Liquid storage tanks are a crucial type of structures. They are used to store various types of liquids and liquefied gases in different situations. In seismic regions, functionality of these structures after severe earthquakes is an important factor in their design. In earthquake-prone regions, the sloshing phenomena has an important role in the design procedure. Current design codes and guidelines (e.g. ACI 350.3 and ASCE 7) are based on analytical studies that in some cases can be inaccurate in prediction of forces and pressures. Since a long time ago scientists have studied the sloshing phenomena in liquid storage tanks with different methods including analytical, numerical and experimental studies. In the current study, rectangular ground-supported tanks are studied and the effect of seismic loading on them is investigated both experimentally and numerically. For the experimental tests, the tanks were placed on a shaking table and using high-speed HD cameras, tests were filmed and later analyzed frame by frame to capture the critical moments. To investigate the bi-lateral effect of base excitation on the tanks, they were oriented on the table with four different angles. In the numerical study, a computational fluid dynamics tool - OpenFOAM - was used to simulate the tank motion and finally the results were compared with the experiment in order to develop a reliable model.

Cases of dam failure occur around the world almost each year. Dam failures can result in the formation and propagation of fast-moving unsteady flows that can cause loss of life as well as significant environmental and economic consequences in downstream flooded areas. The initial stages of a dam break are important due to wave-breaking front and the associated turbulence. Furthermore, characteristics of the river bed downstream of the dam (topography and bathymetry) as well as the presence of obstacles in the dam break wave path such as man-made or natural obstacles like bridges, trees, and local sills affect flow dynamics, which can lead to the formation of hydraulic jumps and the reflection of the flood wave. Accordingly, the precise prediction of flood parameters such as arrival times, free surface profiles, and flow velocity profiles is essential in order to mitigate flood hazards. This study aimed to assess the performance of various turbulence models in predicting and estimating dam-break flows and related positive and negative flood wave characteristics over different downstream bed conditions. Three-dimensional (3-D) Computational Fluid Dynamics (CFD) models were created to solve the unsteady Reynolds equations in order to determine the initial stages of the free surface profiles over dry and wet beds and to investigate the generation and propagation of dam-break flows and reflected flood waves in the presence of a bed obstacle. The performance of different Reynolds-averaged Navier-Stokes (RANS) turbulence models has been investigated, and the standard k-ε, RNG k-ε, realizable k-ε, k-ω SST, and v^2-f turbulence models have been studied using OpenFOAM software. Dam-breaks were modelled using the Volume of Fluid (VOF) method employing the Finite Volume Method (FVM). Both qualitative and quantitative comparisons of numerical simulations with laboratory experiments were completed in order to assess the suitability of different turbulence models. The results of the first study showed that the RNG k-ε model exhibited better performance in capturing the flood wave free surface profiles over both dry- and wet-bed downstream conditions, while from the second study, it was concluded that the k-ω SST model was able to accurately predict the formation and propagation of reflected waves against a bottom obstacle in terms of free surface profiles and negative bore propagation speeds.

