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The thesis describes state of the art volumetric measurement techniques and applies a 3D measurement technique, 3D Scanning Particle Tracking Velocimetry, to the transitional backward facing step flow. The measurement technique allows the spatial and temporal analysis of coherent structures apparent at the backward facing step. The thesis focusses on the extraction and interaction of coherent flow structures like shear layers or vortical structures.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho abordamos a teoria de Ginzburg-Landau da supercondutividade (teoria GL). Apresentamos suas origens, características e resultados mais importantes. A idéia fundamental desta teoria e descrever a transição de fase que sofrem alguns metais de uma fase normal para uma fase supercondutora. Durante uma transição de fase em supercondutores do tipo II é característico o surgimento de linhas de fluxo magnético em determinadas regiões de tamanho finito chamadas comumente de vórtices. A dinâmica destas estruturas topológicas é de grande interesse na comunidade científica atual e impulsiona incontáveis núcleos de pesquisa na área da supercondutividade. Baseado nisto estudamos como essas estruturas topológicas influenciam em uma transição de fase em um modelo bidimensional conhecido como modelo XY. No modelo XY vemos que os principais responsáveis pela transição de fase são os vórtices (na verdade pares de vórtice-antivórtice). Villain, observando este fato, percebeu que poderia tornar explícita a contribuição desses defeitos topológicos na função de partição do modelo XY realizando uma transformação de dualidade. Este modelo serve como inspiração para a proposta deste trabalho. Apresentamos aqui um modelo baseado em considerações físicas sobre sistemas de matéria condensada e ao mesmo tempo utilizamos um formalismo desenvolvido recentemente na referência [29] que possibilita tornar explícita a contribuição dos defeitos topológicos na ação original proposta em nossa teoria. Após isso analisamos alguns limites clássicos e finalmente realizamos as flutuações quânticas visando obter a expressão completa da função correlação dos vórtices o que pode ser muito útil em teorias de vórtices interagentes (dinâmica de vórtices). / In this work we introduced the Ginzburg-Landau theory of superconductivity (GL theory). We have shown your foundations, features and more important results. The fundamental idea of this theory is to describe the phase transition that some metals undergoes from a normal to a superconductor phase. During a phase transition in superconductors of type II is common the appearance of magnetic flux lines in given regions of finite size called of vortices. The knowledge of the dynamics of these vortices is of great importance in the current cientific community and drives many research centers to study the superconductivity. In view of this we study how these vortices changes a phase transition in a bidimensional model known as XY model.In XY model one can show that the main responsible for the phase transition are the vortices (or still, vortice-antivortice pairs). Villain, noting this fact, realized that could to turn explicit the contribution of theses topological defects in the partition function of XY model making a duality transformation. This model inspired us to study the subject of this master thesis. We presented here a model based in physical considerations about systems of condensed matter. At the same time we used a formalism developed in reference [29] that permits to turn explicit the contribution of these vortices in the original action proposed in our theory. Finally we analysed some classical limits and we looked for the quantum fluctuations to obtain the complete expression of the correlation function of vortices, whose utility is in the study of interacting vortices is wide (vortex dynamics).

To predict fluid properties and interactions is an important task for the industry. It is plagued, however, by being close to impossible to predict analytically. Hence, it is customary to turn to numerical solutions. This in itself comes with many different methods and approaches suitable for different needs. This work focuses on two methods: Tip Vortex Index (TVI) and Diffused Vortex Hydrodynamics (DVH). TVI is a method to predict when a marine propeller will experience cavitation of tip vortices and is based on calculations from a Boundary Element Method (BEM). DVH is a particle method for simulating the circulation of a fluid in two dimensions and three dimensions. The aim is to investigate an implemented TVI model based on MPUF-3A for different marine propeller series, with different sub-designs for a total of 28 unique propellers, and implement the DVH method and test it for 3 different bodies. The results of this thesis show that the implemented TVI model is non-functional for the 28 different propellers, but the DVH method is successfully implemented and able to handle 2 different bodies. / Att förutspå fluid egenskaper och interaktioner är en viktig uppgift för industrin. Det plågas dock av att vara näst intil omöjligt att förutspå analytiskt. Det är därför vanligt att vända sig till numeriska lösningar. Detta kommer i sig med många olika metoder och tillvägagångssätt som passar olika behov. Detta arbete fokuserar på två metoder: Tip Vortex Index (TVI) och Diffused Vortex Hydrodynamics (DVH). TVI är en metod för att förutsäga när en marin propeller kommer att uppleva kavitation av spetsvirvlar och baseras på beräkningar från en Boundary Element Method (BEM). DVH är en partikelmetod för att simulera cirkulationen i fluid i två dimensioner och tre dimensioner. Syftet är att undersöka en implementerad TVI-modell baserad på MPUF-3A för olika marina propellerserier, med olika underdesigner, för totalt 28 unika propellrar, och implementera DVH-metoden och testa den för 3 olika kroppar. Resultaten av denna avhandling visar att den implementerade TVI-modellen är icke-funktionell för de 28 olika propellrarna, men DVH-metoden är framgångsrikt implementerad och kan hantera 2 olika kroppar

Ce travail de thèse porte sur les propriétés de stabilité des tourbillons hélicoïdaux, structures que l'on retrouve notamment dans le sillage des rotors d'hélicoptères et d'éoliennes.Dans une première partie, le développement spatio-temporel de l'instabilité d'appariement est caractérisé à l'aide d'un code numérique pseudo-spectral pour une allée infinie d'anneaux tourbillonnaires. On montre que ce modèle axisymétrique d'écoulement est en effet une bonne approximation du système hélicoïdal dans la limite des grands rayons et petits pas d’hélice. Dans ces conditions, et en utilisant un adimensionnement judicieux, on obtient également que le résultat théorique pour le taux de croissance spatio-temporel obtenu pour une double allée de tourbillons ponctuels s’avère être une bonne prédiction pour le cas hélicoïdal.Dans une seconde partie, on décrit comment un ou plusieurs tourbillons hélicoïdaux ont pu être générés de façon très peu perturbée à l’aide de modèles réduits de rotors dans le canal hydrodynamique du laboratoire. Grâce à l’introduction de perturbations d’amplitudes et de fréquences soigneusement contrôlées, le taux de croissance de l’instabilité d’appariement a pu être mesuré et comparé aux résultats théoriques. L’évolution non linéaire de ces perturbations ainsi que d’autres modes instables, à plus petites longueurs d’onde, ont également pu être observés expérimentalement pour la première fois.Enfin, ces résultats ont été appliqués au cas des rotors d’hélicoptères pour la prédiction du régime de Vortex Ring State (VRS) et à la transition vers la turbulence du sillage des éoliennes. / This thesis is devoted to the stability properties of helical vortices, which are of interest for applications such as helicopter and wind turbine wakes.In a first part, the spatio-temporal development of the pairing instability is characterised for an infinite array of vortex rings, using a pseudo-spectral numerical code. We show that this axisymmetric flow model is indeed a good approximation of the helical system in the limit of large helix radius and small pitch. Under these assumptions, and by using appropriate dimensionless variables, we also show that the theoretical result concerning the spatio-temporal growth rate for a double row of point vortices represents a good prediction for the helical case.In a second part, we describe how one or several helical vortices were generated in a carefully controlled way using small-scale rotor models in the water channel of the laboratory. Introducing perturbations with well-defined amplitudes and frequencies, the growth rate of the pairing instability could be measured experimentally and compared to theoretical predictions. The non-linear evolution of these perturbations, as well as other unstable modes of smaller wavelengths, were also observed experimentally for the first time.Finally, these results were applied to helicopter wakes for the prediction of the Vortex Ring State (VRS) regime and to the transition to turbulence in wind turbine wakes.

