With many decades of research devoted to the intriguing nature of vortex-induced vibrations, the offshore industry is constantly looking for new effective solutions in predicting VIV of slender marine structures such as riser pipes. These structures are very sensitive to excitations induced by vortex shedding, which results in vibrations that in certain combinations of waves and current develop into a structural resonance phenomenon known as lock-in. This kind of vibrations can be destructive to some structures and lead to collapse. Many of VIV aspects are far from being understood and advanced modelling is required to investigate the impact of the phenomenon, which significantly affects the service life of marine structures. The main objective of this research is to contribute to the family of semi-empirical models used for prediction of vortex-induced vibrations, with emphasis on low mass ratio elastically supported cylinders capable of moving in cross-flow and in-line directions. In this work a new two degree-of-freedom wake oscillator model has been developed, where vortex-induced lift and drag forces were modelled with two nonlinear self-excited oscillators of van der Pol type. Phenomena exclusive for two degreeof- freedom motion at low mass ratios were examined in detail. Computational fuid dynamics was applied to the problem in order to tune the model parameters. Twodimensional flow past an elastically supported cylinder was considered, and CFD simulation results were used to calibrate the wake oscillator model predictions of the complex fluid-structure interaction.

This thesis work started as an attempt to create a computational tool to model hydrodynamics problems involving lifting flows. The method employed to solve the problem is potential flow theory. Despite the fast evolution of computers and the latest developments in Navier-Stokes solvers, such as the Ranse methods; potential flow theory offers the possibility to create or use existing computational tools, which allow us modeling hydrodynamics problems in a simpler manner. Navier-Stokes solver can be very expensive from the computational point of view, and require a high level of expertise in order to achieve reliable models. Based on the above, we have developed a lifting flow modeling tool that we hope can serve as the starting point of a more elaborated method, and a valuable alternative, for the solution of different hydrodynamics problems. Key words highlighting important concepts related to this thesis work are: Vortex, circulation, potential flow, panel methods, Sources, doublets.

For several years, we have been developing vortex phase masks based on sub-wavelength gratings, known as Annular Groove Phase Masks. Etched onto diamond substrates, these AGPMs are currently designed to be used in the thermal infrared (ranging from 3 to 13 pm). Our AGPMs were first installed on VLT/NACO and VLT/VISIR in 2012, followed by LBT/LMIRCam in 2013 and Keck/NIRC2 in 2015. In this paper, we review the development, commissioning, on-sky performance, and early scientific results of these new coronagraphic modes and report on the lessons learned. We conclude with perspectives for future developments and applications.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, May 2017 / Flow around flat plates that were uniformly accelerated from rest with acceleration of 13g is analysed with overset mesh from Star CCM+ commercial CFD software. The particular interest is more on the vortices shed from the plate edges. Three 8mm thick plates of the same cross-sectional areas (108mm length equilateral triangular, 71mm length square and 80mm diameter circular) were simulated. The validation of the numerical method was achieved by using laser vapor sheet method to visualize the flow profiles of accelerating circular plate and comparing the CFD and experimental results. The CFD and experimental results were consistent with each other. It was found that when a plate accelerated in air, it displaced air particles out of its way. The shear layers of air separated from the front edges of the plate and rolled around a vortex core forming a primary vortex ring in the plate wake. The size of the primary vortex increased with Reynolds number (Re) that was increasing with time. This was because as Re increased, more fluid particles were displaced from the front face of the plate at a time. More displacement of the fluid particles led to shear layers separating from the plate edges with stronger momentum resulting in larger vortex ring. The shape of the primary vortex depended on the shape of the accelerating plate. For the circular plate, all the points on the front edge being equidistant from the plate centroid, fluid particles were evenly displaced from that separation edge. The result was an axis-symmetric ring of primary vortex around a circular vortex core. The asymmetric plates (triangular and square) did not evenly displace air particles from their edges of separation. The result was an asymmetric vortex ring. More air particles separated from the plate at separation points closest to the plate centroid and led to the largest vortical structure there. That is; the primary vortex ring was largest at the midpoints of the plate edges because they were the closest points of separation from the plate centroid. The size of the primary vortex continuously reduced from the mid-points of the plate edges to the corners. The corners had the smallest primary vortical structure due to being furthest points of separation from the plate centroid. The parts of the vortex ring from the two edges of the plate interacted at the corner connecting those edges. / MT 2017

