京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18782号 / 理博第4040号 / 新制||理||1582(附属図書館) / 31733 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 佐々 真一, 教授 山本 潤, 准教授 荒木 武昭 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Background. The A* algorithm is a well-established pathfinding technique frequently used in video game development. However, a disadvantage of the A* algorithm is that it becomes computationally inefficient and impractical to utilize whenthousands of agents demand an optimal path. A solution to mitigate this issue isthe use of the flow field algorithm. This algorithm employs a goal-based pathfindingstrategy, which allows for the movement of a large number of units through the useof a single direction map (flow field map) that indicates the direction units must take to progress toward their goal. Objectives. The main objective of this study is to examine the performance and scalability of precomputed flow field maps with regard to execution time and memory utilization, with the objective of determining the feasibility of precomputed maps as an alternative to maps generated at runtime. Furthermore, the study implements and investigates compression techniques to minimize the memory footprint of precomputed flow field maps. Methods. The study adopts an experimental research design to assess the performance of the two implementations under various conditions of grid size and movement system. Performance evaluation is accomplished through the measurement and comparison of execution time and memory consumption. Additionally, a directional accuracy test is performed to quantify the potential loss of accuracy in the vectors stored in the precomputed flow field maps. Results. The precomputed flow field maps provide constant access time, with a time complexity of O(1), regardless of the grid size and the type of movement system. The memory usage of these maps increases significantly with the growth of the grid size. A doubling of the grid size leads to a more than fifteen-fold increase in memory usage. The techniques proposed in the study significantly reduced the memory footprint by a factor of thirty-two and by a factor of eight, depending on the selected movement system. The average loss in accuracy was approximately 0.35 degrees for the proposed any-angle compression technique. Conclusions. The use of precomputed flow field maps has limitations, including being limited to static environments and having high memory requirements. On the other hand, they provide constant access time for pathfinding information, independent of the grid dimensions, movement system, and the number of target nodes. Whether precomputed flow field maps are better than the traditional runtime implementation depends on the intended use.

Repairs and modifications were made to a flow velocity measurement system designed to measure a planar area of unsteady three component velocities in a single realization using a velocity measurement technique referred to as Doppler Global Velocimetry (DGV). Several hardware components in the system were modified and new hardware was added. Significant improvements were made to the procedures used in acquiring DGV data as well as the procedures used in reducing the acquired DGV data. Though hardware problems were encountered, successful iodine cell calibrations were acquired and attempts were made to acquire DGV velocity data from a calibration wheel and in the wake of a 6:1 prolate spheroid. These attempts were hampered by poor performance of the Nd:YAG laser and one of the digital cameras used in this research. While the magnitudes of the velocities acquired from the calibration wheel were noticeably higher than those calculated from the angular velocity and large fluctuations were present in these reduced velocities, the general trends measured by the VT DGV system matched those calculated from the angular velocity. The attempt to acquire flow field data in the wake of a 6:1 prolate spheroid model was unsuccessful due to insufficient seed particle density in the area where data were to be obtained. The results of this research indicate that while significant improvements have been made to the system, there are still some significant problems to overcome. / Master of Science

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24644号 / 工博第5150号 / 新制||工||1983(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 河瀬 元明, 准教授 中川 浩行, 教授 外輪 健一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Computational fluid dynamics, abbreviated CFD, is a valuable tool for several engineering applications. Applications such as heating, cooling or drying are some examples of areas where CFD is used. The Lattice Boltzmann method, abbreviated LBM, is a popular method for CFD simulations due to its fast simulation time in comparison to traditional methods like the finite volume method. ESS is a company that has supplied LBM simulation software aimed at perfecting the baking process of car carrosserie. The aim of this thesis is to verify and validate their software against earlier works, to ensure that the solver can capture the physical reality of an impinging jet. The verification and validation are conducted over three different cases: experimental free jet, direct numerical simulation and a large eddy simulation of an impinging jet. Identical digital models were created for each case and then simulated with similar conditions for comparison. All three cases were tested with Richardson extrapolation to ensure grid convergence. The Richardson method showed that the highest error among the cases was 2.02 %. The results shows that the LBM solver can accurately predict the flow and thermal field compared to the three cases. The LBM solver is considered verified and validated for flowfield and thermal simulations. / Numeriska strömningsberäkningar är ett värdefullt verktyg för flera ingenjörsapplikationer. Applikationer som uppvärmning, kylning eller torkning är några exempel på områden där CFD används. Lattice Boltzmann-metoden, förkortat LBM, är en populär metod för CFD-simuleringar på grund av dess snabba simuleringshastighet jämfört med traditionella metoder som finita volymsmetoden. ESS är ett företag som har skapat LBM- simuleringsprogramvaran med målet att förbättra bakningsprocessen för bilkarosserier. Målet med denna avhandling är att verifiera och validera deras programvara mot tidigare arbeten för att säkerställa att lösningsmetoden kan fånga den fysiska verkligheten hos en påverkande jetstråle.Verifieringen och valideringen utförs över tre olika fall: Experimentell fri stråle, Direkt numerisk simulering och en Large eddy-simulering av en påverkande jetstråle. Identiska digitala modeller skapades för varje fall och simulerades sedan under liknande förhållanden för att kunna jämföra resultaten. Alla tre fall testades med Richardson-extrapolation för att säkerställa nätkonvergens. Richardson-metoden visade att den högsta felet bland fallen var 2,02 %. Resultaten visar att LBM-lösaren kan förutsäga flödes och termiska fältet noggrant jämfört med de tre fallen. LBM-lösaren anses vara verifierad och validerad för flödesfälts- och termiska simuleringar.

