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[pt] Volume of Fluid (VOF) é um método amplamente empregado na predição de escoamentos multifásicos devido à sua simplicidade, boas características de conservação de massa e natural tratamento de interfaces topologicamente complexas. No entanto, para escoamentos dominados por tensão interfacial, a literatura tem mostrado que a precisão nas estimativas da tensão interfacial ainda é um problema em questão, que pode levar a correntes parasíticas e previsão imprecisa da condição de salto de pressão através das interfaces. Isto ocorre principalmente devido às variações abruptas do campo de fração volumétrica através das interfaces, que leva a imprecisão no cálculo das curvaturas interfaciais. Portanto, diferentes abordagens têm sido apresentadas para mitigar este problema, incluindo funções-altura, suavização da fração volumétrica, ajuste parabólico, entre outros. Este trabalho propõe uma nova abordagem para estimativa de curvatura em VOF, mas não limitado a este, que lança uma nova luz a este problema persistente. A ideia é amostrar a interface por nuvens de pontos e normais na isosuperfície de nível 0.5 do campo de fração volumétrica e calcular a curvatura para cada ponto da nuvem por uma técnica de Computação Gráfica (ajuste de normais). As curvaturas são, então, projetadas na malha Euleriana de maneira tal como no método Front-Tracking. O novo método foi implementado no código padrão de VOF do OpenFOAM (interFoam) resultando em melhorias nas estimativas de salto de pressão e em significativa redução das correntes espúrias. Simulações numéricas foram realizadas e resultados comparados a dados de referência demonstrando a viabilidade da ideia. / [en] Volume of Fluid (VOF) is a widely employed method for multiphase flows prediction for its simplicity, good mass conservation characteristics and natural handling of topologically complex interfaces. For surface tension dominated flows, however, literature has shown that accuracy in surface tension estimations is still an issue, what may cause parasitic currents and inaccurate prediction of pressure jump condition across interfaces. It occurs mainly due to abrupt changes in the volume fraction field across the interfaces, which takes to inaccurate estimates of interfacial curvatures. Therefore, different approaches have been proposed to mitigate this problem including height-functions, volume fraction smoothing, parabolic fittings, among others. This work proposes a novel approach for curvature estimation in VOF, but not limited to it, that sheds a new light on this persistent problem. The idea is to sample the interfaces with clouds of points and normals at the 0.5 level isosurface of the volume fraction field and to compute the curvature for each point of the cloud by a Computer Graphics technique (normal fitting). The curvatures are then projected onto the Eulerian grid in a Front-Tracking fashion. The new method was implemented in the standard OpenFOAM VOF solver (interFoam) resulting in improvements on the pressure jump estimations and in significant reduction of spurious currents. Numerical simulations were performed and results compared to benchmark data showing the feasibility of the idea.

Cavitation can be defined as the breakdown of a liquid (either static or in motion) medium under very low pressure. The hydrodynamic happened in high-speed flow, where local pressure in liquid falls under the saturating pressure thus the liquid vaporizes to form the cavity. During the evolution and collapsing of cavitation bubbles, extreme physical conditions like high-temperature, high-pressure, shock-wave, and high-speed micro-jets can be generated. Such a phenomenon shall be prevented in hydraulic or astronautical machinery due to the induced erosion and noise, while it can be utilized to intensify some treatment processes of chemical, food, and pharmaceutical industries, to shorten sterilization times and lower energy consumption. Advances in the understanding of the physical processes of cavitating flows are challenging, mainly due to the lack of quantitative experimental data on the two-phase structures and dynamics inside the opaque cavitation areas. This dissertation is aimed at finding out the physical mechanisms governing the cavitation instabilities and making contributions in controlling hydraulic cavitation for engineering applications. In this thesis, cavitation developed in various convergent-divergent (Venturi) channels was studied experimentally using the ultra-fast synchrotron X-ray imaging, LIF Particle Image Velocimetry, and high-speed photography techniques, to (1) investigate the internal structures and evolution of bubble dynamics in cavitating flows, with velocity information obtained for two phases; (2) measure the slip velocity between the liquid and the vapor to provide the validation data for the numerical cavitation models; (3) consider the thermodynamic effects of cavitation to establish the relation between the cavitation extent and the fluid temperature, then and optimize the cavitation working condition in water; (4) seek the coherent structures of the complicated high-turbulent cavitating flow to reduce its randomness using data-driven methods. / Doctor of Philosophy / When the pressure of a liquid is below its saturation pressure, the liquid will be vaporized into vapor bubbles which can be called cavitation. In many hydraulic machines like pumps, propulsion systems, internal combustion engines, and rocket engines, this phenomenon is quite common and could induce damages to the mechanical systems. To understand the mechanisms and further control cavitation, investigation of the bubble inception, deformation, collapse, and flow regime change is mandatory. Here, we performed the fluid mechanics experiment to study the unsteady cavitating flow underlying physics as it occurs past the throat of a Venturi nozzle. Due to the opaqueness of this two-phase flow, an X-ray imaging technique is applied to visualize the internal flow structures in micrometer scales with minor beam scattering. Finally, we provided the latest physical model to explain the different regimes that appear in cavitation. The relationship between the cavitation length and its shedding regimes, and the dominant mechanism governing the transition of regimes are described. A combined suppression parameter is developed and can be used to enhance or suppress the cavitation intensity considering the influence of temperature.

Computational fluid dynamics (CFD) modeling was used to predict the hydrodynamics of a column reactor. Bubble columns have applications across many engineering disciplines and improved modeling techniques help to increase the accuracy of numerical predictions. An Eulerian-Eulerian multi-fluid model was used to simulate fluidization and to capture the complex physics associated therewith. The commercial code ANSYS Fluent was used to study two-dimensional gas-liquid bubble columns. A comprehensive parameter study, including a detailed investigation of grid resolution was performed. Specific attention was paid to the bubble diameter, as it was shown to be related to cell size have significant effects on the characteristics of the flow. The parameters used to compare the two-dimensional (2D) cases to experimental results of Rampure, et. al. were then applied to a three-dimensional (3D) geometry. It was demonstrated that the increase in accuracy from 2D to 3D is negligible compared to the increase in CPU required to simulate the entire 3D domain. Additionally, the reaction chamber of a microbial fuel cell (MFC) was modeled and a preliminary parameter study investigating inlet velocity, temperature, and acetate concentration was conducted. MFCs are used in wastewater treatment and have the potential to treat water while simultaneously harvesting electricity. The spiral spacer and chemical reactions were modeled in a 3D geometry, and it was determined that inlet velocity was the most influential parameter that was simulated. There were also significant differences between the 2D and 3D geometries used to predict the MFC hydrodynamics. / Master of Science

Sediment transport is ubiquitous in aquatic environments, and the study of sediment transport is important for both engineering and environmental reasons. However, the understanding and prediction of sediment transport are hindered by its complex dynamics and regimes. In this dissertation, the open-source solver SediFoam is developed for high-fidelity particle-resolving simulations of various sediment transport problems based on open-source solvers OpenFOAM and LAMMPS. OpenFOAM is a CFD toolbox that can perform three-dimensional flow simulations on unstructured mesh; LAMMPS is a massively parallel DEM solver for molecular dynamics. To enable the particle-resolving simulation of sediment transport on an arbitrary mesh, a diffusion-based algorithm is used in SediFoam to obtain the averaged Eulerian fields from discrete particle data. The parallel interface is also implemented for the communication of the two open-source solvers. Extensive numerical simulations are performed to validate the capability of SediFoam in the modeling of sediment transport problems. The predictions of various sediment transport regimes, including `flat bed in motion', `small dune' and `vortex dune', are in good agreement of with the experimental results and those obtained by using interface resolved simulations. The capability of the solver in the simulation of sediment transport in the oscillatory boundary layer is also demonstrated. Moreover, this well-validated high-fidelity simulation tool has been used to probe the physics of particle dynamics in self-generated bedforms in various hydraulic conditions. The results obtained by using SediFoam not only bridge the gaps in the experimental results but also help improve the engineering practice in the understanding of sediment transport. By using the particle-resolving simulation results and the insights generated therein, the closure terms in the two-fluid models or hydro-morphodynamic models can be improved, which can contribute to the numerical modeling of sediment transport in engineering scales. / Ph. D. / The study and prediction of sediment transport are important for both engineering and environmental reasons. However, the understanding of sediment transport is hindered by the complex dynamics of sediment particles in turbulent flow. In this dissertation, the open-source solver SediFoam is developed for the simulations of various sediment transport problems. Both turbulent flow and particle motions can be resolved by using SediFoam, and thus high-fidelity predictions can be provided. The SediFoam is validated extensively with respect to various sediment transport applications, including “flat bed in motion”, “dune generation and migration”, and “sediment transport in oscillatory flow”. The results obtained by using SediFoam are in good agreement of with available data in the literature. By using this well-validated high-fidelity simulation tool, the physics of particle dynamics in sediment bed and self-generated dunes are investigated. Physical insights of sediment transport that have not been captured by experimental measurements are provided by the high-fidelity simulations. Although the domain length in high-fidelity simulations is only 0.1 m, the results can also be used to improve low-fidelity numerical modeling in macro-scale engineering problems.

A pneumatic particulate collection system harnesses the energy associated with the release of a compressed gas to transport particulate to a collection chamber. In an effort to improve the efficiency of a previously designed collection system, high speed imaging in conjunction with computational fluid dynamics (CFD) was utilized to highlight design deficiencies. Areas of recirculation within the collection device as well as impingement of the sampling surface were observed through the testing and CFD analysis. The basis of the improved collection system was conceived through research of pneumatic transport and the deficiencies found through testing and simulation. An improved rectangular-duct-styled system was designed in three main stages. A variety of filters used to contain the desired particulate were characterized through testing for use in simulations as well as fluids calculations. The improved system was then analyzed utilizing compressible and incompressible flow calculations and design iterations were conducted with CFD to determine the final parameters. The final design was simulated with a multiphase flow model to examine the particulate entrainment performance. The improved collection system efficiently expanded and developed the gas flow prior to the collection area to employ the particulate entrainment process. The final design was constructed with an additive manufacturing process and experimentally tested to validate the simulations and flow calculations. The testing proved that the final design operated purely on particulate entrainment and collected only the top layer of particles as simulated. The improved collection system eliminated all areas of flow recirculation and impingement of the particle bed to provide a more efficient sampling device. / Master of Science

Electric vehicles aim to carry the torch into a sustainable future. An optimized cooling system is crucial to an Electric Drive Unit (EDU). A smartly designed cooling system will deliver high-performance, efficient and long-lasting EDUs at lower costs. One way to achieve that is to have an integrated cooling system. When the electric motor and transmission share a common oil, the oil returning from the transmission side is aerated due to spraying and splashing. This aeration affects the pump performance and may reduce the cooling performance of the oil. Thus, this thesis is initiated to understand the impact of aeration on heat transfer.     Oil aeration is the presence of air in oil. This aeration depends on the air content and bubble sizes mixed in the oil. Typically, there is also some amount of dissolved air in any oil. Depending on the type of aeration, the oil will appear lighter than its usual colour and have a very foamy texture, showing a change in the properties of the fluid, for example density, viscosity and heat transfer.    An experimental setup is built in order to replicate and study the effect of aeration on local heat transfer. A flat channel with rectangular cross-section is designed with three parts – a bottom plate, a flow spacer channel and a top transparent plate. The oil and air are mixed before they enter the channel and then heated using thin film heaters. A groove within the bottom plate houses an insulating material, the thin film heater, a thermocouple touching the heater and a thermochromic liquid crystal sheet facing the fluid mixture. The thermocouple gives temperature readings from a single point between the heater and the insulating material. Meanwhile, the liquid crystal sheets come in different desired temperature ranges and change in colour from red to blue to show the surface temperatures over an area. So, the surface temperature of the mixed fluid flow can be recorded visually over an area with the thin film heater under it to calculate the heat transfer coefficients accordingly.    The drop in Nusselt number and heat transfer rates with increased aeration in the working fluid is the main highlight and result. The size of the air bubbles in the channel also determine how fast the heat transfer rate drops.

