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This thesis presents results from experimental and numerical investigations of the hydrodynamic forces on a rigid cylinder moving with prescribed orbits in uniform flow. The hydrodynamic forces are measured in both in-line (IL) and cross- ow (CF) directions. The measurements are processed to nd excitation and added mass coeffcients at discrete frequencies. The numerical simulations are used to illustrate the vortex shedding modes and are compared with the experimental results. The hydrodynamic coeffcients obtained from the harmonic forced motion experiments of a rigid cylinder do not always represent forces on a cross section of a exible beam. The orbits used in the forced motion experiments are therefore extracted from the measured motions of cross sections of a exible pipe under uniform and shear flows. Both periodic and observed orbits within a time window are applied as prescribed motions. Higher order displacement components are present in such orbits. IL response amplitudes from combined IL and CF response are larger than pure IL response amplitudes. The hydrodynamic coefficients obtained from the periodic experiments are often larger than those obtained from the pure IL tests. Higher order displacement components are more common in the IL direction than in the CF direction, and higher order IL displacement components will cause larger hydrodynamic forces in both directions. The hydrodynamic coefficients obtained from periodic motion tests are adequate for representing quasi-periodic observed motions. For chaotic observed motions, periodic orbits will yield hydrodynamic coefficients with larger uncertainties. Results from numerical analyses using large eddy simulation (LES) indicate that this method can be used to identify vortex shedding patterns and predict hydrodynamic forces under certain Re numbers and orbits.

The hydrodynamic properties of a horizontal cylinder which is free to pitch about an off-centred axis are studied and used to derive the equations of motion of a wave energy converter which extracts energy from incoming sea waves with a linear power-take-off mechanism. The present work follows from a recent study which compared the performance of an off-centred cylinder with those of the Edinburgh Duck wave energy converter. The small decrease in performance found is offset by a reduction in the likely costs associated with the manufacturing of the cylindrical cam compared with those of the asymmetric profile. As part of the survivability strategy in very energetic seas-states it had been planned to completely submerge the device so as to reduce the mooring forces. However, experiments with scale models show that a good absorption capacity is retained even when fully-submerged. The hydrodynamic properties of a horizontal cylinder that pierces the free-surface and of one that is fully submerged are therefore of central concern in this study. These properties are well known for the case of very long cylinders but they are now found for cylinders with different widths, drafts, submergence levels and water-depths. The hydrodynamic forces and moments at the off-centred axis are, furthermore, derived through the application of transformation formulae. The equation of motion of the off-centred cylinder is derived for one degree of freedom and its performance as a wave energy converter is analysed. A relationship which relates the resonance of the device with the location of the off-centred axis and its mass distribution is derived and used to optimize the design for average sea conditions attained at a real location. Design cases associated with three diameters of the cylinder are looked into detail for both a fully-submerged and free-surface piercing cylinder. The one degree of freedom model is extended to include a multi-body which has three degrees of freedom in order to describe the dynamics of a proposed wave powered desalination system based on a cylindrical Duck device. This mathematical model is derived through linearised Lagrangian equations of motion in which the hydrodynamic forces are included as generalised external forces. The advantage of such approach is to reduce the number of equations associated with multi-body systems by removing the reaction forces of holomonic constraints from the system of equations to solve. This model is validated through experiments with a scale model performed in the curved tank of the University of Edinburgh with both regular waves and mixed seas.

The diverse applications of advanced marine craft ascribed to their high speed and technological advancements has led to the use of stronger and lighter metals in such crafts. High speed, in effect also increases slamming loads as higher speed increases frequency of wave encounter while operating in waves. The present study is limited to wedge impact models. Fundamentally, the study is thus about two-dimensional (2D) wedge impact in water. In an attempt to predict the structural response to impact hydrodynamic force, a beam element based finite element (FE) computer program is written and the results of the code are presented in the thesis. A computational tool is developed to predict the transient elastic response of a 2D wedge under impact force using two different numerical methods. Both explicit and implicit numerical schemes have also been studied in order to apply to the present work. Explicit forth order Runge-Kutta (RK4) method and implicit Newmark-b (NB) method have been used in the present work. Coupling effects between excitation and response are ignored in the present numerical computations. Both the numerical schemes are validated using simple static solution and also modal expansion technique.

