Dissertation (Ph.D.)--Auburn University,2009. / Abstract. Includes bibliographic references (p.107-112).

<p>This dissertation addresses interface problems simulated with the finite element method (FEM) with mesh adaptivity. More specifically, we concentrate on the strategies that adaptively modify the mesh and the associated data transfer issues. </p><p>In finite element simulations there often arises the need to change the mesh and continue the simulation on a new mesh. Analysts encounter such an issue when they adaptively refine the mesh to reduce the computational cost, smooth distorted elements to improve system conditioning, or introduce new surfaces and change the domain in simulations of fracture problems. In such circumstances, the transfer of data from the old mesh to the new one is of crucial importance, especially for nonlinear problems. We are concerned in this work with contact problems with adaptive re-meshing and fracture problems modeled with the eXtended finite element method (X-FEM). For the former ones, the transfer of surface data is built upon the technique of parallel transport, and the error of such a transfer strategy is investigated through classic benchmark tests. A transfer scheme based on a least squares problem is also proposed to transfer the bulk data when nearly incompressible hyperelastic materials are employed. For the latter type of problems, we facilitate the transfer of internal variables by making partial elements utilize the same quadrature points from the uncut parent elements and meanwhile adjusting the quadrature weights via the solution of moment fitting equations. The proposed scheme helps avoid the complicated remapping procedure of internal variables between two different sets of quadrature points. A number of numerical examples are presented to demonstrate the robustness and accuracy of our proposed approaches.</p><p>Another renowned technique to simulate fracture problems is based upon the phase-field formulation, where a set of coupled mechanics and phase-field equations are solved via FEM without modeling crack geometries. However, losing the ability to model distinct surfaces in the phase-field formulation has drawbacks, such as difficulties simulating contact on crack surfaces and poorly-conditioned stiffness matrices. On the other hand, using the pure X-FEM in fracture simulations mandates the calculation of the direction and increment of crack surfaces at each step, introducing intricacies of tracing crack evolution. Thus, we propose combining phase-field and X-FEM approaches to utilize their individual benefits based on a novel medial-axis algorithm. Consequently, we can still capture complex crack geometries while having crack surfaces explicitly modeled by modifying the mesh with the X-FEM.</p> / Dissertation

This thesis considers the application of the type II third order WENO finite volume reconstruction for unstructured tetrahedral meshes proposed by Zhang and Shu in (CCP, 2009) and the third order multirate Runge-Kutta time-stepping to the solution of Maxwell's equations. The dependance of accuracy of the third order WENO scheme on the small parameter in the definition of non-linear weights is studied in detail for one-dimensional uniform meshes and numerical results confirming the theoretical analysis are presented for the linear advection equation. This analysis is found to be crucial in the design of the efficient three-dimensional WENO scheme, full details of which are presented. Several multirate Runge-Kutta (MRK) schemes which advance the solution with local time-steps assigned to different multirate groups are studied. Analysis of accuracy of three different MRK approaches for linear problems based on classic order-conditions is presented. The most flexible and efficient multirate schemes based on works by Tang and Warnecke (JCM, 2006) and Liu, Li and Hu (JCP, 2010) are implemented in three-dimensional finite volume time-domain (FVTD) method. The main characteristics of chosen MRK schemes are flexibility in defining the time-step ratios between multirate groups and consistency of the scheme. Various approaches to partition the three-dimensional computational domain into multirate groups to maximize the achievable speedup are discussed. Numerical experiments with three-dimensional electromagnetic problems are presented to validate the performance of the proposed FVTD method. Three-dimensional results agree with theoretical and numerical accuracy analysis performed for the one-dimensional case. The proposed implementation of multirate schemes demonstrates greater speedup than previously reported in literature. / February 2016

Volvo Car Corporation (VCC) uses Computational Fluid Dynamics (CFD) and wind tunnel during the aerodynamic development of new vehicles. In the past VCC main focus has been on the drag force correlation to the wind tunnel measurements but in recent years improved methods for lift force correlations has been highly wanted. Three objectives were considered in this study to improve the lift force correlation between the CFD simulations and wind tunnel measurements for geometrical configurations of the V60 and S60 models.Poor mesh resolution for the wall bounded flow existed for the VCC mesh method and therefore prisms layers were considered in this thesis to increase the mesh resolution inside the boundary layer.As slick tyres generally were used in the CFD simulations better geometrical correlation was wanted to be studied as it could improve the lift force correlation between CFD simulations and wind tunnel measurements. Therefore detailed tyres were considered in this study.As the coarsest surface mesh size was used for the underbody and the components inside the engine bay, where some of the highest flow velocities occurred, mesh refinements were investigated for engine bay and underbody in this study.The prisms layers improved the predicted behavior for the boundary layer as it captured the large velocity gradients more accurately. Due to this, the skin friction prediction was also improved. Different flow behavior around the front wheels and rear wake occurred due to earlier separation. The different flow field caused an improved correlation for the lift force but worsened correlation for the drag force due to increased pressure at the rear of the cars. However, the front lift force trend correlation for the considered configurations was improved with the prisms layer mesh method.The detailed tyres caused slight more disturbances for the underbody flow which caused more attached flow around the rear of the car hence lowered pressure. Earlier separation around the front wheels also occurred for the detailed tyre geometry as the disturbed flow around the wheels was increased. Slight improved correlation for the front and rear lift forces to the wind tunnel measurements could be seen with the detailed tyre compared to the slick tyre.The mesh refinements for the engine bay and underbody showed significant differences for the flow at the underbody which had significant impact on the flow at the rear wake for the V60 model. Minor differences could be seen for the aerodynamic forces for the baseline configuration for the V60 model while great differences occurred for the configurations affecting the underbody. Due to this significant improved correlation for the front and rear lift force trends were achieved for the underbody configurations with the refined engine bay and underbody mesh method.Conclusions could be drawn that the prisms layer caused earlier separation due to its increased mesh resolution for the wall bounded flow. However, finer mesh resolution was needed inside the boundary layer to ensure consistent separation behavior for both the considered models. Improved correlation for the front lift force could however be seen. The detailed tyre only had minor effects on the flow field and aerodynamic forces and therefore not so important to include for further studies. The refined engine bay and underbody caused significant improved lift force trend correlation to the wind tunnel measurements and should be considered for future studies. To improve the correlation between CFD simulations and wind tunnel measurements increased mesh resolution for the wall bounded flow should be considered to better capture the large velocity gradients close to the wall.

Techniques described to date for the reproducible and noninvasive quantification of regional cardiac function have been largely relegated to research settings due to time-consuming and cumbersome image acquisition and analysis. In this thesis, feature tracking algorithms are developed for 2-D+Time cardiac magnetic resonance (CMR) and 3-D+Time cardiac computed tomography (CCT) image sequences that are easily acquired clinically, while emphasising reproducibility and automation in their design. First, a commercially-implemented CMR feature tracking algorithm for the analysis of steady state free precession (SSFP) cine series is evaluated in patients with hypertrophic cardiomyopathy (HCM) and arrhythmogenic right ventricular cardiomyopathy (ARVC), which primarily affect the left ventricle (LV) and right ventricle (RV), respectively, and functional impairment compared with control populations is found in both cases. The limitations of this implementation are then used to guide development of an automated algorithm for the same purpose, making use of fully convolutional neural networks (CNN) for segmentation and spline registration across all frames simultaneously for tracking. This study is performed in the subjects with HCM, and functional impairment is again identified in disease subjects. Finally, as myocardial contraction is inherently a 3-D phenomenon, a technique is developed for quantification of regional function from 3-D+Time functional CCT studies using simultaneous registration of automatically generated Loop subdivision surface models for tracking. This study is performed in canine mongrels, and compared with the current state of the art technique for CCT functional analysis. This work demonstrates the feasibility of automated, reproducible cardiac functional analysis from CMR and CCT image sequences. While work remains to be done in extending the principles demonstrated and modular components described to fully automated whole-heart analysis, it is hoped that this thesis will accelerate the clinical adoption of regional functional analysis.

Increasing capacity in wireless mesh networks can be achieved by using multiple channels and radios. By using different channels, two nodes can send packets at the same time without interfering with each other. To utilize diversity of available frequency, a channel assignment scheme is required. Hybrid channel assignment is an interesting approach where at least one radio is tuned to a fixed channel for receiving and the remaining interfaces switch their channels dynamically in order to match the receiving channel at the receiving node. This provides full connectivity, but at the expense of introduced switching costs. Due to hardware limitations it is too costly to switch channels on a per packet basis. Instead, this thesis proposes an anypath routing and forwarding mechanism in order to allow each node along the route to select the best next hop neighbor on a per packet basis. The routing algorithm finds for each destination a set of next hop candidates and the forwarding algorithm considers the state of the channel switch operation when selecting a next hop candidate. Also, in order to allow latency-sensitive packets to be transmitted before other packets, latency-awareness has been introduced to distinguish e.g. VoIP flows from FTP traffic. The ideas have been implemented and tested using real-world experiments, and the results show a significant reduction in latency.

O cenário atual da construção civil tem se caracterizado pelo uso crescente de novos produtos, materiais e tecnologias construtivas. Simultaneamente, a adoção de novas técnicas em detrimento a pouca abordagem científica e às falhas de projeto, execução e planejamento de obra têm colaborado para a ocorrência de manifestações patológicas. O desempenho do edifício resulta da contribuição individual de cada um dos materiais e elementos que o constituem, mas também depende do efeito gerado quando todo o conjunto é solicitado. Caso haja incompatibilidade entre as condições de deformação da estrutura de concreto, a capacidade de acomodar deformações da alvenaria e das camadas de argamassa pode desencadear-se desde pequenas fissuras no revestimento, rupturas na alvenaria até mesmo o colapso total da parede, gerando além de elevados custos e descrédito para as construtoras, principalmente, desconforto ao usuário. Atualmente existem diversas técnicas construtivas de reforço/reabilitação, de forma a distribuir tensões, compatibilizar deformações e garantir o desempenho à tração do revestimento de argamassa. Entre estas técnicas ressalta-se o uso de telas metálicas, que no Brasil é amparado por uma normalização ainda incipiente. Embora este uso seja recomendado, não há consenso quanto aos métodos executivos e parâmetros técnicos da tela metálica mais indicados para cada aplicação. Nesse contexto, o trabalho em questão, julgou importante contribuir para a avaliação do desempenho de revestimentos de argamassa com reforço de tela metálica e apresenta três propostas. Uma delas foi o ensaio de tração por compressão diametral, o qual se mostrou viável para avaliar a eficiência do uso de tela no incremento de resistência da argamassa, no entanto, permite ponderações restritas ao parâmetro de carga de ruptura. O ensaio de tração direta proposto para caracterizar o revestimento de argamassa reforçado com tela metálica, aplicado sobre a interface alvenariaestrutura, embora tenha execução complexa e demande mais tempo, apresentou variabilidade satisfatória e mostrou-se capaz para avaliar além da carga a resistência à tração. Por fim, o ensaio de tração na flexão em 4 pontos foi considerado apto para a caracterizar argamassas reforçadas com tela, diante da fácil aplicabilidade, com tamanho menor dos CP’s e variabilidade reduzida. Então foi usado para avaliar a influência de diferentes tipologias de telas metálicas e posicionamento delas na argamassa no desempenho à tração do revestimento. / The current scenario of the construction has been characterized by the increasing use of new products, materials and construction technologies. Simultaneously, the adoption of new techniques to the detriment a little and scientific approach to design failures, execution and construction management have contributed to the occurrence of pathological manifestations. The building performance results of the individual contribution of each one of the materials and elements constitute, but also depends on the effect generated when the whole set is requested. If there is incompatibility between the deformation conditions of the concrete structure, the ability to accommodate deformations of the masonry and layers of mortar, this may unleash from small cracks in the coating, breaks in the masonry even total collapse of the wall, generating high costs and discredit to the builders, mainly discomfort to the user. Nowadays there are many construction techniques reinforcement / rehabilitation, in order to distribute tensions, compatible deformations and guarantee performance to pull the mortar coating. Among these techniques emphasizes the use of metal mesh, which in Brazil is supported by an incipient normalization. Although this use is recommended, there is no consensus on the executive methods and technical parameters of the metal mesh best suited for each application. In this context, the work in question, considered it important to contribute to the evaluation of mortar coatings performance with metal mesh reinforcement and presents three proposals. One was the indirect tensile (Brazilian) test, which proved feasible to evaluate the use efficiency screen on mortar strength increase, however, allows considerations restricted to the breaking load parameter. The tensile direct test proposed to characterize the reinforced mortar coating metal screen applied to the masonry structure interface, although it has complex execution and demands more time presented satisfactory variability and shown to be able to evaluate the load and also the resistance to traction. Finally, the tensile test in bending at 4 points was considered suitable for the characterize mortars reinforced with screen, due to the easy applicability, have smaller size specimens and reduced variability. Then it was used to evaluate the influence of different types of metal mesh and positioning them in the mortar in performance to coating traction.