Swirling flows abound in nature and numerous engineering applications. Under conditions which are not completely understood, the swirling cores could undergo a sudden enlargement of their vortex core, leading to a ’vortex breakdown’. The physics of vortex breakdown and strategies to control it have been active areas of research for nearly half a century. There are many competing theories of vortex breakdown in the literature; broadly, these are surmised on similarities to flow separation, hydrodynamic instability or transition from a supercritical to a subcritical state. However, a rational criterion for vortex breakdown continues to be elusive. One of the most well known criteria in the literature is the one due to Brown and Lopez (1990) based on an inviscid vortex dynamics model which suggests that the helix angle of the velocity vector should enclose the helix angle of the vorticity vector. However it appears that this only suggests that the stream surface would diverge and not necessarily constitute a condition for breakdown. In this work, we propose a new criterion based on helicity (scalar product of velocity and vorticity vectors) for characterizing breakdown since it has fundamental topological interpretations relating to change in linkages of vortex lines. In particular, it is suggested that the breakdown location corresponds to the location where helicity becomes zero. We study the problem of vortex breakdown in a cylindrical container with a rotating top lid in order to clarify and elucidate our hypothesis. We present results from Direct Numerical Simulation of this problem for three different Reynolds numbers and evaluate the utility of our proposed helicity criterion. Our studies indicate that helicity is indeed a better choice for characterizing vortex breakdown.

In this research, an elastic cylinder that utilized vortex-induced vibration (VIV) was applied to improve convective heat transfer rates by disrupting the thermal boundary layer. Rigid and elastic cylinders were placed across a fluid channel. Vortex shedding around the cylinder led to the periodic vibration of the cylinder. As a result, the flow-structure interaction (FSI) increased the disruption of the thermal boundary layer, and therefore, improved the mixing process at the boundary. This study aims to improve convective heat transfer rate by increasing the perturbation in the fluid flow. A three-dimensional numerical model was constructed to simulate the effects of different flow channel geometries, including a channel with a stationary rigid cylinder, a channel with a elastic cylinder, a channel with two elastic cylinders of the same diameter, and a channel with two elastic cylinders of different diameters. Through the numerical simulations, the channel maximum wall temperature was found to be reduced by approximately 10% with a stationary cylinder and by around 17% when introducing an elastic cylinder in the channel compared with the channel without the cylinder. Channels with two-cylinder conditions were also studied in the current research. The additional cylinder with the same diameter in the fluid channel only reduced the surface wall temperature by 3% compared to the channel without any cylinders because the volume of the second cylinder could occupy some space, and therefore, reduce the effect of the convective heat transfer. By reducing the diameter of the second cylinder by 25% increased the effect of the convection heat transfer and reduced the maximum wall temperature by around 15%. Compared to the channel with no cylinder, the introduction of cylinders into the channel flow was found to increase the average Nusselt number by 55% with the insertion of a stationary rigid cylinder, by 85% with the insertion of an elastic cylinder, by 58% with the insertion of two cylinders of the same diameter, and by approximately 70% with the insertion of two cylinders of different diameters (the second cylinder having the smaller diameter). Furthermore, it was also found that the maximum local Nusselt number could be enhanced by around 200%-400% at the entrance of the fluid channel by using the elastic cylinders compared to the channel without cylinders.

