Depuis la découverte des modes asymétriques dans le sillage d'un corps simplifié d'automobile, réminiscents d'une bifurcation à bas nombre de Reynolds, se posent des questions propres au développement aérodynamique des véhicules terrestres telles que l'influence du vent latéral, de l'assiette et du rétreint d'arrière-corps couramment utilisé en phase d'optimisation. Notre travail s'attache à répondre expérimentalement à ces questions pour des géométries simplifiées mais aussi réelles. Les essais sont réalisés en soufflerie industrielle à l'échelle 2/5 pour le corps académique et en pleine échelle pour les monospaces. Nous montrons que le désalignement du véhicule par rapport à l'écoulement incident n'a pour effet que de modifier l'orientation du mode asymétrique sans en changer l'intensité. Nous construisons un modèle simple prédisant non seulement cette orientation mais aussi les conséquences sur les efforts aérodynamiques transverses. La contribution de l'instabilité sur les coefficients aérodynamiques de portance ou d'effort latéral est de l'ordre de 0,02 indépendamment du vent de travers et de l'assiette du véhicule. Les rétreints d'arrière-corps affectent également la dynamique du sillage et son orientation, mais l'instabilité n'est jamais supprimée. Ces résultats sont retrouvés pour des véhicules réels de type monospace dont le sillage est donc également soumis au même mode asymétrique, révélé sans ambigüité par des expériences de sensibilité en assiette. Nos résultats indiquent que, pour tous les véhicules considérés, le mode asymétrique de sillage est systématiquement présent dans l'enveloppe de conduite. Le contrôle ou la suppression de ce mode devrait offrir de nouvelles perspectives d'optimisation des véhicules à culot droit de type monospaces ou SUV. / Since the recent discovery of asymmetric modes in the wake of a simplified vehicle geometry, reminiscent from a bifurcation at low Reynolds numbers, some questions related to the aerodynamic development of ground vehicles such as the influence of lateral wind, pitch and afterbody boat-tail classically used during shape optimization remain unanswered. Our work is devoted to assess those questions experimentally for simplified but also real geometries. The tests are conducted in an industrial wind-tunnel, at the 2/5-scale for the academic body and at the full scale for the minivans. We show that the vehicle's misalignment only modifies the asymmetric mode's orientation without affecting its intensity. We build a model predicting not only this orientation but also the consequences on the cross-flow aerodynamic loading. The contribution of the instability to the lift or side force coefficients is of the order of 0,02 independently of lateral wind or of the vehicle's pitch. Afterbody boat-tails also impact the wake dynamics and its orientation but the instability is never suppressed. These results are retrieved for real vehicles such as minivans, whose wake is then subjected to the same asymmetric mode as well, revealed unambiguously with pitch sensitivity experiments. Our results indicate that, for all considered vehicles, the asymmetric wake mode is systematically present in the driving envelope. The control or the suppression of this mode should offer new optimization's perspectives for blunt based vehicles such as minivans or SUVs.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21444号 / 理博第4437号 / 新制||理||1638(附属図書館) / 京都大学大学院理学研究科数学・数理解析専攻 / (主査)准教授 前川 泰則, 教授 上 正明, 教授 堤 誉志雄 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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This project focused on the relationship between instream flows and smallmouth buffalo (Ictiobus bubalus) recruitment in the Gulf Coastal Plain of Texas. The flow regime is the dominant factor in lotic systems and, consequently, the relationship between instream flows, including impacts to natural flow regimes, and life-history is a subject of growing interest. Smallmouth buffalo is a good model to investigate the relationship between river flows and variable interannual recruitment success of periodic life-history strategist fish species. Smallmouth buffalo were collected from the Brazos, Colorado, and Guadalupe Rivers of Texas, U.S.A., and otoliths were extracted from individuals in the field and sectioned and photographed in the lab. Photographs of sectioned otoliths were used to estimate age and thus the year in which the individual was spawned by counting back from the time of capture. Population age structure (i.e. a ‘state' or condition at a point in time) was used to infer effects of flow variation on a rates-based process (i.e. recruitment). After controlling for mortality using recruitment index values, interannual variation in recruitment was modeled using multiple components of the flow regime quantified as indicators of hydrologic alteration (IHA) variables based on daily discharge data from USGS gaging stations in each river system. Model selection followed a two-tier approach, first fitting models using only flow attributes associated with the spawning season then adding additional informative parameters from the pre-spawn and post-spawn periods. The primary finding from model selection was that duration of high flow pulses during the spawning season is a critical component of the flow regime associated with successful Smallmouth Buffalo recruitment. These findings have implications for river management and conservation of ecological integrity, in particular populations of periodic life-history strategist species.

