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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
71

COPS: Cluster optimized proximity scaling

Rusch, Thomas, Mair, Patrick, Hornik, Kurt January 2015 (has links) (PDF)
Proximity scaling (i.e., multidimensional scaling and related methods) is a versatile statistical method whose general idea is to reduce the multivariate complexity in a data set by employing suitable proximities between the data points and finding low-dimensional configurations where the fitted distances optimally approximate these proximities. The ultimate goal, however, is often not only to find the optimal configuration but to infer statements about the similarity of objects in the high-dimensional space based on the the similarity in the configuration. Since these two goals are somewhat at odds it can happen that the resulting optimal configuration makes inferring similarities rather difficult. In that case the solution lacks "clusteredness" in the configuration (which we call "c-clusteredness"). We present a version of proximity scaling, coined cluster optimized proximity scaling (COPS), which solves the conundrum by introducing a more clustered appearance into the configuration while adhering to the general idea of multidimensional scaling. In COPS, an arbitrary MDS loss function is parametrized by monotonic transformations and combined with an index that quantifies the c-clusteredness of the solution. This index, the OPTICS cordillera, has intuitively appealing properties with respect to measuring c-clusteredness. This combination of MDS loss and index is called "cluster optimized loss" (coploss) and is minimized to push any configuration towards a more clustered appearance. The effect of the method will be illustrated with various examples: Assessing similarities of countries based on the history of banking crises in the last 200 years, scaling Californian counties with respect to the projected effects of climate change and their social vulnerability, and preprocessing a data set of hand written digits for subsequent classification by nonlinear dimension reduction. (authors' abstract) / Series: Discussion Paper Series / Center for Empirical Research Methods
72

Topologieoptimierung im Creo-Umfeld mit ProTopCI

Simmler, Urs 22 July 2016 (has links) (PDF)
Wikipedia umschreibt die Topologieoptimierung als ein computerbasiertes Berechnungsverfahren, durch welches eine günstige Grundgestalt (Topologie) für Bauteile unter mechanischer Belastung ermittelt werden kann. Durch die Verwendung von 3D-Druck-Verfahren wird die Gestaltung der Komponenten revolutioniert, weil diese nicht mehr abhängig vom Fertigungsverfahren sind. Dabei werden auch optimale Gitterstrukturen innerhalb der Komponenten immer wichtiger. Diese neuen Herausforderungen können im Creo Umfeld mit ProTopCI (Hersteller CAESS, PTC Partner Advantage, Silver) elegant gelöst werden. Im Vortrag (mit Live-Demonstration) werden die neuen Möglichkeiten dieser innovativen Lösung beleuchtet: Modellerzeugung in Creo Simulate (FEM-Mode): - Verschiedene Lastfälle, - Kontakte, - Schraubenverbindungen, - CAD-Geometrie, - zu optimierende Bereiche, ... Technologische Randbedingungen zur Berücksichtigung des Fertigungsverfahren Innovatives Erzeugen/Optimieren der Gitterstrukturen Glätten, Exportieren der optimierten Geometrie
73

Pharmacokinetic-Pharmacodynamic Evaluations and Experimental Design Recommendations for Preclinical Studies of Anti-tuberculosis Drugs

