O método ISPH (do inglês, Incompressible Smoothed Particle Hydrodynamics) é um método de aproximação livre de malha que, através de um conjunto finito de partículas e uma formulação completamente Lagrangeana, permite a solução de diversos tipos de escoamentos. Entretanto, sua aplicação para escoamentos bifásicos ainda é um desafio, principalmente no que refere-se à manutenabilidade da interface entre fluidos. Diante disso, nesta tese é apresentado o desenvolvimento de um código numérico baseado no método ISPH, sendo propostas duas técnicas de tratamento de interface. Para tanto é realizado um estudo a cerca do método, considerando diferentes metodologias, e analisando pontos específicos, tais como a solução do campo de pressões. São apresentados resultados que mostram a eficácia do método, tanto em escoamentos monofásicos, quanto em escoamentos multifásicos, onde, neste caso, são destacadas as melhorias obtidas através das técnicas de tratamento de interface propostas. Por fim, é realizado um estudo do comportamento de misturas bifásicas, com referência ao fenômeno da inversão de fase / Incompressible Smoothed Particle Hydrodynamics (ISPH) method is a meshless approximation that has been used to simulate several types of fluid flows, through a finite particle set and fully lagrangian formulation. The application of ISPH method in two-fluid flow simulations however, has presented many challenges, specially related to the presence of the interface between different fluids. Thus, we present in this study the development of a numerical code based on ISPH, introducing novel interface treatment techniques. A thorough study about this method is provided, considering different methodologies and analysing specific points such as the position of the interface and the obtained pressure field. Results have been presented to show the methods developed in this thesis efficiently simulate two-fluid flows, illustrating the improvements achieved by the proposed interface treatment techniques. Finally, a study of biphasic mixture behavior is carried out with reference to phase inversion phenomena

This thesis deals with a theoretical and an experimental exploration of squeeze film levitation (SFL) of light objects. The investigations aimed to find the important design parameters controlling this levitation mechanism and also to suggest an alternative way to implement SFL. The study, through computer modelling and experimental validation, focused on Poisson's contraction effect for generating SFL. A finite element model (ANSYS) was verified by experimental testing of five different plate designs. Each plate was subjected to a uniaxial plain stress by an arrangement of two hard piezoelectric actuators (PZT) bonded to the bottom of the plate and driven with DC or AC voltages. It was observed that pulsation of a dimple or crest shaped elastic deformation along the longitudinal axis in the central area of the plate was created because of Poisson's contraction. This Poisson's effect generated the squeeze-film between the plate and the levitated object. The separation distance between a floating lightweight object and the plate was analysed using computational fluid dynamics (ANSYS CFX) through creation of a modelling model for the air-film entrapped between the two interacting surfaces - a typical three-dimensional fluid-solid interaction system (FSI). Additionally, the levitation distance has been experimentally measured by a Laser Sensor. A satisfactory agreement has been found between model predictions and experimental results. Two levitation systems, one based on a horn transducer (Langevin type) and the other one in the form of a plain rectangular plate made of Aluminium and firmly fastened at both ends with a surface-mounted piezoelectric actuator, were compared in this thesis. Both devices were based on SFL mechanism. Evidently, the performances of both designs were greatly influenced by the design structure and in particular by the driving plate characteristics such as plate size and geometry as well as the driving boundary conditions. To this end, physical experiments were carried out and it was found that the device utilising horn-type transducer yields better levitation performance. Ultimately, the research explained the confusion between three approaches to non-contact levitation through literature review and also pointed out some essential parameters like piezoelectric actuators location, material of the driving structure, coupled-field between the actuators and the driving structure and the fluid-solid interface that was existed between the excited plate and the levitated object.

