Dissertations / Theses on the topic 'Flow gradients'

Nano-scale fluid flow is unlike transport on the macro-scale. Pressure gradients typically dominate effects on a large scale while thermal gradients contribute negligibly to the motion of fluid. The situation entirely reverses on the nano-scale. At a microscopic level, flows induced by thermal gradients are caused by forces that act on atoms or molecules near an interface. These thermo-osmotic forces cannot, at present, be derived analytically or measured experimentally. Clearly, it would be useful to calculate these forces via molecular simulations, but direct approaches fail because in the steady-state, the average force per particle vanishes, as the thermo-osmotic force is balanced by a gradient in shear stress. In our journey to indirectly calculate the osmotic force, we met another unknown in the field of molecular theory at interfaces: the microscopic pressure tensor. The latter is an open problem since the microscopic pressure near an interface is not uniquely defined. Using local thermodynamics theories, we relate the thermo-osmotic force to the gradient of the microscopic pressure tensor. Yet, because the pressure is not uniquely defined, we arrive at multiple answers for the thermo-osmotic force, where at most one can be correct. To resolve the latter puzzle, we develop a direct, non-equilibrium simulation protocol to measure the thermo-osmotic force, whereby a thermal gradient is imposed and the osmotic force is measured by eliminating the shear force. Surprisingly, we find that the osmotic force cannot be derived from the gradient of well-known microscopic pressure expressions. We, therefore, derive a thermodynamic expression that gets close. In this work, we report the first, direct calculation of the thermo-osmotic force while simultaneously showing that standard microscopic pressure expressions fail to predict pressure gradients.

Internal combustion engines are significant contributors to air pollution. To meet the future legislative particu1ate matter (PM) emissions targets for diesel engines there is a need for aftertreatment of the exhaust gases. Previous investigations have shown that diesel particulate filters (DPFs) are a potential exhaust aftertreatment technology for the reduction of PM emissions. DPF systems generally contain two elements; one or more filters (i.e. porous media) and a means of regenerating (i.e. cleaning) the filter(s). The filter must be regenerated intermittently or continuously to prevent imposing high exhaust back pressures on the engine. This thesis presents the study of fluid flow through Gelcast ceramic foams that are a potential candidate filter material for use in DPF systems.

A systematic asymptotic analysis is used to devise a model for distributed boundary-layer receptivity for flow fields with pressure gradients. The model predicts the generation of a Tollmien-Schlichting (TS) wave due to the interaction of a time-harmonic free-stream disturbance with distributed wall waviness and the subsequent evolution of this TS wave under the influence of wall waviness. The model is restricted to two dimensions and the Reynolds number is assumed to be large. Unlike previous models, the present model allows a nonzero pressure gradient in the base flow, and accounts for nonparallel-flow effects. A Green's-function approach was employed. The interaction between the free-stream disturbance and a point source at the wall was first examined in a local region near the point source. Only the component corresponding to the largest-growing instability wave was determined. The downstream evolution of this wave was then investigated over a region that extended for many TS wavelengths. For this reason it was necessary to account for the nonparallel-flow effects. By solving for the dispersion relation in this downstream region, the evolution of the eigenmode was determined. A match between the local-region and the downstream solution led to a uniform approximation for the TS wave emerging from the point source. Summing over a region of point-source solutions and approximating the resulting integral using the method of steepest descents yielded the instability wave for the wavy wall. The principal results of the present work included a solution to the dispersion relation for a general base flow with a nonzero pressure gradient beyond leading order. A region of maximal growth for TS waves that were generated from point sources located in a region of width O (Re-3/16) surrounding the lower branch neutral stability point (LBNSP) was identified. Here Re is the global Reynolds number. For adverse pressure gradients, it was also determined that, when the wall waviness and the TS wave are in exact resonance, the wave produced from distributed receptivity is significantly smaller than the wave generated by a point source at the LBNSP. For the case of a strong favorable pressure gradient the reverse is true. Finally, an investigation of the effect of pressure gradient on detuning was carried out. It showed that complete detuning occurs when the relative difference between the wavenumber of the TS wave at the LBNSP and the wavenumber of the sinusoidal wall is O (Re-3/16). It also revealed that the response to detuning varies with pressure gradient. For situations with a strong favorable pressure gradient, growth rates are highly sensitive to an exact match between the wavenumbers of the wall and the TS wave. On the other hand, as the pressure gradient grows less favorable, sensitivity to detuning decreases.

Shallow groundwater flow in the critical zone of steep headwater mountain catchments is often assumed to mimic surface topography. However, groundwater flow is influenced by other variables, such as the elevation of the water table and subsurface hydraulic conductivity, which can result in temporal variations in both magnitude and direction of flow. In this study, I investigated the temporal variability of groundwater flow in the soil zone (solum) within the critical zone of a headwater catchment at the Hubbard Brook Experimental Forest in North Woodstock, NH. Groundwater levels were continuously monitored throughout several seasons (March 2019 to Jan 2020) in a network of wells comprising three hillslope transects within the upper hillslopes of the catchment. Five clusters of three wells per cluster were screened from 0.18 – 1.1 m depth at the base of the solum. Water levels were also monitored in five deeper wells, screened from 2.4 - 6.9 m depth within glacial sediments of the C horizon. I conducted 47 slug tests across the well network to determine hydraulic properties of the aquifer materials surrounding each well. In addition, our team conducted a large-scale auger investigation mapping soil horizon depths and thicknesses. Results show that the magnitude of hydraulic gradients and subsurface hydrologic fluxes varied at each site with respect to changing water-table elevation, having a maximum range of 0.12 m/m and 9.19 x 10-6 m/s, respectively. The direction of groundwater flow had an arithmetic mean deviating from surface topography by 2-10 degrees, and a total range that deviated from surface topography by as much as 51 degrees. During lower water table regimes, groundwater flow direction deviated from the ground surface, but under higher water table regimes, in response to recharge events, flow direction mimicked surface topography. Within most of the well clusters, there is an observable connection between the slope direction of the top of the C horizon and the direction of groundwater flow during lower water table regimes. Slug test results show the interquartile range of saturated hydraulic conductivity (K¬sat¬) within the C horizon (1.5×10-7 to 9.8×10-7 m/s) is two orders of magnitude lower than the interquartile range of K¬sat¬ values within the solum (2.9×10-5 to 5.2×10-5 m/s). Thus, the C horizon is on average a confining unit relative to the solum that may constrict groundwater flow below the solum. Additionally, results from the larger scale auger investigation suggest that deviations in subsurface topography of the C horizon may be generalizable at the larger hillslope scale. Overall, these results indicate that 1) shallow groundwater flow direction and magnitude within this headwater catchment are dynamic and can deviate from surface topography, and 2) the subsurface topography of the C horizon can influence groundwater flow direction. These results imply that temporal dynamics of groundwater flow direction and magnitude should be considered when characterizing subsurface flow in critical zone studies. Additionally, knowledge of subsurface topography of confining units may provide constraints on the temporal variability of groundwater flow direction.
M.S.
Streams that originate at higher elevations (defined as headwater streams) are important drinking water sources and deliver water and nutrients to maintain freshwater ecosystems. Groundwater is a major source of water to these streams, but little is known about how groundwater flows in these areas. Scientists delineate watersheds (areas of land that drain water to the same point) using surface topography. This approach works well for surface water, but not as well for groundwater, as groundwater may not flow in the same direction as surface water. Thus, assuming that the ground-watershed is the same as the surface watershed can lead to errors in hydrologic studies. To obtain more accurate information about groundwater flow in headwater areas, I continuously measured groundwater levels in forest soils at the Hubbard Brook Experimental Forest in North Woodstock, NH. My main objective was to determine if there is variability in the direction and amount of groundwater flow. I also measured the characteristics of the soils to identify the thicknesses of soil units and the permeability of those units. I used these data to evaluate the relationship between groundwater flow direction, surface topography, and the permeability of soil units. Overall, I found that groundwater flow direction can differ significantly from surface topography, and groundwater flow direction was influenced by the groundwater levels. When groundwater levels were high (closer to the land surface), groundwater flow was generally in the same direction as surface topography. However, when groundwater levels were lower, flow direction typically followed the slope of the lowest permeability soil unit. These results suggest that scientists should not assume that groundwater flow follows the land surface topography and should directly measure groundwater levels to determine flow direction. In addition, results from this study show that characterizing soil permeability can help scientists make more accurate measurements of groundwater flow.

This master's thesis prepared the programme which is able to follow the trajectories of the movement of people and based on this to create various statistics. In practice it is an effective marketing tool which can be used for instance for customer flow analyses, optimal evaluation of opening hours, visitor traffic analyses and for a lot of other benefits. Histograms of oriented gradients, SVM classificator and optical flow monitoring were used to solve this problem. The method of multiple hypothesis tracking was selected for the association data. The system's quality was evaluated from the video footage of the street with the large concentration of pedestrians and from the school's camera system, where the movement in the corridor was monitored and the number of people counted.

