O estudo do escoamento ao redor de corpos submersos tem sido foco de muitas pesquisas. Escoamentos ao redor de cilindros constituem sistemas de grande interesse prático em vários campos da engenharia e outras ciências. Neste trabalho procurou-se estudar a dinâmica do escoamento ao redor de um cilindro circular submetido a um movimento forçado analisando as alterações ocorridas na formação da esteira de vórtices. Para a realização das simulações foi utilizada a técnica de Simulação Numérica Direta (DNS), com um esquema de diferenças finitas compacto de sexta ordem, em associação com um método de representação virtual das fronteiras do cilindro. Quatro tipos de trajetória foram estudadas: uma trajetória com oscilação transversal, onde o cilindro é forçado a mover-se na direção transversal em relação ao escoamento, uma trajetória elíptica, uma trajetória em arco e uma trajetória em oito. Buscou-se identificar as principais características destes escoamentos, como os modos e freqüências de desprendimento de vórtices na esteira formada, coeficientes hidrodinâmicos e parâmetros estatísticos pertinentes ao fenômeno estudado. As simulações foram realizadas com diferentes amplitudes longitudinais de movimento sendo que a freqüência de movimento do cilindro foi mantida constante para as trajetórias elíptica, em arco e em oito. Os resultados para o caso do cilindro com oscilação transversal representaram bem os fenômenos físicos ocorridos neste tipo de escoamento. Foi possível identificar as fronteiras da região de lock-in primário através das variações bruscas nos coeficientes hidrodinâmicos e do ângulo de fase entre o deslocamento do cilindro e a força de sustentação. No caso das simulações com trajetória elíptica e número de Reynolds Re = 140, alterações significativas foram identificadas na esteira de vórtices à medida que a amplitude longitudinal do movimento era modificada. Nas simulações com Re = 400 e trajetória transversal, notou-se a formação de uma esteira com dois vórtices e duas camadas cisalhantes. Comparando esta simulação com os resultados das simulações em trajetória elíptica, foram observadas alterações no padrão de desprendimento, sendo formado um par de vórtices na região superior da esteira e um vórtice simples na região inferior (padrão assimétrico P+S). Comparando as simulações com trajetória elíptica e sentidos de movimento opostos, notou-se que o escoamento se inverte, evidenciando um efeito de memória deste escoamento que é função do sentido adotado para a trajetória. Tanto a amplitude de deslocamento longitudinal quanto o tipo de trajetória imposta ao cilindro influenciam significativamente os padrões da esteira de vórtices formada. / The study of flow around submerged bodies has been the focus of many researches. Flow around cylinders are systems of great practical interest in many fields of engineering and other sciences. This work aims to study the flow dynamics around a circular cylinder under a forced movement by analysing changes in the patterns of the wake. The simulations were performed through the Direct Numerical Simulation technique (DNS), with a sixth-order compact finite differences scheme associated to a virtual boundary method to represent the cylinder surface. Four trajectories were studied: a trajectory with transversal oscillation; a trajectory on elliptical curve; a trajectory in arc curve and a trajectory in eight curve. The main features of these flows were identified, e.g. modes and vortex shedding frequencies, hydrodynamic coefficients and statistical parameters. The simulations were performed with different streamwise movement amplitude, where the movement frequency was held constant for the elliptical, arc and eight trajectories. The results for the cylinder in transversal oscillation successfully represented the physical phenomena. It was possible to identify the boundaries of the primary lock-in region through jumps in the hydrodynamical coefficients and the phase angle between the cylinder displacement and the lift force. For the simulations with elliptical trajectory and Reynolds number Re = 140, significant changes were identified in the wake as the streamwise displacement of the cylinder was modified. In simulations with Re = 400 and transversal trajectory, two vortex and two shear layers could be observed. Comparing this simulation with simulations in elliptical trajectory, changes in the vortex shedding pattern were observed. A vortex pair was formed in the top of the wake and a single vortex in the lower wake ( asymmetrical mode P+S). Comparing the simulations with elliptical trajectory and opposite directions of movement, it was noted that the flow is reversed showing some effect of memory due to the direction of the trajectory. Both the amplitude of streamwise displacement and the trajectory type may result in different patterns in the wake.

O estudo do escoamento ao redor de corpos submersos tem sido foco atual de muitas pesquisas. Em especial, escoamentos ao redor de obstáculos rombudos constituem sistemas de grande interesse prático em vários campos da engenharia e outras ciências como, por exemplo, na engenharia automobilística, na concepção de edificações submetidas à ação do vento, no estudo de correntes fluviais e marítimas sobre pilares de pontes, em risers, cabos de ancoragem e plataformas de captação de petróleo. O presente trabalho teve como objetivo principal o estudo por Simulação Numérica Direta do escoamento ao redor de um cilindro com dois graus de liberdade, submetido a Vibração Induzida por Vórtices (VIV), buscando o esclarecimento a cerca dos fenômenos tridimensionais de formação e evolução do escoamento em função da interação fluido-cilindro. As equações de Navier-Stokes e da continuidade são resolvidas numericamente usando esquemas de diferenças finitas compacto de sexta ordem na discretização espacial, em associação com um método de representação das fronteiras do cilindro, denominado Método de Fronteiras Imersas. Inicialmente foram realizadas simulações para os casos de cilindro fixo e em movimento forçado, onde os resultados mostraram que o código computacional consegue modelar satisfatoriamente os fenômenos físicos de acordo com os resultados encontrados na literatura. Para as simulações com cilindro fixo, em movimento forçado e em VIV, nota-se que as estruturas do escoamento tridimensional apresentam modos com comprimentos de onda alterados em função do movimento imposto. Os resultados das simulações em VIV com dois graus de liberdade, para Re = 1250 mostraram que as estruturas tridimensionais do escoamento sofrem significativas alterações espaciais e temporais em função dos parâmetros do modelo dinâmico. Foram identificados diferentes fenômenos de formação e emparelhamento dos vórtices que são modificados em função das condições impostas ao modelo massa-mola amortecimento. / The study of flow around submerged bodies has been the focus of many researches. In special, the flow around bluff bodies are systems of great practical interest in many fields of engineering and other sciences, e.g., the flow around vehicles, buildings submitted to the action of the wind, river and sea currents over pillars of the bridges, risers and cables in the oil platforms. The present work aims to study through Direct Numerical Simulation (DNS) the flow dynamics around a circular cylinder under Vortex-Induced Vibration with two degrees of freedom, searching for identify the changes in the three-dimensional structures of the street wake imposed by the interactions between the flow and the immersed cylinder. The simulations were performed using a sixth-order compact finite differences scheme associated to a Immersed Boundary Method (IBM) to represent the cylinder surface. Firstly, simulations of the flow around fixed and forced movement were accomplished. The computational code Incompact3d presented results in agreement with the literature. The validation of the code shown that is possible to obtain satisfactory results relate to the street wake behind cylinder and the force coefficients are in agreement with others experimental and numerical works. In case of free oscillating cylinder, several changes in the three-dimensional vortex shedding modes are observed as the parameters of the oscillating dynamic model was changed. It was observed that different three-dimensional structures are formed in the wake depending on the initial conditions of the mass-stiffness damping model. The results of the simulations at Reynolds numbers equal to 1250 presenting some hydrodynamic phenomena as paring and turbulent structures in the flow that are modifying during the time and in the computational domain.

O estudo do escoamento ao redor de corpos submersos tem sido foco atual de muitas pesquisas. Em especial, escoamentos ao redor de obstáculos rombudos constituem sistemas de grande interesse prático em vários campos da engenharia e outras ciências como, por exemplo, na engenharia automobilística, na concepção de edificações submetidas à ação do vento, no estudo de correntes fluviais e marítimas sobre pilares de pontes, em risers, cabos de ancoragem e plataformas de captação de petróleo. O presente trabalho teve como objetivo principal o estudo por Simulação Numérica Direta do escoamento ao redor de um cilindro com dois graus de liberdade, submetido a Vibração Induzida por Vórtices (VIV), buscando o esclarecimento a cerca dos fenômenos tridimensionais de formação e evolução do escoamento em função da interação fluido-cilindro. As equações de Navier-Stokes e da continuidade são resolvidas numericamente usando esquemas de diferenças finitas compacto de sexta ordem na discretização espacial, em associação com um método de representação das fronteiras do cilindro, denominado Método de Fronteiras Imersas. Inicialmente foram realizadas simulações para os casos de cilindro fixo e em movimento forçado, onde os resultados mostraram que o código computacional consegue modelar satisfatoriamente os fenômenos físicos de acordo com os resultados encontrados na literatura. Para as simulações com cilindro fixo, em movimento forçado e em VIV, nota-se que as estruturas do escoamento tridimensional apresentam modos com comprimentos de onda alterados em função do movimento imposto. Os resultados das simulações em VIV com dois graus de liberdade, para Re = 1250 mostraram que as estruturas tridimensionais do escoamento sofrem significativas alterações espaciais e temporais em função dos parâmetros do modelo dinâmico. Foram identificados diferentes fenômenos de formação e emparelhamento dos vórtices que são modificados em função das condições impostas ao modelo massa-mola amortecimento. / The study of flow around submerged bodies has been the focus of many researches. In special, the flow around bluff bodies are systems of great practical interest in many fields of engineering and other sciences, e.g., the flow around vehicles, buildings submitted to the action of the wind, river and sea currents over pillars of the bridges, risers and cables in the oil platforms. The present work aims to study through Direct Numerical Simulation (DNS) the flow dynamics around a circular cylinder under Vortex-Induced Vibration with two degrees of freedom, searching for identify the changes in the three-dimensional structures of the street wake imposed by the interactions between the flow and the immersed cylinder. The simulations were performed using a sixth-order compact finite differences scheme associated to a Immersed Boundary Method (IBM) to represent the cylinder surface. Firstly, simulations of the flow around fixed and forced movement were accomplished. The computational code Incompact3d presented results in agreement with the literature. The validation of the code shown that is possible to obtain satisfactory results relate to the street wake behind cylinder and the force coefficients are in agreement with others experimental and numerical works. In case of free oscillating cylinder, several changes in the three-dimensional vortex shedding modes are observed as the parameters of the oscillating dynamic model was changed. It was observed that different three-dimensional structures are formed in the wake depending on the initial conditions of the mass-stiffness damping model. The results of the simulations at Reynolds numbers equal to 1250 presenting some hydrodynamic phenomena as paring and turbulent structures in the flow that are modifying during the time and in the computational domain.

O estudo do escoamento ao redor de corpos submersos tem sido foco de muitas pesquisas. Escoamentos ao redor de cilindros constituem sistemas de grande interesse prático em vários campos da engenharia e outras ciências. Neste trabalho procurou-se estudar a dinâmica do escoamento ao redor de um cilindro circular submetido a um movimento forçado analisando as alterações ocorridas na formação da esteira de vórtices. Para a realização das simulações foi utilizada a técnica de Simulação Numérica Direta (DNS), com um esquema de diferenças finitas compacto de sexta ordem, em associação com um método de representação virtual das fronteiras do cilindro. Quatro tipos de trajetória foram estudadas: uma trajetória com oscilação transversal, onde o cilindro é forçado a mover-se na direção transversal em relação ao escoamento, uma trajetória elíptica, uma trajetória em arco e uma trajetória em oito. Buscou-se identificar as principais características destes escoamentos, como os modos e freqüências de desprendimento de vórtices na esteira formada, coeficientes hidrodinâmicos e parâmetros estatísticos pertinentes ao fenômeno estudado. As simulações foram realizadas com diferentes amplitudes longitudinais de movimento sendo que a freqüência de movimento do cilindro foi mantida constante para as trajetórias elíptica, em arco e em oito. Os resultados para o caso do cilindro com oscilação transversal representaram bem os fenômenos físicos ocorridos neste tipo de escoamento. Foi possível identificar as fronteiras da região de lock-in primário através das variações bruscas nos coeficientes hidrodinâmicos e do ângulo de fase entre o deslocamento do cilindro e a força de sustentação. No caso das simulações com trajetória elíptica e número de Reynolds Re = 140, alterações significativas foram identificadas na esteira de vórtices à medida que a amplitude longitudinal do movimento era modificada. Nas simulações com Re = 400 e trajetória transversal, notou-se a formação de uma esteira com dois vórtices e duas camadas cisalhantes. Comparando esta simulação com os resultados das simulações em trajetória elíptica, foram observadas alterações no padrão de desprendimento, sendo formado um par de vórtices na região superior da esteira e um vórtice simples na região inferior (padrão assimétrico P+S). Comparando as simulações com trajetória elíptica e sentidos de movimento opostos, notou-se que o escoamento se inverte, evidenciando um efeito de memória deste escoamento que é função do sentido adotado para a trajetória. Tanto a amplitude de deslocamento longitudinal quanto o tipo de trajetória imposta ao cilindro influenciam significativamente os padrões da esteira de vórtices formada. / The study of flow around submerged bodies has been the focus of many researches. Flow around cylinders are systems of great practical interest in many fields of engineering and other sciences. This work aims to study the flow dynamics around a circular cylinder under a forced movement by analysing changes in the patterns of the wake. The simulations were performed through the Direct Numerical Simulation technique (DNS), with a sixth-order compact finite differences scheme associated to a virtual boundary method to represent the cylinder surface. Four trajectories were studied: a trajectory with transversal oscillation; a trajectory on elliptical curve; a trajectory in arc curve and a trajectory in eight curve. The main features of these flows were identified, e.g. modes and vortex shedding frequencies, hydrodynamic coefficients and statistical parameters. The simulations were performed with different streamwise movement amplitude, where the movement frequency was held constant for the elliptical, arc and eight trajectories. The results for the cylinder in transversal oscillation successfully represented the physical phenomena. It was possible to identify the boundaries of the primary lock-in region through jumps in the hydrodynamical coefficients and the phase angle between the cylinder displacement and the lift force. For the simulations with elliptical trajectory and Reynolds number Re = 140, significant changes were identified in the wake as the streamwise displacement of the cylinder was modified. In simulations with Re = 400 and transversal trajectory, two vortex and two shear layers could be observed. Comparing this simulation with simulations in elliptical trajectory, changes in the vortex shedding pattern were observed. A vortex pair was formed in the top of the wake and a single vortex in the lower wake ( asymmetrical mode P+S). Comparing the simulations with elliptical trajectory and opposite directions of movement, it was noted that the flow is reversed showing some effect of memory due to the direction of the trajectory. Both the amplitude of streamwise displacement and the trajectory type may result in different patterns in the wake.

Abstract In this thesis, the structure and the dynamics of vortices are studied from the standpoint of the hydrodynamical theory of superfluids. In the hydrodynamical theory a superfluid is described by a continuous order parameter field. In the case of superfluid helium-4 this field is a complex-valued function of position and time. However, in superfluid helium-3 the order parameter is a complex-valued 3 × 3 matrix. The first part of this work consists of studies on structures that appear in the order parameter field, when a vessel filled with superfluid helium-3 in the A phase (3He-A) is rotated in an external magnetic field. Among the most common of these structures are the so-called continuous vortices. They exist in several different forms. In addition to vortices, other possible structures include the vortex sheet of 3He-A that was discovered at the Low Temperature Laboratory of Helsinki University of Technology (currently Aalto University) in late 1993. In this thesis, these structures were studied by finding stationary vortex configurations that minimize the free energy of the superfluid. An algorithm for minimizing the free energy was implemented by writing a computer program. This program was then used to study the structure of a few vortex types, inferred to be the most probable ones. In addition, regular lattices formed by these vortices, including the vortex sheet, were studied. A phase diagram for vortex lattices was constructed by comparing the free energy of various lattice structures as a function of rotational velocity and external magnetic field. The study of vortex structures also lead to a discovery of a new type of vortex in 3He-A, later named the LV3 vortex. In the second part of the work, the dynamics of vortices was studied using a filament model of vortex motion, which also has its theoretical justification in the hydrodynamical model of superfluids, but where the detailed structure of the vortex core is not relevant. The specific problem under consideration here was the motion of a quantized vortex in a rotating elongated cylinder filled with superfluid, and how the motion of the vortex depends on temperature and the rotational velocity of the vessel. The study of vortex motion was simplified using scaling laws. A new type of scaling law was discovered, which both simplified the specific problem under study, and made the results more general. In summary, the research in this thesis touched upon two somewhat complementary areas, i.e. the structure of continuous vortices in 3He-A and the dynamics of thin vortex lines, which is more applicable to superfluid 4He or to the B phase of superfluid helium-3 (3He-B). However, these areas complement each other in advancing the general scientific understanding about the properties of superfluids.

In this dissertation, we provide an accurate description of spin transport in magnetic textures and in particular, we investigate in detail, the nature of spin torque and magnetic damping in such systems. Indeed, as will be further discussed in this thesis, the current-driven velocity of magnetic textures is related to the ratio between the so-called non-adiabatic torque and magnetic damping. Uncovering the physics underlying these phenomena can lead to the optimal design of magnetic systems with improved efficiency. We identified three interesting classes of systems which have attracted enormous research interest (i) Magnetic textures in systems with broken inversion symmetry: We investigate the nature of magnetic damping in non-centrosymmetric ferromagnets. Based on phenomenological and microscopic derivations, we show that the magnetic damping becomes chiral, i.e. depends on the chirality of the magnetic texture. (ii) Ferromagnetic domain walls, skyrmions and vortices: We address the physics of spin transport in sharp disordered magnetic domain walls and vortex cores. We demonstrate that upon spin-independent scattering, the non-adiabatic torque can be significantly enhanced. Such an enhancement is large for vortex cores compared to transverse domain walls. We also show that the topological spin currents owing in these structures dramatically enhances the non-adiabaticity, an effect unique to non-trivial topological textures (iii) Antiferromagnetic skyrmions: We extend this study to antiferromagnetic skyrmions and show that such an enhanced topological torque also exist in these systems. Even more interestingly, while such a non-adiabatic torque inuences the undesirable transverse velocity of ferromagnetic skyrmions, in antiferromagnetic skyrmions, the topological non-adiabatic torque directly determines the longitudinal velocity. As a consequence, scaling down the antiferromagnetic skyrmion results in a much more efficient spin torque.

Görtler vortices develop in boundary layer flows over concave surfaces due to the imbalance between centrifugal forces and the wall-normal pressure gradient. These vortices can be efficient precursors to transition in boundary layers exposed to free-stream disturbance or surface non-uniformities, because they can alter the mean flow causing the laminar flow to breakdown into turbulence. In this thesis, a control technique aimed at reducing the energy associated with Görtler vortices that develop in supersonic boundary layers is introduced and tested. The control algorithm is based on distributed blowing and suction, with sensors placed either in the flow or at the wall. The result show that there is a dependence between the efficiency of the control algorithm and the spanwise separation of the vortices, that is the energy reduction is more significant for larger spanwise separations. The efficiency of the control algorithm seems to be insensitive to the variation of the Mach number.

No description available.

This dissertation work is a study of positronium formation for positron-hydrogen and positron-helium collisions in the Ore gap (the energy region between the threshold for ground-state positronium formation and the first excitation level of the target atom) using variational K-matrices. We have fitted the K-matrices using multichannel effective range theories and using polynomials. Using the variational K-matrices and their fits, we have located zeros in the positronium-formation scattering amplitude and corresponding deep minima in the positronium-formation differential cross section. The zeros are related to the vortices in the extended velocity field associated with the positronium-formation scattering amplitude. For positron-hydrogen collisions, we have found two zeros in the positronium-formation scattering amplitude, and corresponding deep minima in the positronium-formation differential cross section, while we have obtained a zero in the positronium-formation scattering amplitude for positron-helium collisions. We have connected the zeros in the positronium-formation scattering amplitude to vortices in the extended velocity fields. Our work shows that vortices can occur for charge exchange in atomic collisions.

No description available.