Le mélange de fluides représente un élément important du domaine de la dynamique des fluides, ce qui rend la compréhension de ce sujet si significative du point de vue fondamental et appliqué (p. ex., les processus industriels). Dans les géométries miniaturisées (dans des conditions typiques) le mélange est un processus lent, difficile et inefficace. Cela en raison du caractère naturellement laminaire de ces écoulements, qui oblige l'homogénéisation de différents éléments fluides à se produire par diffusion moléculaire au lieu d'un transport advectif, à l'action plus rapide. Cependant, des études expérimentales récentes sur les écoulements viscoélastiques à faible nombre de Reynolds ont montré qu'un mélange efficace peut être déclenché dans plusieurs configurations géométriques (y compris les dispositifs à l'échelle microscopique), par le phénomène de la turbulence élastique. La première partie de cette thèse est consacrée à la compréhension et à l'investigation des défis numériques présents dans le domaine de la dynamique des fluides non newtonienne, en se concentrant plus particulièrement au problème du haut nombre de Weissenberg. Ce dernier se manifeste par une rupture du schéma numérique, lorsque les équations d'évolution d'extra-contraintes polymériques sont évaluées de façon directe. Ceci pose des limites importantes à la possibilité de simuler avec précision des écoulements turbulents-élastiques. Nous fournissons des preuves numériques de l'effet bénéfique (en termes de gain en stabilité) de la décomposition en racine carrée de l'extra-contrainte dans une implémentation en volumes finis des équations régissant l'écoulement dans un canal bidimensionnel. La deuxième partie de la thèse traite de l’émergence et de la caractérisation d’instabilités purement élastiques dans des simulations numériques de fluides Oldroyd-B à nombre de Reynolds zéro dans une géométrie du type cross-slot bidimensionnel. Grâce à un travail numérique approfondi, nous présentons une caractérisation détaillée des instabilités purement élastiques. Ces instabilités apparaissant dans le système pour de larges plages d'élasticité du fluide et de concentration des polymères. Pour les solutions concentrées et des nombres de Weissenberg assez grands, nos simulations indiquent l’apparition d’un écoulement désordonné pointant vers la turbulence élastique. Nous analysons le passage à une dynamique irrégulière et caractérisons les propriétés statistiques de tels écoulements très élastiques, en discutant des similitudes et des différences avec les résultats expérimentaux de la littérature. / Fluid mixing represents an important component of the field of fluid dynamics, what makes the understanding of this subject so meaningful from both the fundamental and applied (e.g. industrial processes) point of view. In miniaturised geometries, under typical conditions, mixing is a slow, difficult and inefficient process due to the naturally laminar character of these flows, which forces the homogenisation of different fluid elements to occur via molecular diffusion instead of faster-acting advective transport. However, recent experimental studies on low-Reynolds-number viscoelastic flows have shown that efficient mixing can be triggered in several geometrical configurations (including micro-scale devices), by the phenomenon of elastic turbulence. The first part of this thesis is devoted to the understanding and investigation of numerical challenges present in the domain of non-Newtonian fluid dynamics, focusing in particular on the high-Weissenberg number problem. The latter manifests as a breakdown of the numerical scheme when the polymeric extra-stress evolution equations are implemented in a direct way, which poses severe limits to the possibility to accurately simulate elastic turbulent flows. We provide numerical evidence of the beneficial effect (in terms of increased stability) of the square-root decomposition of the extra-stress in a finite-volume-based implementation of the governing equations in a two-dimensional channel. The second part of the thesis reports about the emergence and characterisation of purely-elastic instabilities in numerical simulations of zero-Reynolds-number Oldroyd-B fluids in a two-dimensional cross-slot geometry. By means of extensive numerical work, we provide a detailed characterisation of the purely-elastic instabilities arising in the system for wide ranges of both the fluid elasticity and the polymer concentration. For concentrated solutions and large enough Weissenberg numbers, our simulations indicate the emergence of disordered flow pointing to elastic turbulence. We analyse the transition to irregular dynamics and characterise the statistical properties of such highly elastic flows, discussing the similarities and differences with experimental results from the literature.

Treated and untreated liquid that is discharged from industrial and desalination plants is one of the main factors that break the ecological balance and destroys aquatic habitat in lakes, rivers and coastal areas where the effluent is discharged. Positively and negatively buoyant jets are two categories of outfalls which are generated because of the destiny difference between the effluent and ambient fluid. In order to ensure minimal impact of the effluent on the environment, it is necessary to estimate the dilution of the jet and compare it with environmental regulations on the level of required dilution to ensure that the concentration of the effluent is diluted quickly enough and the concentration of the effluent at different points does not exceed the allowed concentrations. This study investigated the positively buoyant jet, which happens near the coastal and near water area. For instance, cooling water that flows out from a power plant or factory, wasted water that is discharged from an industrial plant near river, submerged drainage from civil municipal sewer systems and treated water from desalination plant in coastal area. Density difference, velocity and inclined angle of the jet were considered as the main factors that contribute to the jet spreading and were compared to develop the best solution for its dilution. The jet was discharged inclined downward to allow for more mixing and dilution of the effluent with the ambient water. In order to simulate a positive jet, tap water was injected in saline ambient. A large number of experiments were conducted in the laboratory and using camera imaging. The jet trajectory was estimated from the images using image processing and the impact of various parameters such as Froude number and jet velocity were investigated. The opensource software OpenFOAM, was employed for numerical simulations which is a finite volume model ensures mass conservation and allows for flexible mesh size for further accuracy and optimization of computational cost. Using this Computational Fluid Dynamics (CFD) model, the numerical simulations were performed, and the results were compared with laboratory experiments. A Reynold-Averaged Navier-Stokes (RANS) approach was employed in the numerical simulations which offers a good balance between accuracy and computational cost. It was found that the numerical model in conjunction with the second order turbulence model called Launder-Reece- Rodi model (LRR) had a reasonable agreement with the experimental data.

Within the course of the master thesis project, two thermal phase change models for direct contact conden-sation were developed with different modeling approaches, namely interfacial heat transfer and combustionanalysis approach.After understanding the OpenFOAM framework for two phase flow solvers with phase change capabilities,a new solver, including the two developed models for phase change, was implemented under the name of interPhaseChangeCondenseTempFoam and analyzed in a series of 18 tests in order to determine the physicalbehavior and robustness of the developed models. The solvers use a volume-of-fluid (VOF) approach withmixed fluid properties.It has been shown that the approach with inter-facial heat transfer shows physical behavior, a strong timestep robustness and good grid convergence properties. The solver can be used as a basis for more advancedsolvers within the phase change class.

