The commercially available Computational Fluid Dynamics (CFD) software ANSYS-CFX version 11 (2008) is employed to predict the distribution of the bed and sidewall shear stresses in trapezoidal open channels. The investigation includes computation of wall shear stress (1) directly, using CFD for a range of channels layouts (straight, turning, with ridges), and (2), building on the division line concept initially formulated by Leighly in 1932 and later by Einstein in 1942, through the evaluation of the Guo and Julien (2005) equations, as proposed by Cacqueray et al. (2009). These equations include the two complex integral terms II and III, pertaining to viscous and secondary current effects, have been analysed for each cross section, for straight channels. The CFD predictions are validated first against the experimental results of Tominaga et al. (1989) to ensure that the models used are appropriate. Once this is done, the impact of (1) the variation of the slant angle of the sidewalls, (2) the channel aspect ratio and (3) the composite roughness on the shear stress distribution in straight prismatic channels is analysed directly based on the CFD predictions. In wide open channels the lines of boil, consisting of low speed streaks, periodically in the transverse direction, is believed to be due to the initiation of sand ridges on the bed; in other words due to the coupled interaction between moving bed and flow. A numerical analysis on the flow structures created and the distribution of shear stresses on the bed and sidewalls of channel sections having ridges on its bed is therefore carried out to clarify this point and assess the potential consequences on our predictions. Finally, because of obvious practical relevance, as most rivers follow a winding course, numerical simulations on the distribution of shear on the boundaries inside a channel bend is also presented.

Long, thin, flexible cylindrical elements of large scale structures are heavily influenced by the fluid flow around them. Equally, their movement has an appreciable effect on the fluid flow. This two-way interaction leads to complex dynamic behaviour that can cause fatigue and thus reduce operational lifetime. As demand for longer span bridges and drilling in deeper marine environments increases, research into the best modelling practice of this scenario gains importance. The work described in this thesis establishes a suitable method to model in CFD aero/hydro-elastic behaviour of slender cylindrical elements in large scale structures. In order to achieve this outcome, the author has: modelled the drag crisis on a static cylindrical element; developed a suitable FSI coupling program; combined the drag crisis model with the FSI coupling program and validate against published experimental data. The turbulence formulation used was carefully chosen taking into account the flow features that are important to the onset of the drag crisis. An LES formulation capable of adapting the model constant of the SGS model according to local shear conditions was identied as the best candidate to achieve this aim. The fluid and structural solvers used were loosely coupled by an explicit method that achieved a balance of kinetic energy as well as matching displacement at the moving fluid/solid interface. The integration method and implementation of this coupling strategy was verified by running a test case at low Reynolds number that produced a regular sinusoidal lift function on the cylinder that was kept stationary. The displacement, velocity, and acceleration response produced by the structural solver was compared against a closed solution and found to match with an acceptable level of error. A number of FSI simulations with the cylinder free to move in the cross-flow direction only was carried out. The displacement response was compared against published numerical and experimental data and the importance of having a sufficient spanwise dimension of flow domain was highlighted. Simulations with the cylinder free to move in the along-flow direction aswell as cross-flow direction were carried out. In some simulations where lock-in was observed, the effect of the drag crisis was clearly seen. Energy entered into the system as a result of low drag on the upstream motion of the cylinder caused by the drag crisis. More simulations at different velocities are recommended to define a displacement response curve and make further new observations.

Elongated bubbles which are constrained by the walls of a pipe are commonly known as Taylor bubbles. Taylor bubbles are prevalent in industrial gas-liquid flow, where they are commonly found in buoyancy driven fermenters, the production and transportation of hydrocarbons in the oil and gas industry, the boiling and condensing process in thermal power plants, and the emergency cooling of nuclear reactors. These bubbles also exist in the natural world, and are the driving force behind certain types of volcanic eruption. An analysis of the literature identified a paucity of experimental or numerical studies investigating the rise of Taylor bubbles in pipes with a diameter in excess of 0.12 m or in pipes which contain a change in geometry. The aim of this thesis was to gain a better understanding of the behaviour of Taylor bubbles in flow conditions which have not previously been studied. To achieve this, a CFD model was used to simulate the rise of single Taylor bubbles and a set of experiments conducted. The CFD model was validated against the results of published experimental studies, empirical correlations and theoretical predictions. Further validation was conducted using the results of the experimental study which investigated the rise of Taylor bubbles in a pipe of diameter 0.29 m. These experiments confirmed that the theoretically predicted stability and rise rate of the bubble were correct. Bubbles were also shown to exhibit oscillatory behaviour. Sets of parametric simulations replicated the behaviour observed in the experiments and predicted by theoretical models for a wide range of conditions. The qualitative and quantitative experimental behaviour of a Taylor bubble rising through an expansion in pipe geometry was replicated by the CFD model. Bubbles of sufficient length were observed split as they rose through the expansion in diameter, which produced pressure oscillations. The effects of a variation in a number of parameters, including the angle of expansion, the ratio of the upper to lower pipe diameters and the liquid viscosity, were explored.

