Los flujos con interfaces móviles (problemas de superficie libres y de dos fluidos) aparecen en numerosas aplicaciones de ingeniería. Los métodos presentados en esta tesis están orientados principalmente a la simulación del proceso de llenado de molde. Sin embargo la metodología es suficientemente general para ser aplicada a la mayoría de los flujos de superficie libres y de dos fluidos. El modelado numérico proporciona un modo eficiente de analizar los fenómenos físicos que ocurren durante procesos de inyección y fundición. Permite comprender detalles del flujo que de otra forma serían muy difíciles de observar.Se usa un método de elementos finitos de malla fija, donde la posición de interfaz es capturada por la función de Level Set. Los flujos a bajo número de Froude son particularmente desafiantes para los métodos de malla fija. Una representación precisa es necesaria en los elementos cortados por el frente. Se proponen dos alternativas. La primera alternativa usa el modelo de flujo de dos fases típico, enriqueciendo las funciones de forma de presión, para permitir una mejor aproximación de la discontinuidad en el gradiente de presión en la interfaz. La mejora de la representación del gradiente de presión es el ingrediente clave para el correcto modelado de tales flujos.La influencia del segundo fluido puede ser ignorada en una amplia variedad de aplicaciones para terminar con un modelo de superficie libre que es más simple que el modelo de flujo de dos fases. La discontinuidad en el gradiente de presión desaparece porque sólo se simula un fluido. La particularidad de este segundo método es que se usa una malla fija. Las condiciones de contorno son aplicadas exactamente usando integración mejorada e integrándose sólo en la parte llena de los elementos cortados por el frente. Se desarrolla un método ALE de malla fija para tener en cuenta correctamente que el dominio se mueve a pesar de que se usa una malla fija. Los métodos de segregación de presión son explorados como una alternativa a la discretización monolítica de las ecuaciones de Navier Stokes. Ellos desacoplan las incógnitas de velocidad y presión, conduciendo a subproblemas más pequeños y mejor condicionados. Los métodos de corrección de presión y corrección de velocidad son presentados y comparados numéricamente. Usando un Laplaciano discreto se obtiene un método de corrección de velocidad de tercer orden numéricamente estable.Los métodos son aplicados a problemas de llenado de moldes tridimensionales tomados directamente de la fundición con resultados muy satisfactorios. El modelo monolítico con superficial libre resulta ser la opción más robusta y eficiente. La comparación con un código comercial muestra la exactitud y la eficacia del método que proponemos. / Flows with moving interfaces (free surface and two-fluid interface problems) appear in numerous engineering applications. The methods presented in this thesis are oriented mainly to the simulation of mould filling process. Nevertheless the methodology is sufficiently general as to be applied to most free surface and two-fluid interface flows. Numerical modeling provides an efficient way of analyzing the physical phenomena that occur during casting and injection processes. It gives insight into details of the flow that would otherwise be difficult to observe.A fixed mesh finite element method, where the interface position is captured by the Level Set function, is used. Low Froude number flows are particularly challenging for fixed grid methods. An accurate representation is needed in the elements cut by the interface for such flows. Two alternatives are proposed.The first alternative is to use the typical two-phase flow model enriching the pressure shape functions so that the discontinuity in the pressure gradient at the interface can be better approximated. The improvement in the representation of the pressure gradient is shown to be the key to ingredient for the successful modeling of such flows.The influence of the second fluid can be neglected on a wide range of applications to end up with a free surface model that is simpler than the twophase flow model. The discontinuity in the pressure gradient disappears because only one fluid is simulated. The particularity of this second approach is that a fixed mesh is used. Boundary conditions are applied accurately using enhanced integration and integrating only in the filled part of cut elements.A fixed mesh ALE approach is developed to correctly take into account that the domain is moving despite a fixed mesh is used.Pressure segregation methods are explored as an alternative to the monolithic discretization of the Navier Stokes equations. They uncouple the velocity and pressure unknowns, leading to smaller and better conditioned subproblems. Pressure correction and velocity correction methods are presented and compared numerically. Using a discrete Laplacian a numerically stable third order velocity correction method is obtained.The methods are applied to three dimensional mould filling problems borrowed directly from the foundry with very satisfactory results. The free surface monolithic model turns out to be the most robust and efficient option. The comparison with a commercial code shows the accuracy and efficiency of the method we propose.

FROTA, Diego Araújo. Eficiência dos arranjos de poços na recuperação de óleo pesado usando combustão in situ. 2010. 66 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2010. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-04T18:03:06Z No. of bitstreams: 1 2010_dis_dafrota.pdf: 2643650 bytes, checksum: 047ea9b4e4de40134f2d48e5d47887bc (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-07T14:46:34Z (GMT) No. of bitstreams: 1 2010_dis_dafrota.pdf: 2643650 bytes, checksum: 047ea9b4e4de40134f2d48e5d47887bc (MD5) / Made available in DSpace on 2015-05-07T14:46:34Z (GMT). No. of bitstreams: 1 2010_dis_dafrota.pdf: 2643650 bytes, checksum: 047ea9b4e4de40134f2d48e5d47887bc (MD5) Previous issue date: 2010 / Este trabalho contém um estudo comparativo sobre a eficiência de vários arranjos de poços para a recuperação de óleo de reservatórios de hidrocarbonetos usando a técnica de combustão in-situ. Conforme a demanda por energia aumenta a cada dia com o crescimento da economia mundial e o desenvolvimento das nações, a matriz energética mundial ainda depende fortemente de fontes fósseis e principalmente do petróleo, o qual, com o decorrer de sua exploração exige o aperfeiçoamento de métodos para a sua extração. Vários métodos foram criados para facilitar a extração de petróleo das rochas porosas, incluindo injeção de fluidos e/ou reações químicas para aumentar a pressão e/ou reduzir a viscosidade do óleo. Com o avanço tecnológico tornou-se possível também a perfuração de poços horizontais, que passam pelo reservatório paralelamente a superfície. Com isso, aparecem questões sobre determinar qual combinação do método de extração e configuração de poços é a mais favorável para um determinado tipo de reservatório, cujas características são especificadas pelas propriedades de suas rochas e seus fluidos. Nesta dissertação apresenta-se uma breve compilação dos conceitos e equações diferenciais usados na modelagem matemática dos escoamentos multi-fásicos de fluidos e do transporte de calor em meios porosos. Faz-se também um estudo comparativo de várias configurações de poços horizontais e verticais (VIHP, VI2HP, HIHP, HI2HP e 2VIHP) para injetar e extrair fluidos em reservatórios de óleo pesado com dimensões de 300x100x20 metros. Ar é então injetado em seu interior a fim de criar uma frente de combustão que se propaga do poço injetor ao produtor. As simulações numéricas são realizadas através do simulador térmico de reservatórios STARS do CMG. As propriedades físicas e químicas do reservatório e as reações químicas são baseadas no reservatório Wolf Lake. Foi investigado também a influência dos parâmetros que controlam a dependência da viscosidade do óleo com a temperatura. Os resultados mostram que a configuração 2VIHP obtém a melhor recuperação entre as configurações estudadas independentemente dos parâmetros estudados.

Orientador: Gustavo Martin Dalpian / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-graduação em Nanociências e materiais avançados, 2010.

Wormlike micelles are viscoelastic fluids that present an intermediate behavior between solids and ordinary liquids since they are elastic at short time scales but flow easily at large time scales. In opposition to Newtonian fluids, which have constant viscosity, these fluids usually exhibit a non-Newtonian response with a rate-dependent shear viscosity. Wall-bounded oscillatory flows of Newtonian and complex fluids are found in many practical situations. Oscillatory pipe flows are especially important in physiology in connection with the circulatory and respiratory systems of human beings, as well as in industrial processes such as fluid pumping, secondary oil recovery or filtration, and in acoustics. Pulsating flows are of particular interest also in the rheological characterization of complex fluids. We analyze the laminar oscillatory flow of viscoelastic fluids using the Maxwell and Oldroyd-B models. We have shown that in wall-bounded oscillatory flows of viscoelastic fluids the two characteristic lengths of the Ferry waves, the damping length and wavelength, together with the characteristic separation of the walls, define all the flow properties for fluid models with a linear shear-stress equation in unidirectional flow. In wall-bounded settings there exists the possibility that shear waves generated at different locations superpose themselves before decaying so that the shear waves interfere, giving rise to a resonant flow at well defined frequencies of driving. The theoretical predictions obtained for the laminar velocity profiles are validated by carrying out time-resolved Particle Image Velocimetry (PIV) experiments in a vertical pipe at small driving amplitudes. The oscillatory pipe flow has been investigated in the whole range of experimentally accessible driving frequencies and amplitudes, and classified in three main flow regimes: laminar, vortical, and non-axisymmetric vortical. By ramping up and down the driving amplitude at constant frequency we have been able to characterize the transition from laminar to more complex flows, under controlled driving conditions. The first hydrodynamic instability occurs when the laminar base flow becomes unstable against the formation of axisymmetric toroidal vortices that appear distributed along the cylinder. The calculation of root-mean-square fluctuations in the vertical direction, of the vertical and radial components of the velocity (averaged in time or over the tube diameter) has allowed to determine the critical amplitude at which the instability sets in with high accuracy. In the vortical flow an abrupt increase of the fluctuations is observed, that accounts for the loss of the vertical translational symmetry and the formation of vortices in the flow. This transition exhibits hysteresis when the driving amplitude is ramped up and down, which makes us presume that the bifurcation from the laminar flow has a subcritical nature. A second hydrodynamic instability occurs when the vortical flow loses the axial symmetry. In this flow regime the vortices are heavily distorted and no longer axisymmetric. The velocity and vorticity maps of the vortical flow measured in a meridional plane of the tube appear periodic in time, on time scales comparable to the driving period. Interestingly, the vortex formation is favored in the acceleration phases of the piston oscillation. Besides, we have uncovered a spatio-temporal dynamics on long time scales (much larger than the relaxation time of the fluid) that substantially modifies the flow organization. This slow dynamics is more effective in the bottom half of the cylinder, specially next to the driving piston. A global inspection of the vortical flow along the tube length reveals that the instability takes place earlier in the bottom part of the tube, in the vicinity of the driving piston. At increasing the driving amplitude the boundary between laminar and vortical flow progressively raises towards the top regions. And above a critical driving amplitude the entire fluid flow is vortical. The mechanism triggering the hydrodynamic instability from the laminar to the axisymmetric vortical flow is not yet clear. / L'objectiu d'aquesta Tesi és estudiar el flux oscil•latori vertical en fluids micel•lars. Els fluids micel•lars són fluids complexos amb propietats viscoelàstiques, de manera que mostren un comportament intermedi entre els sòlids i els líquids: són elàstics a escales de temps curtes però flueixen a escales de temps més llargues. En contraposició als fluids Newtonians, que tenen una viscositat constant, els fluids complexos mostren un comportament no-Newtonià, amb una viscositat que depèn del ritme de deformació. El fluxos oscil•latoris de fluids Newtonians o complexos en geometries confinades són especialment importants en fisiologia, en relació amb el sistema circulatori i respiratori d'éssers humans, i també en processos industrials com el bombejat de fluids, l'extracció de petroli, i en particular són interessants en la caracterització reològica de fluids complexos. Primer estudiem el flux oscil•latori des d'una perspectiva teòrica i analitzem el flux laminar de fluids viscoelàstics utilitzant els models de Maxwell i Oldroyd-B en un tub vertical. Hem mostrat que en fluxos confinats existeix la possibilitat que les ones de cisalla generades a les diferents parets se sobreposin abans d'esmorteir-se i que eventualment donin lloc a un fenomen de ressonància. Les prediccions teòriques obtingudes pel flux laminar són validades duent a terme experiments de Velocimetria d'Imatges de Partícules (PIV) en un tub vertical, per amplituds petites del forçament oscil•latori. Quan s'incrementa l'amplitud de l'oscil•lació el flux laminar evoluciona cap a fluxos que presenten una dependència espai-temporal més complexa. Fent rampes d'amplitud creixent a una freqüència fixada hem pogut caracteritzar experimentalment la transició del flux laminar a aquests fluxos més complexos, sota condicions de forçament ben controlades. La primera inestabilitat apareix quan el flux laminar esdevé inestable amb la corresponent formació d'anells de vorticitat apilats al llarg del tub. Es manifesta una segona inestabilitat per amplituds del forçament més grans, per la qual el flux vortical perd la simetria axial. En aquest nou règim els vòrtex estan fortament distorsionats i no són axisimètrics. Fent rampes d’amplitud creixent i decreixent hem observat que aquestes dues transicions presenten histèresi, i que per tant són de caràcter subcrític.

Traditionally, the sea has been considered as the ultimate sink of most pollutions, due to its huge volume and assimilating capacities, and the coastal waters, because of their intermediate position between deep seas and human activities, has been acting as a buffer zone. However, during the past years, the interest in environmental issues in general, and in the conservation of the water quality in coastal regions, in particular, has steadily increased. For coastal waters, this growing interest can be related to the increase in population and industrial activity along rivers and on coastal areas, which poses an important threat on coastal ecosystems, and may limit the use of coastal waters for human activity. This problem is drastically increased during the tourist season, when a large number of tourists tend to favour coastal resorts. Although at different scales, both the discharge of wastewater from a marine outfall or the discharge of nutrient- or sediment-laden freshwater from a river play an important role in many physical, chemical and biological processes, which in turn may affect the human use (recreational, economical, etc), of coastal waters. It is important, therefore, to be able to model these processes in order to optimise the multiple uses of coastal waters, accommodating the economical interests to marine production and recreation. The present thesis describes the development, validation and application to real cases of a three-dimensional numerical model for substance (pollution) transport in coastal waters, which can be applied to a wide range of situations, as marine outfall discharges, river freshwater discharges or sediment transport. The three-dimensional advection-diffusion equation is solved using a random walk Lagrangian particle formulation, in which the substance to be transported is divided into discrete "packets" that are moved around due to the effects of the transport mechanisms relevant in coastal waters. Transport induced by currents, waves, and turbulent diffusion is included in the model calculations, together with other mechanisms such as sediment settling and resuspension, buoyancy effects due to density differences, and microbiological inactivation. Specific formulations have been developed to account for the transport due to the initial discharge velocity, for buoyant transport in the frame of particle models in waterbodies, for sediment resuspension, and to increase the efficiency of the model by uncoupling the nearfield modelling from that of the farfield. Concentration distributions are obtained at the end of the simulation by mapping the position of the resulting set of discrete particles onto the nodes of a grid, that may be deformable, employing different optional methods. This transport model has been validated using several test cases of known analytical solution, and has been applied to a series of real cases for which numerical results could be compared to observational evidence (such as experimental data, video recordings and satellite images). The comparison yields a good agreement between observational evidence and modelled data, and reveals that the model can be applied to a wide range of transport problems in various environments.