O método de Floquet foi utilizado para estudar a transição da esteira de dois cilindros posicionados lado a lado. Quando os cilindros estão afastados, cada cilindro possui sua própria esteira e para espaçamentos grandes (T/D > 4) a principal influência do cilindro ao lado é a sincronização das esteiras em anti-fase. A esteira em anti-fase corresponde a um escoamento simétrico em relação à linha central, igualmente espaçada em relação aos dois cilindros. Para espaçamentos com T/D < 3, a esteira em fase visualizada por Williamson (1985) é observada. A análise de Floquet da esteira em anti-fase simétrica encontrou um modo que quebra a simetria e induz a transição para esteira em fase. Reduzindo o espaçamento diversos modos complexos apresentam multiplicadores de Floquet que crescem e se tornam dominantes para T/D = 2 coincidindo com o surgimento de esteira irregular. A aproximação dos cilindros também afeta o surgimento de tridimensionalidades no escoamento. Os modos A e B de Williamson (1996b) observados para o cilindro isolado ainda existem, mas observa-se uma supressão do modo B. Por outro lado, o modo A se torna instável para Re menores. A esteira em fase também apresenta os modos A e B, mas neste caso, o modo B se torna instável para Re menores que o observado por Williamson. / The Floquet method was used to study the transition in the wake of two side-by-side circular cylinders. When the cylinders are far apart, each cylinder presents its own wake and for large spacing (T/D > 4) the main influence from the neighboring cylinder is an anti-phase synchronization of the wakes. The antiphase wake corresponds to a symmetric flow in relation to the mid-line between cylinders. When T/D < 3 the in-phase wake described by Williamson (1985) is also observed. The Floquet analysis of the symmetric anti-phase wake found an unstable mode that breaks the symmetry and triggers the transition to in-phase vortex-shedding. As the spacing is further reduced, several complex modes have Floquet multipliers that grow with the decreasing spacing and when T/D = 2 they become dominant suggesting an important role on the irregular wake regime. The presence of the neighboring cylinder also affects the transition to three-dimensional wake. Modes A and B described by Williamson (1996b) for the isolated circular cylinder are still observed but mode B is suppressed as the spacing decreases. On the other hand, mode A becomes unstable for lower Re numbers. The in-phase wake also presents modes A and B but in this case, mode B becomes unstable for lower Re numbers when compared to the single circular cylinder.

This thesis is aimed to calculate the effective permeability tensor and to simulate the fluid flow in naturally fractured reservoirs. This requires an understanding of the mechanisms of fluid flow in naturally fractured reservoirs and the detailed properties of individual fractures and matrix porous media. This study has been carried out to address the issues and difficulties faced by previous methods; to establish possible answers to minimise the difficulties; and hence, to improve the efficiency of reservoir simulation through the use of properties of individual fractures. The methodology used in this study combines several mathematical and numerical techniques like the boundary element method, periodic boundary conditions, and the control volume mixed finite element method. This study has contributed to knowledge in the calculation of the effective permeability and simulation of fluid flow in naturally fractured reservoirs through the development of two algorithms. The first algorithm calculates the effective permeability tensor by use of properties of arbitrary oriented fractures (location, size and orientation). It includes all multi-scaled fractures and considers the appropriate method of analysis for each type of fracture (short, medium and long). In this study a characterisation module which provides the detail information for individual fractures is incorporated. The effective permeability algorithm accounts for fluid flows in the matrix, between the matrix and the fracture and disconnected fractures on effective permeability. It also accounts for the properties of individual fractures in calculation of the effective permeability tensor. The second algorithm simulates flow of single-phase fluid in naturally fractured reservoirs by use of the effective permeability tensor. This algorithm takes full advantage of the control volume discretisation technique and the mixed finite element method in calculation of pressure and fluid flow velocity in each grid block. It accounts for the continuity of flux between the neighbouring blocks and has the advantage of calculation of fluid velocity and pressure, directly from a system of first order equations (Darcy???s law and conservation of mass???s law). The application of the effective permeability tensor in the second algorithm allows us the simulation of fluid flow in naturally fractured reservoirs with large number of multi-scale fractures. The fluid pressure and velocity distributions obtained from this study are important and can considered for further studies in hydraulic fracturing and production optimization of NFRs.

