[pt] Elementos combustíveis de reatores nucleares são, em geral, constituídos por feixes de barras, através dos quais o refrigerante escoa axialmente. A confiabilidade do projeto termohidráulico destes elementos está ligada a um conhecimento detalhado do campo (principal e secundário) de velocidades. Um modelo estatístico de turbulência (K-E) é aplicado na determinação dos escoamentos principal e secundário, tensões de cisalhamento na parede e coeficiente de atrito para regime turbulento desenvolvido permanente, com fluido incompressível escoando axialmente através de feixes de barras em arranjo triangular ou quadrado. O método numérico emprega a vorticidade e a função corrente na descrição do campo de velocidades. Os resultados ovtidos estão, para diferentes números de Reynolds e diferentes razões de aspecto (P/D) de acordo com os resultados experimentais e analíticos de diversos investigadores. / [en] Reactor fuel elements generally consist of rod bundles with the coolant flowing axially through the region between the rods. The confiability of the thermohydraulic design of such elements is related to a detailed description of the velocity field. A two-equation statistical model (K-E) of turbulence is applied to compute main and secondary flow fields, wall shear stress distributions and friction factors of steady, fully developed turbulent flows, with incompressible, temperature independent fluid flowing axially through triangular or square arrays of rod bundles. The numerical procedure uses the vorticity and the stream function to describe the velocity field. Comparison with experimental and analytical data of several investigations is presented. Results are in good agreement.

In der Reaktorsicherheitsforschung sind auftriebsgetriebene Strömungen von Relevanz für Störfall-szenarien mit Verdünnung der Borkonzentration und für thermische Schockbelastungen des Reak-tordruckbehälters. In der numerischen Simulation der Strömungen werden neben der Berücksichtigung der Auftriebskräfte Quell- und Korrekturterme in die Bilanzgleichungen für die turbulente Energie und die turbulente Dissipation eingeführt. Es wurden erweiterte Modelle entwickelt, in die zusätzliche Gleichungen für die Turbulenzgrößen turbulenter Massenstrom und Dichtevarianz eingehen. Die Modelle wurden in den CFD-Code ANSYS-CFX implementiert. Die Validierung der Modelle erfolgte an einem speziellen Versuchsaufbau (VeMix-Versuchsanlage), mit Einspeisung von Fluid höherer Dichte in eine Vorlage. Als Kriterien für die Validierung wurde der Umschlag zwischen impulsdominiertem Strömungsregime mit vertikalem Jet oder ein vertikales Absinken bei Dominanz von Dichteeffekten herangezogen sowie lokale Konzentrationsmessungen mit Hilfe eines speziell entwickelten Leitfähigkeits-Gittersensors. Eine Verbesserung der Simulation dichtedominierter Vermischungsprozesse mit den erweiterten Turbulenzmodellen konnte allerdings nicht nachgewiesen werden, da die Unterschiede zwischen den Rechnungen mit verschiedenen Turbulenzmodellen zu gering sind. Andererseits konnte jedoch die Simulation der Stratifikation von Fluiden unterschiedlicher Dichte im kalten Strang einer Reaktoranlage deutlich verbessert werden. Anhand der Nachrechnung von Ver-suchen am geometrisch ähnlichen Reaktor-Strömungsmodell ROCOM wurde gezeigt, dass diese Stratifikation von bedeutendem Einfluss auf die Vermischung und somit letztendlich auch auf die Temperatur- bzw. Borkonzentrationsverteilung innerhalb des Reaktordruckbehälters ist. Sie lässt sich nur korrekt simulieren, wenn ausreichend große Abschnitte des kalten Stranges mit modelliert werden. Somit konnte doch eine bessere Vorhersagegenauigkeit der Simulation der Vermischung erreicht werden. In reactor safety research, buoyancy driven flows are of relevance for boron dilution accidents or pressurised thermal shock scenarios. Concerning the numerical simulation of these flows, besides of the consideration of buoyancy forces, source and correction terms are introduced into the balance equations for the turbulent energy and its dissipation rate. Within the project, extended turbulence models have been developed by introducing additional balance equations for the turbulent quantities turbulent mass flow and density variance. The models have been implemented into the computati-onal fluid dynamics code ANSYS-CFX. The validation of the models was performed against tests at a special experimental set-up, the VeMix facility, were fluid of higher density was injected into a vertical test section filled with lighter fluid. As validation criteria the switching-over between a momentum controlled mixing pattern with a horizontal jet and buoyancy driven mixing with