Doctor of Philosophy / We investigate the linear growth and vertical structure of the magnetorotational instability (MRI) in weakly ionised, stratified accretion discs. The magnetic field is initially vertical and perturbations have vertical wavevectors only. Solutions are obtained at representative radial locations from the central protostar for different choices of the initial magnetic field strength, sources of ionisation, disc structure and configuration of the conductivity tensor. The MRI is active over a wide range of magnetic field strengths and fluid conditions in low conductivity discs. For the minimum-mass solar nebula model, incorporating cosmic ray and x-ray ionisation and assuming that charges are carried by ions and electrons only, perturbations grow at 1 AU for B < 8G. For a significant subset of these strengths (200mG < B < 5 G), the growth rate is of order the ideal MHD rate (0.75 Omega). Hall conductivity modifies the structure and growth rate of global unstable modes at 1 AU for all magnetic field strengths that support MRI. As a result, at this radius, modes obtained with a full conductivity tensor grow faster and are active over a more extended cross-section of the disc, than perturbations in the ambipolar diffusion limit. For relatively strong fields (e.g. B > 200 mG), ambipolar diffusion alters the envelope shapes of the unstable modes, which peak at an intermediate height, instead of being mostly flat as modes in the Hall limit are in this region of parameter space. Similarly, when cosmic rays are assumed to be excluded from the disc by the winds emitted by the magnetically active protostar, unstable modes grow at this radius for B < 2 G. For strong fields, perturbations exhibit a kink at the height where x-ray ionisation becomes active. Finally, for R = 5 AU (10 AU), unstable modes exist for B < 800 mG (B < 250 mG) and the maximum growth rate is close to the ideal-MHD rate for 20 mG < B < 500 mG (2 mG < B < 50 mG). Similarly, perturbations incorporating Hall conductivity have a higher wavenumber and grow faster than solutions in the ambipolar diffusion limit for B < 100 mG (B < 10 mG). Unstable modes grow even at the midplane for B > 100 mG (B ~ 1 mG), but for weaker fields, a small dead region exists. When a population of 0.1 um grains is assumed to be present, perturbations grow at 10 AU for B < 10 mG. We estimate that the figure for R = 1 AU would be of order 400 mG. We conclude that, despite the low magnetic coupling, the magnetic field is dynamically important for a large range of fluid conditions and field strengths in protostellar discs. An example of such magnetic activity is the generation of MRI unstable modes, which are supported at 1 AU for field strengths up to a few gauss. Hall diffusion largely determines the structure and growth rate of these perturbations for all studied radii. At radii of order 1 AU, in particular, it is crucial to incorporate the full conductivity tensor in the analysis of this instability, and more generally, in studies of the dynamics of astrophysical discs.

No description available.

No description available.

Diss. (sammanfattning) Göteborg : Univ. , 2007. / Härtill 4 uppsatser.

Diss. (sammanfattning) Stockholm : Karol. inst., 2005. / Härtill 5 uppsatser.

Dissertation (Ph.D.).--University of Texas Graduate School of Biomedical Sciences at San Antonio, 2007. / Vita. Includes bibliographical references.