We present predictions for the statistical error due to finite sampling in the presence of thermal fluctuations in molecular simulation algorithms. Specifically, we present predictions for the error dependence on hydrodynamic parameters and the number of samples taken. Expressions for the common hydrodynamic variables of interest such as flow velocity, temperature, density, pressure, shear stress and heat flux are derived using equilibrium statistical mechanics. Both volume-averaged and surface-averaged quantities are considered. Comparisons between theory and computations using direct simulation Monte Carlo for dilute gases, and molecular dynamics for dense fluids, show that the use of equilibrium theory provides accurate results. / Singapore-MIT Alliance (SMA)

The objective of this dissertation is to investigate the non-Darcy flow effects on field base data by considering gas viscosity, gas deviation factor and gas density as variables. To achieve it, different correlations from the literature and field data have been combined to Sawyer-Brown Method, thus a contribution has been achieved. Production history of selected gas field has been implemented to a numerical simulator. To find out non-Darcy effects quantitatively, Darcy flow conditions have also been run in the simulator for each scenario in addition to non-Darcy flow correlation runs. Extracted data from simulation runs have been analyzed on the basis of Sawyer-Brown Method by introducing several correlations to consider gas viscosity, gas deviation factor and gas density as variables. Engineering and scientific research on non-Darcy flow is still being conducted in order for better understanding the nonlinear flow behavior of fluids through porous media. The deviations from Darcy&rsquo / s Law are attributed to the occurrence of all or alternating combinations of factors that can be categorized as the anisotropy of porosity and permeability, multi-phase flow of fluids in varying phases, magnitude of pressure drop and the subsequent phase change in fluids, and the change in flow regime at elevated rates of flow in porous media. Throughout this dissertation, the factors causing deviations from Darcy flow behavior have been investigated.

Problems involving coupled thermo-poro-chemo-mechanical processes are of great importance in geothermal and petroleum reservoir systems. In particular, economic power production from enhanced geothermal systems, effective water-flooding of petroleum reservoirs, and stimulation of gas shale reservoirs are significantly influenced by coupled processes. During such procedures, stress state in the reservoir is changed due to variation in pore fluid pressure and temperature. This can cause deformation and failure of weak planes of the formation with creation of new fractures, which impacts reservoir response. Incorporation of geomechanical factor into engineering analyses using fully coupled geomechanics-reservoir flow modeling exhibits computational challenges and numerical difficulties. In this study, we develop and apply efficient numerical models to solve 3D injection/extraction geomechanics problems formulated within the framework of thermo-poro-mechanical theory with reactive flow. The models rely on combining Displacement Discontinuity (DD) Boundary Element Method (BEM) and Finite Element Method (FEM) to solve the governing equations of thermo-poro-mechanical processes involving fracture/reservoir matrix. The integration of BEM and FEM is accomplished through direct and iterative procedures. In each case, the numerical algorithms are tested against a series of analytical solutions. 3D study of fluid injection and extraction into the geothermal reservoir illustrates that thermo-poro-mechanical processes change fracture aperture (fracture conductivity) significantly and influence the fluid flow. Simulations that consider joint stiffness heterogeneity show development of non-uniform flow paths within the crack. Undersaturated fluid injection causes large silica mass dissolution and increases fracture aperture while supersaturated fluid causes mineral precipitation and closes fracture aperture. Results show that for common reservoir and injection conditions, the impact of fully developed thermoelastic effect on fracture aperture tend to be greater compare to that of poroelastic effect. Poroelastic study of hydraulic fracturing demonstrates that large pore pressure increase especially during multiple hydraulic fracture creation causes effective tensile stress at the fracture surface and shear failure around the main fracture. Finally, a hybrid BEFEM model is developed to analyze stress redistribution in the overburden and within the reservoir during fluid injection and production. Numerical results show that fluid injection leads to reservoir dilation and induces vertical deformation, particularly near the injection well. However, fluid withdrawal causes reservoir to compact. The Mandel-Cryer effect is also successfully captured in numerical simulations, i.e., pore pressure increase/decrease is non-monotonic with a short time values that are above/below the background pore pressure.

No description available.

