An arterial traffic control system influences the travel time characteristics of a corridor, including the average corridor travel time and the travel time reliability. However, reliability measures have typically been outside of the focus of arterial control system performance evaluation studies. To assess the effectiveness of arterial traffic control performance evaluation studies are normally limited to average measures of travel time, speed, or delay. As an advanced traffic management system, adaptive traffic control has been developed to address real time demand variability. Thus, an evaluation of the adaptive traffic control system based on reliability may be as important as evaluation based on average travel time or delay. In addition, arterial control systems may also affect the performance of side street traffic as well as arterial corridor traffic. The performance of side street traffic is another measure that should be used in the assessment of the effectiveness of any arterial traffic control system. Finally, an arterial's operational performance often changes throughout a day and over the arterial length. Thus, a system-wide measure that reflects the range of observed operations is needed to thoroughly assess the performance. Given these issues the goal of this research is the development of procedures to evaluate adaptive traffic control's effect on arterial characteristics such as travel time distribution, reliability, side street performance, and system-wide performance. The developed procedures were applied to the evaluation of an adaptive traffic control system, SCATS (Sydney Coordinated Adaptive Traffic System) in Cobb County, Georgia that replaced a semi-actuated coordinated control system. After the procedures were applied, it was found that SCATS produced a less extreme shape of travel time distribution, possibly due to the adaptive feature, but that it did not make statistically significant changes in the selected overall analysis measures. Also, it was found that the results of the performance evaluation can vary depending on the measures selected or the study period and location.

This thesis is concerned with queueing models where demand is allowed to exceed the system capacity, and also with the capacity sizing and pricing problem for heterogeneous products and resources under demand uncertainty. Our aim is to improve productivity and profitability. In the first part of the thesis, we consider the dynamic assignment of servers to tasks in queueing networks where demand may exceed the capacity for service. The objective is to maximize the system throughput. We use fluid limit analysis to show that several quantities of interest, namely the maximum possible throughput, the maximum throughput for a given arrival rate, the minimum arrival rate that will yield a desired feasible throughput, and the optimal allocations of servers to classes for a given arrival rate and desired throughput, can be computed by solving linear programming problems. We develop generalized round robin policies for assigning servers to classes for a given arrival rate and desired throughput, and show that our policies achieve the desired throughput as long as this throughput is feasible for the arrival rate. We conclude with numerical examples that illustrate the points discussed and provide insights into the system behavior when the arrival rate deviates from the one the system is designed for. In the second part of the thesis, we consider the effects of inspection and repair stations on the production capacity and product quality in a serial line with possible inspection and repair following each operation. We consider multiple defect types and allow for possible inspection errors that are defect dependent. We construct a profit function that takes into account inspection, repair, and goodwill costs, as well as the capacity of each station. Then we compare the profitability of different inspection plans and discuss how to identify the optimal inspection plan. Finally, in the third part of the thesis, we consider the capacity and pricing decisions made by a monopolistic firm producing two heterogeneous products under demand uncertainty. The objective is to maximize profit. Our model incorporates dedicated and flexible resources, product substitutability, and processing rates that may depend on the product and on the resource type. We provide the optimum prices and production quantities as functions of resource capacities and demand intercepts. We also show that investment in flexible capacity is only desirable when it is optimal to invest in dedicated capacities for both products, and obtain upper bounds for the costs of the dedicated capacities that need to be satisfied for investment in the flexible resource. We conclude with numerical examples that illustrate the points discussed and provide insights into how the optimal capacities and expected production quantities, prices, and profit depend on various model parameters.

In this paper we present detailed Euler-Euler Large Eddy Simulations (LES) of dispersed bubbly flow in a rectangular bubble column. The motivation of this study is to investigate potential of this approach for the prediction of bubbly flows, in terms of mean quantities. The set of physical models describing the momentum exchange between the phases was chosen according to previous experiences of the authors. Experimental data, Euler-Lagrange LES and unsteady Euler-Euler Reynolds-Averaged Navier-Stokes model are used for comparison. It was found that the presented modelling combination provides good agreement with experimental data for the mean flow and liquid velocity fluctuations. The energy spectrum made from the resolved velocity from Euler-Euler LES is presented and discussed.

A generalized model for bubble coalescence and breakup has been developed, which is based on a comprehensive survey of existing theories and models. One important feature of the model is that all important mechanisms leading to bubble coalescence and breakup in a turbulent gas-liquid flow are considered. The new model is tested extensively in a 1D Test Solver and a 3D CFD code ANSYS CFX for the case of vertical gas-liquid pipe flow under adiabatic conditions, respectively. Two kinds of extensions of the standard multi-fluid model, i.e. the discrete population model and the inhomogeneous MUSIG (multiple-size group) model, are available in the two solvers, respectively. These extensions with suitable closure models such as those for coalescence and breakup are able to predict the evolution of bubble size distribution in dispersed flows and to overcome the mono-dispersed flow limitation of the standard multi-fluid model. For the validation of the model the high quality database of the TOPFLOW L12 experiments for air-water flow in a vertical pipe was employed. A wide range of test points, which cover the bubbly flow, turbulent-churn flow as well as the transition regime, is involved in the simulations. The comparison between the simulated results such as bubble size distribution, gas velocity and volume fraction and the measured ones indicates a generally good agreement for all selected test points. As the superficial gas velocity increases, bubble size distribution evolves via coalescence dominant regimes first, then breakup-dominant regimes and finally turns into a bimodal distribution. The tendency of the evolution is well reproduced by the model. However, the tendency is almost always overestimated, i.e. too much coalescence in the coalescence dominant case while too much breakup in breakup dominant ones. The reason of this problem is discussed by studying the contribution of each coalescence and breakup mechanism at different test points. The redistribution of the gaseous phase from the injection position at the pipe wall to the whole cross section is overpredicted by the Test Solver especially for the test points with high superficial gas velocity. Besides the models for bubble forces, the simplification of the Test Solver to a 1D model has an influence on the redistribution process. Simulations performed using CFX show that a considerable improvement is achieved with comparison to the results delivered by the standard closure models. For the breakup-dominant cases, the breakup rate is again overestimated and the contribution of wake entrainment of large bubbles is underestimated. Furthermore, inlet conditions for the liquid phase, bubble forces as well as turbulence modeling are shown to have a noticeable influence, especially on the redistribution of the gaseous phase.

In CANDU reactors, under normal operating conditions, the inlet headers collect and distribute single-phase liquid flow (heavy water) to the fuel cooling channels via the feeders. However, under some postulated loss of coolant accidents, the inlet headers may receive two-phase fluid (steam/water) and the fluid forms a stratified region inside the header. To predict the thermalhydraulic behaviour of headers for the reactor safety analysis, the two-phase flow distribution within the headers and through the feeders must be modelled. In order to analyse the two-phase flow behaviour of a scaled CANDU inlet header / a transparent and instrumented version of a header with 5 feeders was previously built in the Mechanical Engineering Department of Middle East Technical University (METU-Two Phase Flow Test Facility / METU-TPFTF). The aim of this study is to investigate two-phase flow distribution through multiple outlets from such a horizontal drum both numerically and experimentally. For this purpose, three-dimensional incompressible finite difference equations in cylindrical coordinates were derived by using two-fluid model to simulate adiabatic two-phase flow (air/water) in the header numerically. The discretized equations were then programmed into a computer code which was developed specifically for modelling the header type geometry. A method based on the principles of Implicit Multi Field (IMF) technique has been utilised to solve those equations. The solution algorithm was tested by using some numerical benchmark problems. A number of experimental tests covering single and two-phase flow distribution through outlet pairs from the header were performed. Void fractions and flow rates obtained from these tests are in good agreement with the code results. The code also predicts the void fraction and pressure distribution in the header satisfactorily.

[pt] Escoamentos bifásicos no regime anular são caracterizados pela formação de um filme de líquido ao redor das paredes do duto com a fase gasosa escoando na área central do duto. O presente trabalho consiste na simulação numérica de um escoamento anular em tubulação horizontal, com e sem transferência de calor através de um código unidimensional baseado no Modelo de Dois Fluidos. São considerados dois pares de fluidos, sendo o primeiro ar-água, o qual é vastamente estudado na literatura e um fluido típico encontrado na produção de petróleo formado de gás natural e óleo. Parâmetros característicos do padrão de escoamento anular como gradiente de pressão, fator de atrito da interface e espessura do filme de líquido são determinados e comparados com dados experimentais e numéricos, apresentando boa concordância. O gás natural é modelado como gás real, através da aplicação da Equação de Estado de Peng-Robinson e comparado com a modelagem utilizando Equação de Gases Ideais. Para o fluido típico selecionado, este efeito é muito pequeno tanto com relação aos parâmetros hidrodinâmicos como velocidades das fases e queda de pressão como nos parâmetros térmicos como campo de temperatura, perda de calor para o ambiente e coeficiente bifásico de troca de calor. / [en] Annular two-phase flow is characterized by the formation of a liquid layer spread around the pipe circumference with gas flowing in the core area of the pipe. The present work consists in the numerical simulation of an annular flow in horizontal pipe, with and without heat transfer through a one-dimensional code based on the Two Fluid Model. Two pairs of fluids are considered, being the first air-water, which is widely studied in the literature and a typical natural gas and oil fluid from production oil fields. Characteristics parameters of annular flow such as pressure drop, interface friction factor e liquid film height are obtained and compared with experimental and numerical data, showing in both cases good agreement. The natural gas is modeled as real gas, using the Peng-Robinson Equation of State, and compared with the ideal gas modeling. For the typical fluid selected, this effect is quite small on the hydrodynamics parameters such as phases velocities and pressure drop and on the thermal parameters such as temperature, heat loss for the environment and heat exchange coefficient.

De nombreux procédés industriels nécessitent la manipulation de matériaux sous forme pulvérulente. L’émission de poussières générée par leur manipulation peut s’avérer dangereuse pour la santé des travailleurs ou bien causer un risque d’explosion. Afin de mieux comprendre les mécanismes de dispersion des poussières, le cas de la décharge d’un silo est étudié par simulation numérique avec une approche Euler-Euler. Deux configurations ont été étudiées au cours de cette thèse. La première, sans silo, a permis d’étudier l’influence de perturbations de vitesses imposées à l’entrée de la chambre de dispersion en lieu et place du silo. Cette étude a révélé que ces perturbations peuvent influencer l’élargissement du panache de poudre. Seules les perturbations avec une corrélation temporelle ont généré une ouverture importante du jet tombant semblable à celle relevée expérimentalement. Dans la deuxième configuration, le silo et la chambre de dispersion sont représentés afin d’étudier le couplage entre la dispersion du jet et l’écoulement dans le silo. L’une des difficultés de ces simulations est de prédire les différents régimes d’écoulements granulaires, allant de l’état quasi-statique dans le silo au régime très dilué lors de la dispersion du jet tombant, en passant par le régime collisionnel à la sortie du silo. La théorie cinétique permet de modéliser le régime dilué et collisionnel. En revanche pour la partie quasi-statique un modèle semi-empirique a été utilisé, implémenté et validé sur différentes configurations. La seconde étude a montré l’importance du rapport entre le diamètre de l’orifice et le diamètre des particules sur la structure du jet. En effet, lorsque ce paramètre est faible, le coeur du jet se contracte immédiatement après la sortie du silo puis s’ouvre en aval. Pour des valeurs grandes, l’ouverture du jet est négligeable. Cependant, il semblerait que l’angle du silo modifie le comportement de l’écoulement, ce qui nécessitera des études supplémentaires. / A wide range of industrial processes requires the handling of granular material in a pulverulent form. The subsequent dust emissions due to these processes can be harmful to the health of workers or hazardous explosion risks. In order to understand dust dispersion mechanisms, a case of a free falling granular jet discharged from a silo is studied by numerical simulations using an Euler-Euler approach. Two types of numerical simulation are conducted. First, the influence of velocity fluctuations at the inlet chamber is studied on the plume behavior, instead of the silo. This study reveals that fluctuations are enable to reproduce the jet expansion. It is established that only fluctuations with temporal correlation generate a large jet opening similar to the experiment. The second type of setup shows the coupling between the silo and the chamber. One of the major challenges is the ability to predict the different flow regimes going from quasi-static regime inside the silo, to the very dilute regime in the dust spread and include the collisional regime occurs through the silo. Kinetic theory allows modeling of the dilute and collisional regime. By contrast, frictional models have been used, implemented and validated in different cases. The second study highlights the key role of the ratio defined by the orifice diameter on the particle diameter. Indeed, when this parameter is small, the jet powder core contracts immediately after the exit of the silo dump plane and expands downstream. For high values, the granular jet does not exhibit dispersion anymore. This study suggests that the silo half-angle has an impact on the flow field which justifies the need for further investigations.

Émissions polluantes, les motoristes souhaitent contrôler au mieux l’atomisation du carburant, injecté généralement sous forme de jets ou de nappes liquides. Les essais étant long et coûteux, leur remplacement par un outil numérique capable de simuler le processus d’atomisation permettrait non seulement une réduction des coûts importante mais faciliterait également la phase de conception. Toutefois, en raison du caractère multi-échelle du phénomène, il est difficile de le décrire dans son ensemble avec les approches habituellement utilisées en mécanique des fluides numérique.L’objectif de cette thèse est de concevoir une nouvelle approche qui permettra à terme de simuler l’atomisation pour une configuration industrielle complète. Celle-ci consiste à coupler deux types de modèles. Le premier, dit modèle bifluide, est un modèle à deux fluides compressibles basé sur les équations de Navier-Stokes diphasiques. Celui-ci permet de décrire les grandes échelles du phénomène d’atomisation correspondant à la formation de ligaments et d’amas liquides dans la zone proche de l’injecteur. Le second, dit modèle de spray, est basé sur une équation cinétique. Dans la zone située en aval de l’injecteur, ce dernier permet de décrire de manière statistique l’évolution du brouillard de gouttelettes issues de la fragmentation primaire du jet de carburant. Le point délicat, à la fois sur le plan de la modélisation et sur celui de l’algorithmique, réside dans le couplage des deux modèles. Celui ci a été réalisé grâce à l’introduction de deux modèles auxiliaires permettant de traiter le transfert de liquide entre le modèle bifluide et le modèle de spray par atomisation ou ré-impact.L’approche proposée a été appliquée à la simulation numérique de nappes liquides cisaillées. Les comparaisons entre les résultats numériques et des résultats expérimentaux montrent que le modèle bifluide permet de prévoir l’influence de la géométrie et des conditions d’injection sur l’atomisation primaire de la nappe liquide. Le modèle d’atomisation permet quant à lui, de reproduire le caractère instationnaire des mécanismes de production de gouttes lors du transfert de la phase liquide depuis le modèle bifluide vers celui de spray. Des cas de ré-impact valident également la robustesse et la généralité de la méthodologie de couplage. / In order to improve efficiency of aircraft combustion chambers and reduce polluting emissions,engine manufacturers try to achieve a better control on fuel atomization, which is usually injectedas a jet or liquid sheet. As experiments are expensive and time consuming, a numerical tool able to simulate atomization would be a powerful asset in engine conception design. However, simulation ofthe whole atomization process with commonly used approach in computational fluid dynamics is still prohibitive due to the multi-scale nature of the phenomenon.The objective of this thesis is to develop a new approach allowing the simulation of the spray formation for a industrial configuration in the near future. This involves coupling of two types of models.The first one, called two-fluid model, is based on the Navier-Stokes equations for two immiscible compressible fluids. This one is used to describe the large scales of the atomization mechanism corresponding to the formation of ligaments and liquids blobs in the near-injector area. The second one,called spray model, is based on a kinetic equation. Further downstream from the injector, this model describes statistically the evolution of the droplet cloud produced by the primary fragmentation of liquid jet. The main difficulty, in terms of both modeling and algorithmic, is the coupling of these twomodels.This has been achieved by introducing an atomization and an impact models which ensure liquid transfer between the two-fluid model and the spray model.This new approach was applied to the numerical simulation of sheared liquid sheets. Comparisons between numerical and experimental results show how the two-fluid model predicts the influence of injector geometry and injection conditions on the primary atomization of the liquid sheet. Concerning droplets production, the atomization model is able to reproduce the unsteady nature of this mechanism when transferring liquid phase from the two-fluid model to the spray model. Test cases for the impact model also validate the robustness and generality of the coupling approach.

The production from an oil reservoir is a mixture of liquids (oil and water) and gas, and is often maintained by using a pump placed in the well to ensure a continuous flow to the surface. Electrical Submersible Pumps consist of stacked centrifugal pump stages, each comprising a bladed impeller (rotating part) and diffuser (stationary part). In multiphase conditions, the gas tends to accumulate in the impeller, severely reducing the pressure produced by the pump. Radial-flow pumps operate in a plane perpendicular to their rotation axis, while mixed-flow pumps are characterised by a lower meridional angle (generally 40 to 80 degrees), and are generally better at handling gas-liquid mixtures. We first describe the impact of gas on the whole pumping system, from the reservoir to the storage facility, and give context to the subject. The available literature shows that the size of the gas bubbles present in the fluid is critical to the pump performance. A transparent, full-scale pump was built in order to explore the flow features in single and multiphase flows. Laser Doppler Velocimetry and high speed imaging in single phase flow showed a high turbulence level in the wake of the impeller blades, and recirculation cells at low flow rates. In gas-liquid conditions, we demonstrated that the bubble size varies within a pump stage, as break-up occurs at the impeller tip, and coalescence is dominant in the diffuser, especially because of recirculation. The first impeller acted as a mixer, and at moderate to high gas fractions (10 to 30%), the flow patterns at the stage level alternated between bubbly and radially separated flows. Finally, a dispersed-gas model was developed to predict the pressure rise in a mixed-flow pump impeller under gas-liquid conditions. This model based on the forces acting on a single spherical gas bubble, was implemented with a simplified, parametric representation of the flow field in a mixed-flow impeller. In the meridional direction, the Coriolis force opposes the centrifugal force and the adverse pressure gradient. Both forces tend to retain the gas bubble within the impeller. The relative magnitude of the drag force strongly depends on the maximal bubble diameter, which was determined as a function of the flow conditions and used to calculate the gas velocity through the impeller. This method resulted in a better agreement with the experimental data than a one-dimensional two-fluid model where the gas phase follows the same path as the liquid. We used the dispersed-gas model to give quantitative evidence that low blade and meridional angles reduce the gas accumulation and the associated performance degradation.

[pt] Diversos processos práticos apresentam uma seqüência de diferentes configurações de escoamentos bifásicos, com diferentes padrões de escoamento. Dependendo das razões entre as velocidades superficiais do gás e do líquido, diferentes padrões de escoamento bifásico podem ser encontrados. O padrão de escoamento slug pode ser formado a partir do padrão estratificado, devido ao crescimento das instabilidades hidrodinâmicas ou devido às ondulações no terreno onde se instala a tubulação. O presente trabalho consiste na análise do escoamento bifásico no padrão slug, ao longo de tubulações horizontais, mediante a aplicação do modelo de dois fluidos em sua forma transiente e unidimensional. Através de uma análise de estabilidade de Kelvin-Helmholtz para escoamento estratificado, estima-se a possível faixa de operação para a obtenção de um modelo matemático bem posto. Diferentes técnicas de discretização foram implementadas e testadas. Comparações com dados disponíveis na literatura foram realizadas. / [en] Several practical processes show a sequence of different configurations of two-phase flows, with different flows pattern. Depending upon the relation between the superficial velocities of gas and liquid, different two- phase flows patterns can be found. Slug flow can be formed from the stratified flow, due to growth of hydrodynamic instabilities or due to undulations of the surface where the pipes are installed. The present work consists in the analysis of two-phase flow in slug flow through horizontal pipes using the two-fluid model in its transient and one-dimensional form. By means of a Kelvin-Helmholtz stability analysis for stratified flows, the operation range for obtaining of a well-posed mathematical model is estimated. Different techniques of discretization were implemented and tested. Comparisons with result obtained in the literature were done.