An important aspect in the process of architectural design is the testing of solution alternatives in order to evaluate them on their appropriateness within the context of the design problem. Computational Fluid Dynamics (CFD) analysis is one of the approaches that have gained popularity in the testing of architectural design solutions especially for purposes of evaluating the performance of natural ventilation strategies in buildings. Natural ventilation strategies can reduce the energy consumption in buildings while ensuring the good health and wellbeing of the occupants. In order for natural ventilation strategies to perform as intended, a number of factors interact and these factors must be carefully analysed. CFD simulations provide an affordable platform for such analyses to be undertaken. Traditionally, these simulations have largely followed the direction of Best Practice Guidelines (BPGs) for quality control. These guidelines are built around certain simplifications due to the high computational cost of CFD modelling. However, while the computational cost has increasingly fallen and is predicted to continue to drop, the BPGs have largely remained without significant updates. The need to develop a CFD simulation framework that leverages the contemporary and anticipates the future computational cost and capacity can, therefore, not be overemphasised. When conducting CFD simulations during the process of architectural design, the variability of the wind flow field including the wind direction and its velocity constitute an important input parameter. Presently, however, in many simulations, the wind direction is largely used in a steady state manner. It is assumed that the direction of flow downwind of a meteorological station remains constant. This assumption may potentially compromise the integrity of CFD modelling as in reality, the wind flow field is bound to be dynamic from place to place. In order to improve the accuracy of the CFD simulations for architectural design, it is therefore necessary to adequately account for this variability. This study was a two-pronged investigation with the ultimate objective of improving the accuracy of the CFD simulations that are used in the architectural design process, particularly for the design and analysis of natural ventilation strategies. Firstly, a framework for integrated meso-scale and building scale CFD simulations was developed. Secondly, the newly developed framework was then implemented by deploying it to study the variability of the wind flow field between a reference meteorological station, the Virginia Tech Airport, and a selected localized building scale site on the Virginia Tech campus. The findings confirmed that the wind flow field varies from place to place and showed that the newly developed framework was able to capture this variation, ultimately, generating a wind flow field characterization representative of the conditions prevalent at the localized building site. This framework can be particularly useful when undertaking de-coupled CFD simulations to design and analyse natural ventilation strategies in the building design process. / Doctor of Philosophy / The use of natural ventilation strategies in building design has been identified as one viable pathway toward minimizing energy consumption in buildings. Natural ventilation can also reduce the prevalence of the Sick Building Syndrome (SBS) and enhance the productivity of building occupants. This research study sought to develop a framework that can improve the usage of Computational Fluid Dynamics (CFD) analyses in the architectural design process for purposes of enhancing the efficiency of natural ventilation strategies in buildings. CFD is a branch of computational physics that studies the behaviour of fluids as they move from one point to another. The usage of CFD analyses in architectural design requires the input of wind environment data such as direction and velocity. Presently, this data is obtained from a weather station and there is an assumption that this data remains the same even for a building site located at a considerable distance away from the weather station. This potentially compromises the accuracy of the CFD analyses as studies have shown that due to a number of factors such the urban built form, vegetation, terrain and others, the wind environment is bound to vary from one point to another. This study sought to develop a framework that quantifies this variation and provides a way for translating the wind data obtained from a weather station to data that more accurately characterizes a local building site. With this accurate site wind data, the CFD analyses can then provide more meaningful insights into the use of natural ventilation in the process of architectural design. This newly developed framework was deployed on a study site at Virginia Tech. The findings showed that the framework was able to demonstrate that the wind flow field varies from one place to another and it also provided a way to capture this variation, ultimately, generating a wind flow field characterization that was more representative of the local conditions.