The Transmission Control Protocol (TCP) has successfully governed the Internet congestion control for two decades. It is by now, however, widely recognized that TCP has started to reach its limits and that new congestion control protocols are needed in the near future. This has spurred an intensive research effort searching for new congestion control designs that meet the demands of a future Internet scaled up in size, capacity and heterogeneity. In this thesis we derive network fluid flow models suitable for analysis and synthesis of window based congestion control protocols such as TCP. In window based congestion control the transmission rate of a sender is regulated by: (1) the adjustment of the so called window, which is an upper bound on the number of packets that are allowed to be sent before receiving an acknowledgment packet (ACK) from the receiver side, and (2) the rate of the returning ACKs. From a dynamical perspective, this constitutes a cascaded control structure with an outer and an inner loop. The first contribution of this thesis is a novel dynamical characterization and an analysis of the inner loop, generic to all window based schemes and formed by the interaction between the, so called, ACK-clocking mechanism and the network. The model is based on a fundamental integral equation relating the instantaneous flow rate and the window dynamics. It is verified in simulations and testbed experiments that the model accurately predicts dynamical behavior in terms of system stability, previously unknown oscillatory behavior and even fast phenomenon such as traffic burstiness patterns present in the system. It is demonstrated that this model is more accurate than many of the existing models in the literature. In the second contribution we consider the outer loop and present a detailed fluid model of a generic window based congestion control protocol using queuing delay as congestion notification. The model accounts for the relations between the actual packets in flight and the window size, the window control, the estimator dynamics as well as sampling effects that may be present in an end-to-end congestion control algorithm. The framework facilitates modeling of a quite large class of protocols. The third contribution is a closed loop analysis of the recently proposed congestion control protocol FAST TCP. This contribution also serves as a demonstration of the developed modeling framework. It is shown and verified in experiments that the delay configuration is critical to the stability of the system. A conclusion from the analysis is that the gain of the ACK-clocking mechanism dramatically increases with the delay heterogeneity for the case of an equal resource allocation policy. Since this strongly affects the stability properties of the system, this is alarming for all window based congestion control protocols striving towards proportional fairness. While these results are interesting as such, perhaps the most important contribution is the developed stability analysis technique. / QC 20100813

Intentional amendment of soil with charcoal (called biochar) is a promising new approach to sequester atmospheric carbon dioxide and increase soil fertility. However, the environmental properties of biochars can vary with production conditions, making it challenging to engineer biochars that are simultaneously optimized for carbon sequestration, nutrient storage, and water-holding capacity. For this reason, I have undertaken a systematic study to (a) determine the pyrolysis conditions that lead to biochars with desired chemical and physical properties, and (b) find how these properties affect the water-holding capacity and nutrient adsorption in biochar-soil mixtures. First, a library of biochars was produced in a custom-built pyrolysis reactor under precisely controlled conditions. The chemical and physical structures of the produced biochars were characterized with various analytical techniques including 13C NMR, XPS, EA and BET pore surface analysis. My results suggest that the chemical composition and pore structure of biochars are determined not just by the maximum heat treatment temperature, but also by several other factors that include the pyrolysis heating rate, treatment time at the maximum temperature and particle size. I also tested a new approach that combines thermogravimetric reactivity measurements, diffusion-reaction theory and structural models to achieve a better characterization of the complicated multi-scale pore structure of biochars. The structural models treat biochars as porous solids having micro- and macropores of different shapes and exhibiting widely ranging pore-size distributions. Simulations results are then compared to experimental data to identify the presence of ordered or random pore networks and test their size distributions and connectivity. I then developed a multi-solid one-dimensional model that can use experimentally determined biochar properties to predict their field performance in beds packed with soil/biochar mixtures. The model used a system of coupled partial differential equations to describe the dynamic adsorption/elution of ammonium nitrate, a model fertilizer, in columns packed with biochar/soil mixtures and perfused with aqueous solutions of the fertilizer. The PDE system was solved using orthogonal collocation on finite elements. My chromatographic model accounted for all the important processes occurring in this system, including external mass transfer between the fluid phase and the solid particles, as well as intraparticle diffusion and adsorption of the solute on the pore surface area of the sorbents. To our knowledge, this is the first chromatographic model that accounted explicitly for the presence of two solid phases with widely different pore structures and adsorption capacities. A systematic parametric study was carried out to determine the importance of each system parameter. The adsorption equilibrium parameters and the intraparticle effective diffusivity of ammonium had the most significant effect on environmental performance. To complete the theoretical analysis, I also developed a model to describe the saturation and drainage of water from the packed column. The model accounted for all the important processes occurring in this system: (a) water exchange between the interstitial pore region and two different smaller pore regions and (b) water flow inside the larger pore region and the two different smaller pore regions. The transient mass balances led to a system of partial differential equations that was solved using block centered finite difference.