The wall effect is known to present difficulties when attempting to predict the pressure drop over randomly packed beds. The Nuclear Safety Standard Commission, “Kerntechnischer Auss-chuss" (KTA), made considerable efforts to develop an equation which predicts the pressure drop over cylindrical randomly packed beds consisting of mono-sized spheres. The KTA was able to estimate a limiting line, which defines the region for which the wall effect is negligible, however the theoretical basis for this line is unclear. The goal of this investigation was to determine the validity of the KTA limiting line, using an explicit approach. Packed beds were generated using Discrete Element Modelling (DEM), and the flow through the beds simulated using Computational Fluid Dynamics (CFD). STAR-CCM+R was used for both DEM and CFD operations, and the methods developed for this explicit approach were validated with empirical data. The KTA correlation predictions for friction factors were com- pared with the CFD results, as well as the predictions from a few other correlations. The KTA correlation predictions for friction factors did not correspond well with the CFD results at low aspect ratios and low modified Reynolds numbers, due to the influence of the wall effect. The KTA limiting line was found to be valid, but not exact. A new limiting line for the KTA correlation was suggested, however the new limiting line improved little on the existing line and was the result of some major assumptions. In order to improve the determination of the position of the KTA limiting line further, criteria need to be established which determine how small the error in predicted friction factor must be before the KTA correlation can be accepted as accurate. / MIng (Nuclear Engineering), North-West University, Potchefstroom Campus, 2014

published_or_final_version / abstract / Applied Geosciences / Master / Master of Science

Sabe-se que o Método dos Elementos Finitos em sua forma convencional é uma ferramenta poderosa no cálculo estrutural moderno. Porém, se o problema apresenta singularidades, como os efeitos de borda tipicamente introduzidos pelos vínculos nas estruturas em casca, a análise pode exigir alto refinamento da malha. Procurando resolver mais eficientemente esse tipo de problema, especificamente em estruturas com simetria de revolução como os tubos cilíndricos e as cascas esféricas, apresentam-se neste trabalho alternativas não convencionais para o emprego do Método dos Elementos Finitos. Dadas as simetrias de forma e carregamento, a abordagem pode ser feita em campo unidimensional. São apresentadas as respostas analíticas, em termos de deslocamentos e esforços, para as estruturas citadas, partindo-se de suas equações diferenciais governantes. Em seguida, as formas fracas correspondentes são resolvidas pelo Método dos Elementos Finitos, incorporando-se alguns tipos de enriquecimento que aproveitam a estrutura deste método. Por fim, são comparados os resultados aproximados entre si e com relação aos analíticos, comprovando o grande potencial das alternativas sugeridas. / It is known that the Finite Element Method in its conventional form is a powerful technique in the modern structural calculus. However, if the problem has singularities, as boundary effects typically introduced by the support into shell structures, the analysis may demand high refinement of the mesh. In order to resolve this type of problem more efficiently, particularly in structures with symmetry of revolution as cylindrical tubes and spherical shells, this dissertation presents non-conventional alternatives for the employment of the Finite Element Method. Due to the form and load symmetry, the treatment can be made in one-dimensional system. The analytical responses, in terms of displacements and efforts, are recovered to the mentioned structures, from their governing differential equations. Next, corresponding weak forms are resolved by the Finite Element Method and some types of enrichment that utilize the structure of this method are incorporated. Finally, the approach results are compared among themselves and with analytical results, proving the great potential of the suggested alternatives

Neste trabalho, estudamos a ação dos grupos split metacíclicos \'D IND. (2h+1) POT. 2 nas esferas. Encontramos uma região fundamental dos espaços quocientes, chamados de Formas Espaciais Esféricas Metacíclicas, que foi utilizada para construirmos um conveniente complexo de cadeias destas formas com o qual calculamos o anel de cohomologia e a torção de Reidemeister. Obtivemos também uma relação entre as diferentes torções encontradas / In this work, we study the action of the split metacyclic groups \'D IND. (2h+1) POT. 2 on the spheres. We find a fundamental domain of the quotient spaces, called Metacyclic Spherical Space Forms. Through this region we have built a convenient chain complex of these spaces and we used it to calculate their cohomology ring and Reidemeister torsion. We obtained also a relation between the different torsions found

Made available in DSpace on 2016-04-27T16:57:07Z (GMT). No. of bitstreams: 1 Maria Lucia Torelli Doria de Andrade.pdf: 1813485 bytes, checksum: d412ceceb706049da015f5eeb9437db2 (MD5) Previous issue date: 2011-05-31 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The objective of the current work is to investigate ownership of Spherical Geometry elementary concepts by students in the second year of high school, from a teaching sequence. In addition, it aimed to solve the activities of this sequence, the subjects carry out research treatments and Semiotic Registers Representation conversions relevant to the mathematical Spherical Geometry objects to be studied. We seek to answer the following question: How a didactic sequence articulating different Registers Representation can evaluate high school students in learning Spherical Geometry concepts? Therefore, we applied to two high school students teaching sequence. We based it on Raymond Duval s Semiotic Registers Representation Theory and Guy Brousseau s Didactic Situations Theory as theoretical foundation of this research. The research approach was qualitative and, as methodology we adopted Engineering Curriculum assumptions. Watching the subjects research production in the course of the activities sequence, we found out these individuals performed conversions and registers treatments by semiotic representation according to Duval (2009), as well as they did the record conversion in natural language (activity statement) to record material (Styrofoam ball), which led them to solve the activity and understand Spherical Geometry straight line concept. In this sense, we could highlight semiotic representation material record, and infer that it contributed to the concepts ownership required by the research subjects / O presente trabalho tem como objetivo investigar a apropriação de conceitos elementares de Geometria Esférica por alunos do 2º ano do Ensino Médio, a partir de uma sequência de ensino. Além disso, objetivava que ao resolver as atividades desta sequência, os sujeitos de pesquisa realizassem os tratamentos e conversões dos Registros de Representação Semiótica pertinentes aos objetos matemáticos da Geometria Esférica a serem estudados. Buscamos responder a seguinte questão de pesquisada: Como uma sequência didática articulando diferentes registros de representação pode avaliar alunos do Ensino Médio na aprendizagem de conceitos de Geometria Esférica? Assim, aplicamos junto a dois alunos do Ensino Médio a sequência de ensino. Nos embasamos na Teoria dos Registros de Representação Semiótica de Raymond Duval e na Teoria das Situações Didáticas de Guy Brousseau como fundamentação teórica dessa pesquisa. A abordagem desta investigação foi qualitativa e, como metodologia adotamos pressupostos da Engenharia Didática. Observando as produções dos sujeitos de pesquisa no decorrer das atividades da sequência, constatamos que esses realizaram as conversões e os tratamentos dos registros de representação semiótica de acordo com Duval (2009), como quando fizeram a conversão do registro em língua natural (enunciado da atividade) para o registro material (bola de isopor), o que os levou a resolver a atividade e compreender o conceito de reta na Geometria Esférica. Nesse sentido, pudemos destacar o registro material de representação semiótica, e inferir que esse contribuiu para a apropriação dos conceitos requeridos por parte dos sujeitos de pesquisa

The human cerebral cortex is a highly foliated structure that supports the complex cognitive abilities of humans. The cortex is divided by its cytoarchitectural characteristics that can be approximated by the folding pattern of the cortex. Psychiatric and neurological diseases, such as Huntington's disease or schizophrenias, are often related with structural changes in the cerebral cortex. Detecting structural changes in different regions of cerebral cortex can provide insight into disease biology, progression and response to treatment. The delineation of anatomical regions on the cerebral cortex is time intensive if performed manually, therefore automated methods are needed to perform this delineation. Magnetic Resonance Imaging (MRI) is commonly used to explore the structural change in patients with psychiatric and neurological diseases. This dissertation proposes a fast and reliable method to automatically parcellate the cortical surface generated from MR images. A fully automated pipeline has been built to process MR images and generate cortical surfaces associated with parcellation labels. First, genus zero cortical surfaces for each hemisphere of a subject are generated from MR images. The surface is generated at the parametric boundary between gray matter and white matter. Geometry features are calculated for each cortical surface to as scalar values to drive a multi-resolution spherical registration that can align two cortical surfaces together in the spherical domain. Then, the labels on a subject's cortical surface are evaluated by registering a subject's cortical surface with a population atlas and combining the information of prior probabilities on the atlas with the subject's geometry features. The automated parcellation has been tested on a group of subjects with various cerebral cortex structures. It shows that the proposed method is fast (takes about 3 hours to parcellate at one hemisphere) and accurate (with the weighted average Dice ~0.86). The framework of this dissertation will be as follows: the first chapter is about the introduction, including motivation, background, and significance of the study. The second chapter describes the whole pipeline of the automated surface parcellation and focuses on technical details of every method used in the pipeline. The third chapter presents results achieved in this study and the fourth chapter discusses the results and draws a conclusion.