Within the course of the master thesis project, two thermal phase change models for direct contact conden-sation were developed with different modeling approaches, namely inter-facial heat transfer and combustionanalysis approach. After understanding the OpenFOAM framework for two phase flow solvers with phase change capabilities, a new solver, including the two developed models for phase change, was implemented under the name of interPhaseChangeCondenseTempFoam and analyzed in a series of 18 tests in order to determine the physical behavior and robustness of the developed models. The solvers use a volume-of-fluid (VOF) approach withmixed fluid properties. It has been shown that the approach with inter-facial heat transfer shows physical behavior, a strong timestep robustness and good grid convergence properties. The solver can be used as a basis for more advanced solvers within the phase change class.

Within the course of the master thesis project, two thermal phase change models for direct contact conden-sation were developed with different modeling approaches, namely interfacial heat transfer and combustionanalysis approach.After understanding the OpenFOAM framework for two phase flow solvers with phase change capabilities,a new solver, including the two developed models for phase change, was implemented under the name ofinterPhaseChangeCondenseTempFoam and analyzed in a series of 18 tests in order to determine the physicalbehavior and robustness of the developed models. The solvers use a volume-of-fluid (VOF) approach withmixed fluid properties.It has been shown that the approach with inter-facial heat transfer shows physical behavior, a strong timestep robustness and good grid convergence properties. The solver can be used as a basis for more advancedsolvers within the phase change class.

This analysis calculates the velocity profile and recirculation ratio in the aspirator region of an enhanced once-through steam generator of the Babcock & Wilcox design. This information is important to the development of accurate RELAP5 models, steam generator level calculations, steam generator downcomer models, and flow induced vibration analyses. The OpenFOAM CFD software package was used to develop the three-dimensional model of the EOTSG aspirator region, perform the calculations, and post-process the results. Through a series of cases, each improving upon the modeling accuracy of the previous, insight is gained into the importance of various modeling considerations, as well as the thermal-hydraulic behavior in the steam generator downcomer. Modeling the tube support plates and tube nest is important for the accurate prediction of flow rates above and below the aspirator port, but has little affect on the aspirator region itself. Modeling the MFW nozzle has minimal influence on the incoming steam velocity, but does create a slight azimuthal asymmetry and alter the flow pattern in the downcomer, creating recirculation patterns important to inter-phase heat transfer. Through the development of a two-phase solution that couples the aspirated steam and liquid feedwater, it was found that the ratio of droplet surface area to volume plays the most important role in determining the rate of aspiration. Calculations of the velocity profile and recirculation ratio are compared against those of historical calculations, demonstrating the possibility that these parameters were previously underpredicted. Such a conclusion can only be confidently made once experimental data is made available to validate the results of this analysis. / Master of Science

>Magister Scientiae - MSc / In this thesis, I explore the subjects of fluid dynamics and the Hot-Wire Chemical Vapour Deposition (HWCVD) process. HWCVD, in its simplicity, is one of the more powerful and elegant deposition techniques available in thin film research which allows for both the growth and post deposition treatments of functional thin films. In the HWCVD process, the quality of the final films is determined by a fixed set of deposition parameters namely: temperature, pressure and the gas flow rate. Finding the optimal combination of these parameters is key to obtaining the desired film specifications during every deposition. Conducting multiple trial experiments to determine said parameters can be expensive and time consuming, this is where simulation methods come into play. One such simulation method is Computational Fluid Dynamics (CFD) modelling

Improving the design of flow channels in a liquid-cooled heat sink is critical for boosting the capabilities of electronic components as well as reducing energy usage by the pump. This work explores the use of topology optimization to minimize the pressure difference across a heat sink and consequently, the energy used to supply the liquid. Topology optimization involves solving mathematical equations to obtain the optimal design for a defined cost function, here the total pressure difference between the inlet and outlet. A design variable called the porosity is defined for each cell in the mesh. The porosity features in a sink term in the momentum equation, which 'solidifies' cells by velocity suppression when deemed to be counterproductive to the cost function. The adjoint method of topology optimization, in particular, is a well-established tool for use in flow network problems and includes non-physical parameters such as the adjoint velocity and pressure. The method isn't without its drawbacks, such as the numerical instability of the adjoint equations, and the absence of boundary layers or wall functions at the interface of high and low porosity. The strength of the adjoint method lies in the ease with which it calculates the gradient of the cost function with respect to the porosity. When applied to the geometries in this work, it is observed that the problem is non-convex and results in multiple optimums with similar cost values. Thus the objective becomes seeking solutions with the simplest shape and at the same time having a minimized pressure difference. Interesting techniques are tested, namely an interpolation function, a velocity tolerance, and a volume constraint. The work is accomplished by modifying an existing adjoint optimization solver in the open-source CFD software, OpenFOAM.