This research is directed toward the investigation of a low temperature direct propane fuel cell (DPFC). Modeling included a parametric study of a direct propane fuel cell using computational fluid dynamics (CFD), specifically FreeFem++ software. Polarization curves predicted by the CFD model were used to understand fuel cell performance. The predictions obtained from the computational fluid dynamics mathematical model for the fuel cell were compared with experimental results. The computational work identified some critical parameters (exchange current density, pressure, temperature) for improving the overall performance of the fuel cell. The model predictions clearly highlighted the role of catalysts in significantly enhancing the overall performance of a DPFC. Experiments were performed using commercial Nafion-Pt based membrane electrode assemblies (MEAs) to obtain a basis for comparison. It is the first report in the literature that a Pt-Ru (Platinum-Ruthenium) MEA was used in the investigation of a DPFC. Also, it was the first study that fed liquid water continuously to a DPFC by using interdigitated flow field (IDFF) at the anode to humidify the dry propane feed gas. During the experiments oscillations were observed at very low current densities i.e. in nA/cm2, which is a rare case and not reported in the literature to date. This observation has raised serious concerns about the existence of absolute open-circuit cell potential difference for a DPFC. The cycling behaviour observed with DPFC indicated the presence of a continuous degradation-regeneration process of the catalyst surface near open-circuit potential. The experimental work further evaluated the performance of fuel cell by measurement of polarization curves.

Tethered polymer layers at solid-fluid interfaces are used widely in a variety of surface science applications. Although many of these applications require exposure to dynamic flow conditions, flow field penetration into densely grafted polymer brushes, is still a question open to debate despite the fact that it is a fundamental process crucial to mass transport through these polymer films. Although most theoretical work has indicated flow field penetration into polymer films, with varying predicted penetration depths predicted, the limited experimental attempts to investigate this phenomenon have resulted in inconsistent conclusions due to lack of a proper analytical method. To help resolve this controversy, in this Dissertation, a new spectroscopic method, FRET-TIRFM, based on a combination of Förster resonance energy transfer (FRET) and total internal reflectance fluorescence microscopy (TIRFM), is developed to provide the first direct, quantitative measurements on flow field penetration by measuring linear diffusion coefficients of small molecules through densely grafted, thin poly(N-isopropylacryl-amide) (pNIPAM) films. Decay curves from FRET of the acceptor with a donor covalently attached at the substrate surface were fit to a combined Taylor-Aris-Fickian diffusion model to obtain apparent linear diffusion coefficients of the acceptor molecules for different flow rates. These values can then be used to obtain quantitative estimates of flow field penetration depths. For a pNIPAM surface of 110 nm dry thickness, with a 0.6 chain/nm² grafting density, apparent diffusion coefficients ranging from 1.9-9.1 × 10-12 cm²/s were observed for flow rates ranging from 100 to 3000 μL/min. This increase in apparent diffusion coefficient with applied fluid flow rate is indicative of flow field penetration of the polymer film. The depth of penetration of the flow field is estimated to range from ~6% of the polymer film thickness to ~57% of the film thickness in going from 100 to 3000 μL/min flow rate of a good solvent. Factors other than flow rate that may impact flow field penetration were also investigated using this new FRET-TIRFM method. Solvent quality and polymer brush grafting density are the two most important parameters due to the fact that they influence changes in tethered polymer chain conformation. This work demonstrates that polymer films are most penetrable in a good solvent and least penetrable in a poor solvent under identical flow conditions. These findings are consistent with polymer chain conformational changes going from extended brushes to compact globules. For flow rates ranging from 100 to 3000 μL/min, flow field penetration depth ranges from ~6% of the film thickness to ~57% of film thickness for a good solvent compared to ~4% to ~19% for a poor solvent. Thus, by simply changing solvent quality from good to poor, flow field penetration decreases by about 38%. Grafting density has a less pronounced effect than solvent quality on penetration depth, probably due to the small range of grafting densities chosen for study. However, a roughly 10-20% difference in penetration depth was observed between high density (0.60 chain/nm²) and low density (0.27 chain/nm²) pNIPAM surfaces in the same solvent. Changes in grafting density have a less significant impact in a good solvent compared to a poor solvent. This is most likely caused by the fact that grafting density impacts polymer chain conformation mainly through polymer-polymer repulsion, which becomes less significant in a solvent that better solvates the polymer. For the two extreme cases studied here at flow rates ranging from 100 to 3000 μL/min, the penetration depth is estimated to range from ~4-19% of the original solvent-swollen film thickness for high density pNIPAM films in a poor solvent and from ~7-67% for low density films in a good solvent. One important assumption that underlies all of this work is that the dominant mass transport mechanism for small molecules in dense polymer brushes is diffusion. This assumption was further validated through the use of two different small molecule quenchers, RhB and 2-nitrobenzylalcohol. These molecules are significantly different in size, charge, and structure, and operate by different quenching mechanisms. Despite these differences, the results for flow field penetration are statistically the same for both. These observations validate the assumption of diffusive mass transport in these films.