The study of particle morphology plays an important role in understanding the micromechanical behavior of cohesionless soil. Shear strength and liquefaction characteristics of granular soil depend on various morphological characteristics of soil grains such as their particle size, shape and surface texture. Therefore, accurate characterization and quantification of particle shape is necessary to study the effect of grain shape on mechanical behavior of granular assembly. However, the theoretical and practical developments of quantification of particle morphology and its influence on the mechanical response of granular assemblies has been very limited due to the lack of quantitative information about particle geometries, the experimental and numerical difficulties in characterizing and modeling irregular particle morphology. Motivated by the practical relevance of these challenges, this research presents a comprehensive approach to model irregular particle shape accurately both in two and three dimensions. To facilitate the research goal, a variety of natural and processed sand samples is collected from various locations around the world. A series of experimental and analytical studies are performed following the sample collection effort to characterize and quantify particle shapes of various sand samples by using Fourier shape descriptors. As part of the particle shape quantification and modeling, a methodology is developed to determine an optimum sample size for each sand sample used in the analysis. Recently, Discrete Element Method (DEM) has gained attention to model irregular particle morphology in two and three dimensions. In order to generate and reconstruct particle assemblies of highly irregular geometric shapes of a particular sand sample in the DEM environment, the relationship between grain size and shape is explored and no relationship is found between grain size and shape for the sand samples analyzed. A skeletonization algorithm is developed in this study in order to automate the Overlapping Discrete Element Cluster (ODEC) technique for modeling irregular particle shape in two and three dimensions. Finally, the two-dimensional and three-dimensional particle shapes are implemented within discrete element modeling software, PFC2D and PFC3D, to evaluate the influence of grain shape on shear strength behavior of granular soil by using discrete simulation of direct shear test.

<p>New methods for global shape description of three-dimensional digital objects are presented. The shape of an object is first represented by a digital surface where the faces are either triangles or quadrilaterals. Techniques for computing a high-quality parameterization of the surface are developed and this parameterization is used to approximate the shape of the object. Spherical harmonics are used as basis functions for approximations of the coordinate functions. Information about the global shape is then captured by the coefficients in the spherical harmonics expansions.</p><p>For a starshaped object it is shown how a parameterization can be computed by a projection from its surface onto the unit sphere. An algorithm for computing the position at which the centre of the sphere should be placed, is presented. This algorithm is suited for digital voxel objects. Most of the work is concerned with digital objects whose surfaces are homeomorphic to the sphere. The standard method for computing parameterizations of such surfaces is shown to fail on many objects. This is due to the large distortions of the geometric properties of the surface that often occur with this method. Algorithms to handle this problem are suggested. Non-linear optimization methods are used to find a mapping between a surface and the sphere that minimizes geometric distortion and is useful as a parameterization of the surface. </p><p>The methods can be applied, for example, in medical imaging for shape recognition, detection of shape deformations and shape comparisons of three-dimensional objects.</p>

New methods for global shape description of three-dimensional digital objects are presented. The shape of an object is first represented by a digital surface where the faces are either triangles or quadrilaterals. Techniques for computing a high-quality parameterization of the surface are developed and this parameterization is used to approximate the shape of the object. Spherical harmonics are used as basis functions for approximations of the coordinate functions. Information about the global shape is then captured by the coefficients in the spherical harmonics expansions. For a starshaped object it is shown how a parameterization can be computed by a projection from its surface onto the unit sphere. An algorithm for computing the position at which the centre of the sphere should be placed, is presented. This algorithm is suited for digital voxel objects. Most of the work is concerned with digital objects whose surfaces are homeomorphic to the sphere. The standard method for computing parameterizations of such surfaces is shown to fail on many objects. This is due to the large distortions of the geometric properties of the surface that often occur with this method. Algorithms to handle this problem are suggested. Non-linear optimization methods are used to find a mapping between a surface and the sphere that minimizes geometric distortion and is useful as a parameterization of the surface. The methods can be applied, for example, in medical imaging for shape recognition, detection of shape deformations and shape comparisons of three-dimensional objects.

This thesis examines the advantages to nonlinear least-squares (NLS) fitting of diffusion-weighted MRI data over the commonly used linear least-squares (LLS) approach. A modified fitting algorithm is proposed which accounts for the positive bias experienced in magnitude images at low SNR. For b-values in the clinical range (~1000 s/mm2), the increase in precision of FA and fibre orientation estimates is almost negligible, except at very high anisotropy. The optimal b-value for estimating tensor parameters was slightly higher for NLS. The primary advantage to NLS was improved performance at high b-values, for which complex fibre architectures were more easily resolved. This was demonstrated using a model-selection classifier based on higher-order diffusion models. Using a b-value of 3000 s/mm2 and magnitude-corrected NLS fitting, at least 10% of voxels in the brain exhibited diffusion profiles which could not be represented by the tensor model.