Spin-vortices in vertically spaced pairs of thin elliptical Permalloy nanoparticles are investigated. The two vortex cores with parallel out-of-plane magnetization exhibit a strong monopole-like attraction through the spacer much thinner than the core length, thus forming a bound core-core pair. The material of the spacer is designed to suppress both direct and indirect exchange interactions, so the remaining inter-vortex coupling is purely dipolar. In the investigated vortex pairs, the in-plane magnetization in the vortex periphery, outside the vortex cores, curl in opposite directions (have opposite chirality). As a result, the two cores move in opposite directions in response to an in-plane magnetic field, the Zeeman effect of which acts to decouple the core-core pair. This leads to unique dynamics of the spin-vortex parallel-core/antiparallel-chirality pair, which strongly depend on whether the pair is coupled or decoupled. In the coupled state, the cores are held close together by the core-core attraction, which results in short-radius oscillations and a resonance frequency of about 2 GHz for the main rotational eigen-mode. In the decoupled state, the cores are separated by a distance much greater than the core length and gyrate independently with a resonance frequency of the order of 100 MHz. The dynamics of the vortex pair are investigated at 77 K, where there is a bistability between the coupled and decoupled core states. Resonant excitations are used to decouple the cores with pulses of ∼10 Oe in amplitude and ∼100 ps in duration. The ability to decouple a vortex pair using such fast low-power pulses can be useful for multifrequency oscillators and vortex based memory. A search for quantum effects is undertaken at sub-Kelvin temperatures using a dilution refrigerator. Square pulses of 100 ns duration and amplitudes of the order of 1 Oe are applied in-plane to bring the system closer to decoupling, giving the cores a chance to tunnel through the barrier between the coupled and decoupled states. The amplitude required for decoupling is measured as a function of temperature and a leveling off in the decoupling probability is seen below 400 mK, giving some indication of core tunneling. Macroscopic quantum tunneling of magnetization is interesting from the fundamental physics point of view, e.g., as a model system for studying the measurement paradox in quantum mechanics, as well as for current and future computer technology in terms of understanding the ultimate limitations of miniaturizing magnetic memory elements. / I detta arbete studeras spinnvirvlar i elliptiska skivor av Permalloy ordnade i vertikala par. Kärnor av parallell vertikal magnetisering attraherar varandra likt monopoler genom en film mycket tunnare än kärnorna och bildar därmed ett sammankopplat par. Materialet i filmen mellan virvlarna är designat för att förhindra både direkt och indirekt utbytesväxelverkan och lämnar endast kärnornas dipolväxelverkan. I de virvelpar som studeras går den plana magnetiseringen i virvlarnas periferi runt kärnorna åt olika håll (de har motsatt kiralitet). På grund av detta rör sig kärnorna åt olika håll vid applikation av magnetfält i planet (Zeeman effekten) vilket kan leda till att de kopplas isär. Detta ger virvelpar med parallella kärnor och antiparallell kiralitet unika dynamiska egenskaper som ändras med deras tillstånd, sammankopplade eller isärkopplade. I det sammankopplade tillståndet hålls kärnorna ihop av monopolattraktionen vilket gör att de bara kan röra sig i små banor kring sitt magnetiska masscentrum, med en resonansfrekvens på circa 2 GHz. I det isärkopplade tillståndet är kärnorna separerade med ett avstånd som är mycket större än kärnornas diameter, och de rör sig oberoende av varandra med en resonansfrekvens i storleksordningen 100 MHz. Virvelparets dynamik undersöks vid 77 K, där det finns en bistabilitet mellan det sammankopplade och det isärkopplade tillståndet. Pulser med längd ∼100 ps och styrka ∼10 Oe i resonans med det sammankopplade tillståndet används för att koppla isär kärnorna. Att kunna koppla isär dem med så korta lågeffektspulser kan vara användbart för virvelbaserade minnen och multifrekvensoscillatorer. Ett sökande efter kvanteffekter påbörjas vid temperaturer under 1 K med hjälp av en utspädningskyl. Fyrkantsvågor med en längd på 100 ns och en styrka i storleksordningen 1 Oe, orienterade i planet, används för att ge kärnorna en chans att tunnla genom barriären mellan det sammankopplade och det isärkopplade tillståndet. Den vågamplitud som krävs för att koppla isär kärnorna plottas mot temperaturen och kan ses plana ut under 400 mK, vilket ger viss indikation av tunnling. Dessa undersökningar av makroskopisk kvanttunnling av magnetisering kan vara användbar i grundforskning för att studera mätparadoxen i kvantmekanik, men också i modern datorteknologi för att förstå de absoluta begränsningarna i hur små magnetiska minneselement kan göras.

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