Thesis (Ph.D.) - University of St Andrews, June 2009.

Thesis (Ph. D.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains xii, 99 p. : ill. Vita. Includes abstract. Includes bibliographical references (p. 79-82).

Thesis (M.S. in Aerospace Engineering) -- University of Dayton. / Title from PDF t.p. (viewed 10/06/09). Advisor: Aaron Altman. Includes bibliographical references (p. 140-142).

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Made available in DSpace on 2014-06-11T19:23:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-06-28Bitstream added on 2014-06-13T19:50:16Z : No. of bitstreams: 1 simoes_rp_me_bauru.pdf: 20203043 bytes, checksum: b1791cdffb4735eb5cba45bb735dc2f7 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O estudo dos regimes dinâmicos de vórtices em supercondutores do tipo II tem atraído grande interesse do ponto de vista teórico e experimental, por um lado pela riqueza de comportamentos que é apresentado e, por outro, pela compreensão dos mecanismos de pinning, o qual é utilizado para aumentar e manter a densidade de corrente crítica, permitindo seu uso em aplicações práticas. Usando simulações de dinâmica molecular, analisamos os efeitos de pinning artificiais na dinâmica de vórtices em filmes finos supercondutores do tipo II. Além da distribuição aleatória de centros de pinning, dois tipos de arranjos periódicos foram considerados: hexagonal e kagomé. Para o arranjo kagomé analisamos a dinâmica para a força externa aplicada nas direções transversal e longitudinal em relação à simetria da rede. Em cada caso são observados comportamentos dinâmicos distintos da rede de vórtices, uma vez que existem simetrias diferentes para cada direção. Diferentemente dos resultados obtidos para a distribuição aleatória e hexagonal de pinnings, para o arranjo de kagomé, a rede de vórtices apresenta uma dinâmica e um segundo pico na resistência diferencial ainda não descritos na literatura. Quando a força é aplicada na direção longitudinal, são observados regimes dinâmicos complexos da rede de vórtices. Para esse caso, além dos regimes ordenados típicos de redes periódicas de pinnings, os vórtices se movem em canais desordenados quando a corrente de transporte é aumentada, lembrando o regime smectic visto em simulações com pinning aleatório. Subsequentemente, para altas correntes, os vórtices atingem um regime dinâmico onde eles novamente se movem em canais bem definidos, revelando uma ordem transversal. Para o arranjo hexagonal, os vórtices sempre se movem em linhas retas. Apresentamos também um estudo da densidade de corrente crítica para diferentes valores de densidade de vórtices. / The study of current-driven vortex lattices in type II superconductors has attracted a great interest from both experimental and theoretical point of view, on the one side for the riches of behaviors that is showed and, otherwise, for the comprehension of pinning mechanisms, which has been used to increase and to maintain the critical current density, to allow their use in practical applications. Using molecular dynamics simulations, we analyze the effects of artificial pinning on the vortex dynamics of type II superconductors thin films. Yonder random distribution of pinning centers, two types of periodic pinning arrays are considered: the hexagonal and kagomé. For the kagomé array, we analyze the dynamics for the applied external force in the transversal and longitudinal directions to the lattice symmetry. For each case are observed different dynamical behaviors of vortex lattice, once there are different symmetries for each direction. Differently than the results obtained for the random and hexagonal distribution of pinnings, for the kagomé array, the vortex lattice show a dynamic and a second peak in the differential resistance still not reported in the literature. When the force is applied in the longitudinal direction, the dynamical behavior of vortices is much more complex. For this case, yonder the ordered regimes, typically observed in systems with periodic pinning, the vortices moving in disordered channels when the transport current is increased, resembling the smetic dynamical regime that was found in simulations for random pinning distributions. Subsequently, for high currents, the vortices reach dynamical regime where they move in well defined channels again, revealing transversal ordering. For the hexagonal array, the vortices always move in straight trajectories. We also show a study of critical current density for different values of vortex density. For all densities, the hexagonal pinning network shows higher critical currents.

