Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / In RF power transistor characterisation, the designer is confronted with low impedance measurements (typically from 1 Ohm to 12 Ohm). These transistors are contained in metal-ceramic packages of which the lead widths vary with power capability. This thesis presents a high-quality fixture design with low impedance TRL calibration standards for characterisation of an LDMOS transistor. Pre-matching networks are used to transform to the low impedance environment. Since these pre-matching networks are independent of the termination impedance, the low impedance port can always be designed to comply with the same dimension as the device which is being measured.

Le but de ce travail était la modélisation mécanique et thermique du laminage asymétrique. Celle-ci a été effectuée à l'aide de diverses méthodes. Deux approches, l'une analytique et l'autre semi-analytique, ont été utilisées pour la modélisation mécanique et deux autres, celles des différences finies et des éléments finis, pour la modélisation thermique. La première approche est celle des champs uniformes ; on a développé un modèle simple qui peut donner des ordres de grandeurs des différents paramètres du procédé. Une deuxième méthode plus complète mais plus complexe se base sur un champ de vitesse déduit d'une estimation parabolique de la forme des trajectoires. L’avantage de ce modèle est qu’il est capable de prévoir des grandeurs locales. Enfin la méthode des éléments finis a été aussi utilisée pour la modélisation mécanique. Dans un deuxième temps, l’aspect thermique du laminage asymétrique a été étudié. Pour la modélisation du procédé, nous avons alors adopté deux méthodes numériques, les différences finies et les éléments finis, du fait de la complexité de celui-ci. Le modèle thermique développé par différences finies a été ensuite intégré dans notre modèle mécanique (méthode des trajectoires). Celui-ci a donc pu être étendu pour pouvoir utiliser des lois d’écoulement qui dépendent de la température et donc pouvoir simuler le laminage asymétrique à chaud. Les prédictions du champ de température ont été confrontées aux résultats de calculs par éléments finis obtenus avec le code Abaqus. Finalement, une étude simple du développement de la texture cristallographique au cours du laminage a été réalisée. / The aim of this work was to propose a mechanical and thermal modelling of asymmetric rolling. This was achieved using various methods. Two approaches, the first one analytical and the second semi-analytical, were employed for mechanical modeling. Two others methods, the finite difference and finite element methods, were used for thermal modeling. The first analytical model is an upper bound method based on a uniform strain field; a simple model was developed that can give orders of magnitude of the various process parameters. The second model uses a more refined analytical velocity field based on the classical parabolic estimation of the material flow lines in rolling. The advantage of this model is its ability to provide local quantities. Finally, the finite element method was also used for mechanical modeling.The thermal aspects of asymmetric rolling were investigated in turn by two methods. For modeling the process, we adopted two numerical approaches, the finite difference and finite element methods, because of the complexity of the latter. The thermal model developed by finite differences was integrated into our mechanical model (flow line model). It was then extended to investigate asymmetrical hot rolling. The predictions of the temperature field were compared with results from finite element calculations obtained with the Abaqus code. Finally, a simple study of crystallographic texture development was achieved.

The manufacturing process ‘long line method’ has shown many advantages when producing prestressed concrete sleepers, such as rapid production and low cost. However, there have been certain difficulties when cutting the 100 m long concrete blocks into sleepers. The sleepers tend to crack when the diamond cutting blade cuts through the last strands. Moreover, the shape and size of the cracks varies from one sleeper to another. Although these cracks may not affect the load carrying capacity, they will not be aesthetically pleasing. No earlier studies within the area have been found, i.e. diamond cutting of prestressed concrete blocks. As a result, there have been certain difficulties when approaching the problem. Finite element analysis has been proven to be a useful tool when analyzing stress variation. Throughout this project, the cutting simulation has been analyzed using the finite element analysis software ABAQUS. To summarize, stress variation has been examined during different cutting depths. The results from the FE model shows that no critical values were reached in the areas expected to have cracks. The true cause of the problem could not be specified. Therefore, further studies are needed yet this thesis could be a good foundation.

Made available in DSpace on 2014-12-17T14:55:12Z (GMT). No. of bitstreams: 1 RobertoRCF.pdf: 849885 bytes, checksum: 4d53af5d41e5eb5a8eb261ca52870951 (MD5) Previous issue date: 2009-03-10 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / This work presents a theoretical and numerical analysis of parameters of a rectangular microstrip antenna with bianisotropic substrate, and including simultaneously the superconducting patch. The full-wave Transverse Transmission Line - TTL method, is used to characterize these antennas. The bianisotropic substrate is characterized by the permittivity and permeability tensors, and the TTL gives the general equations of the electromagnetic fields of the antennas. The BCS theory and the two fluids model are applied to superconductors in these antennas with bianisotropic for first time. The inclusion of superconducting patch is made using the complex resistive boundary condition. The resonance complex frequency is then obtained. Are simulated some parameters of antennas in order to reduce the physical size, and increase the its bandwidth. The numerical results are presented through of graphs. The theoretical and computational analysis these works are precise and concise. Conclusions and suggestions for future works are presented / Este trabalho apresenta a an?lise te?rica e num?rica dos par?metros de uma antena de micro?ta do tipo retangular sobre substrato bianisotr?pico e tamb?m incluindo simul-taneamente supercondutor na antena. ? aplicada a teoria de onda completa do m?todo da Linha de Transmiss?o Transversa - LTT, para a caracteriza??o das grandezas do substrato e obten??o das equa??es gerais dos campos eletromagn?ticos. ? realizado um estudo atrav?s da teoria bianisotr?pica com o intuito de obter alguns par?metros. Os mesmos s?o caracterizados atrav?s de tensores permissividade e permeabilidade, chegando-se ?s equa??es gerais para os campos eletromagn?ticos da antena. ? apresentado um estudo das principais teorias que explicam o fen?meno da super-condutividade. As teorias BCS, Equa??es de London e modelo dos Dois Fluidos s?o usadas no estudo nas antenas de microfita com estrutura bianisotr?pica pela primeira vez. A inclus?o do patch supercondutor ? feita utilizando-se a condi??o de contorno com-plexa resistiva. Em seguida ? obtida a freq??ncia de resson?ncia complexa. S?o simu-lados v?rios par?metros de antenas com o intuito de diminuir as dimens?es f?sicas e aumentar a largura de banda das mesmas. Os resultados s?o apresentados atrav?s de gr?ficos. A an?lise te?rico-computacional desse trabalho se mostra precisa e relati-vamente concisa. S?o apresentadas as conclus?es e sugest?es para trabalhos futuros

Made available in DSpace on 2014-12-17T14:55:19Z (GMT). No. of bitstreams: 1 MarinaldoPSN_TESE.pdf: 2731148 bytes, checksum: 7c2caa3355d4d42f0702baf0d147bf97 (MD5) Previous issue date: 2014-04-25 / Universidade Federal do Rio Grande do Norte / Metamaterials have attracted a great attention in recent years mostly due to their electromagnetic properties not found in nature. Since metamaterials began to be synthesized by the insertion of artificially manufactured inclusions in a medium specified host , it provides the researcher a broad collection of independent parameters such as the electromagnetic properties of the material host. In this work was presents an investigation of the unique properties of Split Ring Resonators and compounds metamaterials was performed. We presents a theoretical and numerical analysis , using the full-wave formalism by applying the Transverse Transmission Line - LTT method for the radiation characteristics of a rectangular microstrip antenna using metamaterial substrate, as is successfully demonstrated the practical use of these structures in antennas. We experimentally confirmed that composite metamaterial can improved the performance of the structures considered in this thesis / Os metamateriais tem atra?do uma grande aten??o nas ?ltimas d?cadas, principalmente devido as suas propriedades eletromagn?ticas n?o encontradas na natureza. Desde que os metamateriais passaram a ser sintetizados atrav?s da inser??o de inclus?es artificialmente fabricadas num meio hospedeiro especificado, isto propicia ao pesquisador uma larga cole??o de par?metros independentes, tais como as propriedades eletromagn?ticas do material hospedeiro. Neste trabalho foi realizada uma investiga??o das propriedades ?nicas dos Ressoadores em Anel Partido (Split Ring Ressonators - SRR) e dos metamateriais compostos. Apresentou-se uma an?lise te?rica e num?rico-computacional, utilizando o formalismo de onda completa atrav?s da aplica??o do m?todo da Linha de Transmiss?o Transversa LTT, para as caracter?sticas ressonantes de uma antena de microfita com patch retangular utilizando substrato metamaterial, assim como ? demonstrado com sucesso ? utiliza??o pr?tica dessas estruturas em antenas. Esta utiliza??o pr?tica ? confirmada experimentalmente

Made available in DSpace on 2014-12-17T14:55:43Z (GMT). No. of bitstreams: 1 HumbertoDA_DISSERT.pdf: 1369294 bytes, checksum: 4b7817dfceebc22263e36739d877c8f2 (MD5) Previous issue date: 2010-06-30 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Recently, planar antennas have attracted interest due to its characteristics as well as the advantages they offer compared to other types of antennas. In the area of mobile communications the need for such antennas has become increasingly intense due to development, which requires antennas that operate in multifrequency and broadband. The microstrip antennas have narrow bandwidth due to losses in the dielectric caused by irradiation. Another limitation is the radiation pattern degradation due to generation of surface waves in the substrate. Some techniques are being developed to minimize this bandwidth limitation, as is the case in the study of type materials PBG - Photonic Band Gap, to compose the dielectric material. The analysis developed in this work were performed with use of the method LTT - Transverse Transmission Line, in the field of Fourier transform that uses a component propagating in the y direction (transerve real direction of propagation z), thus treating the general equations of the fields electric and magnetic fields as a functions of y E and Hy . This work has as main objective the method LTT structures resonator line slot with four layers of material photonic PBG, for obtaining the complex resonant frequency and efficiency of this structure. PBG theory is applied to obtain the relative permittivity for the substrate biases sep compounds photonic material. Numerical-computational results in graph form in two dimensions for all the analysis are presented for the proposed structures that have photonic materials, as substrates / Recentemente as antenas planares tem despertado interesses devido as suas caracter?sticas e vantagens que oferecem quando comparadas com os demais tipos de antenas. Na ?rea de comunica??es m?veis a necessidade de antenas desse tipo, tem se tornado cada vez mais utilizadas, devido ao intenso desenvolvimento, que necessita de antenas que operem em multifrequ?ncia e em banda larga. As antenas de microfita apresentam largura de banda estreita devido ?s perdas no diel?trico geradas pela irradia??o. Outra limita??o ? a degrada??o do diagrama de irradia??o devido ? gera??o de ondas de superf?cie no substrato. Algumas t?cnicas est?o sendo desenvolvidas para minimizar esta limita??o de banda, como ? o caso do estudo de materiais do tipo PBG Photonic Band Gap, para compor o material diel?trico. Este trabalho tem como objetivo principal a aplica??o do m?todo LTT ?s estruturas ressoadoras retangulares de fenda com quatro camadas de material fot?nico PBG, para a obten??o da freq??ncia de resson?ncia complexa e a efici?ncia de radia??o dessa estrutura. As an?lises desenvolvidas neste trabalho foram realizadas com utiliza??o do m?todo LTT Linha de Transmiss?o Transversa, no dom?nio da Transformada de Fourier que utiliza uma componente de propaga??o na dire??o y (transversa ? dire??o real de propaga??o z), tratando assim as equa??es gerais dos campos el?tricos e magn?ticos em fun??o de yEe yH. A teoria PBG ser? aplicada para a obten??o da permissividade relativa para as polariza??es s e p dos substratos compostos de material fot?nico. Resultados num?rico-computacionais s?o apresentados em forma de gr?fico em duas dimens?es para todas as an?lises realizadas para as estruturas propostas que tem como substratos, materiais fot?nicos

Made available in DSpace on 2014-12-17T14:55:47Z (GMT). No. of bitstreams: 1 DaviBB.pdf: 1037005 bytes, checksum: 218c25a038f89588a46ddca7509b0705 (MD5) Previous issue date: 2006-07-06 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The aim of this work is to characterize and use the characteristic parameters of the planar structures constructed with fin lines looking for their applications in devices, using PBG Photonic Band Gap photonic materials as substrate, operating in the millimeter and optic wave bands.The PBG theory will be applied for the relative permittivity attainment for the PBG photonic substrate s and p polarizations. The parameters considered in the structures characterization are the complex propagation constant and the characteristic impedance of unilateral and bilateral fin lines that were obtained by the use of the TTL Transverse Transmission Line Method, together with the Method of the Moments. The final part of this work comprises studies related to the behavior of the asymmetric unilateral fin line coupler with photonic substrate. This research opens perspectives for new works in this modern area. Numerical results are shown by means of bi-dimensional and three-dimensional graphics. Conclusions and suggestions for future works are also presented / O presente trabalho tem como objetivo caracterizar e utilizar os par?metros de estruturas planares constru?das com linhas de l?minas visando a sua utiliza??o em circuitos, com materiais fot?nicos do tipo PBG Photonic Band Gap como substrato, operando nas faixas de ondas milim?tricas e ?pticas. A teoria PBG ser? aplicada para a obten??o da permissividade relativa para as polariza??es s e p dos substratos compostos de material fot?nico PBG. Os par?metros considerados na caracteriza??o das estruturas s?o a constante de propaga??o complexa e a imped?ncia caracter?stica, de linhas de l?mina unilaterais e bilaterais, que foram obtidos atrav?s da utiliza??o do m?todo da Linha de Transmiss?o Transversa LTT com o aux?lio do M?todo dos Momentos. Nesse trabalho foi realizado ainda, um estudo do funcionamento do acoplador com linha de l?mina unilateral assim?trica com substrato fot?nico. Esta pesquisa abre perspectivas para novos trabalhos nesta moderna ?rea. A an?lise te?rica computacional desse trabalho se mostrou precisa, com compara??es de outros trabalhos, podendo ser empregada em outros dispositivos que utilizem a linha de l?mina como estrutura b?sica, e materiais ?pticos. Resultados num?rico-computacionais em forma de gr?fico em duas e tr?s dimens?es para todas as an?lises realizadas s?o apresentados, para as estruturas propostas que tem como substratos materiais fot?nicos. S?o apresentadas conclus?es e sugest?es para a continuidade deste trabalho

Made available in DSpace on 2014-12-17T14:56:00Z (GMT). No. of bitstreams: 1 RossanaMSC.pdf: 1964809 bytes, checksum: 4bf5121999d212cf5e61c5b931343038 (MD5) Previous issue date: 2005-08-15 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work, the transmission line method is explored on the study of the propagation phenomenon in nonhomogeneous walls with finite thickness. It is evaluated the efficiency and applicability of the method, considering materials like gypsum, wood and brick, found in the composition of the structures of walls in question. The results obtained in this work are compared to those available in the literature, for several particular cases. A good agreement is observed, showing that the performed analysis is accurate and efficient in modeling, for instance, the wave propagation through building walls and integrated circuit layers in mobile communication and radar system applications. Later, simulations of resistive sheets devices such as Salisbury screens and Jaumann absorbers and of transmission lines made of metal-insulator-semiconductor (MIS) are made. Thereafter, it is described a study on frequency surface selective structures (FSS). It is proposed the development of devices and microwave integrated circuits (MIC) of such structures, for the accomplishment of experiments. Finally, future works are suggested, for instance, on the development of reflectarrays, frequency selective surfaces with dissimilar elements, and coupled frequency selective surfaces with elements located on different layers / Neste trabalho, ? utilizado o M?todo da Linha de Transmiss?o, para a investiga??o do fen?meno de propaga??o em paredes n?o-homog?neas e de espessura finita. A avalia??o da efici?ncia e aplicabilidade do m?todo da linha de transmiss?o ? realizada, considerando materiais como gesso, madeira e tijolo, encontrados na composi??o das estruturas de paredes em quest?o. Posteriormente, s?o apresentadas simula??es para superf?cies resistivas, como telas de Salisbury e absorvedores Jaumann, e para linhas de transmiss?o do tipo metal-isolante-semicondutor (MIS), al?m do estudo sobre superf?cies seletivas de freq??ncia (FSS). Em seguida, ? proposto o desenvolvimento de dispositivos e circuitos integrados de microondas (MIC) de tais estruturas, para a realiza??o de experimentos. Os resultados obtidos demonstram que a an?lise efetuada neste trabalho ? eficiente e precisa. Para diversas estruturas e aplica??es em circuitos, os valores num?ricos obtidos para os par?metros analisados foram comparados com os valores te?ricos e experimentais, inclusive de outros autores. Nestes casos, observa-se uma excelente concord?ncia. Estes resultados indicam o potencial da t?cnica adotada para a an?lise da propaga??o de ondas eletromagn?ticas atrav?s de estruturas de camadas m?ltiplas, com aplica??es em sistemas de comunica??es m?veis e radar. Finalmente, s?o apresentadas propostas para a realiza??o de trabalhos futuros relacionados, por exemplo, com o desenvolvimento de reflectarrays, superf?cies seletivas de freq??ncia com elementos dissimilares, localizados na mesma interface, e superf?cies seletivas de freq??ncias acopladas, com elementos localizados sobre camadas distintas

Understanding the flow around wind turbines is a highly relevant research question due to the increased interest in harvesting energy from renewable sources. This thesis approaches the topic by means of numerical simulations using the actuator line method and the incompressible Navier–Stokes equations in the spectral element code Nek5000. The aim is to gain enhanced understanding of the wind turbine wake structure and wind turbine wake interaction. A verification study of the method and implementation is performed against the finite volume solver EllipSys3D using two types of turbines, an idealized constant circulation turbine and the Tjæreborg turbine. It is shown that Nek5000 requires significantly lower resolution to accurately compute the wake development, however, at the cost of a smaller time step.The constant circulation turbine is investigated further with the goal of establishing guidelines for the use of the actuator line method in spectral element codes, where the mesh is inherently non-equidistant and currently used guidelines of force distribution based on Gaussian kernels are difficult to apply. It is shown that Nek5000 requires a larger kernel width in the fixed frame of reference to remove numerical instabilities. Further, the impact of different Gaussian widths on the wake development is investigated in the rotating frame of reference, showing that the convection velocity and the breakdown of the spiral tip and root vortices are dependent on the Gaussian width. In the second part, the flow around single and multiple wind-turbine setups at different operating conditions is investigated and compared with experimental results. The focus is placed on comparing the power and thrust coefficients and the wake development based on the time-averaged streamwise velocity and turbulent stresses. Further the influence of the tower model is investigated both upstream and downstream of the turbine. The results show that the wake is captured accurately in most cases. The loading exhibits a significant dependence on the Reynolds number at which the airfoil data is extracted. When the helical tip vortices are stable the turbulent stresses at the tip vortices are underestimated in the numerical simulations. This is due to the finite resolution and the projection of the actuator line forces in the numerical domain using a prescribed Gaussian width, which leads to lower induced velocities in the helical vortices. / <p>QC 20170523</p>

In the present thesis the wake flow behind wind turbines is analyzed numerically using large-eddy simulations. The wind turbine rotors are modeled by using either the actuator disc method or the actuator line method in which the blades are represented by body forces computed with airfoil data. Using these models, the boundary layers of the turbine blades are not resolved and most of the computational power is preserved to simulate the wake flow. The actuator disc method is used for the wake interaction studies of the Lillgrund wind farm. In this study the power production is simulated for two different wind directions and compared to measurements. A grid sensitivity study and a turbulence intensity study are also performed. As a last step the front row turbines are derated in an attempt to increase the total production of the farm. The results show that it is important to impose atmospheric conditions at the inlet in the simulations, otherwise production will be unrealistically low for some turbines in the farm. The agreement between the simulated and measured power is very good. The study based on derating the front row turbines does not show any positive increase on the farm production. The actuator line method is used for near wake analysis of the MEXICO rotor. In this study the near wake is simulated for five different flow cases and compared with particle image velocimetry (PIV) measurements. The analysis is performed by comparing size and circulation of the tip vortices, the radial and streamwise velocity distributions, the spatial expansion of the wake and the axial induction factor. The simulations and measurements generally are in agreement. In some cases, however, the measurements are affected by tunnel effects which are not captured in the simulations. In connection to the actuator disc method a power control strategy for operating conditions below rated power is implemented and tested. The strategy is first validated using an in-house developed blade element momentum code and then is implemented in the actuator disc method used in the EllipSys3D code. The initial tests show that the strategy responds as expected when changing the moment of inertia of the rotor and when varying the inlet conditions. Results from the implementation of the strategy in the actuator disc method in EllipSys3D show that the turbine adapts to the conditions it is operating in by changing its rotational velocity and power output when the inlet conditions are varied. / <p>QC 20130111</p>