Chen, Chunli January 2017 (has links)
Tuberculosis is an ancient infectious disease and a leading cause of death globally. Preclinical research is important for defining drugs and regimens which should be carried forward to human studies. This thesis aims to characterize the population pharmacokinetics and exposure-response relationships of anti-tubercular drugs alone and in combinations, and to suggest experimental designs for preclinical settings. The population pharmacokinetics of rifampicin, isoniazid, ethambutol and pyrazinamide were described for the first time in two mouse models. This allowed for linking the population pharmacokinetic model to the Multistate Tuberculosis Pharmacometric (MTP) model for biomarker response, which was used to characterize exposure-response relationships in monotherapy. Pharmacodynamic interactions in combination therapies were quantitatively described by linking the MTP model to the General Pharmacodynamic Interaction (GPDI) model, which provided estimates of single drug effects together with a quantitative model-based evaluation framework for evaluation of pharmacodynamic interactions among drugs in combinations. Synergism (more than expected additivity) was characterized between rifampicin and ethambutol, while antagonism (less than expected additivity) was characterized between rifampicin and isoniazid in combination therapies. The new single-dose pharmacokinetic design with enrichened individual sampling was more informative than the original design, in which only one sample was taken from each mouse in the pharmacokinetic studies. The new oral zipper design allows for informative pharmacokinetic sampling in a multiple-dose administration scenario for characterizing pharmacokinetic-pharmacodynamic relationships, with similar or lower bias and imprecision in parameter estimates and with a decreased total number of animals required by up to 7-fold compared to the original design. The optimized design for assessing pharmacodynamic interactions in the combination therapies, which was based on EC20, EC50 and EC80 of the single drug, provided lower bias and imprecision than a conventional reduced four-by-four microdilution checkerboard design at the same total number of samples required, which followed the 3Rs of animal welfare. In summary, in this thesis the population pharmacokinetic-pharmacodynamic models of first-line drugs in mice were characterized through linking each population pharmacokinetic model to the MTP model. Pharmacodynamic interactions were quantitatively illustrated by the MTP-GPDI model. Lastly, experimental designs were optimized and recommended to both pharmacokinetic and pharmacodynamic studies for preclinical settings.
74

House prices, capital inflows and macroprudential policy

Mendicino, Caterina, Punzi, Maria Teresa 12 1900 (has links) (PDF)
This paper evaluates the monetary and macroprudential policies that mitigate the procyclicality arising from the interlinkages between current account deficits and financial vulnerabilities. We develop a two-country dynamic stochastic general equilibrium (DSGE) model with heterogeneous households and collateralised debt. The model predicts that external shocks are important in driving current account deficits that are coupled with run-ups in house prices and household debt. In this context, optimal policy features an interest-rate response to credit and a LTV ratio that countercyclically responds to house price dynamics. By allowing an interest-rate response to changes in financial variables, the monetary policy authority improves social welfare, because of the large welfare gains accrued to the Savers. The additional use of a countercyclical LTV ratio that responds to house prices, increases the ability of borrowers to smooth consumption over the cycle and is Pareto improving. Domestic and foreign shocks account for a similar fraction of the welfare gains delivered by such a policy. (authors' abstract)
75

Análise de risco de obras subterrâneas em maciços rochosos fraturados / Risk analysis of underground structures in fractured rock masses

Napa García, Gian Franco 11 June 2015 (has links)
Nesta tese o autor estabelece um método sistemático de quantificação de risco em obras subterrâneas em maciço rochoso fraturado utilizando de maneira eficiente conceitos de confiabilidade estrutural. O método é aplicado a um caso de estudo real da caverna da Usina Hidrelétrica Paulo Afonso IV, UHE-PAIV. Adicionalmente, um estudo de otimização de projeto com base em risco quantitativo também é apresentado para mostrar as potencialidades do método. A estimativa do risco foi realizada de acordo com as recomendações da Organização de Auxílio contra Desastres das Nações Unidas, UNDRO, onde o risco pode ser estimado como a convolução entre as funções de perigo, vulnerabilidade e perdas. Para a quantificação da confiabilidade foram utilizados os métodos de aproximação FORM e SORM com uso de acoplamento direto e de superfícies de resposta polinomial quadráticas. A simulação de Monte Carlo também foi utilizada para a quantificação da confiabilidade no estudo de caso da UHE-PAIV devido à ocorrência de múltiplos modos de falha simultâneos. Foram avaliadas as ameaças de convergência excessiva das paredes, colapso da frente de escavação e a queda de blocos. As funções de perigo foram estimadas em relação à intensidade da ameaça como razão de deslocamento da parede ou volume do bloco. No caso da convergência excessiva, um túnel circular profundo foi estudado com o intuito de comparar a qualidade de aproximação da técnica numérica (FLAC3D com acoplamento direto) em relação à solução exata. Erros inferiores a 0,1% foram encontrados na estimativa do índice de confiabilidade ß. Para o caso da estabilidade de frente foram comparadas duas soluções da análise limite da plasticidade contra a solução obtida numericamente. Já no caso de queda de bloco, verificou-se que as recomendações de parcialização do sistema de classificação geomecânica Q incrementa consideravelmente a segurança da escavação conduzindo a padrões da prática mais avançada, por exemplo, de um ß de 2,04 para a escavação a seção plena até 4,43 para o vão recomendado. No estudo de caso, a segurança da caverna da UHE-PAIV foi estudada perante a queda de blocos utilizando o software Unwedge. A probabilidade de falha individual foi integrada no comprimento da caverna e o conceito de sistema foi utilizado para estimar a probabilidade de falha global. A caverna apresentou uma probabilidade de falha global de 3,11 a 3,22% e um risco de 7,22x10-3 x C e 7,29x10-3 x C, sendo C o custo de falha de um bloco de grandes dimensões. O bloco mais crítico apresentou um ß de 3,63. No estudo de otimização foram utilizadas duas variáveis de projeto, a espessura do concreto projetado e o número de tirantes por metro quadrado. A configuração ótima foi encontrada como o par [t, nb] que minimiza a função de custo total. Também, um estudo de sensibilidade foi realizado para avaliar as influências de alguns parâmetros no projeto ótimo da escavação. Finalmente, os resultados obtidos sugerem que as análises quantitativas de risco, como base para a avaliação e gestão de risco, podem e devem ser consideradas como diretriz da prática da engenharia geotécnica, uma vez que estas análises conciliam os conceitos básicos de projeto como eficiência mecânica, segurança e viabilidade financeira. Assim, a quantificação de risco é plenamente possível. / In this thesis the author establishes a systematic method for quantifying the risk in underground structures in fractured rock masses using structural reliability concepts in an efficient way. The method is applied to the case study of the underground cavern of Paulo Afonso IV Hydroelectrical Power Station UHE-PAIV. Additionally, an optimization study was conducted in order to show a potential application of the method. The estimation of the risk was done according to the recommendations of the United Nations Disaster Relief Organization UNDRO where risk can be estimated as the convolution between the hazard, vulnerability and losses functions. FORM and SORM were used as approximation methods for the reliability quantification by means of Direct Coupling and Quadratic Polynomial Response Surfaces. A Monte Carlo simulation was also used to quantify the reliability of the cavern UHE-PAIV because of the presence of multiple failure modes in the numerical model. In this study 3 types of threads were evaluated: excessive wall convergence, face stability and wedge block fall. Hazard functions were built relative to the thread intensities such as wall convergence ratio or block size. In the case of excessive wall convergence a deep circular tunnel was studied meaning to compare the quality of the approximation of the reliability technique (FLAC3D with direct coupling) to the exact solution. Errors below 0.1% were found in the reliability index ß estimation. The reliability of the face stability was evaluated using two limit analysis solutions against the numeric estimation. For the block stability it was verified that the sequential excavation recommended by the Q system increases considerably the reliability of the excavation leading safety to modern standard levels, e.g. from a ß equal to 2.04 for a full section excavation to 4.43 for a partial excavation. In the case study of the UHE-PAIV, the reliability of the underground cavern was estimated using the commercial software Unwedge. The probability of failure of individual blocks was integrated along the length of the cavern and the concept of structural system was used to estimate the global probability of failure. The cavern presented a probability of failure of 3.11% to 3.22% and a risk of 7.22x10-3 x C and 7.29x10-3 x C - where C is the cost of failure of a large block. The critical individual block showed a ß equal to 3.63. The optimization was performed considering two design variables − liner thickness and number of bolt per square meter. The optimal design was found as the pair, [t, nb] which minimizes the total cost function. Also, a sensibility analysis was conducted to understand the influence of some parameters in the location of the optimal excavation design. Concluding, the results obtained here suggest that the quantitative risk analyses, as a base for the risk assessment and management, can and must be considered as a north for the practice of geotechnical engineering owing that these analyses reconcile the basic concepts of mechanical efficiency, safety and financial feasibility. Thus, risk quantification is fully affordable.
76

Influência dos métodos de lubri-refrigeração na retificação do aço ABNT 4340 usando rebolo de CBN