The electron microscope is an ideal tool to prepare an electron into a specified quantum state, entangle that state with states in a specimen of interest, and measure the electron final state to indirectly gain information about the specimen. There currently exist excellent technologies to prepare both momentum eigenstates (transmission electron microscopy) and position eigenstates (scanning transmission electron microscopy) in a narrow band of energy eigenstates. Similarly, measurement of the momentum and position final states is straightforward with post-specimen lenses and pixelated detectors. Measurement of final energy eigenstates is possible with magnetic electron energy loss spectrometers. In 2010 and 2011, several groups independently showed that it was straightforward to prepare electrons into orbital angular momentum eigenstates. This disseratation represents my contributions to the toolset we have to control these eigenstates: preparation, application (interaction with specimen states), and measurement. My collaborators and I showed that phase diffraction gratings efficiently produce electron orbital angular momentum eigenstates; that control of orbital angular momentum can be used to probe chirality and local magnetic fields; and that there are several routes toward efficient measurement.

The combustion process of a fuel is a challenging subject when it comes to analyze its performance and resultant emissions. The main task of this study is to optimize the selection of a hydrogen-rich fuel based on its performance and emissions. Computational Fluid Dynamics analysis is performed to test the combustion performance and emissions from the vortex trapped combustor when natural gas fuel (methane) is replaced with renewable and alternative fuels such as hydrogen and synthesis gas. Correlation graphs for the trapped vortex combustor performance and NOx, CO, and CO2 emissions for various types of fuels with different compositions and heat of combustion values were established. Methane, Hydrogen and 10 different syngas fuels were analyzed in this study using computational fluid dynamics numerical method. The trapped vortex combustor that represents an efficient and compact combustor for flame stability was investigated. The TVC consists of a fore body and two after body disks . These components are all encircled with a Pyrex tube. The purpose of the after body disks is to create the vortex wakes that will enhance the combustion process and minimize the NOx emissions. The TVC CFD model was validated by comparing the CFD model results using propane fuel with existing experimental results that were established in Rome, Italy. The static temperature distribution and NOx, CO emissions, combustor efficiency and total pressure drop results of the three dimensional CFD model were similar to the experimental data. Effects of H2/CO and H2/CH4 ratios and the mass fraction of each constituent of syngas fuels and Hydrogen-Methane fuel mixture on the TVC performance and emissions were investigated. / Moreover, the fuel injector Reynolds number and Lower heating values for Methane, Hydrogen and 10 syngas fuels on the TVC performance and emissions were also investigated. Correlation plots for the NOx, CO and CO2 emissions versus the fuel injector Reynolds number and low heating value were established. These correlation curves can be used as a fair design diagram to optimize the fuel selection process for aerospace and electrical power plant applications. / by Khaled Zbeeb. / Thesis (Ph.D.)--Florida Atlantic University, 2011. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2011. Mode of access: World Wide Web.

O objetivo deste trabalho foi a análise do movimento do fluido dentro de um segmento de vórtice em anel gerado através da ação magnetohidrodinâmica e o monitoramento do seu movimento resultante, de modo a oferecer condições de visualização usando velocimetria a laser com técnica de processamento de imagens de partículas envolvidas na circulação. O objetivo final desta linha de pesquisa é que através de análises de dados como as velocidades envolvidas na circulação obtida, possa modelar corpos em movimento em fluidos, de forma que o arrasto devido ao seu deslocamento possa ser reduzido. O movimento circulatório a que foi submetido o fluido sem meios mecânicos, ou seja, por meio de uma força resultante da ação de campos conjugados, teve a função de criar campos de baixa pressão tendo, para isso, acelerado o fluido em um circuito fechado restrito pela atuação da força e das paredes da célula de ensaio. O movimento do fluido, com a leitura da variação de velocidades em pontos diferentes do circuito teve o auxílio de um equipamento ótico que forneceu todas as variações, mesmo as mínimas, mostrando que tal aparato pode ser desenvolvido para a obtenção de melhores resultados. / The aim of this work involves the analysis of fluid flow, using sea water stimulated by localised magnetohydrodynamic induction within an experimental glass sided receptacle. The above mentioned apparatus simulated one segment of a three dimensional apparatus capable of creating a flow pattern very similar to that of a vortex ring. Particle Image Velocimetry techniques using a copper vapour laser with computerised image and data acquisition system were used to analyse the flow in the experimental segment. The final aim of this line of research is to be able to eventually model the shape of bodies in movement within a fluid when combined with the use of a magnetohydrodynamically induced flow field in such a way as to significantly reduce fluid drag forces on the body. Circulatory movement was induced in the fluid within the confines of the experimental tank using a locally applied Lorentz field force between electrodes to accelerate the fluid. Both the electrical and magnetic fields were controlled independently using separate external rectified a.c. circuits. The aim of the fluid movement described above was to induce low static pressure over a specific surface area. The analysis of the fluid movement, including the measurement of velocities in different points of the circuit was attained using specialised optical equipment linked to a computerised data acquisition and analysis system. The results, which included exceptionally low velocity readings, showed thatsuch equipment possesses the potential for developed for the attainment of better results in the future.

Direct Numerical Simulation of channel flow was utilized to study the evolution of various vortex configurations presented as flow initial conditions. Simulations of longitudinally, laterally and cross-flow oriented vortices suggested that the predominant form of turbulent structure was the half hairpin vortex. This vortical structure was dominant in the simulations seen in this as well as other investigations. In all cases hairpin vortices quickly degenerated to half hairpin or inclined vortical structures. It is hypothesized that these structures function as the predominant momentum transfer mechanism within the boundary layer, entraining fluid into the vortex cores like miniature tornados and transporting this fluid to the top of the boundary layer while simultaneously dragging fluid viscously around the inclined core of the vortex causing mixing of low-speed and high-speed flows.

Wingtip vortices are an important problem in aerodynamic and hydrodynamic engineering because of their contribution to induced drag, tip cavitation, and wake turbulence. These effects decrease equipment efficiency and lifespan, which increases application costs. Biology provides an inspiring solution to this problem in avian flight through the spreading of primary feathers. Previous studies have shown increased lift to drag ratio and efficiency of wings and propeller blades through modified wingtip geometry. The goal of this project is to optimize the tip geometry (primary feather angle) of a test wing for minimal tip vortex strength using genetic algorithms to mimic natural design evolution. Ultrasonic transducers are used to measure the wing tip vortex circulation in wind tunnel tests for each candidate design. Although neither angle of attack series converged completely, there was partial convergence in each. Due to the fluctuations in the low angle of attack tests, the parent selection algorithm was altered for the high angle of attack series, which resulted in improved convergence trends. A genetic algorithm that used uniform crossover breeding, a 20% mutation rate, and roulette wheel parent selection methods was used to generate an improved tip geometry at a low angle of attack of 6° and a freestream velocity of 15.25 m/s over the course of 17 generations. This improved design consisted of three key features, a staggered leading edge, a drastic mid-section vertical separation, and an upswept trailing edge. A second algorithm, which employed uniform crossover, a 20% mutation rate, and an elitist selection roulette parent selection, provided an improved tip geometry for a 12° angle of attack at a freestream velocity of 11.5 m/s. This improved design consisted of three key features, a downswept leading edge, a drastic mid-section vertical separation, and an upturned trailing edge. Both results showed that the wing tip vortex strength can be reduced by approximately 20% by manipulating tip geometry and that the trailing edge traits produce the most prominent effects on vortex strength.

The impact of buoyancy on the development of starting flows in the near field was experimentally investigated using the Digital Particle Image Velocimetry and Planar Laser Induced Flourescence techniques. The experiments were conducted by releasing cylindri-cal columns of fluid into a glass water tank. Two diameters (0.95 and 1.9 cm) and four aspect ratios, ranging from 2 to 8, were examined. The fluid was released by bursting the thin latex membrane that held it in the tube. The buoyant fluid had a density difference of 4.7%. The flow was imaged at 60 Hz up to 7 diameters downstream. For the aspect ratio of 2, the flow developed into a single buoyant vortex ring (BVR), and was compared to a purely momentum driven vortex ring (MVR) generated with the same setup. For the aspect ratios of 4, 6, and 8, the flow was similar to a starting plume, with a vortical cap, followed by a columnar tail. The BVR's diameter grew linearly in space, with a full spreading angle of 18 degrees, while the MVR's diameter remained constant. The BVR started out as an axis touching ring, and transitioned to non-axis touching, opposite of the behavior of the MVR. The total circulation for the BVR was more than twice the amount predicted by the slug flow model, and the impulse grew linearly in time. The impulse of the MVR decayed slightly after the intial growth. The flows began to transition to thermal behavior at down-stream distance proportional to the cube root of the initial fluid volume. For all aspect ratios the impulse grew linearly in time. The growth rate was roportional to the initial buoyant force. The circulation generated by the addition of buoyancy was proportional to the square root of the initial buoyant force. Also the addition of buoyancy suppressed the separation of a starting vortex.