vertical sinking down of the heavier fluid was used. Additionally, measurement data gained from an especially developed conductivity wire mesh sensor were used. However, an improvement of the modelling of buoyancy driven mixing by use of the extended models could not be shown, because the differences between calculations with the different models were not relevant. On the other hand, the modelling of the stratification of fluids with different density in the cold leg of a reactor primary circuit could be significantly improved. It has been shown on calculations of experi-ments at the ROCOM mixing test facility, a scaled model of a real reactor plant, that this stratification is relevant as a boundary condition for the mixing process inside the reactor pressure vessel. It can be correctly simulated only if sufficient large parts of the cold legs are included in the modelling. On this way, an improvement of the accuracy of the prediction of mixing processes was achieved.

Two active flow control methods are investigated numerically to understand the mechanism by which they control aerodynamics in the presence of severe flow separation on an airfoil. In particular, synthetic jets are applied to separated flows generated by additional surface feature (the actuators) near the trailing edge to obtain Coanda-like effects, and an impulse jet is used to control a stalled flow over an airfoil. A moving-grid scheme is developed, verified and validated to support simulations of external flow over moving bodies. Turbulent flow is modeled using detached eddy simulation (DES) turbulence models in the CFD code CDP (34) developed by Lopez (54). Synthetic jet actuation enhances turbulent mixing in flow separation regions, reduces the size of the separation, deflects stream lines closer to the surface and changes pressure distributions on the surface, all of which lead to bi-directional changes in the aerodynamic lift and moment. The external flow responds to actuation within about one convective time, which is significantly faster than for conventional control surfaces. Simulation of pitching airfoils shows that high-frequency synthetic jet affects the flow independently of the baseline frequencies associated with vortex shedding and airfoil dynamics. These unique features of synthetic jets are studied on a dynamically maneuvering airfoil with a closed-loop control system, which represents the response of the airfoil in wind-tunnel experiments and examines the controller for a rapidly maneuvering free-flight airfoil. An impulse jet, which is applied upstream of a nominal flow separation point, generates vortices that convect downstream, interact with the separating shear layer, dismantle the layer and allow following vortices to propagate along the surface in the separation region. These following vortices delay the separation point reattaching the boundary layer, which returns slowly to its initial stall condition, as observed in wind-tunnel experiments. A simple model of the impulse jet actuator used herein is found to be sufficient to represent the global effects of the jet on the stalled flow because it correctly represents the momentum injected into the flow. / text

This thesis presents the implementation of several two-equation turbulence models into a finite difference, two- and three-dimensional Navier-Stokes Solver. Theories of turbulence modeling and the historical development of these theories are briefly investigated. Turbulence models that are defined by two partial differential equations, based on k-&amp / #969 / and k-&amp / #949 / models, having different correlations, constants and boundary conditions are selected to be adapted into the base solver. The basic equations regarding the base Navier-Stokes solver to which the turbulence models are implemented presented by briefly explaining the outputs obtained from the solver. Numerical work regarding the implementation of turbulence models into the base solver is given in steps of non-dimensionalization, transformation of equations into generalized coordinate system, numerical scheme, discretization, boundary and initial conditions and limitations. These sections of implementation are investigated and presented in detail with providing every steps of work accomplished. Certain trial problems are solved and outputs are compared with experimental data. Solutions for fluid flow over flat plate, in free shear, over cylinder and airfoil are demonstrated. Airfoil validation test cases are analyzed in detail. For three dimensional applications, computation of flow over a wing is accomplished and pressure distributions from certain sections are compared with experimental data.