Bipolarplatten stellen eine Schlüsselkomponente hinsichtlich Fertigungskosten eines Brennstoffzellenstacks dar und sind komplexen Anforderungen ausgesetzt. Sie stützt die anderen biegeempfindlichen Stackkomponenten, verteilt die Reaktionsgase über die aktive Zellfläche und sorgt für die flächige elektrische Kontaktierung der in Reihe geschalteten Einzelzellen, Ableitung der Reaktionswärme und sowie Trennung der Einzelzellen. Daher muss sie einen niedrigen Kontaktwiderstand zur benachbarten Gasdiffusionslage, hohe Biegesteifigkeit, gute Wärmeleitfähigkeit und hohe Lebensdauer der Zelle gewährleisten. Die Funktion der Gasverteilung über die aktive Zellfläche übernimmt das Flussfeld. Deren Kanalstruktur- und Geometrie beeinflusst entscheidend den Wirkungsgrad der Brennstoffzelle und des Gesamtsystems. Um Fertigungsaufwand und damit verbundene Kosten zu senken, ist es Ziel aktueller Forschungsarbeit, die Geometrie so einfach wie möglich zu gestalten. Dabei wird ein minimal geringerer Wirkungsgrad in Kauf genommen, wenn im Gegenzug die Kosten stark sinken. Mit Hilfe einer CFD-Simulation wird das Flussfeld dahingehend optimiert, die Produktion zu vereinfachen und die Funktionalität weiterhin zu gewährleisten. Im Rahmen dieser Präsentation werden Modellbildung, Randbedingungen und erste Ergebnisse vorgestellt.

Increasing environmental concerns as well as diminishing fossil fuel reserves call for a new generation of energy conversion technologies. Fuel cells, which convert the chemical energy of a fuel directly to electrical energy, have been identified as one of the leading alternative energy conversion technologies. Fuel cells are more efficient than conventional heat engines with minimal pollutant emissions and superior scalability. Proton Exchange Membrane Fuel Cells (PEMFCs) which produce electricity from hydrogen have been widely investigated for transportation and stationary applications. The focus of this study is on the Direct Propane Fuel Cell (DPFC), which belongs to the PEMFC family, but consumes propane instead of hydrogen as feedstock. A drawback associated with DPFCs is that the propane reaction rate is much slower than that of hydrogen. Two ideas were suggested to overcome this issue: (i) operating at high temperatures (150-230oC), and (ii) keeping the propane partial pressure at the maximum possible value. An electrolyte material composed of zirconium phosphate (ZrP) and polytetrafluoroethylene (PTFE) was suggested because it is an acceptable proton conductor at high temperatures. In order to keep the propane partial pressure at the maximum value, interdigitated flow-fields were chosen to distribute propane through the anode catalyst layer. In order to evaluate the performance of a DPFC which operates at high temperature and uses interdigitated flow-fields, a computational approach was chosen. Computational Fluid Dynamics (CFD) was used to create two 2-D mathematical models for DPFCs based on differential conservation equations. Two different approaches were investigated to model species transport in the electrolyte phase of the anode and cathode catalyst layers and the membrane layer. In the first approach, the migration phenomenon was assumed to be the only mechanism of proton transport. However, both migration and diffusion phenomena were considered as mechanisms of species transport in the second approach. Therefore, Ohm's law was used in the first approach and concentrated solution theory (Generalized Stefan-Maxwell equations) was used for the second one. Both models are isothermal. The models were solved numerically by implementing the partial differential equations and the boundary conditions in FreeFEM++ software which is based on Finite Element Methods. Programming in the C++ language was performed and the existing library of C++ classes and tools in FreeFEM++ were used. The final model contained 60 pages of original code, written specifically for this thesis. The models were used to predict the performance of a DPFC with different operating conditions and equipment design parameters. The results showed that using a specific combination of interdigitated flow-fields, ZrP-PTFE electrolyte having a proton conductivity of 0.05 S/cm, and operating at 230oC and 1 atm produced a performance (polarization curve) that was (a) far superior to anything in the DPFC published literature, and (b) competitive with the performance of direct methanol fuel cells. In addition, it was equivalent to that of hydrogen fuel cells at low current densities (30 mA/cm2).