Alves, Manoel Cleber de Sampaio [UNESP] 19 October 2007 (has links) (PDF)
Made available in DSpace on 2014-06-11T19:31:04Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-10-19Bitstream added on 2014-06-13T18:41:35Z : No. of bitstreams: 1 alves_mcs_dr_bauru.pdf: 4541684 bytes, checksum: 03e1d27ea6b721a409bebc9862b0e67f (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / É analisada a influência das técnicas de mínima quantidade de lubrificante (MQL), refrigeração otimizada e refrigeração convencional, com diferentes vazões e velocidade de aplicação do fluido de corte, na qualidade das peças produzidas (acabamento, erros de forma, trincas, queima, perda da dureza) com aços endurecidos, no processo de retificação cilíndrica externa de mergulho com a utilização de rebolos superabrasivos com baixa concentração de CBN. Utilizou-se fluidos de corte de origem vegetal, que representam menos riscos à saúde e ao meio. A análise da qualidade das peças foi realizada através da avaliação das variáveis de saída do processo como o comportamento da força tangencial de corte, energia específica de retificação, rugosidade, desvio de circularidade, emissão acústica, tensão residual, análise de microscopias eletrônicas de varredura e microdureza. Com a análise das formas de aplicação e das várias vazões e velocidades do fluido de corte utilizadas encontraram-se condições de lubri-refrigeração que propiciam a diminuição do volume de fluido de corte, diminuição do tempo de usinagem sem prejudicar os parâmetros geométicos, dimensionais, o acabamento superficial e a integridade superficial dos componentes. Em relação as diferentes formas de aplicação do fluido de corte notou-se o melhor desempenho da aplicação otimizada para maiores velocidades mostrando a eficiência de um novo conceito de bocal utilizado. O processo otimizado e o processo MQL foram capazes de manter a dureza superficial e a integridade superficial das peças produzidas. Exceção somente para a condição MQL com vazão de fluido de corte de 40ml/h que produziu trincas e retêmpera superficial. O desvio de circularidade e a rugosidade proporcionada pelos métodos estão dentro de valores recomendados para a retificação sendo que a técnica otimizada... / The influences on surface integrity of the parts (finishing, geometrical errors, cracks, burn, microhardness) and grinding wheel (wear) have been analyzed in this research by employing the minimum quantity of lubrication (MQL), optimized and conventional cooling for hardened parts of steel and superabrasives grinding wheel in the cylindrical plunge grinding process. This study was carried out through the assessment of the process output variables such as the behavior of the tangential cutting force, specific energy, surface roughness, circularity errors, acoustic emission, residual stress, scanning electronic microscopy (SEM) and microhardness. The cutting fluid derived from vegetal oil has been utilized in order to comply with the environmental requirements and thus offering less risk to the health. It could be observed from the analysis on different ways of cutting fluid applications that there are cooling conditions which facilitate the decrease in cutting fluid volume, decrease in grinding time without impairing the geometrical and dimensional parameters, the surface finish and surface integrity of the parts. It could be noted the optimized application for higher velocities has presented the best performance regarding the different applications of cutting fluids. The optimized and MQL processes were able to maintain the hardness and surface integrity of the ground parts. Exception occurend only for MQL condition with flow rate of 40ml/h, which led to cracks and quench on the workpiece surface. Roundness errors and surface roughness are within recommended valus for grinding with the best results obtained for the optimized method (2'mü' for roundness error and 0.5'mü'm for surface roughness) compared with the MQL technique (6.8'mü'm for roundness error and 1.2'mü'm for surface roughness). The process of minimum quantity of lubrication, however, has showed possible... (Complete abstract click electronic access below)
77

Diagnóstico otimizado de transformadores de potência mediante a integração de técnicas preditivas / Optimized diagnosis of power transformers via the integration of predictive techniques