Wakes are present in many engineering flows. These flows include internal flows such as mixing chambers and turbomachinery as well as external flows like flow over high-lift or multi-element airfoils. Many times these wakes are exposed to flow conditions such as adverse pressure gradients and streamline curvature that alter the mean flow and turbulent structure of the wake. The ability to understand how pressure gradients and streamline curvature affects the structure of the wake is essential to predicting how the wake will affect the performance of the application in which it is found. The effects of pressure gradients and curvature of low-speed wakes has been extensively documented. As the transonic flow regime is becoming of more interest as gas speeds in turbomachinery increase this work fills a void in the body of wake knowledge pertaining to curved wakes in high speed flows. An under-resolved direct numerical simulation of transonic wake flow being shed by a cambered airfoil in the presence of adverse pressure gradients and streamline curvature is therefore presented here. It was observed that the turbulence characteristics arising from the cambered airfoil that generates the wake dominate the evolution of the wake for different distances downstream depending on the component of the Reynolds stresses that is being considered. These characteristics dissipated the most quickly in the shear stresses and endured the longest in the tangential normal stresses. Previous work in low-speed wakes has indicated that curvature creates new production terms that translate into asymmetry in the profiles of the wake. Curvature was observed to have limited influence on the evolution of the streamwise normal stresses and an extensive impact on the tangential normal stresses. The transport of the Reynolds shear stresses indicate that the asymmetry in this stress is caused indeed by curvature but through turbulent diffusion and not production. The k-ε turbulence model overpredicted the effect of curvature on the turbulence stresses in the wake. This led to accelerated wake decay and spread compared to the UDNS data.

The main focus of urban stormwater runoff disposal has traditionally been to provide structurally-sound drainage systems to carry runoff from many different surfaces without considering water quality at outfall. This has contributed to the decline of water quality in rivers and lakes and other receiving bodies. According to Lord (1987), "stormwater management is primarily concerned with limiting future flood damages and environmental impacts due to development, where as flood control aims at reducing the extent of flooding that occurs under current conditions". Recent developments in stormwater pollutant trap (SPTs), which are generally end-of-the-line devices designed to capture and store gross pollutants, for subsequent removal and disposal.During the last few decades, use of SPTs as a source of collecting and removing pollutants from stormwater (which carries many different types of chemicals and nonchemical pollutants that contaminates our rivers, lakes and other receiving bodies) has increased considerably. Wide-ranging efforts and attempts have been made in both academic and industrial research to improve the quality of stormwater by improving the use of gross pollutant traps (GPTs – known as hydrodynamic separators) by utilising and improving available experimental and modelling techniques. The use of vortex phenomena has always been a challenging problem and available data is rare and complicated in the literature. This research focuses on detailed investigation by experimental means. The generated vortex in this experiment is created in a cylindrical chamber above the level of a cylindrical screening basket. In addition, the research analyses the processes involved in this separation technique.One scale model of a Versa Trap (Type A) was experimentally analysed to investigate and establish the relationship between headloss and flow rate and hydraulic characteristics of a weir in a diversion weir pit. The Versa trap Type A storm pollutant traps are usually used as off-line traps in city and urban areas to capture and store debris – especially those which are captured from surfaces such as rooftops, paved streets, highways, parking lots, lawns, and paved and gravelled roads (Allison et al., 1998). The Versa Trap Type A utilises an upstream diversion weir pit to divert the design treatment flow (DTF) into the treatment chamber. Treated flow returns to the diversion pit downstream of the weir, where it re-enters the drainage system. Peak flow in excess of the DTF bypasses the SPT over the weir into the pipeline downstream.It has been demonstrated that the aggregate of all flows of three months average recurrence interval (ARI) and less represented the majority (up to 97.5%) of the total flow generated by a stormwater drainage catchment (Works, 2006). There is some conjecture as to the veracity of the ‘first flush’ theory, which holds that most of the pollutants in the catchments are transported during the first flush of the storm event (Lee et al., 2007). However, it is generally accepted that SPTs should be sized so as to treat only a portion of the peak flow, with excess flows bypassing the trap. The three month ARI peak flow is commonly taken as appropriate for establishing the minimum DTF required of the SPT.The measurement of headloss across a scale model of a VT Type A storm pollutant trap at a range of flow rates through the SPT, provide data from which a mathematical relationship between flow rate and the headloss cab be established for the device.The resultant relationship then can be used in another part of the experiment to establish the hydraulic characteristics of a weir across a cylindrical chamber, as used for the upstream diversion weir pit in conjunction with the Type A VT range of SPTs. By varying the weir height in a scale model of a diversion weir pit and measuring the flow rates associated with headlosses determined from the previously established relationship, the relationship between weir height and diverted flow can be established. This allows the designer to specify the weir height required to divert the flow rate associated with a specific peak flow or treatment flow of SPT design.Two main characteristics which determine the performance of a gross pollutant trap are trapping efficiency and required maintenance. The trapping efficiency is defined as the portion of the total mass of gross pollutant transported by stormwater that is retained by the trap. A low trapping efficiency means that gross pollutants pass through the trap and reach downstream waters. A poorly-maintained trap will be inefficient at trapping pollutants and is also a potential source of pollutants as trapped materials break down.The experiment parts of this project were tested at Curtin University of Technology’s Hydraulic Laboratory. To replicate typical in-situ conditions, the VT Type A was tested for 0, 22, 33, 44, 55, 66 and 77% simulated blocked screen conditions for trapping efficiency. Data analysis has demonstrated that the headloss increases in proportion to flow rates and screen blockage condition. The results were scaled up to provide data on the full range of unit sizes. This research describes the testing and scaling methodologies in detail, with graphical representation of headloss and other hydraulic parameters at various conditions. The study’s findings have capabilities to optimise any other types of stormwater treatment systems. These types of traps’ are used in commercial and residential environment.This experiment is in continuation of the experiment which was conducted by Muhammad Ismail on industrial gross pollutant traps using double basket to trap the debris for industrial application.Also another good reference for pollutant build up and wash off modelling of impervious surfaces in Perth area, is done by Saadat Ashraf in his PhD thesis. For more information refer to references.

En el caso del almacenamiento de los residuos radioactivos los flujos osmóticos pueden ser relevantes y requieren un análisis en detalle. El residuo nuclear bituminizado (BW) será almacenado mediante contenedores en cavidades excavadas en la Boom Clay, que es una arcilla marina que presenta propiedades favorables para limitar y retrasar la migración de los contaminantes radioactivos. La interacción entre los dos materiales es un proceso acoplado químico-hidro-mecánico y depende de la respuesta hidromecánica de la Boom Clay y del BW. En condiciones de almacenamiento, el contacto del BW, que contienen cantidades importantes de NaNO3, con el agua subterránea induce la hidratación por gradientes osmóticos y el consiguiente hinchamiento, además de la difusión de la sal disuelta hacia la Boom Clay. Se pueden distinguir dos tipos de afecciones: la perturbación geomecánica causada por el hinchamiento del BW y el aumento de presión en el BW y cambio de las distribución de tensiones en la roca, y la perturbación físico química por la migración de grandes cantidades de sales. El objetivo de esta tesis es: (i) Mejorar la comprensión de los procesos que controlan la absorción de agua y el consecuente hinchamiento del BW que contengan sales (NaNO3), y (ii) Investigar los posibles efectos de la concentración de fluidos de los poros sobre el hinchamiento, la compresibilidad y comportamiento de corte de la Boom Clay. En primer lugar, se ha desarrollado una formulación para el análisis de la deformación inducida por la disolución de sales en medio poroso con contacto con agua. Las ecuaciones planteadas incluyen los flujos acoplados de agua y soluto. Se presenta también un trabajo teórico que ayuda a la comprensión del comportamiento mecánico del BW. Se considera este material como una mezcla de bitumen y cristales de NaNO3. Se ha desarrollado un modelo elasto-viscoplástico que describe el comportamiento de fluencia del BW considerando el comportamiento de fluencia de sus constituyentes. El modelo constitutivo elasto-viscoplástico ha sido implementado en el programa CODE_BRIGHT. Los resultados se han comparado con observaciones experimentales. Se ha estudiado el comportamiento a largo plazo del BW en contacto con agua al simular ensayos de hinchamiento por absorción de agua bajo condiciones confinadas. El análisis numérico ha demostrado ser capaz de proporcionar una representación satisfactoria de los principales patrones observados en su comportamiento. En lo que respecta al segundo objetivo de la tesis, se ha propuesto una formulación para el análisis de las deformaciones inducidas por procesos osmóticos en un medio poroso de doble estructura. Esta formulación distingue dentro del material un nivel micro-estructural y otro macro-estructural con cambios químicos que tienen un efecto significativo en la micro-estructura. Se han obtenido las ecuaciones básicas que describen los flujos acoplados de agua y solutos y el transporte de sus componentes a través de los macroporos así como las ecuaciones de balance de masa para agua y soluto en los macroporos y microporos. La formulación propuesta ha sido aplicada particularmente para analizar cualitativamente el efecto de la succión osmótica sobre el hinchamiento de los suelos arcillosos. Se han analizado los efectos a corto y largo plazo. Se ha investigado también la influencia del aumento de la concentración del fluido en los poros sobre las propiedades geotécnicas y el comportamiento de la Boom Clay no saturada. Se ha llevado a cabo un programa sistemático de investigación experimental, con control de succión osmótica y matricial, con el fin de investigar el efecto del incremento de la concentración del fluido de poros sobre la resistencia de corte y el cambio volumétrico bajo condiciones edométricas. Se ha observado, que bajo condiciones parcialmente saturadas, un cambio en la salinidad provoca una disminución en la compresibilidad y en la resistencia de corte del material.
For deep storage of high-level nuclear waste osmotic flows can be significant and so require a careful analysis. In Belgium, The bituminized nuclear waste (BW) named Eurobitum contained in metallic drums will be placed inside a tunnel or a shaft excavated in the Boom Clay, which is 100 m thick marine clay presenting favourable properties to limit and delay the migration of the leached radionuclides over extended periods of time. In Geological disposal conditions, contact of the bituminized radioactive waste which contains high amounts of highly soluble salt (NaNO3) with groundwater will result in water uptake and swelling of the waste and in subsequent diffusion of the dissolved salt through the host clay formation. Basically, two types of disturbance can be distinguished: A geo-mechanical perturbation, caused by the swelling of the waste and the increase of the pressure in and around the waste and a physico-chemical perturbation by the release of large amounts of NaNO3 and other soluble salts. In this context the aim of this thesis is: (i) to improve the understanding of the processes controlling the water uptake and the subsequent swelling of bituminized waste containing soluble salts (NaNO3), and (ii) to investigate of the possible effects of the increase of pore fluid concentration on swelling, compressibility and shear behaviour of Boom Clay. A formulation has been proposed for the analysis of deformation induced by dissolution of salts in porous media in contact with water. The equations include the effect of coupled transport phenomena and the formulation has been included as an extension in the coupled THM program CODE_BRIGHT. A theoretical and experimental work aiming at understanding the mechanical behaviour of the Bituminized Waste has been presented.This material is considered for this purpose as a mixture of bitumen and crystals of NaNO3. An elasto-viscoplastic model has been developed that describes the creep behaviour of BW considering the constituents' creep behaviour. The elasto-viscoplastic constitutive model has been implemented into CODE_BRIGHT. The modelling results have been compared with the experimental data. The impact of osmotic forces on the swelling of the material has been investigated by simulating water uptake swelling tests under confined conditions and comparing the predictions with experimental results. The numerical analysis has proven to be able to furnish a satisfactory representation of the main observed patterns of the behaviour. In regard to the second objective of this thesis, a formulation has been proposed for the analysis of deformations induced by osmotic processes in double structure porous media. The formulation is based on the distinction within the material of a microstructural and a macrostructural levels with chemical changes having a significant effect on the microstructure. A macroscopic description of the system is provided. Then the basic equations describing coupled flows of water and solutes and the transport of its components through macropores and mass balance equations for water and solute in macro and micro pores have been obtained. The proposed formulation has been particularly applied to analyze qualitatively the effect of osmotic suction on swelling of clayey soils. Transient and long term effects have been analyzed. The influence of pore fluid concentration on the geotechnical properties and behavior of Boom Clay under partially saturated conditions has been investigated. A systematic experimental research program involving osmotic suction and matric suction controlled experiments has been carried to investigate the effect of the increase of pore fluid concentration on shear strength and on the volume change behaviour under odometer stress state conditions. It has been observed that under partially saturated conditions a change in salinity causes a decrease in compressibility and shear strength.