Os motores a jato são componentes importantes e complexos de engenharia cuja eficiência e desempenho estão diretamente ligados à temperatura de operação na câmara de combustão, ou seja, quanto maior a temperatura dos gases nesta região, maior a eficiência global do processo, resultando em economia de combustível e maior potência. A superliga aeronáutica a base de níquel é o material atualmente utilizado nestes componentes mais críticos das turbinas, mas o seu desenvolvimento está chegando cada vez mais próximo do limite e, por isso, existe uma demanda de materiais que possam substituí-las. Os materiais constituídos de microestruturas multifásicas, como as ligas MR-Si-B (MR= Metal Refratário) apresentam potencial para a substituição das superligas nestas aplicações. Estudos recentes no sistema Ta-Si-B identificaram a existência de uma fase ainda não catalogada, denominada φ. No entanto, a sua estrutura cristalina permanece indefinida devido à dificuldade em se obter uma amostra com grande fração volumétrica de φ. A substituição do silício por germânio nesse sistema pode aumentar o campo de estabilidade de φ possibilitando assim, a obtenção de uma amostra através da qual a estrutura cristalina dessa fase possa ser determinada. Entretanto, para o estudo do ternário Ta-Ge-B, é importante o conhecimento dos binários que o compõem (Ta-Ge, Ta-B, Ge-B) e não foi encontrada na literatura uma proposta para o diagrama binário Ta-Ge nem informações suficientes para construí-lo. Este trabalho, portanto, tem como objetivo propor um diagrama de fases para o sistema Ta-Ge, e investigar a possível existência da fase φ no sistema ternário Ta-Ge-B. Para isto foram confeccionadas amostras, a partir de matérias primas (tântalo, germânio e boro) de alta pureza, em toda a extensão composicional do sistema Ta-Ge e na região rica em tântalo do sistema Ta-Ge-B. As amostras, em seu estado bruto de fusão bem como após tratamentos térmicos, foram caracterizadas por microscopia eletrônica de varredura (MEV), microanálise eletrônica por energia dispersiva (EDS) e difratometria de raios X (DRX). Com os resultados obtidos neste trabalho foi proposto um diagrama para o sistema Ta-Ge. A temperatura do eutético na região rica em Ta (2444°C) foi medida através de pirometria ótica. Devido a dificuldades encontradas em fundir a arco amostras com altos teores de Ge, novos experimentos serão necessários para melhor determinar esta região do diagrama de fases. Os resultados obtidos nas amostras ternárias sugerem que a fase ternária φ é estável no sistema Ta-Ge-B. / Jet engines are important and complex engineering devices, whose efficiency and performance are directly related to the operating temperature of the combustion chamber, the higher the gas temperature in this region, the greater the overall efficiency of the process, resulting in economy of fuel and more efficiency. Ni-based superalloys are currently used in the most critical jet engine components. Their development is coming close to a temperature limit and, therefore, there is a demand for new materials to replace them. Materials with multiphase microstructures, such as MR-Si-B alloys (MR = Refractory Metal), have a good potential for substituting superalloys in these applications. Recent studies in the Ta-Si-B system shows the stability of a phase φ whose crystal structure remains unknown due to the difficulty in obtaining a sample with large volume fraction of φ. The substitution of silicon by germanium in this system may increase the stability of φ, enabling the production of a sample by which the crystal structure of this phase can be determined. The study of the Ta-Ge-B ternary system requires the knowledge of the phase relations on its limiting binaries. However, there is no Ta-Ge phase diagram available in the literature, nor enough experimental information to propose one. Therefore, the present work aims at the experimental investigation of the phase relations on the Ta-Ge system, and of the stability of phase φ on the Ta-rich region of the Ta-Ge-B ternary system. Samples were prepared by arc melting high purity materials (tantalum, germanium and boron) under Ti-gettered argon. Their microstructures, in the as cast state as well as after heat treatments were characterized by scanning electron microscopy (SEM), energy dispersive electron probe microanalysis (EDS) and X-ray diffraction (XRD). A Ta-Ge binary phase diagram is proposed based on the results of the present work. The temperature of the Ta-rich eutetic (2444°C) was measured by optical pirometry. Due to the difficulty of arc melting samples with high Germanium contents, further investigation is still necessary for this region of the phase diagram. The results for the ternary samples suggest the stability of the φ phase in the Ta-Ge-B system.