Vertical up-flow of supercritical fluid in the subchannel of a heated rod bundle was numerically simulated using the Computational Fluid Dynamics (CFD) codes ANSYS CFX and ANSYS FLUENT. A total of seven cases from three different sets of experiments were simulated. Three-dimensional steady-state predictions of fluid velocity, pressure, and temperature were made using five versions of two-equation RANS turbulence models with accompanying wall treatments. In addition, the temperature distribution in a solid region comprising a heater and sheathing was also computed in some cases. The k-epsilon turbulence model, implemented using CFX and scalable wall functions, provided the numerical results that have the smallest overall deviation from experimental results for three of the seven cases, and predicts the experimental data of the remaining four cases reasonably well, unlike other turbulence models that severely over-predict the experimental data for wall surface temperature. / February 2016

Com o avanço tecnológico dos computadores e o desenvolvimento de programas de simulação de fluidos, propiciando resultados cada vez mais sofisticados e eficazes, ocorreu uma redução significativa no tempo e nos custos computacionais para conceber veículos mais seguros e com menos agressão ao meio ambiente, devido ao seu menor consumo de combustível. Contudo, mesmo com o uso da Dinâmica dos Fluidos Computacional bastante disseminado, ainda são divergentes as informações sobre o nível de concordância esperado entre os resultados obtidos, por meio da simulação, quando comparada com os resultados medidos experimentalmente. Este trabalho apresenta uma sequencia metodológica para determinação das forças de arrasto sobre um veículo de transporte de pessoas (ônibus), de maneira a transmitir a compreensão dos fenômenos que envolvem o problema, a sequencia necessária de simulações e conceitos que devem ser atribuídos para obtenção de respostas coerentes. Primeiramente é abordada a análise de problemas com respostas experimentais disponíveis e assim validando a metodologia de simulação para cada parâmetro abordado, domínio, malha, discretização da camada limite e modelos de turbulência. Com base nas definições destes parâmetros são realizadas as simulações de um problema em escala real do escoamento de fluidos sobre um ônibus. A metodologia empregada para estimar os parâmetros de simulação é apresentada em forma de uma sequencia de cálculos, de fácil utilização. Os resultados demonstram que o uso do método numérico na criação de veículos pode ser ampliado, com significativa redução de ensaios experimentais. A análise numérica apresentada ratifica a metodologia numérica como importante ferramenta para aprimorar o projeto de veículos, com menor coeficiente de arrasto e maior estabilidade aerodinâmica, com isto proporcionando a redução do consumo de combustível, somados com benefícios secundários significativos, tais como, baixo acúmulo de sujeira, melhorando a visibilidade, ruído aerodinâmico reduzido e até menor fadiga do condutor. / The technological advancement of computers and the development of fluid simulation software providing results increasingly sophisticated and effective, has resulted in a significant reduction in time and computational cost to develop safer vehicles with less harm to the environment due to its lower fuel consumption. However, even with quite widespread Computational Fluid Dynamics use, there is still conflicting information on the expected level of agreement among the results obtained by simulation, when compared with the experimentally measured results. This work presents a methodological sequence to determine the drag force on a vehicle for people transportation (bus ), in order to convey the understanding of phenomena involving the problem , the required sequence of simulations, and concepts that should be assigned to obtain coherent answers. At first, one approaches the analysis of problems with available experimental answers, thus validating the simulation methodology for each parameter approached, domain , mesh ,discretization of the boundary layer and turbulence models. Based on the definitions of these parameters, simulations of an actual scale problem of fluid flow on a bus are performed. The methodology used to estimate the simulation parameters is presented in the form of a sequence of calculations easy to use. The results show that the use of the numerical method for creating vehicles can be expanded with a significant reduction in experimental tests. The numerical analysis presented confirms the numerical methodology as an important tool to upgrade the design of vehicles, with lower drag coefficient and greater aerodynamic stability, thus providing a reduction of fuel consumption, added to significant secondary benefits, such as low dirt accumulation, improving visibility, reduced aerodynamic noise and even less driver fatigue.

Com o avanço tecnológico dos computadores e o desenvolvimento de programas de simulação de fluidos, propiciando resultados cada vez mais sofisticados e eficazes, ocorreu uma redução significativa no tempo e nos custos computacionais para conceber veículos mais seguros e com menos agressão ao meio ambiente, devido ao seu menor consumo de combustível. Contudo, mesmo com o uso da Dinâmica dos Fluidos Computacional bastante disseminado, ainda são divergentes as informações sobre o nível de concordância esperado entre os resultados obtidos, por meio da simulação, quando comparada com os resultados medidos experimentalmente. Este trabalho apresenta uma sequencia metodológica para determinação das forças de arrasto sobre um veículo de transporte de pessoas (ônibus), de maneira a transmitir a compreensão dos fenômenos que envolvem o problema, a sequencia necessária de simulações e conceitos que devem ser atribuídos para obtenção de respostas coerentes. Primeiramente é abordada a análise de problemas com respostas experimentais disponíveis e assim validando a metodologia de simulação para cada parâmetro abordado, domínio, malha, discretização da camada limite e modelos de turbulência. Com base nas definições destes parâmetros são realizadas as simulações de um problema em escala real do escoamento de fluidos sobre um ônibus. A metodologia empregada para estimar os parâmetros de simulação é apresentada em forma de uma sequencia de cálculos, de fácil utilização. Os resultados demonstram que o uso do método numérico na criação de veículos pode ser ampliado, com significativa redução de ensaios experimentais. A análise numérica apresentada ratifica a metodologia numérica como importante ferramenta para aprimorar o projeto de veículos, com menor coeficiente de arrasto e maior estabilidade aerodinâmica, com isto proporcionando a redução do consumo de combustível, somados com benefícios secundários significativos, tais como, baixo acúmulo de sujeira, melhorando a visibilidade, ruído aerodinâmico reduzido e até menor fadiga do condutor. / The technological advancement of computers and the development of fluid simulation software providing results increasingly sophisticated and effective, has resulted in a significant reduction in time and computational cost to develop safer vehicles with less harm to the environment due to its lower fuel consumption. However, even with quite widespread Computational Fluid Dynamics use, there is still conflicting information on the expected level of agreement among the results obtained by simulation, when compared with the experimentally measured results. This work presents a methodological sequence to determine the drag force on a vehicle for people transportation (bus ), in order to convey the understanding of phenomena involving the problem , the required sequence of simulations, and concepts that should be assigned to obtain coherent answers. At first, one approaches the analysis of problems with available experimental answers, thus validating the simulation methodology for each parameter approached, domain , mesh ,discretization of the boundary layer and turbulence models. Based on the definitions of these parameters, simulations of an actual scale problem of fluid flow on a bus are performed. The methodology used to estimate the simulation parameters is presented in the form of a sequence of calculations easy to use. The results show that the use of the numerical method for creating vehicles can be expanded with a significant reduction in experimental tests. The numerical analysis presented confirms the numerical methodology as an important tool to upgrade the design of vehicles, with lower drag coefficient and greater aerodynamic stability, thus providing a reduction of fuel consumption, added to significant secondary benefits, such as low dirt accumulation, improving visibility, reduced aerodynamic noise and even less driver fatigue.

