[pt] Malhas são amplamente usadas na discretização de domínios geométricos em aplicações na engenharia, como simulações de fluxo, transmissão de calor e deformação mecânica. O problema de geração de malhas é bem conhecido e estudado, mas a geração automática de malhas para um domínio físico com geometrias complexas, criando elementos que obedeçam a forma do objeto, e de tamanho e qualidade adequados, ainda é um desafio. Neste trabalho, foram estudados e implementados métodos para gerar malhas com restrições arbitrárias. O gerador implementado é do tipo de Delaunay, que constrói malhas Delaunay com restrições, e utiliza as propriedades da malha para inserir novos vértices e melhorar a qualidade dos elementos. / [en] Meshes are widely used in the discretization of geometric domains for engineering applications such as fluid flow simulator, heat transfer simulations and mechanical deformation. The mesh generation problem is well known and studied, nevertheless the automatic generation of meshes to domains with complex geometry, creating elements that conform to the forms, and of adequate size and quality, is still a challenge. In this work, mesh generation methods capable of generation mesh of arbitrary restrictions were studied and implemented. The implemented generator is a Delaunay generator, which constructs constrained Delaunay meshes, and utilizes the properties of the mesh to insert new vertices and improve the quality of the elements.

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The vulnerability of a ship to the impact of underwater explosions (UNDEX) and how to incorporate this factor into early-stage ship design is an important aspect in the ship survivability study. In this dissertation, attention is focused on the cost-efficient simulation of the ship response to a far-field UNDEX which involves fluid shock waves, cavitation, and fluid-structural interaction. Traditional fluid numerical simulation approaches using the Finite Element Method to track wave propagation and cavitation requires a high-level of mesh refinement to prevent numerical dispersion from discontinuities. Computation also becomes quite expensive for full ship-related problems due to the large fluid domain necessary to envelop the ship. The burden is aggravated by the need to generate a fluid mesh around the irregular ship hull geometry, which typically requires significant manual intervention. To accelerate the design process and enable the consideration of far-field UNDEX vulnerability, several contributions are made in this dissertation to make the simulation more efficient. First, a Cavitating Acoustic Spectral Element approach which has shown computational advantages in UNDEX problems, but not systematically assessed in total ship application, is used to model the fluid. The use of spectral elements shows greater structural response accuracy and lower computational cost than the traditional FEM. Second, a novel fully automatic all-hexahedral mesh generation scheme is applied to generate the fluid mesh. Along with the spectral element, the all-hex mesh shows greater accuracy than the all-tetrahedral finite element mesh which is typically used. This new meshing approach significantly saves time for mesh generation and allows the spectral element, which is confined to the hexahedral element, to be applied in practical ship problems. A further contribution of this dissertation is the development of a surrogate non-numerical approach to predict structural peak responses based on the shock factor concept. The regression analysis reveals a reasonably strong linear relationship between the structural peak response and the shock factor. The shock factor can be conveniently employed in the design aspects where the peak response is sufficient, using much less computational resources than numerical solvers. / Doctor of Philosophy / The vulnerability of a ship to the impact of underwater explosions (UNDEX) and how to incorporate this factor into early-stage ship design is an important aspect in the ship survivability study. In this dissertation, attention is focused on the cost-efficient simulation of the ship response to a far-field UNDEX which involves fluid shock waves, cavitation, and fluid-structural interaction. Traditional fluid numerical simulation approaches using the Finite Element Method to track wave propagation and cavitation requires a highly refined mesh to deal with large numerical errors. Computation also becomes quite expensive for full ship-related problems due to the large fluid domain necessary to envelop the ship. The burden is aggravated by the need to generate a fluid mesh around the irregular ship hull geometry, which typically requires significant manual intervention. To accelerate the design process and enable the consideration of far-field UNDEX vulnerability, several contributions are made in this dissertation to make the simulation more efficient. First, a Cavitating Acoustic Spectral Element approach, which has shown computational advantages in UNDEX problems but not systematically assessed in total ship application, is used to model the fluid. The use of spectral elements shows greater structural response accuracy and lower computational cost than the traditional FEM. Second, a novel fully automatic all-hexahedral mesh generation scheme is applied to generate the fluid mesh. Along with the spectral element, the all-hex mesh shows greater accuracy than the all-tetrahedral finite element mesh which is typically used. A further contribution of this dissertation is the development of a non-numerical approach which can approximate peak structural responses comparable to the numerical solution with far less computational effort.

