High-temperature superconductors have been intensely studiedsince the discovery, almost 20 years ago. Their layeredstructure, extreme type-II behavior, large anisotropy, andstrong fluctuations have led to a large number of new andinteresting problems. In this work the resistive transitionhave been studied in YBa2Cu3O7-δsingle crystals, from the superconductingfluctuations above Tcdown to the vortex matter near the onset of resistivity.Superconducting fluctuations above Tcare suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, forI||ab. ForH||c, fluctuations in the CuO-chains was not important, andfluctuations in the CuO2 planes seemed isotropic. The in-planecoherence length anisotropy was determined to be close tounity. ForH||ab, Δσbdepended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration. The nature of the vortex phase below Tccan be examined through vortex-correlation measurements.Using a modified pseudo-flux transformer (PFT) geometry forin-plane experiments, the magnetic field could be appliedparallel to both the a, b, and c axis. A strong Lorentz forcewas detected, and observed to be a requirement for the meltingstep feature. The resistive anisotropy close to Tcwas strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution. The effect of anisotropy on vortex-liquid correlation infieldsH||ab, was studied byc-axis transport measurements in oxygen-deficientsamples, using a PFT configuration. Increasing disordersuppressed both longitudinal and transverse correlation. Inheavily underdoped samples, vortex correlation was enhanced formagnetic fields exactly aligned with theabplane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent. Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition. Keywords:high-temperature superconductors, YBa2Cu3O7-δ, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation

<p>High-temperature superconductors have been intensely studiedsince the discovery, almost 20 years ago. Their layeredstructure, extreme type-II behavior, large anisotropy, andstrong fluctuations have led to a large number of new andinteresting problems. In this work the resistive transitionhave been studied in YBa₂Cu₃O_7-δsingle crystals, from the superconductingfluctuations above T<i>c</i>down to the vortex matter near the onset of resistivity.Superconducting fluctuations above T<i>c</i>are suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, for<b>I</b>||<b>ab</b>. For<b>H</b>||<b>c</b>, fluctuations in the CuO-chains was not important, andfluctuations in the CuO2 planes seemed isotropic. The in-planecoherence length anisotropy was determined to be close tounity. For<b>H</b>||<b>ab</b>, Δσ<i>b</i>depended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration.</p><p>The nature of the vortex phase below T<i>c</i>can be examined through vortex-correlation measurements.Using a modified pseudo-flux transformer (PFT) geometry forin-plane experiments, the magnetic field could be appliedparallel to both the a, b, and c axis. A strong Lorentz forcewas detected, and observed to be a requirement for the meltingstep feature. The resistive anisotropy close to T<i>c</i>was strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution.</p><p>The effect of anisotropy on vortex-liquid correlation infields<b>H</b>||<b>ab</b>, was studied by<i>c</i>-axis transport measurements in oxygen-deficientsamples, using a PFT configuration. Increasing disordersuppressed both longitudinal and transverse correlation. Inheavily underdoped samples, vortex correlation was enhanced formagnetic fields exactly aligned with the<i>ab</i>plane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent.</p><p>Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition.</p><p><b>Keywords:</b>high-temperature superconductors, YBa₂Cu₃O_7-δ, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation</p>

Oscillating-foil hydrokinetic turbines have gained interest over the years to extract energy from renewable sources. The influence of the sweep angle on the performance of a fully-passive oscillating-plate hydrokinetic turbine prototype was investigated experimentally in the present work. The sweep angle was introduced to promote spanwise flow along the plate in order to manipulate the leading edge vortex (LEV) and hydrodynamically optimize the performance of the turbine. In the present work, flat plates of two configurations were considered: a plate with a 6° sweep angle and an unswept plate (control), which were undergoing fully-passive pitch and heave motions in uniform inflow at the Reynolds numbers ranging from 15 000 to 30 000. The resulting kinematic parameters and the energy extraction performance were evaluated for both plates. Planar (2D) particle image velocimetry (PIV) was used to obtain patterns of the phase-averaged out-of-plane vorticity during the oscillation cycle. The circulation in the wake was then related to the induced-forces on the plate by calculating the moments of vorticity of the LEV with respect to the pitching axis of the plate. Tomographic (3D) PIV was implemented in evaluating the influence of the spanwise flow on the dynamics of the vortex structure in three-dimensional space. The rate of deformation of the vortex length was quantified by calculating the deformation terms embedded in the vorticity equations, then linked to the stability of the vortex. The results show evidence of delay of the shedding of LEV and increased vortex stability, in the case of the swept plate. The manipulation of the LEV by the spanwise flow was related to the induced kinematics exhibited by the prolonged heave forces experienced by the swept plate, which led to the higher power extraction performance at high inflow velocities. In the presence of spanwise flow, positive vortex stretching along the vortex line increased the stabilization of the vortex core and prevented the onset of helical vortex breakdown, observed in the case of the unswept plate. The use of the sweep profile on the plate has led to the improvement of energy extraction performance of the fully-passive hydrokinetic turbine. / Graduate

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A new vortex model for wind turbines was developed in order to evaluate the loads at the blades and other important characteristics of interest for the wind industry such as power and thrust coefficients. Nowadays, the calculation of these quantaties is done in a reliable and precise manner with LES simulation using actuator line or actuator disk models. However, LES simulations are computationally heavy and the model here developed aims at calculating the same quantities of interest in less time but still giving reliable and accurate results for any wind turbine model. The idea of a vortex model for wind turbines was developed by Segalini &amp; Aöfredsson, J. Fluid Mech., vol. 725, pages 91-116, 2013, using vortex filaments to reproduce the vorticity on the blades and in the wake. Nevertheless, that model had some limitations, among which, the main one, was the impossibility to simluate wind turbines with varying circulation along the blade, something that is always present in reality. With this thesis it is proposed a model based on the one of Segalini &amp; Alfredsson (2013) but with the introduction of a vortex sheet that allows to simulate a vorticity release from the wind turbine blades and hence wind turbines with varying circulation along the blades. The model was validated against a LES simulation of the Tjaereborg wind turbine by Sarmast, KTH Royal Institute of Technology, 2014, that utilized an actuator line model. The results confirmed the improvement of the vortex model compared to the previous one of Segalini &amp; Alfredsson (2013) and gave consistent results regarding the flow field at the rotor plane and the loads on the blades.