Subsea pipelines are subjected to wave and steady current loads which cause pipeline stability problems. Current knowledge and understanding on the pipeline on-bottom stability is based on the research programmes from the 1980’s such as the Pipeline Stability Design Project (PIPESTAB) and American Gas Association (AGA) in Joint Industry Project. These projects have mainly provided information regarding hydrodynamic loads on pipeline and soil resistance in isolation. In reality, the pipeline stability problem is much more complex involving hydrodynamic loadings, pipeline response, soil resistance, embedment and pipe-soil-fluid interaction. In this thesis Computational Fluid Dynamics (CFD) modelling is used to investigate and establish the interrelationship between fluid (hydrodynamics), pipe (subsea pipeline), and soil (seabed). The effect of soil types, soil resistance, soil porosity and soil unit weight on embedment was examined. The overall pipeline stability alongside pipeline diameter and weight and hydrodynamic effect on both soil (resulting in scouring) and pipeline was also investigated. The use of CFD provided a better understanding of the complex physical processes of fluid-pipe-soil interaction. The results show that the magnitude of passive resistance is on the average eight times that of lateral resistance. Thus passive resistance is of greater significance for subsea pipeline stability design hence the reason why Coulomb’s friction theory is considered as conservative for stability design analysis, as it ignores passive resistance and underestimates lateral resistance. Previous works (such as that carried out by Lyons and DNV) concluded that soil resistance should be determined by considering Coulomb’s friction based on lateral resistance and passive resistance due to pipeline embedment, but the significance of passive resistance in pipeline stability and its variation in sand and clay soils have not be established as shown in this thesis. The results for soil porosity show that increase in pipeline stability with increasing porosity is due to increased soil liquefaction which increases soil resistance. The pipe-soil interaction model by Wagner et al. established the effect of soil porosity on lateral soil resistance but did not attribute it to soil liquefaction. Results showed that the effect of pipeline diameter and weight vary with soil type; for sand, pipeline diameter showed a greater influence on embedment with a 110% increase in embedment (considering combined effect of diameter and weight) and a 65% decrease in embedment when normalised with diameter. While pipeline weight showed a greater influence on embedment in clay with a 410% increase. The work of Gao et al. did not completely establish the combined effect of pipeline diameter and weight and soil type on stability. Results also show that pipeline instability is due to a combination of pipeline displacement due to vortex shedding and scouring effect with increasing velocity. As scoring progresses, maximum embedment is reached at the point of highest velocity. The conclusion of this thesis is that designing for optimum subsea pipeline stability without adopting an overly conservative approach requires taking into consideration the following; combined effect of hydrodynamics of fluid flow on soil type and properties, and the pipeline, and the resultant scour effect leading to pipeline embedment. These results were validated against previous experimental and analytical work of Gao et al, Brennodden et al and Griffiths.

A MEMS-based micro-electro-discharge machining technique that is enabled by the actuation of micromachined planar electrodes defined on the surfaces of the workpiece is developed that eliminates the need of numerical control machines. First, the planar electrodes actuated by hydrodynamic force is developed. The electrode structures are defined by patterning l8-µm-thick copper foil laminated on the stainless steel workpiece through an intermediate photoresist layer and released by sacrificial etching of the resist layer. The planer electrodes are constructed to be single layer structures without particular features underneath. All the patterning and sacrificial etching steps are performed using dry-film photoresists towards achieving high scalability of the machining technique to large-area applications. A DC voltage of 80-140 V is applied between the electrode and the workpiece through a resistance-capacitance circuit that controls the pulse energy and timing of spark discharges. The parasitic capacitance of the electrode structure is used to form a resistance capacitance circuit for the generation of pulsed spark discharge between the electrode and the workpiece. The suspended electrodes are actuated towards the workpiece using the downflow of dielectric machining fluid, initiating and sustaining the machining process. Micromachining of stainless steel is experimentally demonstrated with the machining voltage of 90V and continuous flow of the fluid at the velocity of 3.4-3.9 m/s, providing removal depth of 20 µm. The experimental results of the electrode actuation match well with the theoretical estimations. Second, the planar electrodes are electrostatically actuated towards workpiece for machining. In addition to the single-layer, this effort uses double-layer structures defined on the bottom surface of the electrode to create custom designed patterns on the workpiece material. The suspended electrode is electrostatically actuated towards the wafer based on the pull-in, resulting in a breakdown, or spark discharge. This instantly lowers the gap voltage, releasing the electrode, and the gap value recovers as the capacitor is charged up through the resistor. Sequential pulses are produced through the self-regulated discharging-charging cycle. Micromachining of the stainless-steel wafer is demonstrated using the electrodes with single-layer and double-layer structures. The experimental results of the dynamic built-capacitance and mechanical behavior of the electrode devices are also analyzed.

