models implementation is based on the utilization of B-spline like functions to represent the LP parameters waveforms. In order to make a viability study of the proposed model, the performance of a linear predictive vocoder was analyzed, including both the LP model with varying parameters and the conventional LP model with fixed parameters, thus enabling the comparison of their performances. From the results, we concluded that speech coding by time-varying models, although it had not demonstrated clear benefits, can be viewed as another coding way, therefore competing with the already existing methodologies.
- Problems of maximal mean resistance on the planePublication . Plakhov, Alexander; Gouveia, Paulo D.F.A two-dimensional body moves through a rarefied medium; the collisions of the medium particles with the body are absolutely elastic.The body performs both translational and slow rotational motion. It is required to select the body, from a given class of bodies, such that the average force of resistance of the medium to its motion is maximal. There are presented numerical and analytical results concerning this problem. In particular, the maximum resistance in the class of bodies contained in a convex body K is proved to be 1.5 times resistance of K. The maximum is attained on a sequence of bodies with very complicated boundary. The numerical study was made for somewhat more restricted classes of bodies. The obtained values of resistance are slightly lower, but the boundary of obtained bodies is much simpler, as compared to the analytical solutions.
- Computing ODE symmetries as abnormal variational symmetriesPublication . Gouveia, Paulo D.F.; Torres, Delfim F.M.We give a new computational method to obtain symmetries of ordinary differential equations. The proposed approach appears as an extension of a recent algorithm to compute variational symmetries of optimal control problems [P.D.F. Gouveia, D.F.M. Torres, Automatic computation of conservation laws in the calculus of variations and optimal control, Comput. Methods Appl. Math. 5 (4) (2005) 387-409], and is based on the resolution of a first order linear PDE that arises as a necessary and sufficient condition of invariance for abnormal optimal control problems. A computer algebra procedure is developed, which permits one to obtain ODE symmetries by the proposed method. Examples are given, and results compared with those obtained by previous available methods.
- Divisão silábica automática do texto escrito e faladoPublication . Gouveia, Paulo D.F.; Teixeira, João Paulo; Freitas, Diamantino SilvaEste artigo apresenta um algoritmo que permite realizar automaticamente a separação silábica do texto como uma etapa do desenvolvimento de um trabalho mais extenso, que é o estudo de modelos prosódicos para o português europeu, enquadrado no desenvolvimento de um sintetizador de fala. O algoritmo de separação silábica está foi concebido para aplicação em duas situações distintas: na primeira é aplicado ao texto escrito e na segunda à sequência de fonemas realmente produzidos na locução desse mesmo texto. Cada uma das aplicações está envolta nas suas peculiaridades e dificuldades, que são descritas, bem como as soluções adoptadas para a sua resolução. No primeiro caso consegue-se uma taxa de erro de 0,06% e no segundo caso uma taxa de erro de 0,89%. O algoritmo baseia-se na consideração de sílabas dos tipos V, VC, VCC, CV, CVC, CCV e CCVC, sendo V uma vogal ou ditongo e C uma consoante, que se admite cobrirem todas as sílabas existentes em Português.
- «
- 1 (current)
- 2
- 3
- »