Marques, André Pereira 22 February 2018 (has links)
Submitted by Franciele Moreira (francielemoreyra@gmail.com) on 2018-04-04T12:41:38Z No. of bitstreams: 2 Tese - André Pereira Marques - 2018.pdf: 11271921 bytes, checksum: 086b8deaeaa9f31cd4e5ea975724a254 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2018-04-04T13:27:18Z (GMT) No. of bitstreams: 2 Tese - André Pereira Marques - 2018.pdf: 11271921 bytes, checksum: 086b8deaeaa9f31cd4e5ea975724a254 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2018-04-04T13:27:18Z (GMT). No. of bitstreams: 2 Tese - André Pereira Marques - 2018.pdf: 11271921 bytes, checksum: 086b8deaeaa9f31cd4e5ea975724a254 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2018-02-22 / Fundação de Amparo à Pesquisa do Estado de Goiás - FAPEG / The effective application of predictive maintenance techniques for power transformers is essential for the early detection of defects and failures, and in order to trigger scheduled preventive or corrective actions. This precludes the need for urgent and/or emergency interventions, aiming to reduce unscheduled power outages, which are usually more costly and cause great inconvenience to consumers and the electric power system. In this context, this work focuses on the development of an optimized diagnostic methodology for power transformers, by integrating eleven predictive techniques (with 27 quantities). These techniques are analyzed with respect to voltage level, type of electrical connection and age of the transformers – when applicable –, based on the experience of specialists/analysts, on standards and technical guidelines, and on statistical analyses of a database of actual field and laboratory tests. The analyses of the validation of the results are significant and presented through a specialized system, demonstrating their practical application. The novelty of this thesis consists in the development of an original classification method, called Normalized Doubly Weighted Sum (NDWS), and in the detailed description of the predictive techniques that are integrated, their functionalities, the creation of equations, the definition of criteria and parameters, with their weights and scores for the classification of “A” (excellent) to “E” (very poor), and recommended actions aimed at underpinning decision- making, thereby contributing to the body of studies in this field. Hence, it can be concluded that this work offers a comprehensive and efficient tool to aid in the optimized diagnosis of power transformers, insulated with kraft paper and immersed in mineral insulating oil, providing effective diagnostics and maintenance of these devices, and thus increasing the reliability of electric power systems. / A eficiência da aplicação de técnicas preditivas de manutenção em transformadores de potência é fundamental para se detectar defeitos e falhas ainda em estágio incipiente, disparar ações preventivas ou corretivas programadas, e assim evitar intervenções em caráter de urgência e ou emergência, visando a diminuição de interrupções não programadas no fornecimento de energia elétrica, as quais são comumente mais onerosas e causadoras de grandes transtornos aos consumidores e ao sistema elétrico. Neste sentido, os objetivos deste trabalho baseiam-se em apresentar o desenvolvimento de uma metodologia de diagnóstico otimizado destes equipamentos, mediante a integração de onze técnicas preditivas (com vinte e sete grandezas), analisando-as em relação ao nível de tensão, tipo de ligação elétrica e idade dos transformadores – quando aplicável – e de acordo com o seguinte embasamento: na experiência de especialistas/analistas; em normas e orientações técnicas; e em análises estatísticas de uma base de dados de ensaios reais realizados em campo e em laboratório. As análises com a validação dos resultados são significativas e apresentadas por meio de um sistema especialista, mostrando a sua aplicação prática. O ineditismo desta tese refere-se ao desenvolvimento de um método de classificação original, denominado Soma Duplamente Ponderada Normalizada (SDPN), e à descrição detalhada sobre quais técnicas preditivas são integradas, suas funcionalidades, elaboração de equações, a definição de critérios e de parâmetros, com seus pesos e suas notas para a classificação de “A” (excelente) a “E” (péssimo), e ações recomendadas que auxiliam nas tomadas de decisão, contribuindo com os estudos na área. Portanto, conclui-se que este trabalho possui uma aplicação abrangente e eficiente como ferramenta para o subsídio ao diagnóstico otimizado de transformadores de potência, isolados em papel kraft e imersos em óleo mineral isolante, proporcionando diagnósticos e manutenções efetivas nestes equipamentos e, consequentemente, impactando o aumento da confiabilidade do sistema elétrico.
78

Mathematical Models for Investigation of Performance, Safety, and Aging in Lithium-Ion Batteries

Zavalis, Tommy Georgios January 2013 (has links)
Rechargeable lithium-ion batteries have both the power and energy capabilities to be utilized in hybrid electric vehicles and other power demanding applications. However, there are obstacles primarily related to reliability in safety and lifetime. Additionally, there is still room for improvement in the battery performance. In this work, physics-based mathematical models have been successfully set-up and numerically solved to investigate performance, safety, and aging in lithium-ion battery systems. This modeling approach enabled a detailed analysis of the electrochemical processes related to these issues. As the models included many parameters and spatial resolution of several variables with time or frequency, strategies for investigation needed to be developed for most of the work. The accuracy of the investigation was consolidated by the utilization of parameters characterized from experimental work. The performance expressed in terms of polarization was determined for a power-optimized battery cell undergoing various operating conditions. A methodology that separated and quantified the contribution of each process to the polarization was set up, allowing the study of the contributions as a snapshot in time and as an average over a cycle. Mass transport in electrolyte was shown to be a crucial feature to improve especially if the battery is expected to undergo high current-loads for long periods of time. Safety-concerns when a battery cell is short-circuited were investigated for three types of short-circuit scenarios. All scenarios raised the temperature to the point where exothermic side reactions were initiated. The similarities between the scenarios in temperature increase were a result of the limiting current being reached. The differences, however small, were related to the placement of the short-circuit. Especially when the current collectors were not directly connected by the short circuit, an increased electronic resistance was observed which lowered both the generated current and heat. The aging of a battery cell was investigated by model analysis of electrodes harvested from fresh and aged cells. A methodology was used where a frequency-dependent model was fitted to three-electrode impedance experiments by tuning parameters associated to electrode degradation. For cycled cells, electrolyte decomposition products inhibiting the mass transport in the electrolyte and particle cracking in the positive electrode increased the impedance. A similar model was also set up for investigation of the lithium intercalation processes in PAN-based carbon fibers, showing it to have both good mass transport and kinetic capabilities. / Laddningsbara litiumjonbatterier har både ur energi- och effektsynpunkt möjligheten att kunna användas i elhybridfordon och inom andra effektkrävande tillämpningsområden. Batteriets säkerhet och livslängd är dock inte helt tillförlitliga. Dessutom finns det fortfarande utrymme för förbättringar av litiumjonbatteriets prestanda. I det här arbetet har matematiska modeller baserade på fysikaliska egenskaper framgångsrikt ställts upp och lösts numeriskt för att studera prestandan, säkerheten samt åldrandet hos litiumjonbatterisystem. Denna typ av modellering gjorde det möjligt att detaljerat analysera hur de elektrokemiska processerna bidrar. Eftersom modellerna omfattade ett stort antal parametrar och har variabler som förändras i åtminstone en dimension med tid eller frekvens, krävdes det att tydliga strategier för arbetet ställdes upp. Modelleringsstudiens noggrannhet stärktes av att flertalet av de använda parametrarna hade bestämts experimentellt. Polarisationen som ett mått på prestanda bestämdes för ett effektoptimerat batteri under olika laster. En metodik som separerar och beräknar hur mycket varje process bidrar till polarisationen skapades och användes för att studera bidragen över tid eller över en hel lastcykel. Resultaten visade att masstransporten i elektrolyten påverkar till stor del och bör förbättras om batteriet förväntas belastas med hög ström under lång tid. Säkerheten i samband med kortslutning av en battericell undersöktes för tre olika fall av kortslutningar. Alla fall uppvisade en temperaturökning som skulle kunna bidra till att exoterma reaktioner startas och termisk rusning uppstår. Temperaturökningen var liknande i samtliga kortslutningsfall och berodde på att gränsströmmen nåddes inom cellen. Skillnaderna mellan kortslutningsfallen var inte så betydande men kunde härledas till kortslutningens placering. Framförallt fallet då strömtilledarna inte kontakterades av kortslutningen observerades en ökad elektronisk resistans som sänkte både strömmen och värmeproduktionen. Åldringen i en battericell undersöktes genom modellanalys av elektroder som tagits från nya eller åldrade celler. Som metod användes en frekvensberoende modell som anpassades till tre-elektrod-impedansmätningar genom förändring av parametrar som beskriver elektrodnedbrytning. Då cellerna cyklats, visade förändringen av dessa parametrar att impedansen ökar på grund av nedbrytningsprodukter från elektrolyten som hindrar masstransporten och att det aktiva materialet i positiva elektroden spricker. En liknande modell användes också till att undersöka PAN-baserade kolfibrers förmåga att interkalera litium och resultaten visade på att den har mycket goda elektrokemiska egenskaper. / <p>QC 20130520</p>
79