Roller Compacted Concrete (RCC) is a mixture of well graded aggregates, cement and water. It is placed with a high compaction asphalt type paver and compacted to high density by vibratory rollers to provide a high strength and durable pavement structure. RCC requires no formwork, surface finishing, dowelled joints or reinforcement. These characteristics make RCC simple, fast and economical. However, it also presents difficulties with high-speed applications related to surface texture and surface evenness. These difficulties have so far restricted the use of RCC to the lower layers of normal roads. A two-layer RCC pavement system is a type of composite pavement consisting of two concrete layers. The two layers are paved in either a “wet-on-wet” technique or ”wet-on-dry” technique. The bottom layer serves as the main bending-resistant component of the composite slab, while the top lift is generally constructed with higher-quality constituent materials for improved surface characteristics such as noise and skid resistance. The aim of this research is to evaluate and design two-layer RCC systems with different aggregate sizes and types and different placement conditions in order to expand the application of RCC in pavements. Mechanical properties, bond strength properties, durability characteristics, surface properties, fatigue damage, joint deterioration and pavement design are the main. A range of testing equipment, methodologies and tools have been used in this investigation. The findings of this study showed that a two-layer RCC system can achieved good strength and stiffness for each of the mixtures in the two layers. Also, the inter-layer bond was found to be strong when the two layers were placed within one hour, but weaker when the upper layer was placed three hours after the lower layer. Moreover, the durability of the two-layer RCC system was found to be acceptable, especially when the upper layer was placed within an hour of the lower layer. The surface characteristics for the upper layer of RCC showed that the minimum requirement for skid resistance and texture depth have been achieved. However, it is suggested that further investigation is needed, particularly into the evenness of RCC. The investigation into the effect of dynamic load on the two-layer RCC system demonstrated a good fatigue strength for each RCC mixture and for the two layers together, compared to conventional concrete pavements. Also, the results of load transfer stiffness and joint deterioration showed acceptable performance with regard to crack or joint width, shear stress and placement conditions. The effect of other parameters such as moisture and differential temperature requires a separate investigation and is recommended for future work. The results of the design and analysis of two-layer RCC using KENSLAB, a finite element program, indicted that RCC could perform successfully in pavements with a long service life. In conclusion, the results obtained show that the two-layer RCC are a valid alternative for pavements. On one hand, the use of a harder and more resistance aggregate at the top layer guarantees higher skid resistance and durability, while limiting for the use of high quality aggregate. On the other hand, the results show that adequate construction techniques can alleviate the problems arising from the lack bond between layers.