<div>Free surface flows where the shape of the interface separating two fluids is unknown <i>apriori</i> are an important area of interest in fluid dynamics. The study of free surface flows such as the breakup and coalescence of drops, and thinning and rupture of films lends itself to a diverse range of industrial applications, such as inkjet printing, crop spraying, foam and emulsion stability, and nanolithography, and helps develop an understanding of natural phenomena such as sea spray generation in oceans, or the dynamics of tear films in our eyes. In free surface flows, singularities are commonly observed in nite time, such as when the radius of a thread goes to zero upon pinchoff or when the thickness of a film becomes zero upon rupture. Dynamics in the vicinity of singularities usually lack a length scale and exhibit self-similarity. In such cases, universal scaling laws that govern the temporal behavior of measurable physical quantities such as the thickness of a lm can be determined from asymptotic analysis and veried by high-resolution experiments and numerical simulations. These scaling laws provide deep insight into the underlying physics, and help delineate the regions of parameter space in which certain forces are dominant, while others are negligible. While the majority of previous works on singularities in free-surface flows deal with Newtonian fluids, many fluids in daily use and industry exhibit non-Newtonian rheology, such as polymer-laden, emulsion, foam, and suspension flows.</div><div><br></div><div><div>The primary goal of this thesis is to investigate the thinning and rupture of thin films of non-Newtonian fluids exhibiting deformation-rate-thinning (power-law) rheology due to attractive intermolecular van der Waals forces. This is accomplished by means of intermediate asymptotic analysis and numerical simulations which utilize a robust Arbitrary Eulerian-Lagrangian (ALE) method that employs the Galerkin/Finite-Element Method for spatial discretization. For thinning of sheets of power-law fluids, a signicant finding is the discovery of a previously undiscovered scaling regime where capillary, viscous and van der Waals forces due to attraction between the surfaces of the sheet, are in balance. For thinning of supported thin films, the breakdown of the lubrication approximation used almost exclusively in the past to study such systems, is shown to occur for films of power-law fluids through theory and conrmed by two dimensional simulations. The universality of scaling laws determined for rupture of supported films is shown by studying the impact of a bubble immersed in a power-law fluid with a solid wall.</div></div><div><br></div><div><div>Emulsions, which are ne dispersions of drops of one liquid in another immiscible liquid, are commonly encountered in a variety of industries such as food, oil and gas, pharmaceuticals, and chemicals. Stability over a specied time frame is desirable in some applications, such as the shelf life of food products, while rapid separation into its constituent phases is required in others, such as when separating out brine from crude oil. The timescale over which coalescence of two drops of the dispersed phase occurs is crucial in determining emulsion stability. The drainage of a thin film of the outer liquid that forms between the two drops is often the rate limiting step in this process. In this thesis, numerical simulations are used to decode the role played by fluid inertia in causing drop rebound, and the subsequent increase in drainage times, when two drops immersed in a second liquid are brought together due to a compressional flow imposed on the outer liquid. Additionally, the influence of the presence of insoluble surfactants at the drop interface is studied. It is shown that insoluble surfactants cause a dramatic increase in drainage times by two means, by causing drop rebound for small surfactant concentrations, and by partially immobilizing the interface for large surfactant concentrations.</div></div>

The polymers are integral part of vast number of products used in day to day life due to their anomalous viscoelastic behaviour. The remarkable flow behaviour exhibited by the polymeric fluids including rod climbing, extrudate swell, tube-less siphon, viscoelastic jet, elastic recoil and sharkskin instability is attributed to the complex microstructures in the polymeric liquids that arise due to the interactions of long chain polymer molecules with each other and with the surrounding fluid particles. The significance of polymer in transportation, packaging, pharmaceutical, chemical, biomedical, textiles, food and polymer processing industries highlights the requirement to comprehend the complex rheology of polymeric fluids. First, we investigate the flow features exhibited by different shear thinning vis-coelastic fluids in rectangular cavities over a wide range of depth to width ratio. We have developed a viscoelastic flow solver in order to perform numerical simulations of highly elastic flow of viscoelastic fluids. In particular, we discuss the simulations of flows of constant viscosity Boger and shear thinning viscoelastic fluids in the complex flow problems using different constitutive equations. The effects of elasticity and inertia on the flow behaviour of two shear thinning vis-coelastic fluids modeled using Giesekus and linear PTT constitutive equations in rectangular cavities is studied. The size of the primary eddies and critical aspect ratio over which the corner eddies merge to yield a second primary eddy in deep cavities is discussed. We demonstrate that the flow in the shallow and deep cavities can be characterized using Weissenberg number, defined based on the shear rate, and Deborah number, specified based on the convective time scale, respectively. The study of flow in driven cavities is important in understanding of the mixing process during synthesis of blends and composites. Next, we study two phase polymeric flow in confined geometries. Nowadays, polymer processing industries prefer to develop newer polymer with the desired material properties mechanically by mixing and blending of different polymer components instead of chemically synthesizing fresh polymer. The microstructure of blends and emulsions following drop deformation, breakup and coalescence during mixing determines its macroscopic interfacial rheology. We developed a two phase viscoelastic flow solver using volume conserving sharp interface volume-of-fluid (VOF) method for studying the dynamics of single droplet subjected to the complex flow fields. We investigated the effects of drop and matrix viscoelasticity on the motion and deformation of a droplet suspended in a fully developed channel flow. The flow behaviour exhibited by Newtonian-Newtonian, viscoelastic-Newtonian, Newtonian-viscoelastic and viscoelastic-viscoelastic drop-matrix systems is presented. The difference in the drop dynamics due to presence of constant viscosity Boger fluid and shear thinning viscoelastic fluid is represented using FENE-CR and linear PTT constitutive equations, respectively. The presence of shear thinning viscoelastic fluid either in the drop or the matrix phase suppresses the drop deformation due to stronger influence of matrix viscoelasticity as compared to the drop elasticity. The shear thinning viscoelastic drop-matrix system further restricts the drop deformation and it displays non-monotonic de-formation. The constant viscosity Boger fluid droplet curbs the drop deformation and exhibits flow dynamics identical to the shear thinning viscoelastic droplet, thus indicating that the nature of the drop viscoelasticity has little influence on the flow behaviour. The matrix viscoelasticity due to Boger fluid increases drop deformation and displays non-monotonic deformation. The drop deformation is further enhanced in the case of Boger fluid in viscoelastic drop-matrix system. Interestingly, the pressure drop due to the presence of viscoelastic drop in a Newtonian matrix is lower than the single phase flow of Newtonian fluid. We also discuss the effects of inertia, surface tension, drop to matrix viscosity ratio and the drop size on these drop-matrix systems. Finally, we investigate the emulsion rheology by studying the motion of a droplet in the square lid driven cavity flow. The viscoelastic effects due to constant viscosity Boger fluid and shear thinning viscoelastic fluid are illustrated using FENECR and Giesekus rheological relations, respectively. The presence of viscoelasticity either in drop or matrix phase boosts the drop deformation with the drop viscoelasticity displaying intense deformation. The drop dynamics due to the droplet viscoelasticity is observed to be independent of the nature of vis-coelastic fluid. The shear thinning viscoelastic matrix has a stronger influence on the drop deformation and orientation compared to the Boger fluid matrix. The different blood components, cells and many materials of industrial importance are viscoelastic in nature. Thus, the present study has significant applications in medical diagnostics, drug delivery, manufacturing and processing industries, study of biological flows, pharmaceutical research and development of lab-on-chip devices.