Όταν ένα εναλλασσόμενο ηλεκτρικό πεδίο, χαμηλής συχνότητας, εφαρμόζεται κάθετα σε ένα στρώμα νηματικού υγρού κρυστάλλου, με πάχος της τάξης από 10 μm έως 100 μm, παρατηρείται ένας αναπροσανατολισμός του πεδίου του κατευθυντή. Πρόκειται για την περίπτωση της μονοδιάστατης μετάβασης Freedericsz, όπως επίσης και για τη δισδιάστατη ή τρισδιάστατη περίπτωση της ηλεκτροϋδροδυναμικής αστάθειας. Λόγω της διπλοθλαστικότητας των νηματικών υγρών κρυστάλλων, η έναρξη της αστάθειας, και στις δύο περιπτώσεις, προκαλεί σε μονοχρωματική φωτεινή δέσμη, που προσπίπτει στο νηματικό στρώμα, αλλαγή στην ένταση ή/και στη φάση της, με επακόλουθο ιδιαίτερα οπτικά αποτελέσματα. Οι αστάθειες παρατηρούνται όταν η εξωτερικά εφαρμοζόμενη εναλλασσόμενη τάση είναι υψηλότερη από μία τιμή κατωφλίου, ο προσδιορισμός της οποίας έχει ιδιαίτερη σημασία σε πιθανές ηλεκτροοπτικές εφαρμογές των νηματικών υγρών κρυστάλλων. Το κατώφλι της αστάθειας καθορίζεται μέσω της παρατήρησης του οπτικού αποτελέσματος, καθώς η φωτεινή δέσμη προσπίπτει στο νηματικό στρώμα. Όταν η εξωτερικά εφαρμοζόμενη τάση είναι χαμηλότερη ή ίση με την τιμή κατωφλίου, το οπτικό αποτέλεσμα είναι μηδενικό. Καθώς η τιμή της τάσης αυξάνει, σταδιακά εμφανίζεται, όλο και πιο έντονα, το ιδιαίτερο οπτικό αποτέλεσμα της αστάθειας. Έχοντας υπόψη την παραπάνω πειραματική συμπεριφορά, μπορεί κανείς να υπολογίσει την τάση κατωφλίου αρκεί να καθορίσει, μέσω οπτικής παρατήρησης, το σημείο έναρξης της αστάθειας. Στην παρούσα εργασία παρουσιάζεται μία αντικειμενική μέθοδος πειραματικού προσδιορισμού της ηλεκτροϋδροδυναμικής αστάθειας, βασισμένη στην εξάρτηση του χρόνου αποκατάστασης του κατευθυντή, Τ, από την τιμή του εξωτερικά εφαρμοζόμενου ηλεκτρικού πεδίου. Όταν η εφαρμοζόμενη τάση τείνει στην τιμή κατωφλίου, ο ρυθμός εξασθένισης, 1/Τ, του πεδίου του κατευθυντή τείνει στο μηδέν. Επομένως, η γραφική παράσταση του 1/Τ σε συνάρτηση με την εξωτερικά εφαρμοζόμενη τάση καθιστά δυνατό τον προσδιορισμό της τάσης κατωφλίου της αστάθειας. / When an AC electric field in the acoustic frequency range is applied perpendicularly to a nematic liquid crystal layer, with a thickness of the order of 10μm to 100μm, a reorientation of the director of the nematic layer is observed. This is the case of the one dimensional Freedericsz instability, as well as of the two or three dimensional electrohydrodynamic instability. Due to the birefringence of any nematic material, the onset of the instability in both cases causes a monochromatic light beam illuminating the nematic layer, to change its intensity and/or its phase, resulting in spectacular optical effects. On the other hand, the instabilities are observed when the applied AC voltage is larger than a threshold value, the determination of which is of major importance for the potential electro-optical applications of nematic liquid crystals. As a rule, the instability threshold is determined by observing its optical effect on the incident of the monochromatic light beam : When the applied voltage is lower than or equal to its threshold value, the optical effect is zero. Upon further increasing of the applied voltage, the optical effect increases gradually. The measurement of the threshold voltage, based on the above experimental behavior, leads to a subjective estimation of the experimental result. One has to decide which optical result has to be considered to mark the onset of the instability. In what follows, we present a completely objective method for the experimental determination of the instability threshold, based on the dependence of the decay time, T, of the director field on the value of the electric field applied across the nematic layer. When the applied voltage tends to its threshold value, the decay rate, 1/T, of the director field tends to zero. Thus, plotting 1/T as a function of the applied voltage, enables us to graphically determine the threshold value of the latter.