Acoustic and aerodynamic measurements have been carried out on a rotor operating in a planar turbulent boundary layer near a wall for a variety of thrust conditions and yaw angles with respect to the inflow. At the highest thrust condition a strong flow reversal in the wall-rotor tip gap was observed. Average velocity fields filtered by the angular position of the rotor show that the flow reversal is fed by jets of fluid that tend to form below the blade as it passes by the wall. Instantaneous velocity measurements show the presence of strong vortices in the tip gap. These vortices were characterized and found to be both stronger and more numerous on the downstroke side of the tip gap. Additionally, vortices with the same handedness as the bound circulation in the blade were more numerous and only located on the downstroke side of the tip gap. Those with the opposite handedness were found to be only located on the upstroke side. Unexpectedly strong far-field acoustic response at the blade passage frequency at this highest thrust condition and is believed to be due to an interaction of the blade tip with these vortices. At moderate thrust, when the rotor was yawed toward the downstroke side the far field acoustic response at the blade passage frequency was found to increase. The opposite was true as it was yawed toward the upstroke side. At the highest thrust, however the unyawed rotor had the strongest blade passage frequency response which is believed to be due to stronger vortex-tip interaction in this case. / Master of Science

In the recent decades the Fluid-Structure Interaction (FSI) problem has been of great interest to many researchers and a variety of methods have been proposed for its numerical simulation. As FSI simulation is a multi-discipline and a multi-physics problem, its full simulation consists of many details and sub-procedures. On the other hand, reliable FSI simulations are required in various applications ranging from hemo-dynamics and structural engineering to aero-elasticity. In hemo-dynamics an incompressible fluid is coupled with a flexible structure with similar density (e.g. blood in arteries). In aero-elasticity a compressible fluid interacts with a stiff structure (e.g. aircraft wing) or an incompressible flow is coupled with a very light structure (e.g. Parachute or sail), whereas in some other engineering applications an incompressible flow interacts with a flexible structure with large displacement (e.g. oil risers in offshore industries). Therefore, various FSI models are employed to simulate a variety of different applications. An initial vital step to conduct an accurate FSI simulation is to perform a study of the physics of the problem which would be the main criterion on which the full FSI simulation procedure will then be based. In this thesis, interaction of an incompressible fluid flow at low Reynolds number with a flexible circular cylinder in two dimensions has been studied in detail using some of the latest published methods in the literature. The elements of procedures have been chosen in a way to allow further development to simulate the interaction of an incompressible fluid flow with a flexible oil riser with large displacement in three dimensions in future. To achieve this goal, a partitioned approach has been adopted to enable the use of existing structural codes together with an Immersed Boundary (IB) method which would allow the modelling of large displacements. A direct forcing approach, interpolation / reconstruction, type of IB is used to enforce the moving boundary condition and to create sharp interfaces with the possibility of modelling in three dimensions. This provides an advantage over the IB continuous forcing approach which creates a diffused boundary. And also is considered as a preferred method over the cut cell approach which is very complex in three dimensions with moving boundaries. Different reconstruction methods from the literature have been compared with the newly proposed method. The fluid governing equation is solved only in the fluid domain using a Cartesian grid and an Eulerian approach while the structural analysis was performed using Lagrangian methods. This method avoids the creation of secondary fluid domains inside the solid boundary which occurs in some of the IB methods. In the IB methods forces from the Eulerian flow field are transferred onto the Lagrangian marker points on the solid boundary and the displacement and velocities of the moving boundary are interpolated in the flow domain to enforce no-slip boundary conditions. Various coupling methods from the literature were selected and improved to allow modelling the interface and to transfer the data between fluid and structure. In addition, as an alternative method to simulate FSI for a single object in the fluid flow as suggested in the literature, the moving frame of reference method has been applied for the first time in this thesis to simulate Fluid-Structure interaction using an IB reconstruction approach. The flow around a cylinder in two dimensions was selected as a benchmark to validate the simulation results as there are many experimental and analytical results presented in the literature for this specific case.

In this work the dynamics of permalloy circular magnetic islands of 225 nm radius and 10 nm thickness arranged into square lattices was investigated employing micromagnetic simulations.The simulations of the vortex magnetization loops and the ferromagnetic resonance (FMR) spectra were carried out using a free micromagnetics simulation software Mumax3. The obtained data was analyzed using Matlab. The simulations were carried out on a single vortex island as well as on two different lattices. The first lattice is comprised of interacting islands, while the second lattice - of non-interacting islands, separated by 25 nm and by 450 nm edge-to-edge distance, respectively. The magnetization loops were simulated by applying the static magnetic field in-plane of the single island or the lattice. The FMR simulations were carried out by applying the static magnetic field in-plane of the lattice and after the system reached the ground state in that field, the excitation as a sinc pulse was sent out along the out-of-plane direction of the lattice. The analysis of the obtained FMR spectra revealed that the several resonant modes are present for the single vortex island and the lattice, comprised of such islands. However, the physical explanation of the origin of those modes is a subject for further investigations.