Optimal allocation of thermodynamic irreversibility for the integrated design of propulsion and thermal management systems

Maser, Adam Charles 13 November 2012 (has links)
More electric aircraft systems, high power avionics, and a reduction in heat sink capacity have placed a larger emphasis on correctly satisfying aircraft thermal management requirements during conceptual design. Thermal management systems must be capable of dealing with these rising heat loads, while simultaneously meeting mission performance. Since all subsystem power and cooling requirements are ultimately traced back to the engine, the growing interactions between the propulsion and thermal management systems are becoming more significant. As a result, it is necessary to consider their integrated performance during the conceptual design of the aircraft gas turbine engine cycle to ensure that thermal requirements are met. This can be accomplished by using thermodynamic modeling and simulation to investigate the subsystem interactions while conducting the necessary design trades to establish the engine cycle. As the foundation for this research, a parsimonious, transparent thermodynamic model of propulsion and thermal management systems performance was created with a focus on capturing the physics that have the largest impact on propulsion design choices. A key aspect of this approach is the incorporation of physics-based formulations involving the concurrent usage of the first and second laws of thermodynamics to achieve a clearer view of the component-level losses. This is facilitated by the direct prediction of the exergy destruction distribution throughout the integrated system and the resulting quantification of available work losses over the time history of the mission. The characterization of the thermodynamic irreversibility distribution helps give the designer an absolute and consistent view of the tradeoffs associated with the design of the system. Consequently, this leads directly to the question of the optimal allocation of irreversibility across each of the components. An irreversibility allocation approach based on the economic concept of resource allocation is demonstrated for a canonical propulsion and thermal management systems architecture. By posing the problem in economic terms, exergy destruction is treated as a true common currency to barter for improved efficiency, cost, and performance. This then enables the propulsion systems designer to better fulfill system-level requirements and to create a system more robust to future requirements.
80

Range finding in passive wireless sensor networks using power-optimized waveforms

Trotter, Matthew 14 November 2011 (has links)
Passive wireless sensor networks (WSNs) are quickly becoming popular for many applications such as article tracking, position location, temperature sensing, and passive data storage. Passive tags and sensors are unique in that they collect their electrical energy by harvesting it from the ambient environment. Tags with charge pumps collect their energy from the signal they receive from the transmitting source. The efficiency of converting the received signal to DC power is greatly enhanced using a power-optimized waveform (POW). Measurements in the first part of this dissertation show that a POW can provide efficiency gains of up to 12 dB compared to a sine-wave input. Tracking the real-time location of these passive tags is a specialized feature used in some applications such as animal tracking. A passive WSN that uses POWs for the improvement of energy-harvesting may also estimate the range to a tag by measuring the time delay of propagation from the transmitter to the tag and back to the transmitter. The maximum-likelihood (ML) estimator is used for estimating this time delay, which simplifies to taking the cross-correlation of the received signal with the transmitted signal. This research characterizes key aspects of performing range estimations in passive WSNs using POWs. The shape of the POW has a directly-measurable effect on ranging performance. Measurements and simulations show that the RMS bandwidth of the waveform has an inversely proportional relationship to the uncertainty of a range measurement. The clutter of an environment greatly affects the uncertainty and bias exhibited by a range estimator. Random frequency-selective environments with heavy clutter are shown to produce estimation uncertainties more than 20 dB higher than the theoretical lower bound. Estimation in random frequency-flat environments is well-behaved and fits the theory quite nicely. Nonlinear circuits such as the charge pump distort the POW during reflection, which biases the range estimations. This research derives an empirical model for predicting the estimation bias for Dickson charge pumps and verifies it with simulations and measurements.

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