A MEMS-based micro-electro-discharge machining technique that is enabled by the actuation of micromachined planar electrodes defined on the surfaces of the workpiece is developed that eliminates the need of numerical control machines. First, the planar electrodes actuated by hydrodynamic force is developed. The electrode structures are defined by patterning l8-µm-thick copper foil laminated on the stainless steel workpiece through an intermediate photoresist layer and released by sacrificial etching of the resist layer. The planer electrodes are constructed to be single layer structures without particular features underneath. All the patterning and sacrificial etching steps are performed using dry-film photoresists towards achieving high scalability of the machining technique to large-area applications. A DC voltage of 80-140 V is applied between the electrode and the workpiece through a resistance-capacitance circuit that controls the pulse energy and timing of spark discharges. The parasitic capacitance of the electrode structure is used to form a resistance capacitance circuit for the generation of pulsed spark discharge between the electrode and the workpiece. The suspended electrodes are actuated towards the workpiece using the downflow of dielectric machining fluid, initiating and sustaining the machining process. Micromachining of stainless steel is experimentally demonstrated with the machining voltage of 90V and continuous flow of the fluid at the velocity of 3.4-3.9 m/s, providing removal depth of 20 µm. The experimental results of the electrode actuation match well with the theoretical estimations. Second, the planar electrodes are electrostatically actuated towards workpiece for machining. In addition to the single-layer, this effort uses double-layer structures defined on the bottom surface of the electrode to create custom designed patterns on the workpiece material. The suspended electrode is electrostatically actuated towards the wafer based on the pull-in, resulting in a breakdown, or spark discharge. This instantly lowers the gap voltage, releasing the electrode, and the gap value recovers as the capacitor is charged up through the resistor. Sequential pulses are produced through the self-regulated discharging-charging cycle. Micromachining of the stainless-steel wafer is demonstrated using the electrodes with single-layer and double-layer structures. The experimental results of the dynamic built-capacitance and mechanical behavior of the electrode devices are also analyzed.

A força hidrodinâmica em termos dos coeficientes de massa adicional e amortecimento, para obstáculos aproximadamente circulares, finos e submersos sob uma superfície livre aquática, é calculada numericamente usando um método espectral. Primeiramente, é apresentado um modelo matemático para ondas aquáticas de superfície e em seguida, o problema de difração de ondas devido à presença de um obstáculo é descrito. Quando o obstáculo é submerso e fino, o problema pode ser formulado em termos de uma equação integral hipersingular. Usando um mapeamento conforme sobre um disco circular, é mostrado que a solução pode ser obtida através de um método espectral onde a hipersingularidade é avaliada analiticamente em termos de polinômios ortogonais. Os coeficientes da força hidrodinâmica, em função do número de onda, são obtidos para obstáculos quase circulares. A ocorrência de frequências ressoantes ´e observada para submersões suficientemente pequenas e subpicos de ressonância aparecem para valores moderados da submersão, em comparação com o caso do disco circular. / The hydrodynamic force, in terms of the added mass and damping coefficients, for thin and submerged nearly circular obstacles below a water free surface is computed by a spectral method. Firstly, a mathematical model for surface water waves is presented. Next, the diffraction problem of waves due to the presence of an obstacle is described. When the body is thin and submerged, the problem can be formulated in terms of a hypersingular integral equation. Using a conformal mapping over a circular disc, it is shown that the solution can be obtained by means of a spectral method where the hipersingularity is analytically evaluated in terms of orthogonal polynomials. The hydrodynamics coefficients, in function of the wavenumber, are computed and shown for nearly circular obstacles. The occurrence of resonant frequencies is observed for sufficiently small submergences and subpeaks of resonances appear for moderate values of the submergence, in comparison with the case of a circular disc.

A força hidrodinâmica em termos dos coeficientes de massa adicional e amortecimento, para obstáculos aproximadamente circulares, finos e submersos sob uma superfície livre aquática, é calculada numericamente usando um método espectral. Primeiramente, é apresentado um modelo matemático para ondas aquáticas de superfície e em seguida, o problema de difração de ondas devido à presença de um obstáculo é descrito. Quando o obstáculo é submerso e fino, o problema pode ser formulado em termos de uma equação integral hipersingular. Usando um mapeamento conforme sobre um disco circular, é mostrado que a solução pode ser obtida através de um método espectral onde a hipersingularidade é avaliada analiticamente em termos de polinômios ortogonais. Os coeficientes da força hidrodinâmica, em função do número de onda, são obtidos para obstáculos quase circulares. A ocorrência de frequências ressoantes ´e observada para submersões suficientemente pequenas e subpicos de ressonância aparecem para valores moderados da submersão, em comparação com o caso do disco circular. / The hydrodynamic force, in terms of the added mass and damping coefficients, for thin and submerged nearly circular obstacles below a water free surface is computed by a spectral method. Firstly, a mathematical model for surface water waves is presented. Next, the diffraction problem of waves due to the presence of an obstacle is described. When the body is thin and submerged, the problem can be formulated in terms of a hypersingular integral equation. Using a conformal mapping over a circular disc, it is shown that the solution can be obtained by means of a spectral method where the hipersingularity is analytically evaluated in terms of orthogonal polynomials. The hydrodynamics coefficients, in function of the wavenumber, are computed and shown for nearly circular obstacles. The occurrence of resonant frequencies is observed for sufficiently small submergences and subpeaks of resonances appear for moderate values of the submergence, in comparison with the case of a circular disc.

A força hidrodinâmica em termos dos coeficientes de massa adicional e amortecimento, para obstáculos aproximadamente circulares, finos e submersos sob uma superfície livre aquática, é calculada numericamente usando um método espectral. Primeiramente, é apresentado um modelo matemático para ondas aquáticas de superfície e em seguida, o problema de difração de ondas devido à presença de um obstáculo é descrito. Quando o obstáculo é submerso e fino, o problema pode ser formulado em termos de uma equação integral hipersingular. Usando um mapeamento conforme sobre um disco circular, é mostrado que a solução pode ser obtida através de um método espectral onde a hipersingularidade é avaliada analiticamente em termos de polinômios ortogonais. Os coeficientes da força hidrodinâmica, em função do número de onda, são obtidos para obstáculos quase circulares. A ocorrência de frequências ressoantes ´e observada para submersões suficientemente pequenas e subpicos de ressonância aparecem para valores moderados da submersão, em comparação com o caso do disco circular. / The hydrodynamic force, in terms of the added mass and damping coefficients, for thin and submerged nearly circular obstacles below a water free surface is computed by a spectral method. Firstly, a mathematical model for surface water waves is presented. Next, the diffraction problem of waves due to the presence of an obstacle is described. When the body is thin and submerged, the problem can be formulated in terms of a hypersingular integral equation. Using a conformal mapping over a circular disc, it is shown that the solution can be obtained by means of a spectral method where the hipersingularity is analytically evaluated in terms of orthogonal polynomials. The hydrodynamics coefficients, in function of the wavenumber, are computed and shown for nearly circular obstacles. The occurrence of resonant frequencies is observed for sufficiently small submergences and subpeaks of resonances appear for moderate values of the submergence, in comparison with the case of a circular disc.