This thesis is based on four essays in exchange rates and international finance. The first essay, examined in the second chapter, considers the long-run performance of the flexible-price monetary model as well as the real interest differential monetary model to explain the dollar–yen exchange rate during a period of high international capital mobility. We apply the Johansen methodology to quarterly data over the period 1980:01–2009:04 and show that the inadequacy of the two monetary models is due to the breakdown of their underlying building-blocks, money demand stability and purchasing power parity. In particular, modifying the monetary models by adjusting them for real stock prices to capture the stability of money demands on one hand and also for real economic variables such as productivity differential, relative government spending, and real oil price to explain the persistence in the real exchange rate on the other provide long-run relationships that appear consistent with the monetary models. Our findings of long-run weak exogeneity tests also emphasise the importance of the extended models employed here. The second essay, examined in the third chapter, is on the nature of the linkages between stock market prices and exchange rates in six advanced economies, namely the US, the UK, Canada, Japan, the euro area, and Switzerland, using data on the banking crisis between 2007 and 2010. Bivariate GARCH-BEKK models are estimated to produce evidence of unidirectional Granger causality from stock returns to exchange rate changes in the US and the UK, in the opposite direction in Canada, and of bidirectional causality in the euro area and Switzerland. Furthermore, causality-in-variance from stock returns to exchange rate changes is found in Japan and in the opposite direction in the euro area and Switzerland, whilst there is evidence of bidirectional causality-in-variance in the US and Canada. These findings imply limited opportunities for investors to diversify their assets during this period. The third essay, examined in the fourth chapter, considers the impact of net bond and net equity portfolio flows on exchange rate changes. Two-state Markov-switching models are estimated for the exchange rate of the US vis-a-vis Canada, the euro area, Japan and the UK. Our results suggest that the relationship between net portfolio flows and exchange rate changes is nonlinear for all cases considered, except that of the US dollar against the Canadian dollar. The fourth essay, examined in the fifth chapter, considers the impact of exchange rate uncertainty on different components of net portfolio flows, namely net equity and net bond flows, as well as the dynamic linkages between exchange rate volatility and the variability of these two types of flows. Specifically, a bivariate GARCH-BEKK-in mean model is estimated using bilateral data for the US vis-à-vis Australia, the UK, Japan, Canada, the euro area, and Sweden over the period 1988:01-2011:12. The results indicate that the effect of exchange rate uncertainty on net equity flows is negative in the euro area, the UK and Sweden, and positive in Australia, whilst two countries (Canada and Japan) showed insignificant responses. With regard to the impact of uncertainty on net bond flows, it is shown to be negative in all countries, except Canada (where it is positive). Under the assumption of risk aversion, this suggests that exchange rate uncertainty induces investors, especially those of the counterpart countries to the US, to reduce their financing activities to maximise returns and minimise exposure to uncertainty. This evidence is strong for the UK, the euro area and Sweden as opposed to Canada, Australia and Japan. Furthermore, since exchange rate volatility and the variability of flows are interlinked, exchange rate or credit controls on these flows can be used to pursue economic and financial stability.

In this study we investigate the interactions between two co-rotating vortices. These vortices are subject to rapid rotation and stable stratification such as are found in planetary atmospheres and oceans. By conducting a large number of simulations of vortex interactions, we intend to provide an overview of the interactions that could occur in geophysical turbulence. We consider a wide parameter space covering the vortices height-to-width aspect-ratios, their volume ratios and the vertical offset between them. The vortices are initially separated in the horizontal so that they reside at an estimated margin of stability. The vortices are then allowed to evolve for a period of approximately 20 vortex revolutions. We find that the most commonly observed interaction under the quasi-geostrophic (QG) regime is partial-merger, where only part of the smaller vortex is incorporated into the larger, stronger vortex. On the other hand, a large number of filamentary and small scale structures are generated during the interaction. We find that, despite the proliferation of small-scale structures, the self-induced vortex energy exhibits a mean `inverse-cascade' to larger scale structures. Interestingly we observe a range of intermediate-scale structures that are preferentially sheared out during the interactions, leaving two vortex populations, one of large-scale vortices and one of small-scale vortices. We take a subset of the parameter space used for the QG study and perform simulations using a non-hydrostatic model. This system, free of the layer-wise two-dimensional constraints and geostrophic balance of the QG model, allows for the generation of inertia-gravity waves and ageostrophic advection. The study of the interactions between two co-rotating, non-hydrostatic vortices is performed over four different Rossby numbers, two positive and two negative, allowing for the comparison of cyclonic and anti-cyclonic interactions. It is found that a greater amount of wave-like activity is generated during the interactions in anticyclonic situations. We also see distinct qualitative differences between the interactions for cyclonic and anti-cyclonic regimes.