For many years, terrorism has threatened life, property and business. Targets are largely in urban areas where there is a greater density of life and economic value. Governments, insurers and engineers have sought to mitigate these threats through understanding the effects of urban bombings, increasing the resilience of buildings and improving estimates of financial loss for insurance purposes. This has led to a desire for an improved approach to the prediction of blast propagation in urban cityscapes. Urban geometry has a significant impact on blast wave propagation. Presently, only computational fluid dynamics (CFD) methods adequately simulate these effects. However, for large-scale urban domains, these methods are both challenging to use and are computationally expensive. Adaptive mesh refinement (AMR) methods alleviate the problem, but are difficult to use for the non-expert and require significant tuning. We aim to make CFD urban blast simulation a primary choice for governments, insurers and engineers through improvements to AMR and by studying the performance of CFD in relation to other methods used by the industry. We present a new AMR flagging approach based on a second derivative error norm for compressive shocks (ENCS). This is compared with existing methods and is shown to lead to a reduction in overall refinement without affecting solution quality. Significant improvements to feature tracking over long distances are demonstrated, making the method easier to tune and less obtuse to non-experts. In the chapter that follows, we consider blast damage in urban areas. We begin with a validation and a numerical study, investigating the effects of simple street geometry on blast resultants. We then investigate the sensitivity of their distribution to the location of the charge. We find that moving the charge by a small distance can lead to a significant change in peak overpressures and creates a highly localised damage field due to interactions between the blast wave and the geometry. We then extend the investigation to the prediction of insured losses following a large-scale bombing in London. A CFD loss model is presented and compared with simpler approaches that do not account for urban geometry. We find that the simpler models lead to significant over-predictions of loss, equivalent to several hundred million pounds for the scenario considered. We use these findings to argue for increased uptake of CFD methods by the insurance industry. In the final chapter, we investigate the influence of urban geometry on the propagation of blast waves. An earlier study on the confinement effects of narrow streets is repeated at a converged resolution and we corroborate the findings. We repeat the study, this time introducing a variable porosity into the building facade. We observe that the effect of this porosity is as significant as the confinement effect, and we recommend to engineers that they consider porosity effects in certain cases. We conclude the study by investigating how alterations to building window layout can improve the protective effects of a facade. Maintaining the window surface area constant, we consider a range of layouts and observe how some result in significant reductions to blast strength inside the building.