[pt] Este trabalho apresenta uma metodologia para geração de malhas adaptativas de elementos finitos 2D e 3D usando modeladores geométricos com multi-regiões e superfícies paramétricas. A estratégia adaptativa adotada é fundamentada no refinamento independente das curvas, superfícies e sólidos. Inicialmente as curvas são refinadas, no seu espaço paramétrico, usando uma técnica de partição binária da curva (binary-tree). A discretização das curvas é usada como dado de entrada para o refinamento das superfícies. A discretização destas é realizada no seu espaço paramétrico e utiliza uma técnica de avanço de fronteira combinada com uma estrutura de dados do tipo quadtree para gerar uma malha não estruturada de superfície. Essas malhas de superfícies são usadas como dado de entrada para o refinamento dos domínios volumétricos. A discretização volumétrica combina uma estrutura de dados do tipo octree juntamente com a técnica de avanço de fronteira para gerar uma malha sólida não estruturada de elementos tetraédricos. As estruturas de dados auxiliares dos tipos binary-tree, quadtree e octree são utilizadas para armazenar os tamanhos característicos dos elementos gerados no refinamento das curvas, superfícies e regiões volumétricas. Estes tamanhos característicos são definidos pela estimativa de erro numérico associado à malha global do passo anterior do processo adaptativo. A estratégia adaptativa é implementada em dois modeladores: o MTOOL (2D) e o MG (3D), que são responsáveis pela criação de um modelo geométrico, podendo ter, multi-regiões, onde no caso 3D as curvas e superfícies são representadas por NURBS. / [en] This work presents a methodology for adaptive generation of 2D and 3D finite-element meshes using geometric modeling with multi- regions and parametric surfaces. The adaptive strategy adopted in this methodology is based on independent refinements of curves, surfaces and solids. Initially, the model´s curves are refined using a binary-partition algorithm in parametric space. The discratizetion of these curves is used as input for the refinement of adjacent surfaces. Surface discretization is also performed in parametric space and employs a quadtree-based refinement coupled to an advancing-front technique for the generation of an unstructured triangulation. These surface meshes are used as input for the refinement adjacent volumetric domains. Volume discretization combines an octree refinement with an advancing-front technique to generate an unstructural mesh of tetrahedral elements. In all stages of the adaptive strategy, the refinement of curves, surface meshes and solid meshes is based on estimated numerical errors associated to the mesh of the previous step in the adaptive process. In addition, curve and surface refinement takes into account metric distortions between parametric and Cartesian spaces and high curvatures of the model´s geometric entities. The adaptive strategies are implemented in two different modelers: MTOOL (2D) and MG (3D), which are responsible for the creation of a geometric model with multi-regions, where for case 3D the curves and surfaces are represented by NURBS, and for the interactive and automatic finite-element mesh generation associated to surfaces and solid regions. Numerical examples of the simulation of engineering problems are presented in order to validate the methodology proposed in this work.

CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / This work describes a technique for generating three-dimensional tetrahedral meshes using parallel computing, with shared, distributed, or hybrid memory processors. The input for the algorithm is a triangular mesh that models the surface of one of several objects, that might have holes in its interior or internal or boundary cracks. A binary tree structure for spatial partitioning is proposed in this work to recursively decompose the domain in as many subdomains as processes or threads in the parallel system, in which every subdomain has the geometry of a rectangular parallelepiped. This decomposition attempts to balance the amount of work in all the subdomains. The amount of work, known as load, of any mesh generator is usually given as a function of its output size, i.e., the size of the generated mesh. Therefore, a technique to estimate the size of this mesh, the total load of the domain, is needed beforehand. This work uses a refined octree, generated from the surface mesh, to estimate this load, and the decomposition is performed on top of this octree. Once the domain is decomposed, each process/thread generates the mesh in its subdomain by means of an advancing front technique, in such a way that it does not overpass the limits defined by its subdomain, and applies an improvement on it. Some of the processes/threads are responsible for generating the meshes connecting the subdomains, i.e., the interface meshes, in order to generate the whole mesh. This technique presented good speed-up results, keeping the quality of the mesh comparable to the quality of the serially generated mesh. / Este trabalho descreve uma tÃcnica para gerar malhas tridimensionais tetraÃdricas utilizando computaÃÃo paralela, com processadores de memÃria compartilhada, memÃria distribuÃda ou memÃria hÃbrida. A entrada para o algoritmo à uma malha triangular que modela a superfÃcie de um ou vÃrios objetos, que podem conter buracos no interior ou fraturas internas ou na borda. Uma estrutura em forma de Ãrvore binÃria de partiÃÃo espacial à proposta neste trabalho para, recursivamente, decompor o domÃnio em tantos subdomÃnios quantos forem os processos ou threads no sistema paralelo, em que cada subdomÃnio tem a geometria de um paralelepÃpedo retangular. Esta decomposiÃÃo tenta equilibrar a quantidade de trabalho em todos os subdomÃnios. A quantidade de trabalho, conhecida como carga, de qualquer gerador de malha à geralmente dada em funÃÃo do tamanho da saÃda do algoritmo, ou seja, do tamanho da malha gerada. Assim, faz-se necessÃria uma tÃcnica para estimar previamente o tamanho dessa malha, que à carga total do domÃnio. Este trabalho faz uso de uma octree refinada, gerada a partir da malha de superfÃcie dada como entrada, para estimar esta carga, e a decomposiÃÃo à feita a partir dessa octree. Uma vez decomposto o domÃnio, cada processo/thread gera a malha em seu subdomÃnio por uma tÃcnica de avanÃo de fronteira, de forma que ela nÃo ultrapasse os limites definidos pelo seu subdomÃnio, e aplica um melhoramento nela. Alguns dos processos/threads ficam responsÃveis por gerar as malhas conectando os subdomÃnios, ou seja, as malhas de interface, atà que toda a malha tenha sido gerada. Esta tÃcnica apresentou bons resultados de speed-up, mantendo a qualidade da malha comparÃvel à qualidade da malha gerada sequencialmente.

<p>Digitizing objects and environments from real world has become an important part of creating realistic computer graphics. Through the use of structured lighting and laser time-of-flight measurements the capturing of geometric models is now a common process. The result are visualizations where viewers gain new possibilities for both visual and intellectual experiences. </p><p>This thesis presents the reconstruction of the Parthenon temple and its environment in Athens, Greece by using a 3D laser-scanning technique. </p><p>In order to reconstruct a realistic model using 3D scanning techniques there are various phases in which the acquired datasets have to be processed. The data has to be organized, registered and integrated in addition to pre and post processing. This thesis describes the development of a suitable and efficient data processing pipeline for the given data. </p><p>The approach differs from previous scanning projects considering digitizing this large scale object at very high resolution. In particular the issue managing and processing huge datasets is described. </p><p>Finally, the processing of the datasets in the different phases and the resulting 3D model of the Parthenon is presented and evaluated.</p>