This PhD research has proposed novel computer vision and machine learning algorithms for the problem of video based anomalous event detection of individuals. Varieties of Hidden Markov Models were designed to model the temporal and spatial causalities of crowd behaviour. A Markov Random Field on top of a Gaussian Mixture Model is proposed to incorporate spatial context information during classification. Discriminative conditional random field methods are also proposed. Novel features are proposed to extract motion and appearance information. Most of the proposed approaches comprehensively outperform other techniques on publicly available datasets during the time of publications originating from the results.

The vast majority of rivers in the developed world are affected by human alteration, which in turn negatively affects the species that rely on these highly diverse and species rich areas to survive. Homogenization of previously heterogenic areas due to a change in water discharge and the substrate availability in the rivers is often regarded as the main reason for the loss of species richness in rivers and the riparian zone. Because of this, there are a lot of restoration projects which main goal is to increase heterogeneity. The goal of this thesis is to compare hydrology and geomorphology variables between four types of reaches with high flow velocity (rapids in free-flowing reaches, rapids with regulated flow, impounded reaches and reaches in outlet channels below hydropower plants). In this thesis I wanted to find out how changes in geomorphology and hydrology affect fish species richness, riparian vegetation richness and aquatic vascular plants species richness. The result show that both fish species richness and riparian vegetation species richness are negatively affected by the changes in geomorphology and hydrology. There was no significant difference for aquatic vascular plants when comparing the regulation types. The most altered reach type, outlet channels, had significantly fewer fish species compared to the less altered reach type, impounded reaches. Outlet channels also had less riparian vegetation cover compared to all other regulation types, and fewer riparian vegetation species compared to both free-flowing reaches and impounded reaches. In conclusion, hydropower plants and timber floating have both negatively impacted the riverine ecosystem, and in turn caused a decrease in species richness for fish and riparian plants. The reaches studied in this thesis will continue to be altered and the species richness and species composition will change from an ecosystem that relied on the natural flow regime to an ecosystem more used to the flow regime created by the hydropower plants.

On s'intéresse à la géométrie globale et asymptotique de certaines variétés riemanniennes non compactes. Dans une première partie, on étudie la topologie et la géométrie à l'infini des variétés riemanniennes à courbure (de Ricci) positive ayant un rapport asymptotique de courbure fini. On caractérise le cas non effondré via la notion de cône asymptotique et on donne des conditions suffisantes sur le groupe fondamental pour garantir un non effondrement. La seconde partie est dédiée à l'étude des solutions de Type III du flot de Ricci à courbure positive et aux solitons gradients de Ricci expansifs (points fixes de Type III) présentant une décroissance quadratique de la courbure. On montre l'existence et l'unicité des cônes asymptotiques de tels points fixes. On donne également des conditions suffisantes de nature algébrique et géométrique pour garantir une géométrie de révolution de tels solitons. Dans une troisième partie, on caractérise la géométrie des solitons gradients de Ricci stables à courbure positive et à croissance volumique linéaire. Puis, on s'intéresse au non effondrement des variétés riemanniennes de dimension trois à courbure de Ricci positive ayant un rapport asymptotique de courbure fini
We study the global and asymptotic geometry of non-compact Riemannian manifolds. First, we study the topology and geometry at infinity of Riemannian manifolds with nonnegative (Ricci) curvature and finite asymptotic curvature ratio. We focus on the non-collapsed case with the help of asymptotic cones and we give sufficient conditions on the fundamental group to guarantee non-collapsing. The second part is dedicated to the study of (non-negatively curved) Type III Ricci flow solutions. We mainly analyze the asymptotic geometry of Type III self-similar solutions (expanding gradient Ricci soliton) with finite asymptotic curvature ratio. We prove the existence and uniqueness of their asymptotic cones. We also give algebraic and geometric sufficient conditions to guarantee rotational symmetry of such metrics. In the last part, we characterize the geometry of steady gradient Ricci solitons with nonnegative sectional curvature and linear volume growth. Finally, we study the non-collapsing of three dimensional Riemannian manifold with nonnegative Ricci curvature and finite asymptotic curvature ratio

Thesis (Ph.D.)--Case Western Reserve University, 2004
Title from PDF (viewed on 01 October 2009) Department of Mechanical Engineering Includes abstract Includes bibliographical references Available online via the OhioLINK ETD Center

Nous étudions des équations de type Cahn-Hilliard, équation qui fut introduite pour décrire la séparation de phases dans les systèmes multi-composants. Les résultats obtenus dans ce travail ont été motivés par des applications biologiques, notamment la formation de tissus et la croissance de tumeurs, ainsi que par des applications physiques, tels que les écoulements de fluides impliquant des phénomènes de tension de surface. La première partie de cette thèse est une analyse de la relation entre l'équation de Cahn-Hilliard et les modèles Hele-Shaw, qui sont fréquemment utilisés pour modéliser l'écoulement des fluides ou l'évolution de tumeurs cancéreuses dans des espaces confinés. Nous examinons en particulier comment obtenir les modèles Hele-Shaw dans la limite dite incompressible de l'équation de Cahn-Hilliard. La deuxième partie de la thèse se concentre sur l'étude de l'équation de Cahn-Hilliard non-locale (et ses variations) et sa convergence vers l'équation locale. Cette équation, qui peut être dérivée rigoureusement à partir d'un système de particules en interaction, est obtenue en remplaçant le laplacien, qui est un terme local, par une approximation non-locale prenant en compte les interactions à longue distance entre les composants. Nous montrons que la solution de l'équation non-locale converge vers la solution de l'équation locale dans la limite d'interaction à courte distance. La troisième partie de la thèse se penche sur l'étude des modèles de fluides plus classiques, tels que les équations d'Euler et de Navier-Stokes, qui intègrent des phénomènes de tension de surface. Ces modèles sont utilisés pour décrire les écoulements de fluides ou les mouvements de cellules dans lesquels les forces interfaciales jouent un rôle important. La quatrième partie juxtapose la théorie cinétique, traditionnellement employée pour la représentation de phénomènes physiques à une échelle mésoscopique, avec l'équation de Cahn-Hilliard. Notre étude se concentre spécifiquement sur l'équation de Vlasov-Cahn-Hilliard, qui décrit les processus de transition de phase
We study Cahn-Hilliard type equations, an equation that was introduced to describe phase separation in multi-component systems. The results obtained in this work have been motivated by biological applications, such as tissue formation and tumor growth, as well as physical applications, such as fluid flows involving surface tension phenomena. The first part of this thesis is an analysis of the relationship between the Cahn-Hilliard equation and the Hele-Shaw models, which are frequently used to model fluid flow or the evolution of cancerous tumors in confined spaces. In particular, we examine how to obtain Hele-Shaw models in the so-called incompressible limit of the Cahn-Hilliard equation. The second part of the thesis focuses on the study of the non-local Cahn-Hilliard equation (and its variations) and its convergence to the local equation. This equation, which can be derived rigorously from a system of interacting particles, is obtained by replacing the Laplacian, which is a local term, by a non-local approximation taking into account the long range interactions between the components. We show that the solution of the non-local equation converges to the solution of the local equation in the short range interaction limit. The third part of the thesis focuses on the study of more classical fluid models, such as the Euler and Navier-Stokes equations, which incorporate surface tension phenomena. These models are used to describe fluid flows or cell motions in which interfacial forces play an important role. The fourth part juxtaposes kinetic theory, traditionally used to represent physical phenomena on a mesoscopic scale, with the Cahn-Hilliard equation. Our study focuses specifically on the Vlasov-Cahn-Hilliard equation, which describes phase transition processes

Environmental noise has for decades been a well known problem, especially in urban areas. As noise requirements for vehicles are sharpened, noise reducing concepts are needed in early design stages requiring accurate simulations to support the design. Specifically for optimization of noise treatments, the absorber performance must be simulated correctly. So called noise encapsulations are placed below the powertrain on heavy vehicles to enclose the engine and reduce noise radiation. The attenuation of the absorbers on these shields must be represented correctly in simulations, even in environments with complex sound field, cooling flow and high temperature variations which may affect the absorber performance. This thesis studies the performance variation due to different absorber representations and due to these factors and how to include this in simulations. It is shown that the material representation significantly affects the attenuation performance in the simulations. Assuming locally reacting absorbers neglects the full interaction between the sound field and the material, which was shown to affect the noise reduction considerably. A measurement method to determine the angular dependent surface impedance was evaluated. It was shown sensitive to small samples and a method to improve accuracy was suggested. Including the angular dependence, either by full resolution or an angular dependent impedance, the field-absorber interaction is included in the simulations and more accurate results are obtained. The influence of flow and temperature fields on the absorber performance was also investigated. A method to include these effects was developed and the attenuation performance shown significant, especially for materials with bulk reaction. In conclusion, thorough knowledge of the material behavior and the field in the applications is required to choose appropriate material representation to enable reliable simulation results.​

QC 20160311

Ce manuscrit présente l'étude et l'analyse d'écoulements de gaz raréfiés, induits par la transpiration thermique. Le terme de transpiration thermique désigne le mouvement macroscopique d'un gaz raréfié engendré par l'effet du seul gradient de température. L'aspect principal de ce travail est centré autour de la mesure du débit stationnaire déclenché en soumettant un micro tube à un gradient de température appliqué le long de son axe. On a développé à cet effet un appareillage expérimental original ainsi qu'une méthodologie expérimentale innovatrice basée sur la dépendance du phénomène, analysé dans son ensemble, à l'égard du temps. Les résultats obtenus pour le débit stationnaire initial de transpiration thermique et pour les paramètres thermo-moléculaires caractérisant l'équilibre final de débit nul, ont été comparés aux résultats obtenus numériquement par la résolution de l'équation cinétique modèle de Shakhov et par la méthode de simulation directe de Monte-Carlo
This thesis presents the study and analysis of rarefied gas flows induced by thermal transpiration. Thermal transpiration refers to the macroscopic movement of rarefied gas generated by a temperature gradient. The main aspect of this work is centered around the measurement of the mass flow rate engendered by subjecting a micro-tube to a temperature gradient along its axis. In this respect, an original experimental apparatus and an original time-dependent experimental methodology was developed. The experimental results for the initial stationary thermal transpiration mass flow rate and for the final zero- flow thermal molecular parameters were compared with the results obtained from the numerical solution of the Shakhov model kinetic equation and the direct simulation Monte Carlo method

This thesis deals with an automatic counting of cylinders placed on the back of a truck using images taken by a camera mounted above the car. To achieve this goal, an SVM classifier based on HOG image descriptors has been trained to detect the cylinders. Further, a tracking method based on optical flow estimation has been designed to track the cylinders through image sequences. The result of the thesis is an application that counts bottles with precision 93,08 % placed on the truck and visualizes results of the detection.

L'objectif de cette thèse est de fournir des modèles et des outils de simulation pour décrire les échanges de masse entre les circuits de ventilation (galeries) et les milieux poreux des ouvrages souterrains d'enfouissement des déchets nucléaires. La modélisation prend en compte le couplage à l'interface poreux-galerie entre les écoulements liquide gaz compositionnels dans le milieu poreux constituant le stockage et les écoulements gazeux compositionnels dans le milieu galerie libre
The objective of this thesis is to develop models and algorithms to simulate efficiently the mass exchanges occurring at the interface between the nuclear waste deep geological repositories and the ventilation excavated galleries. To model such physical processes, one needs to account in the porous medium for the flow of the liquid and gas phases including the vaporization of the water component in the gas phase and the dissolution of the gaseous components in the liquid phase. In the free flow region, a single phase gas free flow is considered assuming that the liquid phase is instantaneously vaporized at the interface. This gas free flow has to be compositional to account for the change of the relative humidity in the free flow region which has a strong feedback on the liquid flow rate at the interface

Strongly coupled systems are elusive and not suitable to be described by conventional perturbative approaches. However, they are ubiquitous in nature, especially in particle physics. The lattice formulation of quantum field theories provided a unique framework in which the physical content of these systems could be precisely determined. Combined with numerical techniques, the lattice formalism allowed to precisely determined physical quantities describing the thermodynamics, as well as the spectroscopy of strongly interacting theories. In this work, the lattice formulation has been employed to probe the effectiveness of a recently proposed method, which aims at determining the renormalized energy-momentum tensor in non perturbative regimes. The latter plays a fundamental role to quantitatively describe the thermodynamics and fluid-dynamics of hot, dense systems, or to characterize theories that enlarge the actual standard model. In all these aspects, only a non perturbative approach provides physically reliable results: hence a non perturbative determination of the energy momentum tensor is fundamental. The new method consists in defining suitable lattice Ward identities probed by observables built with the gradient flow. The new set of identities exhibits many interesting qualities, arising from the UV finiteness of such probes, and allows to define a numerical strategy for estimating the renormalization constants of the lattice energy-momentum tensor. In this work the method has been tested within two different quantum theories, with the purpose of understanding its effectiveness and reliability.

We report on a scale determination with gradient-flow techniques on the N-f = 2 + 1 + 1 highly improved staggered quark ensembles generated by the MILC Collaboration. The ensembles include four lattice spacings, ranging from approximately 0.15 to 0.06 fm, and both physical and unphysical values of the quark masses. The scales root t(0)/a and their tree-level improvements, root t(0,imp) and root w(0,imp), are computed on each ensemble using Symanzik flow and the cloverleaf definition of the energy density E. Using a combination of continuum chiral-perturbation theory and a Taylor-series ansatz for the latticespacing and strong-coupling dependence, the results are simultaneously extrapolated to the continuum and interpolated to physical quark masses. We determine the scales root t(0) = 0.171416((18)(-5)) fm, and w(0) =.174 ((-15)(-12)), where the errors are sums, in quadrature, of statistical and all systematic errors. The precision of w(0) and root t(0) is comparable to or more precise than the best previous estimates, respectively. We then find the continuum mass dependence of root t(0) and w(0), which will be useful for estimating the scales of new ensembles. We also estimate the integrated autocorrelation length of < E(t)>. For long flow times, the autocorrelation length of < E > appears to be comparable to that of the topological charge.

For this dissertation I studied flow and transport in low gradient Florida streams. Chapter 2 is a statewide analysis of long-term variations in stream discharge. The results from Chapter 2 suggest that changes in mean annual stream discharge are controlled by the Atlantic Multi-Decadal Oscillation (AMO). During the warm phase, mean annual discharge decreases in central Florida and increases in north Florida. The opposite is true during the cool phase, with mean annual discharge increasing in central Florida and decreasing in north Florida. This pattern is observed for both components of stream discharge, base flow and runoff. The following two chapters are part of an analysis of particle transport in low gradient mangrove estuaries. Chapter 3 describes the use of a numerical model to simulate the hydrodynamics of a coastal reach of the Shark River, Florida Everglades and the development of a Lagrangian particle tracking model. The particle tracking model uses the output from the hydrodynamic model to simulate the movement of particles released within the model domain. In Chapter 4, the hydrodynamic and particle tracking models are used to estimate the historical particle residence time in the Shark River Slough Estuary (SRSE) and determine the key factors controlling particle residence time and fate in mangrove estuaries. The mean and median residence times in the model domain are 16 and 8 hours, respectively, and 60% of all particles exit the model domain downstream, towards the Gulf of Mexico. Particle residence time varies greatly depending on the particle release location and timing. The residence time is significantly lower for particles released in the middle of the channel and for particles released during the wet season, spring tides or during upstream flows. Additionally, there is a decreasing trend in mean particle residence time from 1997 through 2017, mirroring an increasing trend in mean annual water levels in the SRSE. The combined results of this dissertation show the impact that a variable climate can have on stream flow and particle transport.

Cette thèse porte sur la modélisation et la discrétisation d’écoulements Darcy dans les milieux poreux fracturés. Nous suivons l’approche des modèles, dits à dimensions hybrides, qui représentent les réseaux de fractures comme des surfaces de codimension 1 immergées dans la matrice. Les modèles considérés prennent en compte les interactions entre matrice et fractures et permettent de traiter des fractures agissant comme conduites ou comme barrières, ce que nécessite de prendre en compte les sauts de pression aux interfaces matrice-fracture. Dans le cas des écoulements diphasiques, nous proposons des modèles, qui prennent en compte les sauts de saturations aux interfaces matrice-fracture, dû à la capillarité. L’analyse numérique est menée dans le cadre général de la méthode de discrétisations gradients, qui est étendue aux modèles considérés. Deux familles de schémas numériques, le schéma Vertex Approximate Gradient et le schéma Volumes Finis Hybrides sont adaptées aux modèles à dimensions hybrides. On prouve via des résultats de densité que ce sont des schémas gradients, pour lesquels la convergence est établie. En diphasique, l’existence d’une solution est obtenue en passant. Plusieurs cas tests sont présentés. En monophasique, on observe la convergence sur des différents types de mailles pour une famille de solutions dans un milieux fracturé hétérogène et anisotrope. En diphasique, nous présentons une série de cas tests afin de comparer les modèles à dimensions hybrides au modèle de référence, dans lequel les fractures ont la même dimension que la matrice. A part quantifier le gain en performance de calcul, ces tests montrent la qualité des différents modèles réduits
This thesis investigates the modelling of Darcy flow through fractured porous media and its discretization on general polyhedral meshes. We follow the approach of hybrid dimensional models, invoking a complex network of planar fractures. The models account for matrix-fracture interactions and fractures acting either as drains or as barriers, i.e. we have to deal with pressure discontinuities at matrix-fracture interfaces. In the case of two phase flow, we present two models, which permit to treat gravity dominated flow as well as discontinuous capillary pressure at the material interfaces. The numerical analysis is performed in the general framework of the Gradient Discretisation Method, which is extended to the models under consideration. Two families of schemes namely the Vertex Approximate Gradient scheme (VAG) and the Hybrid Finite Volume scheme (HFV) are detailed and shown to fit in the gradient scheme framework, which yields, in particular, convergence. For single phase flow, we obtain convergence of order 1 via density results. For two phase flow, the existence of a solution is obtained as a byproduct of the convergence analysis. Several test cases are presented. For single phase flow, we study the convergence on different types of meshes for a family of solutions. For two phase flow, we compare the hybrid-dimensional models to the reference equidimensional model, in which fractures have the same dimension as the matrix. This does not only provide quantitative evidence about computational gain, but also leads to deep insight about the quality of the proposed reduced models

[EN] This Thesis is focused on the development and implementation of efficient numerical methods for the acoustic modelling and design of noise control devices in the exhaust system of combustion engines. Special attention is paid to automotive perforated dissipative silencers, in which significant differences are likely to appear in their acoustic behaviour, depending on the temperature variations within the absorbent material. Also, material heterogeneities can alter the silencer attenuation performance. Therefore, numerical techniques considering all these features are required to guarantee the accuracy of the results. A literature review is carried out, mainly related to one-dimensional models, as well as to acoustic models for absorbent materials and perforated surfaces. However, plane wave model limitations make indispensable using alternative multidimensional methods. In addition, the possibility of using new acoustic elements is explored. These elements have as an objective being a potential alternative to the fibrous absorbent materials, which can have a negative impact on health. The Thesis considers the use of microperforated and sintered surfaces. The latter have, in some cases, a nearly constant acoustic impedance, whose value depends, among others, on the thickness and porosity of the plates. To avoid the limitations of plane wave models, a finite element (FE) approach is proposed for the acoustic analysis of dissipative silencers including a perforated duct with uniform axial mean flow and an outer chamber with a heterogeneous distribution of the absorbent material. On the other hand, property variations can be also produced by temperature gradients. In this case, a hybrid FE model has been derived for perforated dissipative silencers including: (1) Thermal gradients in the central duct and the chamber; (2) A perforated passage carrying non-uniform axial mean flow. A FE approach has been implemented to solve the pressure-based wave equation for a non-moving heterogeneous medium, associated with the chamber. Also, the governing equation in the central duct has been written and solved in terms of an acoustic velocity potential to allow the presence of an axially inhomogeneous flow. The coupling between both regions has been carried out by means of a perforated duct and its acoustic impedance, adapted here to include absorbent material heterogeneities and mean flow effects. It has been found that the presence of non-homogeneities can have a significant influence on the acoustic attenuation of a silencer and should be included in the theoretical models. Optimization techniques for industrial noise control devices are relevant, since they lead to the production of elements with better characteristics. Evolutionary algorithms are emergent techniques able to obtain a solution, even in those problems in which the traditional optimization have difficulties. Optimization techniques are combined with the FE method to achieve the maximum attenuation in the frequency range of interest. A multichamber silencer optimization problem is defined and several analyses are carried out to obtain the most suitable configuration for each application. Under certain assumptions of axial uniformity, several techniques have been considered to reduce the computational effort of a full 3D FE analysis for dissipative silencers with temperature gradients and mean flow. These are based on a decomposition of the acoustic field into transversal and axial modes within each silencer subdomain, and a matching procedure of the modal expansions at the silencer area changes through the continuity conditions of the acoustic fields. The relative computational efficiency and accuracy of predictions for the matching techniques are studied, including point collocation at nodes and Gauss points and also mode-matching with weighted integration. All of them provide accurate predictions of the attenuation and improve the computational cost of a FE calculation
[ES] Esta Tesis se centra en el desarrollo e implementación de métodos numéricos eficientes para el diseño y modelado de componentes de la línea de escape en motores de combustión interna. Merecen especial atención los silenciadores disipativos perforados de automóviles, ya que su comportamiento acústico puede sufrir variaciones importantes debidas a las variaciones de temperatura en el material absorbente, así como a las heterogeneidades de la fibra. Por tanto, se requieren técnicas numéricas que consideren estos casos para garantizar la precisión de los resultados. Se lleva a cabo una revisión bibliográfica que recoge los modelos de onda unidimensionales, así como modelos acústicos de materiales absorbentes y superficies perforadas. Sin embargo, las limitaciones de los primeros hacen indispensable el uso de modelos multidimensionales. Además se explora la posibilidad de usar nuevos elementos acústicos, cuyo objetivo es ser una alternativa potencial a los materiales absorbentes, que pueden tener un efecto negativo sobre la salud. La Tesis considera el uso de superficies microperforadas y sinterizadas. Estas últimas en algunos casos presentan una impedancia casi constante, cuyo valor depende, entre otras cosas, del espesor y la porosidad de las placas. Para evitar las limitaciones de los modelos de onda plana, se propone un enfoque en elementos finitos (EF) para el análisis acústico de silenciadores disipativos que incluyen un conducto con flujo medio axial uniforme y una cámara externa con una distribución heterogénea de material absorbente. Por otro lado, la variación de las propiedades también puede producirse por gradientes térmicos. En este caso, se propone una formulación híbrida de EF para silenciadores disipativos perforados que incluye: (1) Gradientes térmicos en el conducto central y la cámara; (2) Un conducto perforado que canaliza flujo medio axial no uniforme. Se ha implementado una formulación de EF para resolver la ecuación de ondas en términos de presión para el medio estacionario heterogéneo asociado a la cámara. Además, la ecuación asociada al conducto central, expresada en términos de potencial de velocidad acústica, permite la presencia de flujo axial no uniforme. El acoplamiento entre ambas regiones se ha realizado mediante un conducto perforado y su impedancia acústica y se ha adaptado para incluir la citada falta de homogeneidad. Se ha visto que las heterogeneidades pueden influir notablemente en la atenuación acústica de un silenciador, debiéndose incluir en los modelos teóricos. Las técnicas de optimización para componentes industriales de control de ruido son importantes, ya que producen elementos con mejores características. Los algoritmos evolutivos son técnicas emergentes capaces de obtener una solución, incluso cuando la optimización tradicional tiene dificultades. Las técnicas de optimización se combinan con el MEF para conseguir la máxima atenuación posible en el rango de frecuencias de interés. Se ha definido un problema de optimización de un silenciador multicámara y se han llevado a cabo varios análisis para obtener la configuración más adecuada para cada caso. Bajo ciertas hipótesis de uniformidad axial, se han considerado varias técnicas para reducir el coste computacional de un análisis 3D completo para silenciadores disipativos con gradientes de temperatura y flujo medio. Éstas se basan en la descomposición del campo acústico en modos axiales y transversales dentro de cada subdominio, y un procedimiento de acoplamiento de las expansiones modales en los cambios de sección del silenciador mediante las condiciones de continuidad de los campos acústicos. Se estudia la eficiencia computacional y precisión de las predicciones de las técnicas de acoplamiento, incluyendo colocación puntual en nodos y puntos de Gauss, así como ajuste modal. Todos ellos proporcionan predicciones precisas de la atenuación mejorando el coste
[CAT] Aquesta Tesi es centra en el desenvolupament i implementació de mètodes numèrics eficients per al disseny i modelatge de components de la línia d'escapament en motors de combustió interna. Mereixen especial atenció els silenciadors dissipatius perforats d'automòbils, ja que el seu comportament acústic pot patir variacions importants degudes a les variacions de temperatura en el material absorbent, així com a les heterogeneïtats de la fibra. Per tant, es requereixen tècniques numèriques que considerin aquests casos per garantir la precisió dels resultats. Es porta a terme una revisió bibliogràfica que recull els models d'ona unidimensionals, així com models acústics de materials absorbents i superfícies perforades. No obstant això, les limitacions dels primers fan indispensable l'ús de models multidimensionals. A més s'explora la possibilitat d'usar nous elements acústics amb l'objectiu que siguen una alternativa potencial als materials absorbents, que poden tenir un efecte negatiu sobre la salut. La Tesi considera l'ús de superfícies microperforades i sinteritzades. Aquestes últimes en alguns casos presenten una impedància gairebé constant. El seu valor depèn, entre altres coses, del gruix i la porositat de les plaques. Per evitar les limitacions dels models d'ona plana, es proposa un enfocament amb elements finits (EF) per a l'anàlisi acústic de silenciadors dissipatius que inclouen un conducte amb flux mig axial uniforme i una càmera externa amb una distribució heterogènia de material absorbent. D'altra banda, la variació de les propietats també es pot produir per gradients tèrmics. En aquest cas, es proposa una formulació híbrida d'EF per silenciadors dissipatius perforats que inclou: (1) Gradients tèrmics en el conducte central i la càmera; (2) Un conducte perforat que canalitza flux mig axial no uniforme. S'ha implementat una formulació d'EF per resoldre l'equació d'ones en termes de pressió per al medi estacionari heterogeni associat a la càmera. A més, l'equació associada al conducte central, expressada en termes de potencial de velocitat acústica, permet la presència de flux axial no uniforme. L'acoblament entre les dues regions s'ha realitzat mitjançant un conducte perforat i la seva impedància acústica i s'ha adaptat per incloure la esmentada falta d'homogeneïtat. S'ha vist que les heterogeneïtats poden influir notablement en l'atenuació acústica d'un silenciador i s'han d'incloure en els models teòrics. Les tècniques d'optimització per a components industrials de control de soroll són importants, ja que produeixen elements amb millors característiques. Els algoritmes evolutius són tècniques emergents capaces d'obtenir una solució, fins i tot quan l'optimització tradicional té dificultats. Les tècniques d'optimització es combinen amb el mètode d'elements finits (MEF) per aconseguir la màxima atenuació possible en el rang de freqüències d'interès. S'ha definit un problema d'optimització d'un silenciador multicàmera i s'han dut a terme diverses anàlisis per obtenir la configuració més adequada per a cada cas. Sota certes hipòtesis d'uniformitat axial, s'han considerat diverses tècniques per reduir el cost computacional d'una anàlisi 3D complet per silenciadors dissipatius amb gradients de temperatura i flux mig. Aquestes es basen en la descomposició del camp acústic en modes axials i transversals dins de cada subdomini, i un procediment d'acoblament de les expansions modals en els canvis de secció del silenciador mitjançant les condicions de continuïtat dels camps acústics. S'estudia l'eficiència computacional i precisió de les prediccions de les tècniques d'acoblament, incloent col·locació puntual en nodes i punts de Gauss, així com ajust modal. Tots ells proporcionen prediccions precises de l'atenuació millorant el cost computacional d'EF.
Sánchez Orgaz, EM. (2016). Advanced numerical techniques for the acoustic modelling of materials and noise control devices in the exhaust system of internal combustion engines [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/64090
TESIS

Thesis (M.S.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Electrical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

The evolution of temperature in sedimentary basins is a fundamental tool for the evaluation and exploration of hydrocarbons, for the evaluation of geothermal potential, for paleogeographic reconstruction, for carbon sequestration and for the hydrogeological evaluation of a given region. Estimates of heat flow density (HFD) on the surface in the Portuguese Meso Cenozoic basins are difficult to obtain. The small number of HFD estimates in the Meso Cenozoic basins is a consequence of the high drilling costs for determining HFD and strict drilling regulation measures. Most of the temperature data available for estimating HFD is obtained in oil exploration holes; however, the temperature data obtained from them are subject to high uncertainty. Twelve oil exploration holes carried out in Portugal, with temperature records, were considered in this work; only one hole was rejected because they did not present the minimum quality requirements for HFD estimation. The values of thermal conductivity of the rock formations traversed by the various holes were assumed since there are no laboratory determinations for those geological formations. Bottom temperatures (BHT) have been corrected with Zetaware software that uses the Horner method and produces results with acceptable uncertainties. Only three sedimentary basins (Lusitanian, Porto, Alentejo) were identified and possessing a regional HFD estimates ranging from 61 to 174 mWm-2. The average geothermal gradient and average HFD estimates of the Lusitanian basin were found to be 33 ℃ km-1 and 113 mWm-2, Porto (24 ℃ km-1, 78 mWm-2) and Alentejo (21 ℃ km-1, 61 mWm-2) respectively. Compared to previous geothermal and HFD values, the new estimates obtained had a fair correspondence with a high regional sedimentary HFD estimates. Nevertheless, a heat flow density map was generated and an attempt to geothermally characterize the Portuguese Meso Cenozoic basins is made; RESUMO: CONTRIBUIÇÃO PARA A ESTIMATIVA DA DENSIDADE DO FLUXO DE CALOR NAS BACIAS MESO CENOZÓICAS DE PORTUGAL A evolução da temperatura nas bacias sedimentares é uma ferramenta fundamental para a avaliação e exploração de hidrocarbonetos, para a avaliação do potencial geotérmico, para a reconstrução paleogeográfica, para o sequestro de carbono e para a avaliação hidrogeológica de uma determinada região. Estimativas da densidade do fluxo de calor (DFC) na superfície das bacias Meso Cenozóicas Portuguesas são difíceis de obter. O pequeno número de estimativas de DFC nas bacias Meso Cenozóicas é uma consequência dos elevados custos de perfuração para a determinação do DFC e de medidas rigorosas de regulação da perfuração. A maioria dos dados de temperatura disponíveis para estimar o DFC é obtida em furos de prospeção de petróleo; no entanto, os dados de temperatura neles obtidos estão sujeitos a uma elevada incerteza. Neste trabalho foram considerados doze furos de prospeção de petróleo realizados em Portugal com registos de temperatura; apenas um furo foi rejeitado por não apresentar os requisitos mínimos de qualidade para a estimativa do DFC. Assumiram-se os valores de condutividade térmica das formações rochosas atravessadas pelos diversos furos uma vez que não existem determinações laboratoriais para essas formações geológicas. As temperaturas de fundo de furo (BHT) foram corrigidas com o software Zetaware que utiliza o método de Horner e produz resultados com incertezas aceitáveis. Apenas foram identificadas três bacias sedimentares (Lusitanianas, do Porto, do Alentejo) e com uma estimativa regional de DFC que varia entre 61 e 174 mWm-2. Verificou-se que o gradiente geotérmico médio e a DFC média na bacia Lusitaniana são, respectivamente, 33 ℃km-1 e 113 mWm-2 Porto (24 ℃ km-1, 78 mWm-2) e Alentejo (21 ℃ km-1, 61 mWm-2) respectivamente. Em comparação com valores geotérmicos e de DFC anteriores, as novas estimativas obtidas correspondem a uma DFC sedimentar regional elevada. Foi desenhado um mapa da densidade de fluxo de calor e é feita uma tentativa de caracterizar geotermicamente as bacias Meso Cenozóicas Portuguesas.

Aortic stenosis is the commonest valve disorder in the Western World. The natural history of the disease is very well described; with a decreased survival once symptoms occur. There is currently, no medical therapy available to reduce the natural progression of the aortic stenosis, and therefore, aortic valve replacement has been recognised as the standard of care treatment for symptomatic aortic stenosis, with TAVI having merged as alternative for those cases with high/prohibitive surgical risk. All types of aortic stenosis have in common a reduced valve area (AVA <1.0cm2), but depending on the mean gradients and the stroke volume index, there are different types: Normal-Flow High-Gradient (NFHG AS) and Low-Flow Low-Gradient (LFLG AS) aortic stenosis. The latter is further subdivided into Classical and Paradoxical forms depending on the ejection fraction value. NFHG AS is the most common type. The left ventricle is capable of generating a normal flow through the stenotic valve, that it is translated onto high gradients. This type has been thoroughly studied and has an excellent prognostic with aortic valve replacement, with very low in-hospital mortality and long-term survival similar to the age-matched population. LFLG AS, on the other hand, is defined by a reduced stroke volume (SVi < 35 ml/min) and a low gradient (mean gradient < 40mmHg). The Classical form (CLFLG AS) has impaired ventricular function. These patients have dilated ventricles that are not able to generate enough flow through the stenotic valve and hence the low gradient. Dobutamine stress echocardiography is key for the diagnosis of this subtype, as it differentiates it from the Pseudo-Severe aortic stenosis (in which the problem is not in the aortic valve but in the left ventricle, and therefore there is no benefit from aortic valve replacement) and it has also prognostic value by determining the contractile reserve. These patients have been reported to have the highest mortality post aortic valve replacement and a reduced long-term survival; however, those who survive achieve excellent functional class. The other subtype of LFLG, the Paradoxical form (PLFLG AS) has a preserved ventricular function. These patients have a ventricular restrictive physiology, with reduced SVi due to a combination of mechanism such as subendocardial fibrosis, concentric remodeling, impaired diastolic filling and high afterload. It is paramount here to confirm the diagnosis by accurate echocardiography, ruling out measurement errors and other causes of reduced SVi. PLFLG AS patients have worse prognosis than NFHG AS but better prognosis than CLFLG AS patients. The primary hypothesis of our research project was that aortic valve replacement could be performed in patients with LFLG AS with low in-hospital mortality. Therefore, with the objectives of determining operative and mid-term outcomes of surgical intervention in LFLG AS compared to NFHG AS, we conducted a retrospective analysis of all patients who underwent isolated aortic valve replacement in our centre. Primary end-points were mortality (in-hospital, at one and five years) and the secondary end-points analysed were postoperative complications and clinical status at follow-up. Patients in the LFLG AS group were significantly older and had more cardiovascular risk factors and comorbidities than the NFHG AS group. Despite those differences, in-hospital mortality was equivalent and remarkably low in both groups. As expected, LFLG AS patients had a reduced mid-term survival but those who survived remained in an excellent functional class. With the separate analysis of the LFLG AS subgroups, we confirmed that CLFLG AS had higher in-hospital and mid-term mortality than PLFLG AS patients. In both groups, the in-hospital mortality was remarkably low compared to previous literature reports. Aortic valve replacement provided symptomatic relief and excellent functional class during the mid-term follow-up as well as recovery of the ventricular function in most of the patients. Based on our results, we concluded that aortic valve replacement should be recommended for symptomatic severe LFLG AS.
La estenosis aórtica es la enfermedad valvular más frecuente en el tercer mundo. La historia natural de la enfermedad es bien conocida desde hace décadas, siendo una enfermedad con mal pronóstico a medio-corto plazo que hace necesario someter a estos pacientes a recambio valvular aórtico tras la aparición de síntomas. La forma mas común de estenosis aórtica, con flujo normal y gradiente alto, tiene un pronóstico excelente tras el recambio valvular aórtico, con una supervivencia similar a la de la población normal. Sin embargo, la estenosis aórtica de bajo-flujo y bajo-gradiente, es una entidad menos conocida y de peor pronóstico. Estos pacientes tienen una mortalidad mucho mayor tras recambio valvular aórtico y menor supervivencia a largo plazo. El diagnóstico en el bajo-flujo bajo-gradiente es vital para seleccionar correctamente los pacientes con estenosis aórtica que se beneficiarán de tratamiento quirúrgico, teniendo también valor pronóstico, dependiendo de la categoría de bajo flujo (Clásica o Paradójica) y otros determinantes como la presencia/ausencia de reserva contráctil del ventrículo izquierdo. Nuestra hipótesis fue que la estenosis aórtica de bajo-flujo y bajo-gradiente, pueden ser tratada con recambio valvular aórtico con una mortalidad hospitalaria similar a aquellos con flujo normal y alto gradiente. Los objetivos del proyecto fueron el análisis de resultados hospitalarios y a medio plazo (mortalidad hospitalaria, a 1 y 5 años) así como la clase funcional y recuperación de la función ventricular, en pacientes con estenosis aórtica de bajo-flujo bajo-gradiente sometidos a recambio valvular aórtico comparado con flujo normal alto-gradiente. Nuestros resultados nos llevan a la conclusión de que el recambio valvular aórtico en pacientes con estenosis aórtica de bajo-flujo bajo-gradiente se puede lograr con baja mortalidad quirúrgica, comparable con pacientes con flujo normal y alto gradiente. A pesar de que tener una mayor mortalidad a medio-plazo, los supervivientes exhiben una excelente clase funcional y desaparición de síntomas, que apoyan la indicación quirúrgica en estos pacientes.

Navier-Stokes equations are since many years the most important tool for studying viscous fluids. They are quite well established under a physical point of view, providing anyway one of the most challenging problems in Analysis. During the last century a number of variants of the Navier-Stokes equations have been proposed, mainly with the goal of describing some nonlinear phenomena like, e.g. shear thinning, and they got a considerable success in describing features of some biological fluids, like blood. Yet, all those fluids, including of course Navier-Stokes, share one common property: the work expended by the inner forces depends only on first derivatives of the velocity, as it has to be, at least for simple fluids. However, starting first from a very general point of view with the pioneering work of Germain, who introduced in a systematic way the concept of virtual power and its use in the foundations of Continuum Mechanics, and subsequently with many others, it became clearer and clearer that another possibile generalization was available, i.e. the second-gradient fluids. In these fluids the working done by the inner forces depends also on the second derivatives of the velocity field and includes the possibility of a “hyperviscosity” analogous to hyperstress coefficients which appears in the corresponding solid mechanics theories. These fluids have been considered only as an exercise, or an analytical variation of the problem, until the works of Fried and Gurtin , Giusteri and Fried and Giusteri-Marzocchi-Musesti, in which some convincing features of physical materials of this type were described and used. Subsequently, the very important case of isotropic fluids showed that this generalization leads to what we will call Hyperviscous Navier-Stokes problem. In this Thesis we will deal with the general initial and boundary value problem of such a fluid in a bounded or unbounded domain in three spatial dimensions, which is still open (even for bounded domains) for Navier-Stokes, with natural homogeneous boundary conditions. We introduce the second-order fluids trough their derivation from the theory of Virtual Powers: the advantages of considering such fluids are presented, for example the possibility to treat slender bodies moving in viscous fluids. Then, we study the initial boundary value problem for the hyperviscous Navier-Stokes system, that describes the special class of fluids derived in such a theory; in particular, we will consider the flow of these fluids at first in bounded domains and then in exterior domains. In order to do this, the existence of a solution is proved trough the construction of a suitable Galerkin approximated solution that passes to the limit thanks to suitable a priori energy estimates which are independent on the size of the bounded domains, thus allowing also existence in unbounded domains. In these estimates, the terms which do not appear in the Navier-Stokes problem will play a crucial role. The solutions are then proved to be regular, both in time and space, and unique in their functional spaces.

Large Eddy Simulations (LES) are performed to study flows with adverse pressure gradients (APG) produced by different two-dimensional bumps profiles installed on the bottom wall of a flat channel with parallel plates. An initial study considers a bump profile for which previous direct numerical simulation (DNS) results are available at the Reynolds number Re? = 617, based on the channel inlet friction velocity. The results obtained showed that LES is an adequate methodology for that type of simulation, being capable of reproducing accurately DNS results in terms of mean flow variables, Reynolds stresses profiles and turbulent stresses balances, with refinement levels much coarser than those from DNS. Particularly, LES results reproduce the steep increase in wall friction coefficients and strong peaks in Reynolds stresses in the adverse pressure gradient regions of both bottom and top walls of the channel which are observed in DNS results and linked to instability modes of the near wall streaks. LES is used to study the same bump profile at higher Reynolds numbers, Re? = 950 and Re? = 2000, and the results obtained are very similar to those at Re? = 617, including the steep increase in wall friction coefficients and strong peaks in Reynolds stresses at the APG region for both attached and mildly separated flows. Additional bump profiles are generated by modifying the rear portion of the initial bump configuration, which is elongated in order to produce a longer adverse pressure gradient (APG) region and promote smoother curvature changes as compared to the original profile. As in previous cases, a steep increment in Reynolds stresses values is observed at the APG region along with an augmentation of skin friction values. LES results indicate that the near wall streaks instability observed in previous DNS is characteristic of adverse pressure gradient regions located downstream of an initial favorable pressure gradient (FPG). All configurations studied with LES are also analyzed by Reynolds-average Navier-Stokes(RANS) computations with various turbulence closures. In all cases analyzed with LES it is observed that in the initial FPG region the streamwise component of the Reynolds stresses is increased while the other turbulent stresses are strongly reduced which creates a pattern analogous to the pre-transitional stages of a laminar boundary layer, which becomes unstable in the APG region. This strong non-equilibrium conditions present a great challenge for RANS models which is approximately captured by models sensitized for laminar-turbulent transition prediction.

A hydrological modelling effort using MODFLOW was undertaken in order to determine the relative importance of some of the factors influencing hydraulic gradient reversals in peatlands. Model domains were of two types, large raised bog type (LRBT) and kettle bog type (KBT), and were made to undergo various levels of meteorological forcing (water deficit). Substrate, too, was varied in order to determine its importance on reversals. Domain-wide reversals were successfully simulated in LRBT systems, but not in KBT systems. Although simulated flow patterns matched field-observed patterns, both pre- and post-drought, simulated reversals occurred more quickly than in the field. This may be due to insufficiently distributed parameters, such as hydraulic conductivity. Reversals were easily terminated by simulating non-drought conditions. In the LRBT system, reversal duration decreased, and time-to-reversal increased, with a decrease in drought severity. Increasing drought severity in KBT systems had the opposite effect on the duration of semi-reversed flow patterns, suggesting a possibly different/additional mechanism for flow reversals in KBT systems. Hydraulic conductivity had an appreciable effect on flow reversal evolution, though neither changing porosity, nor differences in catotelm layering had a great effect.

It is well known that for small Reynolds numbers, flow around a cylinder is laminar and stable. For larger Reynolds numbers, although the flow regime remains laminar, the formation of complex periodic structures appear downstream. The cyclic nature of this periodic flow is well characterized by the vortex shedding frequency and Strouhal number. However, complexities of these periodic structures downstream continue to be a topic of research. Periodic laminar 2D incompressible viscous flow around a cylinder is simulated using OpenFoam, an open source computational fluid dynamics program. To better understand these complex structures downstream, a customized computer graphical tool, VerFlow-V.01, was created to analyze and study OpenFoam simulation results. This study includes an investigation of calculating the details of drag and lift coefficients for the cylinder using mathematical models that integrate properties in subdomains, an approach not previously explored to the knowledge of the author. Numerical integration is accomplished using a finite difference approach for solving surface and contour integrals in subdomains of interest. Special attention is given to pressure and to the second invariant of the velocity gradient, as they have a clear mathematical relationship, which is consistent with results previously published. A customized visual data analysis tool, called VerFlow-V.01, allowed investigators to compare simulation data variables in a variety of useful ways, revealing details not previously understood. Main subroutines and a userâ s manual are included as appendices to encourage reproducibility and future development of the numerical, analytical and graphical models developed here. Together these models resulted in a new understanding of periodic laminar flow around a cylinder. A unique approach was developed to qualitatively understand the origins of drag and lift coefficients associated with properties mapped as images in subdomains of interest downstream. These results explain the development of convergent, eddy, and stream zones embedded in flow fields downstream.
Master of Science

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Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
The multiphase flow occurrence in the oil and gas industry is common throughout fluid path, production, transportation and refining. The multiphase flow is defined as flow simultaneously composed of two or more phases with different properties and immiscible. An important computational tool for the design, planning and optimization production systems is multiphase flow simulation in pipelines and porous media, usually made by multiphase flow commercial simulators. The main purpose of the multiphase flow simulators is predicting pressure and temperature at any point at the production system. This work proposes the development of a multiphase flow simulator able to predict the dynamic pressure and temperature gradient in vertical, directional and horizontal wells. The prediction of pressure and temperature profiles was made by numerical integration using marching algorithm with empirical correlations and mechanistic model to predict pressure gradient. The development of this tool involved set of routines implemented through software programming Embarcadero C++ Builder? 2010 version, which allowed the creation of executable file compatible with Microsoft Windows? operating systems. The simulator validation was conduct by computational experiments and comparison the results with the PIPESIM?. In general, the developed simulator achieved excellent results compared with those obtained by PIPESIM and can be used as a tool to assist production systems development
Na ind?stria do petr?leo a ocorr?ncia de escoamento multif?sico ? comum em todo o percurso dos fluidos, durante a produ??o, transporte e refino. O escoamento multif?sico ? definido como o escoamento simult?neo composto por duas ou mais fases com propriedades diferentes e imisc?veis. Uma importante ferramenta computacional para o dimensionamento, planejamento e otimiza??o de sistemas de produ??o ? a simula??o de escoamento multif?sico em dutos e meios porosos, normalmente, feita por simuladores comerciais. O objetivo b?sico desses simuladores ? prever a press?o e temperatura em diferentes pontos do sistema de produ??o. Este trabalho prop?e o desenvolvimento de um simulador de escoamento multif?sico em po?os verticais, direcionais e horizontais, capaz de determinar o gradiente din?mico de press?o e temperatura. A determina??o dos perfis de press?o e de temperatura foi feita por meio de integra??o num?rica utilizando o algoritmo de marcha com correla??es emp?ricas e modelo mecanicista para determinar o gradiente de press?o. O desenvolvimento do simulador envolveu o conjunto de rotinas implementadas atrav?s do software de programa??o Embarcadero C++ Builder? vers?o 2010, que permitiu a cria??o de arquivo execut?vel compat?vel com os sistemas operacionais da Microsoft Windows?. A valida??o do simulador foi conduzida por experimentos computacionais e compara??o dos resultados com o simulador de uso comercial PIPESIM?. De modo geral, o simulador desenvolvido alcan?ou excelentes resultados quando comparado com os obtidos pelo PIPESIM, podendo ser utilizado como ferramenta para auxiliar no desenvolvimento de sistemas de produ??o

Desde 1980 que tem vindo a ser medidas temperaturas em furos com o objectivo de criar um mapa de densidade de fluxo de calor (DFC) à superfície e cartas da distribuição da temperatura para diferentes profundidades para a região sul de Portugal Continental. Os resultados destes estudos têm importância tanto a nível do possível aproveitamento da energia geotérmica (de baixa entalpia- águas termais; alta entalpia- para produção de energia eléctrica), como a nível do conhecimento das propriedades térmicas e reológicas da crusta e manto superior, como ainda a nível de estudos de captação de recursos hídricos subterrâneos e de processos de maturação e migração de petróleos. Com este trabalho pretende-se: (1) compilar a informação geotérmica existente para a região sul de Portugal Continental; (2) realizar novas determinações de temperatura em furos de prospecção hidrogeológica e, assim, actualizar a carta de fluxo de calor existente para o Sul de Portugal Continental; (3) obter para alguns dos furos escolhidos a densidade de fluxo de calor; (4) fazer uma interpretação preliminar dos valores da densidade de fluxo de calor obtidos para sete dos nove furos utilizados para estimar a DFC. Para os sete furos referidos a DFC foi calculada através do produto do gradiente geotérmico, calculado para cada um deles, e a condutividade térmica estimada para as formações por eles atravessadas; o gradiente geotérmico foi calculado através das diagrafias de temperatura obtidas nos furos. Para além da DFC foi também estimada a temperatura aos 500 metros de profundidade para a zona onde os furos foram realizados. Para a área de estudo, que inclui a Zona Ossa Morena (ZOM) e a Zona Sul Portuguesa (ZSP), os valores da DFC variam entre 15 e 91 mwm-2; para a ZOM os valores variam entre 15 e 80 mwm-2 e para a ZSP variam entre 61 e 91 mwm-2. Assim, de acordo com os resultados deste trabalho, a DFC apresenta e confirma um aumento médio da DFC da ZOM para a ZSP. Os resultados do trabalho poderão ser utilizados, em trabalhos futuros, para construir modelos geotérmicos para a região sul de Portugal Continental e, assim, compreender melhor o regime térmico da crusta e do manto superior nessa região; ABSTRACT: Since 1980, temperatures have been measured in boreholes in order to create a surface heat flow density (HFD) map and temperature distribution maps for different depths for the southern region of Mainland Portugal. The results of these studies are important to the possible use of geothermal energy (low enthalpy - hot springs; high enthalpy - for power production), to the knowledge of the thermal and rheological properties of the crust and upper mantle, and will be useful for studying groundwater resources and processes of maturation and migration of oil. This study aims to: (1) compile all existing geothermal information for the southern region of Mainland Portugal; (2) make new determinations of temperature in water boreholes, and so update the existing heat flow density map for southern Portugal; (3) determining the heat flow density for chosen boreholes; (4) make a preliminary interpretation of the heat flow density values obtained for seven of the nine new boreholes presented in this work. For the seven boreholes HFD was estimated by multiplying the geothermal gradient, calculated for each borehole, and the estimated thermal conductivity of the borehole geological formations; geothermal gradients were calculated from the temperature logs obtained for each borehole. Besides HFD values, temperatures at 500 m depth were also estimated for each borehole. For the study area, which includes the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ), heat flow density values vary from 15 to 91 mwm-2. For the OMZ heat flow density values vary from 15 to 80 mwm-2 and for the SPZ vary from 61 to 91 mwm-2. So, the HFD values show and confirm an increase from the OMZ towards the SPZ. The results of the work may be used in future work to build geothermal models for southern Portugal and so better understand the thermal regime of the crust and upper mantle in this region.

O interesse em escoamento bifásico líquido-líquido aumentou recentemente, em especial devido às grandes perdas de energia envolvidas no transporte de petróleo, onde comumente uma mistura bifásica óleo-água é deslocada ao longo de grandes distancias. Embora este tipo de escoamento seja comum na indústria, não existem tantos trabalhos na literatura quanto os relacionados ao escoamento gás-líquido. Alguns estudos sobre escoamentos óleo-água reportam uma redução de atrito em dispersões e emulsões em regime turbulento sem adição de qualquer tipo de substancia química, mas a física por trás do fenômeno ainda não é bem compreendida. Neste trabalho, foi estudado o padrão de escoamento disperso óleo-água em tubulação horizontal, visando a obtenção de novos dados experimentais e um melhor entendimento do fenômeno de redução de atrito. Uma série de parâmetros considerados importantes para a caracterização do escoamento foi investigada: queda de pressão, fração volumétrica e sub-padrões de escoamento disperso. Apresentam-se dados de distribuição das fases e fração volumétrica in situ, obtidos através de um moderno sensor intrusivo, do tipo wire-mesh, baseado em medidas de capacitância (permissividade). Câmera de alta velocidade e técnica das válvulas de fechamento rápido foram utilizadas para validar os sinais do sensor. Um modelo prospectivo simplificado foi desenvolvido como uma tentativa de explicar a ocorrência do fenômeno de redução de atrito no padrão disperso óleo-água estudado. O modelo sugere que a presença de uma fina película de água rente a parede hidrofílica/oleofóbica do tubo poderia explicar a diminuição no gradiente de pressão bifásico observada.
The interest in two-phase liquid-liquid flow has increased recently mainly due to the petroleum industry where oil and water are often produced and transported together for long distances and the significant frictional pressure gradient involved. Liquid-liquid flows are present in a wide range of industrial processes; however, they have not been studied as intensively as gas-liquid flows. Drag reduction phenomenon in oil-water flows without the addition of any drag reduction agent has been detected in previous works, but the physics behind the phenomenon is yet not well understood. The aim of the research was to study the dispersed oil-water flow pattern in a horizontal pipe in order to better understand the phenomenon and the obtaining of new experimental data of oil-water dispersed flows. Important issues related to oil-water pipe flow were investigated: pressure drop, volume fraction and flow patterns. Phase distribution and holdup data were obtained by a new wire-mesh sensor based on capacitance (permittivity) measurements. A high-speed video camera and the Quick-Closing-Valves technique were used to compare and validate the signals of the wire-mesh sensor. A simplified mathematical model was proposed to explain the drag reduction phenomenon in the oil-water dispersed flow studied. The model suggests that the presence of a thin water film between the homogenously dispersed flow and the pipe wall could explain the observed decreases of the two-phase pressure gradient.