This study has involved the investigation of the flows in a high efficiency cyclone separator and the performance characteristics of the cyclone when operating under the influence of base suction. It has long been accepted that a bleed taken from the base region of a cyclone could, generally, enhance the overall collection efficiency. However, detail analyses and investigations have been limited. This investigation has involved flow visualisation, laser Doppler anemometry, computational fluid Dynamics, as well as both model particle tests and full scale prototype tests to quantify the effect of base suction and cyclone performance. Flow visualisation has highlighted the extension of the vortices into the solid receiver at the base of the cyclone. The flow patterns in this region have been investigated and quantified using laser Doppler anemometry and this result compared to the predictions from computational fluid dynamics. Agreement between these results tends to be good in the inner vortex but less good in the outer vortex region. Model particle tests have shown that the extension of the vortices into the solid receiver results in the complete destruction of the dust layer in the receiver with the subsequent re-entrainment and carry over of particulate to the vortex finder. These particle tests have shown that this re-entrainment can be suppressed by the application of a suction in the base region and the overall collection efficiency improved. A bleed flow of 10% by volume is shown to give the maximum overall efficiency. Above this percentage the efficiency reduces. This trend in the results was also confirmed by full scale prototype tests.

An experimental parametric sturdy of vane and air-jet vortex generators in a turbulent boundary layer has been carried out. Experiments were carried out in two facilities, one with a free-stream velocity of 20 m/s and a boundary layer thickness (6) of 41.5 mm, and one in a high speed facility at free-stream Mach numbers of between 0.45 and 0.75 and a boundary layer thickness of 20 mm. Cross-stream data were measured at a number of downstream locations using a miniature five-hole pressure probe, such that local cross-stream velocity vectors could be derived. Streamwise vorticity was calculated using the velocity vector data. In the low speed study, vortex generator parameters were as follows: ' Vane vortex generators: thin rectangular vanes with a vane aspect ratio of unity (2h/c = 1), free-stream velocity 20 m/s, incidence (cc = 10', 15', 18', 20'), height-to-boundary- layer- thickness-ratio (h/8 0.554,0.916,1.27,1.639), and strearnwise distance from the vortex generator (x/6 = 3.855,12.048,19.277,26.506). ' Air-jet vortex generators: circular jet nozzles, free-stream velocity = 20 m/s, jet nozzle pitch and skew angles (cc, P= 30', 45', 60'), hole diameter-to-boundary-layer-thickness-ratio (D/5 = 0.098,0.193,0.289), jet-to-free-stream-velocity ratio (VR = 0.7,1.0,1.3,1.6,2.0), and strearnwise distance from the vortex generator (x/8 = 3.855,12.048,19.277,26.506). In the high-speed study, the vortex generator parameters were as follows: Vane vortex generators: thin rectangular vanes with an aspect ratio of unity, incidence ((X 1505 20'), he i ght-to- boundary- I ayer-th i ckne s s-rati o (h/8 = 0.75), strearnwise distance from the vortex generator (x/6 = 8.755 16.25,23.75), and free-stream Mach numbers of 0.45,0.6 and 0.75. Air-jet vortex generators: jet pitch ((x = 30', 45'), jet skew angle (P = 30', 45', 60'), hole diameter-to-boundary-layer-thickness-ratio (D/8 = 0.15,0.3), j et-to- free- strearn-ve loc ity ratio (VR = 1.6), and strearnwise distance from the vortex generator (x/6 = 8.75,16.25,23.75, 31.25), and free-stream Mach numbers of 0.50,0.6 and 0.75. Streamwise vorticity data from the experiment was used to generate prediction techniques that would allow the vorticity profiles, downstream of vane or air-jet vortex generators, to be predicted. Both techniques are based on the approximation of the experimental cross-stream vorticity data to Gaussian distributions of vorticity through the vortex centre. The techniques, which are empirically derived, are simple equations that give the peak vorticity and vortex radius based on the vortex generator parameters. Use of these descriptors allows the assembly of the Gaussian vorticity equation. Both techniques are compared with the experimental data set and were seen to produce peak vorticity results to within 12% and 20% (for the vanes and air-jets respectively), 15% for the radius of the vortex, and 15% and 20% in vortex circulation (for the vanes and air-jets respectively). The two simple prediction techniques allow good prediction of the vortex structure at extremely low computational effort.

Resumo: O estudo dos regimes dinâmicos de vórtices em supercondutores do tipo II tem atraído grande interesse do ponto de vista teórico e experimental, por um lado pela riqueza de comportamentos que é apresentado e, por outro, pela compreensão dos mecanismos de pinning, o qual é utilizado para aumentar e manter a densidade de corrente crítica, permitindo seu uso em aplicações práticas. Usando simulações de dinâmica molecular, analisamos os efeitos de pinning artificiais na dinâmica de vórtices em filmes finos supercondutores do tipo II. Além da distribuição aleatória de centros de pinning, dois tipos de arranjos periódicos foram considerados: hexagonal e kagomé. Para o arranjo kagomé analisamos a dinâmica para a força externa aplicada nas direções transversal e longitudinal em relação à simetria da rede. Em cada caso são observados comportamentos dinâmicos distintos da rede de vórtices, uma vez que existem simetrias diferentes para cada direção. Diferentemente dos resultados obtidos para a distribuição aleatória e hexagonal de pinnings, para o arranjo de kagomé, a rede de vórtices apresenta uma dinâmica e um segundo pico na resistência diferencial ainda não descritos na literatura. Quando a força é aplicada na direção longitudinal, são observados regimes dinâmicos complexos da rede de vórtices. Para esse caso, além dos regimes ordenados típicos de redes periódicas de pinnings, os vórtices se movem em canais desordenados quando a corrente de transporte é aumentada, lembrando o regime smectic visto em simulações com pinning aleatório. Subsequentemente, para altas correntes, os vórtices atingem um regime dinâmico onde eles novamente se movem em canais bem definidos, revelando uma ordem transversal. Para o arranjo hexagonal, os vórtices sempre se movem em linhas retas. Apresentamos também um estudo da densidade de corrente crítica para diferentes valores de densidade de vórtices. / Abstract: The study of current-driven vortex lattices in type II superconductors has attracted a great interest from both experimental and theoretical point of view, on the one side for the riches of behaviors that is showed and, otherwise, for the comprehension of pinning mechanisms, which has been used to increase and to maintain the critical current density, to allow their use in practical applications. Using molecular dynamics simulations, we analyze the effects of artificial pinning on the vortex dynamics of type II superconductors thin films. Yonder random distribution of pinning centers, two types of periodic pinning arrays are considered: the hexagonal and kagomé. For the kagomé array, we analyze the dynamics for the applied external force in the transversal and longitudinal directions to the lattice symmetry. For each case are observed different dynamical behaviors of vortex lattice, once there are different symmetries for each direction. Differently than the results obtained for the random and hexagonal distribution of pinnings, for the kagomé array, the vortex lattice show a dynamic and a second peak in the differential resistance still not reported in the literature. When the force is applied in the longitudinal direction, the dynamical behavior of vortices is much more complex. For this case, yonder the ordered regimes, typically observed in systems with periodic pinning, the vortices moving in disordered channels when the transport current is increased, resembling the smetic dynamical regime that was found in simulations for random pinning distributions. Subsequently, for high currents, the vortices reach dynamical regime where they move in well defined channels again, revealing transversal ordering. For the hexagonal array, the vortices always move in straight trajectories. We also show a study of critical current density for different values of vortex density. For all densities, the hexagonal pinning network shows higher critical currents. / Orientador: André Luiz Malvezzi / Coorientador: Pablo Antonio Venegas Urenda / Banca: Ladir Candido da Silva / Banca: Paulo Noronha Lisboa Filho / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Mestre

This thesis presents an investigation into the dynamics of vortex rings impacting a sharply-stratified density interface. This problem has a long history and is important for understanding how individual eddies in stratified turbulence mix the density field. We tackle this problem using a combination of experimental, numerical and modelling techniques to understand the flow instability and subsequent mixing induced by the impinging vortex ring. Our findings demonstrate that there exists a critical Richardson number, corresponding to a mixing transition, beyond which the mixing efficiency is constant. Using a novel Stereo Particle Image Velocimetry (Stereo-PIV) technique, we analyze a series of vortex ring experiments. By amalgamating an ensemble of these experiments, we measure the full, time-resolved, three-dimensional velocity field of the vortex-ring interaction. These measurements capture the instability that is produced on the baroclinically generated vorticity field. This instability is identified as a Crow-like instability. At low Richardson numbers, the timescale of the interface rebound is faster than that of the instability. As a result, there exists a critical Richardson number below which the Crow-like instability will not have sufficient time to grow to large amplitude. By generating a large number of vortex-ring interactions, we measure the incremental change to the stratification. After an initialization period, there is strong evidence to suggest that the mixing due to each vortex ring becomes constant over a moderate range of Richardson numbers. We suggest that the mixing efficiency of the vortex rings does drop at low Richardson numbers (below unity) in agreement with the analysis of the Stereo-PIV measurements. A model of the system accurately predicts the dependence of the mixing rate on the Richardson number. Based upon our study of the vortex-ring system, we construct a one-dimensional turbulence model that includes the energy advection from the vortex rings. This model is validated with both physical experiments and numerical simulations of repeated vortex-ring generations. The constant mixing efficiency regime is recovered in all three methodologies. Through examining the detailed dynamics of the flow, this work suggests that there exists a critical Richardson number corresponding to a transition between mixing regimes, and that this critical Richardson number is a result of the growth of a Crow-like instability. We have highlighted how to improve current mixing-models to capture this physics. New avenues of future research are currently underway to study the mixing produced by a stratified mixing-box experiment in light of these new developments.

Orientador: José Ricardo Pelaquim Mendes / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica e Instituto de Geociências / Made available in DSpace on 2018-08-24T13:52:12Z (GMT). No. of bitstreams: 1 Storti_FelipeChagas_D.pdf: 7344405 bytes, checksum: c9c2f6d0dd3dea372babaa14915d4b0d (MD5) Previous issue date: 2013 / Resumo: O sistema de separação submarina VASPS (Vertical Annular Separation and Pumping System), que se utiliza tanto do efeito gravitacional quanto do centrífugo é composta por Câmara de Expansão, Helicóide de Separação e Piscina. Com o objetivo de se estudar um separador compacto e de menor custo, foi construído um protótipo da Câmara de Expansão em acrílico. Foram realizados testes de eficiência de separação do protótipo utilizando a mistura ar-água e a mistura ar-glicerina com diferentes viscosidades. Para realizar os testes de eficiência foi construído um circuito fechado todo equipado e instrumentado. Nestes testes de eficiência variou-se o diâmetro de bocal de entrada do separador e o nível de líquido na parte inferior do separador. Foi observado durante os testes experimentais a formação de um vórtice, principalmente para níveis mais baixos de líquido, diminuindo a eficiência do separador. Para solucionar esse problema foi desenvolvido e fabricado um dispositivo chamado Redutor de Vórtice, melhorando a eficiência do protótipo para certas condições de operação. Os resultados mostraram que a eficiência diminui para fluidos mais viscosos e também observou-se que o padrão de escoamento no bocal de entrada do separador teve influencia na eficiência. Resultados do ângulo de inclinação e altura de elevação do filme de líquido na parede do separador também foram obtidos / Abstract: The VASPS (Vertical Annular Separation and Pumping System) is a subsea separation system, which utilizes both the gravitational effect as the centrifugal and is composed by Expansion Chamber, Helicoide Separation and Pool. In order, to study a compact separator and with lower cost, we built a prototype of Expansion Chamber in acrylic. Separation efficiency of the prototype using the air-water mixture and air-glycerin mixture with different viscosities tests were performed. To perform efficiency tests was built a closed circuit all equipped and instrumented. In these tests of efficiency ranged the diameter of the separator inlet nozzle and the liquid level at the bottom of the separator. The formation of a vortex, especially for lower levels of liquid was observed during the experimental tests, decreasing the efficiency of the separator. To solve this problem was developed and manufactured a device called Vortex Reducer, improving the efficiency of the prototype for certain operating conditions. The results showed that the efficiency decreases for more viscous fluids and also observed that the flow pattern at the nozzle inlet separator had an influence in the efficiency. Results of tilt angle and lift height of the liquid film on the wall of the separator were also obtained / Doutorado / Explotação / Doutor em Ciências e Engenharia de Petróleo