No description available.

Numerical simulations are presented for three dimensional viscous incompressible free shear flows. The numerical method is based on solving the vorticity equation using Vortex-In-Cell method. In this method, the vorticity field is discretized into a finite set of Lagrangian elements (particles) and the computational domain is covered by Eulerian mesh. Velocity field is computed on the mesh by solving Poisson equation. The solution proceeds in time by advecting the particles with the flow. Second order Adam-Bashford method is used for time integration. Exchange of information between Lagrangian particles and Eulerian grid is carried out using the M’4 interpolation scheme. The classical inviscid scheme is enhanced to account for stretching and viscous effects. For that matter, two schemes are used. The first one used periodic remeshing of the vortex particles along with fourth order finite difference approximation for the partial derivatives of the stretching and viscous terms. In the second scheme, derivatives are approximated by least squares polynomial. The novelty of this work is signified by using the moving least squares technique within the framework of the Vortex-in-Cell method and implementing it to a three dimensional temporal mixing layer. Comparisons of the mean flow and velocity statistics are made with experimental studies. The results confirm the validity of the present schemes. Both schemes also demonstrate capability to qualitatively capture significant flow scales, and allow gaining physical insight as to the development of instabilities and the formation of three dimensional vortex structures. The two schemes show acceptable low numerical diffusion as well.

The structure of turbulence in the subchannels of a large-scale 60 degree section of a CANDU 37-rod bundle was studied at Reynolds numbers equal to 50,000, 100,000 and 130,000. Measurements were conducted at roughly 33.81 rod diameters from the inlet of the rod bundle using single-point, two-component hot-wire anemometry. Analysis of the axial velocity signal indicated a weak effect of Reynolds number on the axial velocity distribution and a bulging of axial velocity contours toward the narrow gaps. The normalised normal Reynolds stresses and the normalised turbulent kinetic energy were found to decrease as the Reynolds number increased. The radial Reynolds shear stress varied linearly with radial distance from the rod, crossing zero at the location of local maximum of the axial velocity. This stress was symmetric about the central rod whereas the azimuthal Reynolds shear stress was anti-symmetric. The Reynolds number effect was weak but measurable on the integral length scales of the axial and radial velocity fluctuations but negligible on the integral length scale of the azimuthal velocity fluctuations, especially in the gap regions. The Taylor and Kolmogorov microscales increased from the wall toward the centre of the subchannel and decreased as the Reynolds number increased. The wall shear stress stress distribution around the central rod indicated no effect of Reynolds number, when normalized by the corresponding average. The wall shear stress reached local minima at rod-wall and rod-rod gaps and local maxima in the open flow regions. Vortex streets were generated within the subchannels very close to the inlet of the rod bundle. The convection speed and frequency of the vortex street were found to increase proportionately to Reynolds number, whereas the vortex spacing was not affected by the Reynolds number.

Ce projet adresse le problème de détection des structures directionnelles précises dans les champs de courant. L'accent est mis surtout sur la détection des structures tourbillonnaires, puisque les chercheurs dans le domaine de la dynamique des fluides considèrent ces structures comme étant particulièrement importantes. Chapitre 1 présente la motivation de ce projet et fournit les détails du contexte environnemental et mathématique du problème en soulignant les parties essentielles de la théorie qu'on utilise après pour la solution. Chapitre 2 propose une méthode statistique, basée sur la proportion des vraisemblances, pour adresser le problème particulier de la détection des tourbillons et montre l'efficacité de cette méthode sur les données simulées et réelles en discutant aussi les limitations. Chapitre 3 développe les idées discutées dans les chapitres 1 et 2 pour trouver un outil statistique plus général qui répare les défauts de la première méthode et permet la détection de toutes les structures directionnelles qui peuvent intéresser les chercheurs. Tous les outils nécessaires pour l'analyse des données en utilisant les deux méthodes développées dans ce projet se trouvent dans les annexes A et B de la thèse. / This work addresses the problem of detecting specific directional structures in flows of current. Specific emphasis is placed on vortex detection, as scientists studying fluid dynamics consider this structure to be of particular importance. Chapter 1 presents the motivation behind the project and provides details about the environmental and, subsequently, the mathematical context of the problem, highlighting the essential parts of the theory that is later used to propose the solution. Chapter 2 offers a statistical approach, based on a likelihood ratio, to solving the specific problem of vortex detection and demonstrates the effectiveness of the method on simulated and real data, also discussing the limitations of the approach. Chapter 3 expands on the ideas discussed in Chapters 1 and 2 to derive a generalized statistical test that remedies the flaws of the first approach and extends to the problem of detecting any directional structure of interest. All necessary tools for the analysis of data using the two methods developed in this project are given in Appendix A and B.