Submitted by Eduardo Felipe de Campos null (eduardo81010@aluno.feis.unesp.br) on 2018-01-11T17:05:53Z No. of bitstreams: 1 DISSERTACAO.pdf: 18831766 bytes, checksum: c62a5bf82a73cc4a7517b74b6a078fb4 (MD5) / Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2018-01-11T18:55:13Z (GMT) No. of bitstreams: 1 campos_ef_me_ilha.pdf: 18831766 bytes, checksum: c62a5bf82a73cc4a7517b74b6a078fb4 (MD5) / Made available in DSpace on 2018-01-11T18:55:13Z (GMT). No. of bitstreams: 1 campos_ef_me_ilha.pdf: 18831766 bytes, checksum: c62a5bf82a73cc4a7517b74b6a078fb4 (MD5) Previous issue date: 2017-08-15 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Estruturas de paredes finas são muito utilizadas na engenharia moderna devido sua boa relação entre peso e resistência. Sua aplicação vai desde obras civis, como cúpulas em edifícios e pontes, até estruturas aeronáuticas e equipamentos industriais. Uma geometria que é muito comum de ser encontrada nas estruturas de paredes finas são os cilindros, como pode ser observado no corpo dos aviões, tubulações e vasos de pressão. Submetidas a esforços compressivos, essas estruturas ficam suscetíveis ao fenômeno de instabilidade ou flambagem, podendo entrar em colapso repentinamente, mesmo antes de apresentar altas tensões. Assim, compreender o comportamento da instabilidade se torna crucial em um projeto de estruturas de paredes finas. Um dos fatores que influenciam no comportamento da instabilidade é a imperfeição geométrica da estrutura, e qualquer processo de fabricação possui erros associados ao processo. Para iniciar o estudo foi necessário medir os defeitos devido ao processo de fabricação do cilindro. Duas alternativas para realizar as medidas foram propostas, a primeira utilizando LVDTs, que medem a variação do raio do cilindro ao longo de toda sua extensão, e a segunda o fotoprocessamento. Ambos os métodos nos deram uma nuvem de pontos que foram processadas para obter a estrutura 3D CAD, possibilitando sua análise pelo método de elementos finitos e obtenção das respectivas cargas de flambagem numéricas. Além do método numérico foi realizado um ensaio experimental visando validar a análise numérica. O cilindro foi fechado com duas tampas e montado em uma bancada experimental, instrumentado com LVDTs, manômetro e por fim, uma bomba de vácuo foi ligada a uma das tampas. A bomba criou vácuo dentro do cilindro até o mesmo sofrer colapso. As cargas de flambagem numéricas foram superiores a encontrada experimentalmente em 14,48% a geometria foto, 11,72% da geometria LVDT e 39,98% a do cilindro referência. Os resultados mostram que as imperfeições geométricas são de grande relevância na resistência a instabilidade de uma estrutura, e também que grandes imperfeições geométricas tendem a ser o local onde ocorrem as ondas de flambagem. Em relação ao método de medição por fotoprocessamento os resultados mostram que é interessante estudar a técnica para aplicações na área de instabilidade. / Thin-walled structures are widely used in modern engineering because of their good weight-to-strength ratio. Its application ranges from civil works, such as domes in buildings and bridges, to aeronautical structures and industrial equipment. One geometry that is very common to be found in thin-walled structures is the cylinder, as can be seen in the body of airplanes, pipes and pressure vessels. Under compressive stresses, these structures are susceptible to the buckling phenomenon, which can suddenly collapse, even before presenting high stresses. Therefore, understanding the behavior of instability becomes crucial in designing thin-walled structures. One of the factors influencing the behavior of instability is the geometric imperfection of the structure, and any manufacturing process has errors associated with the process. To begin the study it was necessary to measure the defects due to the cylinder manufacturing process. Two alternatives to carry out the measurements were proposed, the first using LVDTs, which measure the variation of the cylinder radius along its length, and the second the photoprocessing. Both methods gave us a point cloud that were processed to obtain the 3D CAD structure, allowing its analysis by the finite element method and obtaining the respective numerical buckling loads. In addition to the numerical method, an experimental test was carried out to validate the numerical analysis. The cylinder was closed with two flat heads and mounted on an experimental bench, instrumented with LVDTs, manometer and finally, a vacuum pump was attached to one of the flat heads. The pump has created a vacuum inside the cylinder until it collapses. Numerical buckling loads were greater than that found experimentally at 14.48 % photo geometry, 11.72 % LVDT geometry and 39.98 % a reference cylinder. The results show that the geometric imperfections are of great relevance in the resistance to buckling of a structure, and also that great geometric imperfections tend to be the place where the buckling waves occur. Regarding the method of measurement by photoprocessing the results show that it is interesting to study the technique for applications in the area of instability. / CAPES: 1544963