Com o avanço tecnológico dos computadores e o desenvolvimento de programas de simulação de fluidos, propiciando resultados cada vez mais sofisticados e eficazes, ocorreu uma redução significativa no tempo e nos custos computacionais para conceber veículos mais seguros e com menos agressão ao meio ambiente, devido ao seu menor consumo de combustível. Contudo, mesmo com o uso da Dinâmica dos Fluidos Computacional bastante disseminado, ainda são divergentes as informações sobre o nível de concordância esperado entre os resultados obtidos, por meio da simulação, quando comparada com os resultados medidos experimentalmente. Este trabalho apresenta uma sequencia metodológica para determinação das forças de arrasto sobre um veículo de transporte de pessoas (ônibus), de maneira a transmitir a compreensão dos fenômenos que envolvem o problema, a sequencia necessária de simulações e conceitos que devem ser atribuídos para obtenção de respostas coerentes. Primeiramente é abordada a análise de problemas com respostas experimentais disponíveis e assim validando a metodologia de simulação para cada parâmetro abordado, domínio, malha, discretização da camada limite e modelos de turbulência. Com base nas definições destes parâmetros são realizadas as simulações de um problema em escala real do escoamento de fluidos sobre um ônibus. A metodologia empregada para estimar os parâmetros de simulação é apresentada em forma de uma sequencia de cálculos, de fácil utilização. Os resultados demonstram que o uso do método numérico na criação de veículos pode ser ampliado, com significativa redução de ensaios experimentais. A análise numérica apresentada ratifica a metodologia numérica como importante ferramenta para aprimorar o projeto de veículos, com menor coeficiente de arrasto e maior estabilidade aerodinâmica, com isto proporcionando a redução do consumo de combustível, somados com benefícios secundários significativos, tais como, baixo acúmulo de sujeira, melhorando a visibilidade, ruído aerodinâmico reduzido e até menor fadiga do condutor. / The technological advancement of computers and the development of fluid simulation software providing results increasingly sophisticated and effective, has resulted in a significant reduction in time and computational cost to develop safer vehicles with less harm to the environment due to its lower fuel consumption. However, even with quite widespread Computational Fluid Dynamics use, there is still conflicting information on the expected level of agreement among the results obtained by simulation, when compared with the experimentally measured results. This work presents a methodological sequence to determine the drag force on a vehicle for people transportation (bus ), in order to convey the understanding of phenomena involving the problem , the required sequence of simulations, and concepts that should be assigned to obtain coherent answers. At first, one approaches the analysis of problems with available experimental answers, thus validating the simulation methodology for each parameter approached, domain , mesh ,discretization of the boundary layer and turbulence models. Based on the definitions of these parameters, simulations of an actual scale problem of fluid flow on a bus are performed. The methodology used to estimate the simulation parameters is presented in the form of a sequence of calculations easy to use. The results show that the use of the numerical method for creating vehicles can be expanded with a significant reduction in experimental tests. The numerical analysis presented confirms the numerical methodology as an important tool to upgrade the design of vehicles, with lower drag coefficient and greater aerodynamic stability, thus providing a reduction of fuel consumption, added to significant secondary benefits, such as low dirt accumulation, improving visibility, reduced aerodynamic noise and even less driver fatigue.

In der Reaktorsicherheitsforschung sind auftriebsgetriebene Strömungen von Relevanz für Störfall-szenarien mit Verdünnung der Borkonzentration und für thermische Schockbelastungen des Reak-tordruckbehälters. In der numerischen Simulation der Strömungen werden neben der Berücksichtigung der Auftriebskräfte Quell- und Korrekturterme in die Bilanzgleichungen für die turbulente Energie und die turbulente Dissipation eingeführt. Es wurden erweiterte Modelle entwickelt, in die zusätzliche Gleichungen für die Turbulenzgrößen turbulenter Massenstrom und Dichtevarianz eingehen. Die Modelle wurden in den CFD-Code ANSYS-CFX implementiert. Die Validierung der Modelle erfolgte an einem speziellen Versuchsaufbau (VeMix-Versuchsanlage), mit Einspeisung von Fluid höherer Dichte in eine Vorlage. Als Kriterien für die Validierung wurde der Umschlag zwischen impulsdominiertem Strömungsregime mit vertikalem Jet oder ein vertikales Absinken bei Dominanz von Dichteeffekten herangezogen sowie lokale Konzentrationsmessungen mit Hilfe eines speziell entwickelten Leitfähigkeits-Gittersensors. Eine Verbesserung der Simulation dichtedominierter Vermischungsprozesse mit den erweiterten Turbulenzmodellen konnte allerdings nicht nachgewiesen werden, da die Unterschiede zwischen den Rechnungen mit verschiedenen Turbulenzmodellen zu gering sind. Andererseits konnte jedoch die Simulation der Stratifikation von Fluiden unterschiedlicher Dichte im kalten Strang einer Reaktoranlage deutlich verbessert werden. Anhand der Nachrechnung von Ver-suchen am geometrisch ähnlichen Reaktor-Strömungsmodell ROCOM wurde gezeigt, dass diese Stratifikation von bedeutendem Einfluss auf die Vermischung und somit letztendlich auch auf die Temperatur- bzw. Borkonzentrationsverteilung innerhalb des Reaktordruckbehälters ist. Sie lässt sich nur korrekt simulieren, wenn ausreichend große Abschnitte des kalten Stranges mit modelliert werden. Somit konnte doch eine bessere Vorhersagegenauigkeit der Simulation der Vermischung erreicht werden. In reactor safety research, buoyancy driven flows are of relevance for boron dilution accidents or pressurised thermal shock scenarios. Concerning the numerical simulation of these flows, besides of the consideration of buoyancy forces, source and correction terms are introduced into the balance equations for the turbulent energy and its dissipation rate. Within the project, extended turbulence models have been developed by introducing additional balance equations for the turbulent quantities turbulent mass flow and density variance. The models have been implemented into the computati-onal fluid dynamics code ANSYS-CFX. The validation of the models was performed against tests at a special experimental set-up, the VeMix facility, were fluid of higher density was injected into a vertical test section filled with lighter fluid. As validation criteria the switching-over between a momentum controlled mixing pattern with a horizontal jet and buoyancy driven mixing with vertical sinking down of the heavier fluid was used. Additionally, measurement data gained from an especially developed conductivity wire mesh sensor were used. However, an improvement of the modelling of buoyancy driven mixing by use of the extended models could not be shown, because the differences between calculations with the different models were not relevant. On the other hand, the modelling of the stratification of fluids with different density in the cold leg of a reactor primary circuit could be significantly improved. It has been shown on calculations of experi-ments at the ROCOM mixing test facility, a scaled model of a real reactor plant, that this stratification is relevant as a boundary condition for the mixing process inside the reactor pressure vessel. It can be correctly simulated only if sufficient large parts of the cold legs are included in the modelling. On this way, an improvement of the accuracy of the prediction of mixing processes was achieved.

Spillways are hydraulic structures that allow dams to release and convey surplus water or flood from the reservoir to the downstream channel. The spillway is a safety structure that prevents the overtopping of the dam. Many dam failure disasters were due to the inadequate capacity of the spillway, which fully illustrates the prominence of spillway design. According to the control structure, spillways can be divided into gated and ungated type. The gated spillway provides better control of the managed water level and reduces the elevation of the top of the dam. Researchers have mostly used experimental models to investigate these two types of spillways in previous literature. In the past few years, following the rapid development of numerical simulation technology, there have been more studies on the numerical modeling of spillways. However, most of the literature was about ungated spillways and most of it considered the case of low head ratios, while the case with gates, especially the case of vertical plane gates, was less investigated. In this study, the hydraulic characteristics, such as velocity, pressure, and discharge coefficient, of the ungated and gated ogee spillways are investigated by means of physical and numerical models for the case of low and high head ratios. The study covered head ratios varying from 1.4 to 4.6 and the relative gate-openings varying from 0.5 to 2. The second main objective of this study was to evaluate the performance of the numerical model to simulate gated and ungated spillways. It mainly employed 2DV OpenFOAM to simulate three turbulence models (realizable k-ε, RNG k-ε, k-ω SST), and the results were compared and calibrated with the experimental results from the physical model tests performed by the author to verify the performance of the numerical model. This study aims to demonstrate that the numerical model can be used as a complementary tool to the physical model to measure the hydraulic performance of ogee spillways.