Orientadores: Amir Ordacgi Caldeira, Newton Cesário Frateschi / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-22T06:59:34Z (GMT). No. of bitstreams: 1 Santos_FelipeGustavodaSilva_M.pdf: 5201395 bytes, checksum: aa675cbe9765e61acda958aff38d1a42 (MD5) Previous issue date: 2013 / Resumo: Dispositivos baseados em junções Josephson tem desempenhado um papel importante tanto em ciências básicas quanto aplicadas. Neste trabalho, descrevemos a fabricação e caracterização DC de junções Josephson do tipo ScS (superconductor-constrição-supercondutor). Os dispositivos exibem uma característica VI que concorda com o bem conhecido modelo RSJ (resistência nula abaixo de uma corrente crítica mas constante e não nula acima dessa corrente) para temperaturas ~ 2 K. Para temperaturas um pouco maiores, observamos vários estados resistivos na curva VI, os quais são tipicamente atribuídos ao fluxo de vórtices de Abrikosov no interior da constrição. As medidas, porém, também sugerem que este pode não ser o caso nesses dispositivos devido à pequena magnitude do campo magnético autoinduzido na constrição, se comparado aos campos típicos que a levariam ao estado misto / Abstract: Josephson junction devices have played a role in fundamental and applied sciences. In this work, we report the fabrication and DC characterization of niobium Josephson junctions of the ScS (superconductor-constriction-superconductor) type. The devices exhibit a VI characteristic in accordance with the well known RSJ model (vanishing resistance below a critical current and constant nonzero resistance above it) for temperatures ~ 2 K. For slightly higher temperatures, we observe several resistive states in the VI curve which are usually attributed to the flow of Abrikosov vortices within the constriction. Our measurements, however, suggest that this might not be the case in these devices due to the smallness of the self-induced magnetic field in the constriction in comparison with the typical fields which drive it into the mixed state / Mestrado / Física / Mestre em Física

Orientador: Silvio Antonio Sachetto Vitiello / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-28T00:18:16Z (GMT). No. of bitstreams: 1 Madeira_Lucas_M.pdf: 4273856 bytes, checksum: 8543c0dd916e2ec3fc638a5d31b32787 (MD5) Previous issue date: 2015 / Resumo: Nessa dissertação nós investigamos dois sistemas de muitos corpos. Na primeira parte nós escolhemos uma abordagem clássica para estudar a adsorção de gases nobres pesados, Ne, Ar, Kr, Xe e Rn, em substratos de grafeno. Nós apresentamos evidências de camadas adsorvidas comensuradas, as quais dependem fortemente da simetria do substrato, para duas estruturas: camadas de Ne na rede sqrt{7} X sqrt{7} e Kr na rede sqrt{3} X sqrt{3}. Para estudar o derretimento nós introduzimos um parâmetro de ordem e sua susceptibilidade. O calor específico e a susceptibilidade em função da temperatura foram calculados para os gases nobres pesados em diversas densidades. A posição e largura característica dos picos do calor específico e da susceptibilidade foram determinadas. Finalmente, nós investigamos a distância dos primeiros vizinhos e a distância entre a camada e o substrato, identificando contribuições relacionadas aos picos do calor específico e da susceptibilidade. A segunda parte da dissertação trata de uma linha de vórtice no gás unitário de Fermi. Gases fermiônicos ultrafrios são notáveis devido à possibilidade experimental de variar as interações interpartículas através de ressonâncias de Feshbach, o que possibilita a observação do crossover BCS-BEC. No meio do crossover encontra-se um estado fortemente interagente, o gás unitário de Fermi. Uma linha de vórtice corresponde a uma excitação desse sistema com unidades de circulação quantizadas. Nós construímos funções de onda, inspiradas na função BCS, para descrever o estado fundamental e também o sistema com uma linha de vórtice. Nossos resultados para o estado fundamental elucidam aspectos da geometria cilíndrica do problema. O perfil de densidade é constante no centro do cilindro e vai a zero suavemente na borda. Nós separamos a contribuição devido à parede da energia do estado fundamental e determinamos a energia por partícula do bulk, epsilon_0=(0.42 +- 0.01) E_{FG}. Nós também calculamos o gap superfluído para essa geometria, Delta=(0.76 +- 0.01) E_{FG}. Para o sistema com a linha de vórtice nós obtivemos o perfil de densidade, o qual corresponde a uma densidade não nula no centro do vórtice, e a energia de excitação por partícula, epsilon_{ex}=(0.0058 +- 0.0003) E_{FG}. Os métodos empregados nessa dissertação, Dinâmica Molecular, Monte Carlo Variacional e Monte Carlo de Difusão, nos dão uma base sólida para a investigação de sistemas relacionados, e outros sistemas, de muitos corpos no futuro / Abstract: In this dissertation we investigated two many-body systems. For the first part we chose a classical approach to study the adsorption of heavy rare-gases, Ne, Ar, Kr, Xe and Rn, on graphene substrates. We presented evidences of commensurate adlayers, which depend strongly on the symmetry of the substrate, for two structures: Ne adlayers in the sqrt{7} X sqrt{7} superlattice and Kr in the sqrt{3} X sqrt{3} lattice. In order to study the melting of the system we introduced an order parameter, and its susceptibility. The specific heat and susceptibility as a function of the temperature were calculated for the heavy noble gases at various densities. The position and characteristic width of the specific heat and susceptibility peaks of these systems were determined. Finally, we investigated the first neighbor distance and the distance between the adlayer and the substrate, identifying contributions related to specific heat and melting peaks. The second part of the dissertation deals with a vortex line in the unitary Fermi gas. Ultracold Fermi gases are remarkable due to the experimental possibility to tune interparticle interactions through Feshbach resonances, which allows the observation of the BCS-BEC crossover. Right in the middle of the crossover lies a strongly interacting state, the unitary Fermi gas. A vortex line corresponds to an excitation of this system with quantized units of circulation. We developed wavefunctions, inspired by the BCS wavefunction, to describe the ground state and also for a system with a vortex line. Our results for the ground state elucidate aspects of the cylindrical geometry of the problem. The density profile is flat in the center of the cylinder and vanishes smoothly at the wall. We were able to separate from the ground state of the system the wall contribution and we have determined the bulk energy as epsilon_0=(0.42 +- 0.01) E_{FG} per particle. We also calculated the superfluid pairing gap for this geometry, Delta=(0.76 +- 0.01) E_{FG}. For the system with a vortex line we obtained the density profile, which corresponds to a non-zero density at the core, and the excitation energy, epsilon_{ex}=(0.0058 +- 0.0003) E_{FG} per particle. The methods employed in this dissertation, Molecular Dynamics, Variational Monte Carlo and Diffusion Monte Carlo, give us a solid basis for the investigation of related and other many-body systems in the future / Mestrado / Física / Mestre em Física / 2012/24195-2 / FAPESP

No description available.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21546号 / 理博第4453号 / 新制||理||1639(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 池田 隆介, 教授 前野 悦輝, 教授 川上 則雄 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

The master´s thesis deals with the flow induced by rotation of cavitating fluid in converging-diverging nozzle, which simulates the vortex rope in impeller of water turbines. Measurement is performed on an experimental circuit in laboratory. Results from experimental measurements are compared with CFD simulation of single and two-phase flow. The main focus is to compare the difference of hydraulic losses and shapes of cavitating structures identified in the experiment and in the simulation.

Vortices are present in many fluid flows and depending on the context they may be either beneficial or harmful for different systems or processes. Planar particle image velocimetry was used to examine the vortex evolution and its decay under different turbulence intensities and vortex circulation. The vortex decayed faster in the presence of high turbulence intensity. Vortex trajectories were impacted by turbulence intensity and vortex strength. Trajectories with no turbulence intensity had less variation. The vortex wandering amplitude decreased with growth of vortex strength. The vortex decay was confined to the core of the vortex, with the tangential velocity at large radial distances from the vortex center being relatively constant in time. The vortex core radius had a greater rate of growth with the low turbulence intensity and lower angle of attack. The amplitude of fluctuation of the core circulation increased for the higher turbulence intensity and weaker vortex.