Orientador: Amarildo Tabone Paschoalini / Resumo: Estruturas de paredes finas são muito utilizadas na engenharia moderna devido sua boa relação entre peso e resistência. Sua aplicação vai desde obras civis, como cúpulas em edifícios e pontes, até estruturas aeronáuticas e equipamentos industriais. Uma geometria que é muito comum de ser encontrada nas estruturas de paredes finas são os cilindros, como pode ser observado no corpo dos aviões, tubulações e vasos de pressão. Submetidas a esforços compressivos, essas estruturas ficam suscetíveis ao fenômeno de instabilidade ou flambagem, podendo entrar em colapso repentinamente, mesmo antes de apresentar altas tensões. Assim, compreender o comportamento da instabilidade se torna crucial em um projeto de estruturas de paredes finas. Um dos fatores que influenciam no comportamento da instabilidade é a imperfeição geométrica da estrutura, e qualquer processo de fabricação possui erros associados ao processo. Para iniciar o estudo foi necessário medir os defeitos devido ao processo de fabricação do cilindro. Duas alternativas para realizar as medidas foram propostas, a primeira utilizando LVDTs, que medem a variação do raio do cilindro ao longo de toda sua extensão, e a segunda o fotoprocessamento. Ambos os métodos nos deram uma nuvem de pontos que foram processadas para obter a estrutura 3D CAD, possibilitando sua análise pelo método de elementos finitos e obtenção das respectivas cargas de flambagem numéricas. Além do método numérico foi realizado um ensaio experimental visando vali... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Thin-walled structures are widely used in modern engineering because of their good weight-to-strength ratio. Its application ranges from civil works, such as domes in buildings and bridges, to aeronautical structures and industrial equipment. One geometry that is very common to be found in thin-walled structures is the cylinder, as can be seen in the body of airplanes, pipes and pressure vessels. Under compressive stresses, these structures are susceptible to the buckling phenomenon, which can suddenly collapse, even before presenting high stresses. Therefore, understanding the behavior of instability becomes crucial in designing thin-walled structures. One of the factors influencing the behavior of instability is the geometric imperfection of the structure, and any manufacturing process has errors associated with the process. To begin the study it was necessary to measure the defects due to the cylinder manufacturing process. Two alternatives to carry out the measurements were proposed, the first using LVDTs, which measure the variation of the cylinder radius along its length, and the second the photoprocessing. Both methods gave us a point cloud that were processed to obtain the 3D CAD structure, allowing its analysis by the finite element method and obtaining the respective numerical buckling loads. In addition to the numerical method, an experimental test was carried out to validate the numerical analysis. The cylinder was closed with two flat heads and mounted on an expe... (Complete abstract click electronic access below) / Mestre

Introdução: Instabilidades adquiridas do ombro são uma afecção comum do membro superior na prática esportiva, ocasionadas particularmente pela posição de abdução e rotação lateral da articulação glenoumeral durante movimentos explosivos. O manguito rotador proporciona grande parte da estabilidade dinâmica do ombro, sendo que nos movimentos esportivos, a estabilização necessita ser rápida para evitar a movimentação excessiva da cabeça umeral. O tempo entre a ativação do músculo e a produção de tensão é conhecida como Retardo Eletromecânico (REM), podendo este estar associado à velocidade a qual um músculo transmite sua tensão à articulação. Objetivo: avaliar o REM do músculo infraespinhal durante a rotação lateral (RL), o REM do músculo Peitoral Maior durante a rotação medial (RM) e o atraso entre a ativação do Peitoral Maior e Infraespinhal durante uma RM e relacionar estes eventos com a presença ou não da instabilidade glenoumeral adquirida. O comportamento mecânico do infraespinhal foi avaliado também por meio de mecanomiografia, possibilitando subdividir o REM em relação aos eventos elétricos e mecânicos. Métodos: Os músculos Peitoral Maior e Infraespinhal foram monitorados por eletromiografia (EMG) de superfície (2 kHz); o sinal mecanomiográfico (MMG) (2 kHz) foi coletado do Infraespinhal e os torques (2 kHz) explosivos isométricos de RM e RL do ombro foram coletados na posição do ombro de abdução e rotação lateral a 90° em um dinamômetro. Após uma avaliação funcional do membro superior, 18 indivíduos do sexo masculino, praticantes de atividade esportiva overhead, participaram do estudo, sendo nove com ombros saudáveis e nove com instabilidade glenoumeral anterior. Após a coleta e armazenamento dos sinais, esses foram filtrados e analisados. Os sinais EMG do peitoral maior e EMG e MMG do infraespinhal foram filtrados (EMG 5-500 Hz e MMG 4-400 Hz) e um envoltório linear foi calculado. O início dos sinais foi identificado usando-se o limiar de repouso + 3 desvios padrão para EMG e MMG e 2% do pico de torque para o limiar de força. Os limiares de ativação foram usados para calcular o início de cada sinal (EMG, MMG e Torque). Os intervalos de tempo entre os eventos foram mensurados e comparados entre os grupos (ombro instável, ombro contra-lateral e ombro saudável). Os picos de torque e taxa de produção do torque de RM e RL foram calculados para todos os grupos. Resultados: O REM do infraespinhal em ombros com instabilidade e nos ombros contralaterais ao instável foi menor do que nos ombros saudáveis. Pico de Torque, Taxa de Produção de Torque, atraso entre EMG do peitoral maior e EMG do infraespinhal e REM do peitoral maior não apresentaram diferenças significativas entre os grupos. Conclusão: Ombros instáveis e contralaterais aos instáveis apresentam adaptação crônica do manguito rotador com diminuição do REM. O REM está possivelmente associado ao aumento da rigidez dos elementos elásticos em série. O aumento da demanda pelos estabilizadores dinâmicos em decorrência da falência dos mecanismos estáticos de estabilização do ombro poderia explicar tal adaptação do infraespinhal. / Introduction: Acquired shoulder instabilities are a common upper limb injury in sports, mostly related to abduction and external rotation of glenohumeral joint during explosive contractions. Most of the dynamic stabilization of the shoulder joint is provided by the rotator cuff. In sports movements the stabilization must be quick to avoid humeral head excessive motion. The time between muscle activation and force production is named Electromechanical Delay (EMD), and is related to the speed of transmited tension to the joint/bone. Objetive: Evaluate the EMD in the infraspinatus muscle during External Rotation (ER), the EMD of Pectoralis Major during an Internal Rotation (IR) and the delay between Pectoralis Major and Infraspinatus activation during an IR and relate those mesurements to the shoulder stability/instability. The mechanical behaviour of the infraspinatus muscle was also assessed by mechanomyography, enabling to subdivide the EMD with respect to the electrical and mechanical events. Methods: Pectoralis Major and Infraspinatus muscles were monitored by surface Electromyography (EMG) (2kHz); the mechanomyographic (MMG) signal was collected from infraspinatus muscle (2kHz) and the isometric explosive IR and ER of shoulder were collected at 90° of abduction and external rotation on a dynamometer. After a functional evaluation of the shoulder, 18 male subjects, overhead sports participate in the study, nine with stable shoulders and nine with anterior shoulders instability. After data collection and storage, the signals were filtered and analysed. The EMG signals from Pectoralis Major and EMG and MMG from infraspinatus were filtered (EMG 5-500 Hz and MMG 4-400 Hz) and a linear envelope was calculated. The signal onset was identified using the threshold of resting signal plus 3 standart deviations for EMG and MMG and 2% of peak torque to torque threshold. The thresholds were used to calculate the beginning of each signal (EMG, MMG and Torque). The time delays between events were measured and compared between the groups (stable, unstable and contralateral to the unstable). Peak Torque and Rate of Torque Production of IR and ER were calculated to all groups. Results: The infraspinatus EMD in the unstable and contralateral to unstable shoulders were smaller than the healthy shoulders. Peak Torque, Rate of Torque Production, delay between pectoralis major EMG and infraspinatus EMG and pectoralis EMD were not different between the groups. Conclusions: Unstable and contralateral to unstable shoulders showed a chronic adaptation of rotator cuff with decrease in EMD. The EMD could be related to increase in stiffness of series elastic components. The increased demand for the dynamic stabilization caused by the loss of static stabilization mechanism could lead to the infraspinatus adaptation.