This report describes an algorithm for computing 3D object hierarchies fit for hlod optimization. The algorithm is used as a pre-processing stage in an hlod pipeline that automatically optimizes 3D models containing multiple meshes. The algorithm for generating hierarchies groups together meshes in a hierarchical tree using operations on bounding spheres of the meshes. The algorithm prioritizes grouping close objects together in the early stages, and relaxes its constraints toward the end, resulting in a tree structure with a single root node. The hierarchical tree is then used by computing proxy meshes, i.e. simplified stand-in meshes, for the inner nodes of the hierarchy. Finally, the resulting proxy meshes, together with the generated hierarchy and the original meshes, are used to render the model using a tree-traversing hlod switching algorithm that renders deeper parts of the tree containing more detailed meshes when more detail is needed. In addition, a minor change to the clustering algorithm is proposed. By swapping the bounding spheres to AABBs (Axis-Aligned Bounding Boxes) in the clustering stage, hierarchies with different properties are generated. This change is shown to generate hierarchies with similar rendering performance as the hierarchies made with bounding spheres, while at the same time resulting in lower space requirements for all proxy meshes. Overall, the proposed automatic hlod pipeline is shown to increase rendering performance for all evaluated scenes in most frames, while never yielding noticeably worse performance than the original model as well.

[pt] Os algoritmos para a localização de falta encontrados na literatura são baseados em equações algébricas obtidas da malha de falta [4], [16]. Para as faltas monofásicas, a resistência de arco, a resistência de pé de torre e a impedância de sequência zero da linha são incluídas. Essa última impedância, por ser composta de uma soma entre as impedâncias longitudinal e de retorno pela terra (multiplicada por três), pode comprometer a precisão desses algoritmos na indicação do local da falta, pois, a resistividade do solo sob a linha varia frequentemente. Dados reais [25] e de testes laboratoriais [50] têm comprovado isso. Apesar da variação dessa resistividade, nenhum dos algoritmos publicados na literatura questionou a utilização dessa impedância no processo de localização de falta. O objetivo deste trabalho é justamente propor um novo algoritmo capaz de localizar qualquer falta, sem se apoiar nos parâmetros de sequência zero da rede. A principal contribuição deste trabalho está na utilização somente dos circuitos de sequência positiva e/ou negativa para a localização de faltas em sistemas de potência. Por não utilizar qualquer parâmetro de sequência zero, o novo algoritmo aqui proposto é capaz de localizar falta, tanto em linha de transmissão com subestações em derivação ao longo de sua extensão quanto em linha sem essas subestações em derivação. Na primeira, a localização de falta não foi contemplada por qualquer algoritmos publicados na literatura, pois, esses tinham como objetivo, o atendimento somente às linhas sem subestações em derivação ao longo de sua extensão. Por isso, além do ineditismo na forma de localização, este novo algoritmo é aplicável à qualquer configuração de linha, o que representa uma contribuição adicional deste trabalho. Esse novo processo visa a localização de falta em sistemas elétricos, mesmo os fortemente malhados. Por isso, constitui o embrião de uma possível e provável nova filosofia de proteção para relés de distância. / [en] Present fault location algorithms are based on algebraic equations derived from the faulty network [4], [16]. Their common characteristic is to use zero-sequence description to apply Kichoff`s Laws. As a result, the are resistance, the tower footing resistance and the zero-sequence impedance of the line are included in the overall system of equations, describing the network. This last impedance, composed by the longitudinal and by the ground retum impedances scaled by three, frequently compromises the these algorithms accuracy. The reason is the underline ground resistance fluctuation with temperature, rain, etc. a very annoying fact, particularly in understudied remote tropical regions, a common situation in Brazil. In spite of the noticed ground-resistance variations, none of the algorithms published in specific literature has questioned the use of this impedance in the fault location producedures. The aim of this work is to propose a new algorithm capable to locate any fault, requiring no support of the network zero-sequence parameters and/or negative-sequence circuit network models. Avoiding the of zero-sequence parameters permits the new fault location algorithm to be applied not only to isolated transmission lines, but to a variety of network topologies, including transmission lines tapped by local distribution substations, a completely new result. The proposed algorithm not only is less sensitive to local physical conditions, but may be applied to complex transmission line configurations, which represents an additional contribution of this work. The new process includes fault location for transmission lines embeded in extremely meshed power networks it, therefore, constitutes the beginning of a new of distance relay protection philosophy.