The wide range of applications of bubbly flows in the industry makes its accurate modelling strongly demanded. The modelling of bubbly flow, however, is not straightforward because it consists of multi-scale structures in both time and space. Furthermore, the experimental verifications of the theoretical and the Direct Numerical Simulations, DNS, available are noticeably scarce especially for bubbles at high Reynolds numbers. Hence, this research aims to make its contribution to addressing the basic roots that make the modelling process difficult. These roots are represented, but not limited, by the bubble dynamics and bubble-bubble interaction. Tracking of single and dual air bubbles in quiescent water is experimentally carried out using a high-speed camera. For the numerical simulations, using ANSYS-FLUENT software with the VOF model, a structured adaptive mesh technique was developed here that is used to achieve a desirable level of refinement of the mesh around the rising bubbles. It has been found that there is a relationship between the lateral migration of an ellipsoidal bubble and the shape oscillation. This relationship, however, has not been observed for the numerical approach. It also overestimated the experimental findings of bubble kinetics and shape oscillation by 30%. Interestingly, this research contributes to awakening the small details in the underlying physics of the interaction between a pair of rising bubbles. It has been found that a slight deviation in the size of a trailing bubble plays an important role in the state of the trailing bubble whether it approaches the leading bubble or separates from at large separation distances. This is considered due to the greater rise velocity that a smaller bubble has in the ellipsoidal regime compared to the larger bubble. Furthermore, when the separation distance between the rising bubbles is decreased, the appreciable acceleration that the trailing bubble owns has led it to approach the leading bubble and coalesce with it. This behaviour is supported by experiments and by the good agreement that the numerical approach showed. An empirical model that predicts the coalescence rate based on the deviation in the size ratio is presented. Finally, the spatial boundaries over which the coalescence of bubble pairs might occur has been numerically and experimentally presented.

An extensive amount of works have been carried out to develop the current understanding in punching shear mechanism noted in reinforced concrete slabs. However, despite the increasing popularity of waffle slabs, the current understanding about punching behaviour is mainly focused on solid flat slabs, and only limited amount of works have been carried out on waffle slabs and in the presence of biaxial moment. Thus, there is a need to carry out a research in this area to aid the understanding about punching mechanism of waffle slabs in the presence of biaxial moment for the internal column and edge column connections. The experimental work carried out in this research included destructive testing of thirty-eight 1/10th scale model waffle slab specimens, which consists of fifteen internal column slabs and twenty-three edge column slabs. The main variables were, for the internal column slab, the principle angles of biaxial moment transfer, the column eccentricity, the column orientation and the size of solid sections, and for the edge column slab, the principle angles of biaxial moment transfer, the column eccentricity, the column location and the size of solid sections. From the experimental investigations, three distinct failure mechanisms were observed: the concentric punching at internal column mechanism; the eccentric punching at internal column mechanism; and the edge punching mechanism. In general, the observed punching shear failure mechanisms of waffle slabs were found identical to solid flat slabs; but the punching shear capacities reduced due to some losses in potential failure surface within the waffle section. The principle angle of biaxial moment transfer was found varying the shear surface area that was being mobilized, thus affecting the punching capacity of the slabs. An analytical study was carried out, using an upper-bound plastic model, to simulate the observed punching shear mechanisms, and hence, to predict the punching capacity of the slabs. A theoretical model was developed for each of the identified failure mechanism. In addition, three design models based on the current UK code, Eurocode 2, have been developed. In all cases, these models have achieved good agreements with the test results.

Os motores a jato são componentes importantes e complexos de engenharia cuja eficiência e desempenho estão diretamente ligados à temperatura de operação na câmara de combustão, ou seja, quanto maior a temperatura dos gases nesta região, maior a eficiência global do processo, resultando em economia de combustível e maior potência. A superliga aeronáutica a base de níquel é o material atualmente utilizado nestes componentes mais críticos das turbinas, mas o seu desenvolvimento está chegando cada vez mais próximo do limite e, por isso, existe uma demanda de materiais que possam substituí-las. Os materiais constituídos de microestruturas multifásicas, como as ligas MR-Si-B (MR= Metal Refratário) apresentam potencial para a substituição das superligas nestas aplicações. Estudos recentes no sistema Ta-Si-B identificaram a existência de uma fase ainda não catalogada, denominada φ. No entanto, a sua estrutura cristalina permanece indefinida devido à dificuldade em se obter uma amostra com grande fração volumétrica de φ. A substituição do silício por germânio nesse sistema pode aumentar o campo de estabilidade de φ possibilitando assim, a obtenção de uma amostra através da qual a estrutura cristalina dessa fase possa ser determinada. Entretanto, para o estudo do ternário Ta-Ge-B, é importante o conhecimento dos binários que o compõem (Ta-Ge, Ta-B, Ge-B) e não foi encontrada na literatura uma proposta para o diagrama binário Ta-Ge nem informações suficientes para construí-lo. Este trabalho, portanto, tem como objetivo propor um diagrama de fases para o sistema Ta-Ge, e investigar a possível existência da fase φ no sistema ternário Ta-Ge-B. Para isto foram confeccionadas amostras, a partir de matérias primas (tântalo, germânio e boro) de alta pureza, em toda a extensão composicional do sistema Ta-Ge e na região rica em tântalo do sistema Ta-Ge-B. As amostras, em seu estado bruto de fusão bem como após tratamentos térmicos, foram caracterizadas por microscopia eletrônica de varredura (MEV), microanálise eletrônica por energia dispersiva (EDS) e difratometria de raios X (DRX). Com os resultados obtidos neste trabalho foi proposto um diagrama para o sistema Ta-Ge. A temperatura do eutético na região rica em Ta (2444°C) foi medida através de pirometria ótica. Devido a dificuldades encontradas em fundir a arco amostras com altos teores de Ge, novos experimentos serão necessários para melhor determinar esta região do diagrama de fases. Os resultados obtidos nas amostras ternárias sugerem que a fase ternária φ é estável no sistema Ta-Ge-B. / Jet engines are important and complex engineering devices, whose efficiency and performance are directly related to the operating temperature of the combustion chamber, the higher the gas temperature in this region, the greater the overall efficiency of the process, resulting in economy of fuel and more efficiency. Ni-based superalloys are currently used in the most critical jet engine components. Their development is coming close to a temperature limit and, therefore, there is a demand for new materials to replace them. Materials with multiphase microstructures, such as MR-Si-B alloys (MR = Refractory Metal), have a good potential for substituting superalloys in these applications. Recent studies in the Ta-Si-B system shows the stability of a phase φ whose crystal structure remains unknown due to the difficulty in obtaining a sample with large volume fraction of φ. The substitution of silicon by germanium in this system may increase the stability of φ, enabling the production of a sample by which the crystal structure of this phase can be determined. The study of the Ta-Ge-B ternary system requires the knowledge of the phase relations on its limiting binaries. However, there is no Ta-Ge phase diagram available in the literature, nor enough experimental information to propose one. Therefore, the present work aims at the experimental investigation of the phase relations on the Ta-Ge system, and of the stability of phase φ on the Ta-rich region of the Ta-Ge-B ternary system. Samples were prepared by arc melting high purity materials (tantalum, germanium and boron) under Ti-gettered argon. Their microstructures, in the as cast state as well as after heat treatments were characterized by scanning electron microscopy (SEM), energy dispersive electron probe microanalysis (EDS) and X-ray diffraction (XRD). A Ta-Ge binary phase diagram is proposed based on the results of the present work. The temperature of the Ta-rich eutetic (2444°C) was measured by optical pirometry. Due to the difficulty of arc melting samples with high Germanium contents, further investigation is still necessary for this region of the phase diagram. The results for the ternary samples suggest the stability of the φ phase in the Ta-Ge-B system.

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The accurate study of characteristics of the flow under gravity waves has become of prime importance due to the growing demand for structural engineering designs in the coastal environment. Although many investigations have been carried out, the progress of fundamental research was slow due to the lack of an adequate velocity measuring instrument. However in recent years, the development of the Laser Doppler Velocimeter has made it possible to observe the orbital velocity very close to a bed without disturbing the flow. This technique was used in this investigation, in which observations of the oscillatory flow under gravity waves were carried out above smooth, two-dimensional and three-dimensional rough beds. For the smooth bed case it was found that the velocity profile throughout the depth was well presented by the Stokes second order shear wave equation, except that the theoretical predictions underestimated the observed results, and a linear relationship was obtained for the velocity coefficients between the two sets of values. As for mean velocity the profile was in close agreement with the Longuet- Higgins conduction solution, and it was found to have a negative value (in opposite direction to wave progression) in the bulk of fluid and always positive values within the boundary layer. The rough beds made little change to the flow in the bulk of fluid. As for inside the boundary layer, the laminar boundary layer was eliminated due to the large size of the rough bed, but for a small size rough bed the flow became laminar at the edge of the boundary layer, and a perturbed laminar boundary layer velocity profile was traceable. However, the two rough beds had similar influences on the flow except for the roughness size and Reynolds number values. Inside the roughness elements of the rough beds vortex formation was clearly observed and the comprehensive range of measurements of these formations are analysed and discussed.