[pt] Problemas de multifluxos surgem como modelos básicos no contexto de várias aplicações de fluxos em redes, tais como redes de telecomunicações, redes de transporte e logística. Em tais aplicações, os fluxos que atravessam a rede compartilham simultaneamente os mesmos recursos disponíveis e são definidos por suas próprias restrições. A cada uma das arestas ligando os pontos da rede está associado um custo, fixo ou proporcional à sua utilização. Este trabalho trata problemas de projeto de redes multifluxos, em que os custos estão associados às capacidades instaladas nas arestas. Particularmente, será estudado o caso em que a função de custo nas arestas possui o comportamento de uma função escada crescente e descontínua, para o qual métodos exatos de resolução são ineficientes. Métodos heurísticos são propostos para a resolução aproximada do problema e sintetizados em um algoritmo de multi-partida com memória adaptativa. Um mecanismo de intensificação, conhecido na literatura como construção de vocabulário, é também explorado e aplicado. Finalmente, experimentos computacionais são realizados e o método de resolução proposto é analisado quanto aos seus resultados e os resultados obtidos pelo método de resolução proposto são analisados. O método obtém as melhores soluções conhecidas para algumas instâncias da literatura. / [en] Multicommodity flow problems arise widely as basic models in the context of network flows applications such as telecommunication networks, transportation problems, and logistic. In these applicatons, the flows that cross the networks share the same avaiable resources simultaneously and are defined by their own constraints. Each edge connecting two nodes in the network has an associated cost that is either fixed or proportional to its use. This work focuses on a network design problem in which the cost are associated with the capacities installed in the edges. Particularly, the network design problem studied has discrete and step increasing cost functions on the edges, for which exact methods are inefficient. Heuristics are proposed for the approximate memory algorithm. An intensification mechanism, known in the literature as vocabulary building, is also explored and applied. Finally, computational experiments are performed and the results obtained with the proposed solution method are evaluated. The method obtains the best known solutions for some instances in the literature.

Fluid flows mainly driven by capillary forces are presented in this thesis. By means of modeling and simulations, interesting dynamics in capillary-driven flows are revealed such as coalescences, breakups, precursor films, flow instabilities, rapid spreading, rigid body motions, and reactive wetting. Diffuse-interface methods model a fluid interface as having a finite thickness endowed with physical properties such as surface tension. Two diffuse-interface models that are based on the free energy of the system are presented. The binary model, more specifically the coupled Navier-Stokes/Cahn-Hilliard equations, was used to study different two-phase flows including problems related to microfluidics. Numerical issues using this model have been addressed such as the need for mesh adaptivity and time-step restrictions. Moreover, the flexibility of this model to simulate 2D, axisymmetric, and 3D flows has been demonstrated. The factors affecting reproducibility of microdroplet depositions performed under a liquid medium are investigated. In the deposition procedure, sample solution is dispensed from the end of a capillary by the aid of a pressure pulse onto a substrate with pillar-shaped sample anchors. In both the experimental and numerical study it was shown that the deposited volume mainly depends on the capillary-substrate distance and anchor surface wettability. Furthermore, a critical equilibrium contact angle has been identified below which reproducible depositions are facilitated. The ternary model is developed for more complicated flows such as liquid phase sintering. With the introduction of a Gibbs energy functional, the governing equations are derived, consisting of convective concentration and phase-field equations which are coupled to the Navier-Stokes equations with surface tension forces. Arbitrary phase diagrams, surface energies, and typical dimensionless numbers are some input parameters into the model. Detailed analysis of the important capillary phenomena in liquid phase sintering such as reactive and nonreactive wetting and motion of two particles connected by a liquid bridge are presented. The dynamics of the wetting is found to match with a known hydrodynamic theory for spreading liquids. Factors affecting the equilibrium configuration of the particles such as equilibrium contact angles and volume ratios are also investigated. / QC 20100823