An important process-design problem in multicomponent separations is separation sequencing, which is concerned with the selection of the best method and sequence for a separation system. Essentially all of the published work on this subject has been limited to high-recovery or sharp separations, in which each component to be separated appears in one and only one product stream. In industrial practice, however, it is often useful to permit components that are being separated to appear in two or more product streams. This type of separation results in products that have overlapping components and is called nonsharp or sloppy separations. The present work proposes and demonstrates a simple and practical approach to the systematic synthesis of sloppy multicomponent separation sequences. The task of synthesizing sloppy multicomponent separation sequences is inherently more complicated than that of synthesizing sharp separation sequences as identification of infeasible splits and stream splitting, and transformation of infeasible product sets into equivalent feasible product sets are examples of some difficult tasks involved. A successful synthesis strategy calls for the development of an effective and flexible framework for representing the synthesis problem and for analyzing the feasibility of component splits. In this thesis, we propose a "component assignment diagram (CAD)" for problem representation. It is shown that the use of a CAD allows the design engineer to consider many alternative solutions (or sequences) and eliminate all infeasible component splits. Further, a "separation specification table (SST)" is proposed for feasibility analysis. In particular, the use of an SST provides a means to : (i) properly define and specify key and nonkey components; (ii) quickly identify feasible and infeasible splits; (iii) effectively deal with fuel products with unmatched compo- nent specifications; and (iv) systematically consider sloppy separations with multiple split points. One difficult problem arising from the design of multicomponent distillation columns for sloppy separations is to appropriately specify the distributions of non-key components in both overhead and bottoms products. Despite the importance of these specifications, there is very little information available on this subject in the literature. This thesis reports the results from a comparative study of rigorous simulation and shortcut modeling of multicomponent distillation columns for sloppy separations. One objective was to obtain improved quantitative understanding and practical design insights into the characteristics of nonkey distributions through a shortcut modeling based upon the Fenske equation. One method proposed in this work for synthesizing sloppy multicomponent products is a heuristic method that involves a two-phase approach. The first phase is concerned with the feasibility analysis of splits pertinent to a CAD with the aid of an SST. The second phase is to specify systematically a subsequent split by applying heuristics, an activity that involves the sequential application of several "rank-ordered" heuristics. A unifying approach is proposed and demonstrated for the synthesis of sloppy multicomponent product sets. Its objective is to generate equally good initial separation schemes, featuring as many as three characteristically different sequences, including all-sharp, all-sloppy, and both sharp and sloppy (i.e., mixed separation). The proposed methods have been applied to a number of industrial separation problems. The results show that the new methods offer an extremely useful means for design engineers to generate a number of good initial sequences for obtaining sloppy multicomponent product sets prior to the ultimate separator optimization and heat integration. / Ph. D.

In the current age of space exploration, the push to reach further to deep space presents a greater need for analysis and verification and validation of rocketry components in the space environment. Due to the nature of space, firings of rocket thrusters in space is a multi-regime problem. With the low density, pressure, and temperature of the environment, the resultant plume structure, seeded with unburnt fuel droplets, extends up to multiple orders of magnitude in distance as compared to a plume structure in the Earth’s atmosphere. The frozen droplets, or particles, create concerns including surface contamination and erosion, calling a cause for study and model development to understand particle behavior in this multi-regime environment. This work intends to develop a model to analyze and understand multiphase flow and particle behavior in this environment utilizing the lower fidelity, but more computationally efficient, RANS turbulence modeling. Particle properties are compared against a regime-defining parameter to understand the trends in behavior. Finally, the work closes out on a preliminary look into implementing fully reacting flow chemistry for the multiphase flow. These results and progress are promising in developing an efficient model that may be integrated into a hybrid model to better predict particle behavior and dispersion in this multi-regime environment.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第25017号 / 工博第5194号 / 新制||工||1991(附属図書館) / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 外輪 健一郎, 教授 松坂 修二, 教授 佐野 紀彰 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM