A computational fluid dynamics model has been developed to examine the separation of an oil film from a spherical oil-coated particle falling through quiescent water due to gravity. Using this model, the separation process was studied as a function of the viscosity ratio of oil to water, R, and the ratio of viscous forces to surface tension, represented by the Capillary number Ca. The governing equations of this flow-induced motion are derived in a non-inertial spherical coordinate system, and discretized using a finite volume approach. The Volume-of-Fluid method is used to capture the oil/water interface. The model predicts two mechanisms for oil separation: at R less than 1, the shear difference between the particle/oil interface and the oil/water interface is not significant and Ca determines whether separation occurs or not; at R larger than 1, the shear difference is considerable, and the Ca effect becomes less dominant.

A computational fluid dynamics model has been developed to examine the separation of an oil film from a spherical oil-coated particle falling through quiescent water due to gravity. Using this model, the separation process was studied as a function of the viscosity ratio of oil to water, R, and the ratio of viscous forces to surface tension, represented by the Capillary number Ca. The governing equations of this flow-induced motion are derived in a non-inertial spherical coordinate system, and discretized using a finite volume approach. The Volume-of-Fluid method is used to capture the oil/water interface. The model predicts two mechanisms for oil separation: at R less than 1, the shear difference between the particle/oil interface and the oil/water interface is not significant and Ca determines whether separation occurs or not; at R larger than 1, the shear difference is considerable, and the Ca effect becomes less dominant.

The use of bearings can be found in virtually all aspects of mechanical systems today. Reliability of these critical components is an important issue. Fatigue performance of bearings is a function of many factors, including service conditions, loading, material properties, environmental factors, and manufacturing processes. Crack nucleation, first spall generation and spall growth in rolling contact fatigue are known to be highly sensitive to the heterogeneity of the microstructure. Yet the current state-of-the-art in the design of high performance bearing materials and microstructures is highly empirical requiring substantial lengthy experimental testing to validate the reliability and performance of these new materials and processes. The approach presented here is designed to determine relative rolling contact fatigue performance as a function of microstructural attributes. A fully three-dimensional finite element modeling allows for end effects to be captured that were not previously possible with two-dimensional plane-strain models, providing for a more realistic assessment of inclusion morphology and arbitrary orientations. The scaling of the finite element models has been optimized to capture the cyclic microplasticity around a modeled inclusion accurately and efficiently. To achieve this, two scales of geometric models were developed to incorporate different sized microstructural phenomena, with both models using traction boundary conditions derived from Hertzian contact stresses. A microstructure-sensitive material model adds additional capability. A hybrid model that includes both martensite and austenite phases with additional internal state variable to track the volume fraction of retained austenite due to stress-assisted transformation were developed. This represents an advance over previous models where transform plasticity and crystal plasticity were not simultaneously accounted for in a homogenized element containing both phases. Important links between microstructural features and fatigue indicator parameters (and relative fatigue performance) were determined. Demonstration cases show the relationship between inclusion orientation and relative fatigue performance, allowing for the identification of critical angles which maximize fatigue and reduce performance. An additional case study showed that increasing initial volume fraction of retained austenite reduces relative fatigue life. The tools developed allow for investigations of the influence of many microstructural aspects on relative fatigue performance with a numerical model that were not previously possible.

This thesis presents a two-phase liquid-solid numerical model using Smoothed Particle Hydrodynamics (SPH). The scheme is developed for multi-phase flows in industrial tanks containing sediment used in the nuclear industry for decommissioning. These two-phase liquid-sediments flows feature a changing interfacial profile, large deformations and fragmentation of the interface with internal jets generating resuspension of the solid phase. SPH is a meshless Lagrangian discretization scheme whose major advantage is the absence of a mesh making the method ideal for interfacial and highly non-linear flows with fragmentation and resuspension. Emphasis has been given to the yield profile and rheological characteristics of the sediment solid phase using a yielding, shear and suspension layer which is needed to predict accurately the erosion phenomena. The numerical SPH scheme is based on the explicit treatment of both phases using Newtonian and non-Newtonian Bingham-type constitutive models. This is supplemented by a yield criterion to predict the onset of yielding of the sediment surface and a suspension model at low volumetric concentrations of sediment solid. The multi-phase model has been compared with experimental and 2-D reference numerical models for scour following a dry-bed dam break yielding satisfactory results and improvements over well-known SPH multi-phase models. A 3-D case using more than 4 million particles, that is to the author’s best knowledge one of the largest liquid-sediment SPH simulations, is presented for the first time. The numerical model is accelerated with the use of Graphic Processing Units (GPUs), with massively parallel capabilities. With the adoption of a multi-phase model the computational requirements increase due to extra arithmetic operations required to resolve both phases and the additional memory requirements for storing a second phase in the device memory. The open source weakly compressible SPH solver DualSPHysics was chosen as the platform for both CPU and GPU implementations. The implementation and optimisation of the multi-phase GPU code achieved a speed up of over 50 compared to a single thread serial code. Prior to this thesis, large resolution liquid-solid simulations were prohibitive and 3-D simulations with millions of particles were unfeasible unless variable particle resolution was employed. Finally, the thesis addresses the challenging problem of enforcing wall boundary conditions in SPH with a novel extension of an existing Modified Virtual Boundary Particle (MVBP) technique. In contrast to the MVBP method, the extended MVBP (eMVBP) boundary condition guarantees that arbitrarily complex domains can be readily discretized ensuring approximate zeroth and first order consistency for all particles whose smoothing kernel support overlaps the boundary. The 2-D eMVBP method has also been extended to 3-D using boundary surfaces discretized into sets of triangular planes to represent the solid wall. Boundary particles are then obtained by translating a full uniform stencil according to the fluid particle position and applying an efficient ray casting algorithm to select particles inside the fluid domain. No special treatment for corners and low computational cost make the method ideal for GPU parallelization. The models are validated for a number of 2-D and 3-D cases, where significantly improved behaviour is obtained in comparison with the conventional boundary techniques. Finally the capability of the numerical scheme to simulate a dam break simulation is also shown in 2-D and 3-D.

abstract: Rotorcraft operation in austere environments can result in difficult operating conditions, particularly in the vicinity of sandy areas. The uplift of sediment by rotorcraft downwash, a phenomenon known as brownout, hinders pilot visual cues and may result in a potentially dangerous situation. Brownout is a complex multiphase flow problem that is not unique and depends on both the characteristics of the rotorcraft and the sediment. The lack of fundamental understanding constrains models and limits development of technologies that could mitigate the adverse effects of brownout. This provides the over-arching motivation of the current work focusing on models of particle-laden sediment beds. The particular focus of the current investigations is numerical modeling of near-surface fluid-particle interactions in turbulent boundary layers with and without coherent vortices superimposed on the background flow, that model rotorcraft downwash. The simulations are performed with two groups of particles having different densities both of which display strong vortex-particle interaction close to the source location. The simulations include cases with inter-particle collisions and gravitational settling. Particle effects on the fluid are ignored. The numerical simulations are performed using an Euler- Lagrange method in which a fractional-step approach is used for the fluid and with the particulate phase advanced using Discrete Particle Simulation. The objectives are to gain insight into the fluid-particle dynamics that influence transport near the bed by analyzing the competing effects of the vortices, inter-particle collisions, and gravity. Following the introduction of coherent vortices into the domain, the structures convect downstream, dissipate, and then recover to an equilibrium state with the boundary layer. The particle phase displays an analogous return to an equilibrium state as the vortices dissipate and the boundary layer recovers, though this recovery is slower than for the fluid and is sensitive to the particle response time. The effects of inter-particle collisions are relatively strong and apparent throughout the flow, being most effective in the boundary layer. Gravitational settling increases the particle concentration near the wall and consequently increase inter-particle collisions. / Dissertation/Thesis / M.S. Aerospace Engineering 2011

Les méthodes d'optimisation de forme s’industrialisent progressivement, elles permettent la conception automatisée de structures aux propriétés optimales. Elles constituent aussi un outil d'exploration majeur pour la conception de nouveaux matériaux.Dans une première partie nous utilisons ces méthodes afin de générer des matériaux architecturés aux propriétés thermoélastiques effectives cibles et extrêmes. En plus de proposer différentes solutions, nous répertorions les différents mécanismes œuvrant au contrôle des ces propriétés. Dans ce contexte nous proposons aussi de prendre en compte l'influence des interfaces comportant un gradient de propriétés sur les architectures obtenues.Nous étudions ensuite les procédés de fabrication pouvant être utilisés afin de réaliser ces matériaux. Les méthodes de fabrication additive, considérées comme le vecteur d'une prochaine révolution industrielle, constituent une piste que nous considérerons tout particulièrement. Nous proposons plusieurs solutions pour prendre en compte les limitations et les effets collatéraux de ces procédés de fabrication au sein de processus d'optimisation de forme. Nous traitons le problème de la prise en compte des propriétés induites par la méthode de fabrication Fiber Deposition Molding (FDM), à savoir des propriétés anisotropes orientées. Nous proposons ensuite une approche pour traiter le problème des dépôts en porte-à-faux à l'aide d'un critère mécanique.Enfin, nous abordons la prise en compte des non-linéarités géométriques au sein de calculs d'optimisation de forme et discutons de leurs apports ainsi que de leurs limitations. Nous présentons plusieurs applications pour la conception automatisée d'actuateurs non linéaires. / Shape optimization methods are promising methods and are gradually becoming industrialized. They provide the ability to automatically design structures with optimal behavior. They are outstanding tools for exploration and design of new materials.We use these methods to generate architectured multi-phased materials with prescribed thermoelastic properties. We first propose several solutions and we classify them by the mechanisms they rely on in order to control the effective properties. We also propose to evaluate the influence of an interface with a gradient of properties on the obtained architectures.Eventually we focus on the plausible manufacturing solution to produce our architectured materials. In this context, additive manufacturing methods (often considered as the support of an incoming industrial revolution) is our main option. We introduce several strategies to circumvent some limitations and side effects of these manufacturing methods during optimization process. We particularly focus on Fiber Deposition Molding, which induce an important mechanical anisotropy in processed parts. Then we consider the problem of overhangings features in design and propose a way to handle them prior to additive manufacturing using a mechanical criteria.Finally we take into account geometrical non linearities in optimization process. We highlight the pros and cons of this new modeling by presenting several applications of non linear actuators design.

abstract: High-efficiency DC-DC converters make up one of the important blocks of state-of-the-art power supplies. The trend toward high level of transistor integration has caused load current demands to grow significantly. Supplying high output current and minimizing output current ripple has been a driving force behind the evolution of Multi-phase topologies. Ability to supply large output current with improved efficiency, reduction in the size of filter components, improved transient response make multi-phase topologies a preferred choice for low voltage-high current applications. Current sensing capability inside a system is much sought after for applications which include Peak-current mode control, Current limiting, Overload protection. Current sensing is extremely important for current sharing in Multi-phase topologies. Existing approaches such as Series resistor, SenseFET, inductor DCR based current sensing are simple but their drawbacks such low efficiency, low accuracy, limited bandwidth demand a novel current sensing scheme. This research presents a systematic design procedure of a 5V - 1.8V, 8A 4-Phase Buck regulator with a novel current sensing scheme based on replication of the inductor current. The proposed solution consists of detailed system modeling in PLECS which includes modification of the peak current mode model to accommodate the new current sensing element, derivation of power-stage and Plant transfer functions, Controller design. The proposed model has been verified through PLECS simulations and compared with a transistor-level implementation of the system. The time-domain parameters such as overshoot and settling-time simulated through transistor-level implementation is in close agreement with the results obtained from the PLECS model. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2017

Ce travail de thèse porte sur l’étude des pertes de pression pour des écoulements monophasiques et diphasiques inertiels au travers de milieux poreux. Son objectif est d’aider à la compréhension et à la modélisation des transferts de quantité de mouvement à l’intérieur de lits de particules, en lien avec la problématique de la gestion d’un accident grave dans un réacteur nucléaire. En effet, lors d’un tel accident, la dégradation du coeur du réacteur peut amener celui-ci à s’effondrer pour former un lit de débris, que l’on peut assimiler à un milieu poreux à haute température et dégageant de la chaleur. Ce travail de thèse s’inscrit dans un projet de recherche en sûreté nucléaire visant à prédire la refroidissabilité d’un lit de débris par injection d’eau, ou « renoyage ». Une étude expérimentale des pertes de pression pour des écoulements monodimensionnels monophasiques et diphasiques à froid est proposée dans des situations représentatives du cas réacteur, en termes de granulométrie, de formes de particules et de vitesses d’écoulement. Les expériences réalisées apportent un complément important aux données existantes, en permettant notamment d’explorer les domaines d’écoulements diphasiques avec nombres de Reynolds liquides non nuls, tout en mesurant le taux de vide, ce qui est essentiel pour une modélisation. Des modèles prédictifs pour les pertes de pression à l’intérieur d’écoulements monophasiques et diphasiques au travers de lits de particules sont établis à partir des structures d’équations obtenues par une prise de moyenne volumique des équations de conservation locales. L’observation des écoulements monophasiques montrent que des lois de type Darcy-Forchheimer avec une correction quadratique en vitesse de filtration sont à même de prédire les pertes de pression avec une précision supérieure à 10%. Une étude numérique a montré que ce résultat est applicable pour un lit désordonné de particules peu rugueuses. L’étude des écoulements diphasiques montre qu’une structure d’équations de type Darcy-Forchheimer généralisée, incluant des termes supplémentaires pour prendre en compte les effets inertiels et les frottements interfaciaux, permet de reproduire le comportement des pertes de pression dans cette situation. Un nouveau modèle est proposé, et comparé aux données expérimentales et aux modèles utilisés dans les codes de simulation des accidents graves. / This work deals with single and two-phase flow pressure losses in porous media. The aim is to improve understanding and modeling of momentum transfer inside particle beds, in relation with nuclear safety issues concerning the reflooding of debris beds during severe nuclear accidents. Indeed, the degradation of the core during such accidents can lead to the collapse of the fuel assemblies, and to the formation of a debris bed, which can be described as a hot porous medium. This thesis is included in a nuclear safety research project on coolability of debris beds during reflooding sequences. An experimental study of single and two-phase cold-flow pressure losses in particle beds is proposed. The geometrical characteristics of the debris and the hydrodynamic conditions are representative of the real case, in terms of granulometry, particle shapes, and flow velocities. The new data constitute an important contribution. In particular, they contain pressure losses and void fraction measurements in two-phase air-water flows with non-zero liquid Reynolds numbers, which did not exist before. Predictive models for pressure losses in single and two-phase flow through particle beds have been established from experimental data. Their structures are based on macroscopic equations obtained from the volume averaging of local conservation equations. Single-phase flow pressure losses can be described by a Darcy-Forchheimer law with a quadratic correction, in terms of filtration velocity, with a better-than-10 % precision. Numerical study of single-phase flows through porous media shows that this correlation is valid for disordered smooth particle beds. Twophase flow pressure losses are described using a generalized Darcy-Forchheimer structure, involving inertial and cross flow terms. A new model is proposed and compared to the experimental data and to the usual models used in severe accident simulation codes.

No description available.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20684号 / 工博第4381号 / 新制||工||1681(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 澤田 純男, 教授 清野 純史, 准教授 後藤 浩之 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM