Dissertations / Theses on the topic 'CFD analysis'

A Pelton design software is currently being developed at the Waterpower laboratory at NTNU. The motivation behind this software is to streamline the parametric design process for Pelton turbines. A numerical flow model is a cornerstone in this application, but the lack of a bucket geometry and model runner has prevented the development of such a model. DynaVec, a turbine producer who specializes on sediment erosion and corrosion problems, offered to help by providing a bucket geometry and a model runner.The objective of this Master's thesis was to develop and validate a CFD model that predicts the torque applied to a non-stationary Pelton bucket, subject to a high-speed water jet. The numerical model was based on a method proposed by DynaVec, and the bucket geometry used in the simulations was identical (1:1) to the model runner.Numerous simulations were conducted, testing mesh dependency and different operational points (e.g. head). Mesh independence occurred at approximately 4.5 million elements. Furthermore, simulations of varying heads showed that the model may be independent of the head (40-80m), but this was not verified properly.Experiments showed that the numerical prediction was fairly accurate. A comparison of the numerical and experimental measurements showed that the CFD model over-predicts the torque by approximately 1.5%. This prediction was validated for the specific geometry used in the simulations, and a head of 75m.Overall, the results suggest that the numerical model is promising as a parametric design tool, but further development is required to obtain a true validation of the model.Task three and four were changed in agreement with Ole Gunnar Dahlhaug, because Solemslie's design program was delayed. In essence, the parametric study proceeded in favor of the development of a CFD model. To ensure that this work would benefit future research, especially students at the Waterpower laboratory, a detailed procedure for the CAD modeling, meshing and physical setup was included in the Appendix.

The quest for more efficient machines is always ongoing in the engineering world. This project is no different. ABB are investigating a new type of propeller that seems to offer increased efficiency compared to normal screw propellers. That is a so called foil wheel propeller. The foil move in a circular pattern with the fluid stream moving in the radial direction of the propeller instead of the axial as in a screw propeller. If the propeller is placed and modeled correctly it can also be used as a thrust vectoring device. This report focuses on the fluid physics of the foil wheel propeller, or as it is called in this report radial flow propeller. First of all the movements and interactions of the blades must be understood. Both to keep the efficiency high to compete with screw propellers, but also to foresee any problems that may occur with such a new device. A scaled down version of the propeller have been commissioned by ABB and will be tested in some time after the work within this report is completed. The effects associated to this will also be analyzed. The tool to compute the flow physics of the radial flow propeller will be computational fluid dynamics. Computational fluid dynamics uses a numerical method to compute the entire fluid field in space and time. The flow around the propeller is highly complex so a detailed analysis is needed if a well functioning control system is to be constructed for instance. The differences between the downscale and the full-scale are great, even when the non dimensional coefficients are considered. The down-scale case will be less efficient, it will be difficulties predicting the performance of the full-scale since the downscale flow is much less powerful than the full-scale case. The interaction between the blades has a large effect. There is a strong relation between angle of attack and the number of blades. The forces that are large change by about 30\% so it must definitely be considered if a model is to be used for a control system.

Focusing on helicopter aerodynamics, it is known that the aerodynamic performance of the retreating side of a rotor disk is mainly dictated by the stall characteristics of the blade. Stall under dynamic conditions (Dynamic Stall) is the dominant phenomenon encountered on heavily loaded fast-flying rotors, resulting in an extra lift and excessive pitching moments. Dynamic stall (DS) can be idealised as the pitching motion of a finite wing and this is the focus of the present work which includes three main stages. At first, comparisons between available experimental data with CFD simulations were performed for 3D DS cases. This work is the first detailed CFD study of 3D Dynamic Stall and has produced results indicating that DS can be predicted and analysed using CFD. The CFD results were validated against all known experimental investigations. In addition, a comprehensive set of CFD results was generated and used to enhance our understanding of 3D DS. Straight, tapered and swept-tip wings of various aspect ratios were used at a range of Reynolds and Mach numbers and flow conditions. For all cases where experimental data were available effort was put to obtain the original data and process these in exactly the same ways as the CFD results. Special care was put to represent exactly the motion of the lifting surfaces, its geometry and the boundary conditions of the problem. Secondly, the evolution of the Ω-shaped DS vortex observed in experimental works as well as its interaction with the tip vortices were investigated. Both pitching and pitching/rotating blade conditions were considered. Finally, the potential of training a Neural network as a model for DS was assessed in an attempt to reduce the required CPU time for modelling 3D DS. Neural networks have a proven track record in applications involving pattern recognition but so far have seen little application in unsteady aerodynamics. In this work, two different NN models were developed and assessed in a variety of conditions involving DS. Both experimental and CFD data were used during these investigations. The dependence of the quality of the predictions of the NN on the choice of the training data was then assessed and thoughts towards the correct strategy behind this choice were laid out.

High speed planing hulls are currently widely used for example in recreational and emergency vessel applications. However, very little CFD research has been done for planing vessels, especially for those with stepped hulls. A validated CFD method for planing stepped hulls could be a valuable improvement for the design phase of such hulls. In this thesis, a CFD method for stepped hulls, with a primary focus on two-step hulls, is developed using STAR-CCM+. As a secondary objective, porpoising instability of two-step hulls is investigated. The simulations are divided into two parts: In the first part a method is developed and validated with existing experimental and numerical data for a simple model scale planing hull with one step. In the second part the method is applied for two two-step hulls provided with Hydrolift AS. A maximum two degrees of freedom, trim and heave, are used, as well as RANS based k-w SST turbulence model and Volume of Fluid (VOF) as a free surface model. The results for the one-step hull mostly corresponded well with the validation data. For the two-step hulls, validation data did not exists and they were first simulated with a fixed trim and sinkage and compered between each other. In the simulations with free trim and heave both hulls experienced unstable porpoising behavior.

The work presented in this thesis aims to provide various tools to be used during design process to make maximum use of the increasing availability of accurate engine blade measurement data for high fidelity fluid mechanic simulations at a reasonable computational expense. A new method for uncertainty propagation for geometric error has been proposed for fluid mechanics codes using adjoint error correction. Inexpensive Monte Carlo (IMC) method targets small uncertainties and provides complete probability distribution for the objective function at a significantly reduced computational cost. A brief literature survey of the existing methods is followed by the formulation of IMC. An example algebraic model is used to demonstrate the IMC method. The IMC method is extended to fluid mechanic applications using Principal Component Analysis (PCA) for reduced order modelling. Implementation details for the IMC method are discussed using an example airfoil code. Finally, the IMC method has been implemented and validated for an industrial fluid mechanic code HYDRA. A consistent methodology has been developed for the automatic generation of the linear and adjoint codes by selective use of automatic differentiation (AD) technique. The method has the advantage of keeping the linear and the adjoint codes in-sync with the changes in the underlying nonlinear fluid mechanic solver. The use of various consistency checks have been demonstrated to ease the development and maintenance process of the linear and the adjoint codes. The use of AD has been extended for the calculation of the complete Hessian using forward-on-forward approach. The complete mathematical formulation for Hessian calculation using the linear and the adjoint solutions has been outlined for fluid mechanic solvers. An efficient implementation for the Hessian calculation is demonstrated using the airfoil code. A new application of the Independent Component Analysis (ICA) is proposed for manufacturing uncertainty source identification. The mathematical formulation is outlined followed by an example application of ICA for artificially generated uncertainty for the NACA0012 airfoil.

A better understanding of turbine performance and its sensitivity to variations in the inletboundary conditions is crucial in the quest of further improving the efficiency of aero engines.Within the research efforts to reach this goal, a high-pressure turbine test rig has been designedby Rolls-Royce Deutschland in cooperation with the Deutsches Zentrum für Luft- und Raumfahrt(DLR), the German Aerospace Center. The scope of the test rig is high-precision measurement ofaerodynamic efficiency including the effects of film cooling and secondary air flows as well as theimprovement of numerical prediction tools, especially 3D Computational Fluid Dynamics (CFD).A sensitivity analysis of the test rig based on detailed 3D CFD computations was carried outwith the aim to quantify the influence of inlet boundary condition variations occurring in the testrig on the outlet capacity of the first stage nozzle guide vane (NGV) and the turbine efficiency.The analysis considered variations of the cooling and rimseal leakage mass flow rates as well asfluctuations in the inlet distributions of total temperature and pressure. The influence of anincreased rotor tip clearance was also studied.This thesis covers the creation, calibration and validation of the steady state 3D CFD modelof the full turbine domain. All relevant geometrical details of the blades, walls and the rimsealcavities are included with the exception of the film cooling holes that are replaced by a volumesource term based cooling strip model to reduce the computational cost of the analysis. Thehigh-fidelity CFD computation is run only on a sample of parameter combinations spread overthe entire input parameter space determined using the optimal latin hypercube technique. Thesubsequent sensitivity analysis is based on a Kriging response surface model fit to the sampledata. The results are discussed with regard to the planned experimental campaign on the test rigand general conclusions concerning the impacts of the studied parameters on turbine performanceare deduced.

Aerodynamic analysis has become one of the most important tools in many engineering applications. In this sense, this thesis work is aimed at performing aerodynamic analysis of different geometries, expanding the available knowledge and obtaining valuable insight from the obtained results. Aerodynamic analysis can be carried out, principally, in two ways: Experimental research and Computational Fluid Dynamics (CFD). The former makes use of prototypes, wind tunnels and test tracks, making it a very expensive option. On the other hand, CFD makes use of numerical tools to solve the Navier-Stokes equations within a computational discretized domain. This latter approach is essentially limited by the available computational power and by the aerodynamicist's experience. This work comprises eight chapters. The first one is an introduction to the type of flows and geometries considered, as well as, the general methodology followed in the posterior studies. The following six chapters are the core of this dissertation, and encompass the numerical resolution of the Navier-Stokes equations in selected geometries, ordered by complexity level. In particular, the contents of these seven chapters have been submitted or published in international journals and conferences. For this reason, they are self contained and few changes have been made. The reader might find that some concepts are repeated along them. The last chapter contains concluding remarks. Finally, appendix 1 describes some applications of aerodynamic studies to some related projects and appendix 2 comprises a list of publications done during the PhD.
El análisis aerodinámico se ha convertido en una de las herramientas más importantes en diversidad de aplicaciones de ingeniería. En este contexto, esta tesis está enfocada a realizar análisis aerodinámicos en diferentes geometrías, contribuyendo datos nuevos a la comunidad científica y extrayendo información útil de los resultados obtenidos. Dichos análisis se pueden realizar, principalmente, de dos maneras: Mediante investigación experimental y mediante simulaciones numéricas (CFD). Para realizar experimentos se han de construir prototipos para su uso en túneles de viento y pistas de prueba, con los altos costos que esto conlleva. En otro extremo se encuentra el CFD, donde haciendo uso de herramientas computacionales se resuelven numéricamente las ecuaciones de Navier-Stokes en un dominio computacional. Este segundo enfoque se ve limitado por la potencia de cálculo disponible y la experiencia del aerodinamicista. Este trabajo se compone de ocho capítulos. En el primer capítulo se realiza una breve introducción a los tipos de flujos y geometrías consideradas en este estudio, así como la metodología a usar en el resto de capítulos . Los siguientes seis capítulos constituyen el cuerpo de este documento, y presentan la solución numérica y posterior análisis de las ecuaciones de Navier-Stokes en algunas geometrías de relevancia. Los contenidos de estos seis capítulos han sido presentados para su publicación en revistas indexadas y congresos. El último capítulo presenta las conclusiones extraídas de la presente tesis. Finalmente, el apéndice 1 presenta el análisis aerodinámico aplicado a problemas industriales reales, mientras que el apéndice 2 presenta la lista de publicaciones realizadas durante el desarrollo del doctorado.

En el ámbito de la Oleohidráulica, las bombas de pistón poseen los diseños más sofisticados, de hecho, las bombas de pistones son las únicos capaces de trabajar a altas presiones, además de poseer el mejor rendimiento de todo el grupo de bombas existentes. Sin embargo, cabe señalar que todos los diseños de las bombas de pistón, se basan principalmente en la experiencia de los diseñadores, por lo tanto no existen herramientas matemáticas para optimizar el diseño de las diferentes partes de las bombas. Por otra parte, en la actualidad hay empresas como Oilgear Towler, que inserta ranuras (surcos) en los patines deslizantes y en los pistones, (dos partes principales de estas bombas), pero no hay ningún estudio científico para analizar sus ventajas o desventajas. Por lo tanto, es necesario comprender matemáticamente las ventajas y desventajas debido a la presencia de ranuras en la superficie de diferentes partes de la bomba. Hay cuatro superficies de deslizamiento en las bombas de pistones, plato inclinado patín deslizante, barrilete y placa de cierre, pistón cilindro y junta esférica entre pistón y patín deslizante. Lubricación entre estas superficies es necesaria, apareciendo por tanto fugas de fluido a bombear entre las mismas. En este proyecto, nuestro objetivo es analizar cada una de estas diferentes superficies de deslizamiento por separado para comprender su diseño y el efecto de los parámetros de diseño en el comportamiento de la bomba. Una vez se tenga un buen entendimiento de las diferentes partes de la bomba de pistones, el objetivo es modelar el comportamiento dinámico de la presión y flujo en la salida de la bomba. En concreto se ha realizado: Conjunto plato inclinado, patín deslizante – Estudio de las características estáticas y dinámicas del patín deslizante, incluyendo la ranura tallada en el patín. Las ecuaciones de Navier Stokes en coordenadas cilíndricas se han aplicado entre el patín y el plato incluyendo la ranura. Los resultados presentados en este trabajo contemplan, distribución de la presión, las fugas de fluido, la fuerza y par sobre el patín, se ha estudiado la variación de dichos parámetros al modificar las dimensiones y posición de la ranura. El comportamiento dinámico del patín se ha tenido también en cuenta. Se estudia la posición de la ranura con el fin de optimizar el comportamiento del patín. Barrilete, placa de cierre.- Se analiza mediante la simulación de las ecuaciones de Reynolds de lubricación por FDM (método de diferencias finitas), la distribución de presiones, las fugas, la fuerza y los pares entre el barril y la placa de cierre. La fuerza total y los pares de torsión sobre el barril, se evalúan partiendo de la presión simulada, mostrando que los pares dinámicos que existen sobre el eje XX son mucho menores que los pares actuantes sobre el eje YY. . Pistón cilindro - Se ha investigado el comportamiento del pistón mediante la modificación del número de ranuras y su posición, la distribución de la presión en el intersticio pistón-cilindro, la fuerza sobre el pistón, las fugas y el par de torsión que actúa sobre el pistón se han analizado. También las zonas donde la cavitación es probable que aparezca se han presentado, se discute la forma de prevenir la aparición de cavitación a través del uso de ranuras. La ecuación de lubricación de Reynolds se ha modelizado en el intersticio pistón-cilindro mediante el uso de volúmenes finitos, la excentricidad y el movimiento relativo pistón-cilindro se han considerado. Diferentes configuraciones de ranuras han sido evaluadas con el fin de encontrar mínimas fugas, máximo par y mínima aparición de cavitación. Se especifican instrucciones de diseño para optimizar el comportamiento del pistón. Modelo dinámico de la bomba.- Se ha presentado un amplio conjunto de ecuaciones explícitas para cada parte con movimiento relativo de la bomba de pistones. Todas las ecuaciones se han validado mediante un análisis numérico y en su caso experimental. Las ecuaciones han sido combinadas para estudiar de forma dinámica las perturbaciones de presión y el caudal de fugas. El efecto de la pulsación de caudal cuando se modifica el diseño de la bomba también es presentado. En esta tesis, un modelo de simulación basado en ecuaciones analíticas se ha desarrollado, modelo que produce resultados muy rápidamente y aclara con mucha precisión el efecto de las fugas a través de los diferentes intersticios de la bomba.
In the field of Fluid Power, piston pumps possess the most sophisticated designs, in fact, pistons pumps are the only ones capable of working at high pressures, besides possessing the best performance (efficiency) of the entire group of existing pumps. However, it is noted that all the designs of piston pumps, are mostly based on the experience of the designers, thus there exist no mathematical tools for optimizing the design of the different parts of the pumps. On the other hand, there are now companies like Oilgear Towler, who inserted slots (grooves) in the slippers and in the pistons, (two major parts of these pumps) but there is no scientific study to analyze its advantages or disadvantages. There is therefore a need to understand mathematically to study the advantages and disadvantages due to the presence of the groove on the surface of different pump parts. There are four sliding surfaces in the piston pump, Slipper-swash plate gap, Barrel-valve plate gap, Piston-barrel chamber gap and Spherical bearing, where lubrication exists and leakages through these channels occur. In this project, our aim is to analyze each of these different sliding surfaces separately to understand its design constrains and the effect of the design parameters on the pump behavior. After having a better understanding of all the different parts of the piston pump, the aim is to model the dynamic behavior of pressure and flow at the outlet of the pump. Slipper plate gap - To understand static and dynamic characteristics of a piston pump slipper with a groove. Three dimensional Navier Stokes equations in cylindrical coordinates have been applied to the slipper/plate gap, including the groove. The results presented in this thesis include, pressure distribution, leakage, force and torque variations when groove dimensions and position are being modified, the effect of slipper tangential velocity and turning speed are also considered. Design instructions to optimize slipper/groove performance are also given. Barrel-valve plate gap - Present thesis, analyses the pressure distribution, leakage, force and torque between the barrel and the port plate of an axial piston pump by simulating Reynolds equations of lubrication by FDM (finite difference method). The overall mean force and torques over the barrel are evaluated from simulated pressure and it shows that the torque over the XX axis is much smaller than the torque over the YY axis. A detailed dynamic analysis is then studied by using the temporal torque calculated by Bergada. Piston-barrel chamber gap - It is being investigated the piston performance by modifying the number of grooves and their position, pressure distribution in the clearance piston-cylinder, leakage force and torque acting over the piston will be discussed, also the locations where cavitation is likely to appear will be presented, discussing how to prevent cavitation from appearing via using grooves. A finite volume based Reynolds equation model has been formulated for the piston-cylinder clearance which considers the piston eccentricity and the relative tangential movement between piston and barrel. Different configurations of the grooves have been evaluated in search of finding minimum leakage, minimum appearance of cavitation and maximum restoring torque. Design instructions to optimize the piston behavior are also given. Full pump Model - An extensive set of explicit equations for every pump gap will be presented. All of the equations will be checked via performing a numerical analysis of the specified pump clearance, the equations will then be combined to study dynamically pressure ripple and leakages. The effect on the flow ripple when modifying the pump design will also be presented. Therefore in present thesis, a simulation model based on analytical equations has been developed which produce very fast results and clarify very precisely the effect of different leakages happened through the pump clearances.

Horizontal-axis wind turbines operate in a complex, inherently unsteady aerodynamic environment. The flow over the blades is dominated by 3-D effects, particularly during stall, which is accompanied by massive flow separation and vortex shedding. There is always bluff-body shedding from the turbine nacelle and support structure which interacts with the rotor wake. In addition, the high aspect ratios of wind turbine blades make them very flexible, leading to substantial aeroelastic deformation of the blades, altering the aerodynamics. Finally, when situated in a wind farm, turbines must operate in the unsteady wake of upstream neighbors. Though computational fluid dynamics (CFD) has made significant inroads as a research tool, simple, inexpensive methods, such as blade element momentum theory, are still the workhorses in wind turbine design and aeroelasticity applications. These methods are unable to accurately predict rotor loads near the edges of the operating envelope. In this work, a range of unstructured grid CFD techniques for predicting wind turbine loads and aeroelasticity has been developed and applied to the NREL Unsteady Aerodynamics Experiment Phase VI rotor. First, a kd-tree based nearest neighbor search algorithm was used to improve the computational efficiency of an approximate unsteady actuator blade method. This method was then shown to predict root and tip vortex locations and strengths similar to an overset method, but without the computational expense of modeling the blade surfaces. A hybrid Reynolds-averaged Navier-Stokes / Large Eddy Simulation (HRLES) turbulence model was extended to an unstructured grid framework and demonstrated to improve predictions of unsteady loading and shedding frequency in massively separated cases. For aeroelastic predictions, a methodology for tight coupling between an unstructured CFD solver and a computational structural dynamics tool was developed. Finally, time-accurate overset rotor simulations of a complete turbine---blades, nacelle, and tower---were conducted using both RANS and HRLES turbulence models. The HRLES model was able to accurately predict rotor loads when stalled. In yawed flow, excellent correlations of mean blade loads with experimental data were obtained across the span, and wake asymmetry and unsteadiness were also well-predicted.

As a small step towards their long-term vision of one day producing emission free vessels, Wallenius em-ployed, in 2009, Mårten Silvanius to carry out his master thesis for them in which he studied five different concepts to reduce the overall fuel consumption using wind powered systems. The vessel on which his study was performed is the 230 m LCTC vessel M/V Fedora. One of the concepts studied was the bow wing which is thought to generate enough force in the ship direction to profitably reduce the overall wind resistance. His calculations showed that the wing would be the preferred method of the different concepts studied since it was determined cheapest to build, had good payback, had good global drag reducing ef-fects and had a predicted performance of a reduction in fuel cost between 3-5% on a worldwide route.This thesis is conducted mainly to verify the results of Silvanius numerical study. The method chosen is to perform a fully viscous 3-D CFD study on the entire flow around the above water portion of the ship in full scale. A 3-D model is created and the wing is placed using suggestions given by Silvanius.One major limitation in this project was the computational capacity available at the time this thesis was conducted. In order to run some of the viscous grids created the grids had to be severely coarsened. This had a negative impact on the reliability on some of the results.Since it has been difficult to obtain satisfactory solutions, no work has been done to optimize the shape and position of the wing.Nevertheless, one it has been shown that the wing does in fact affect the resistance in a positive way, however nowhere near as much as predicted by Silvanius. This effect needs to be further determined through further calculations, both using CFD and also through experimental wind tunnel testing where alternatives to the wing profile should be tested, e.g. replacing the wing with a vortex generator to further delay the point of separation.

This thesis reports the numerical investigation of the automotive ventilated disc brake rotor. Disc brakes operate on the principle of friction by converting kinetic energy into heat energy. The main objective of a disc brake rotor is to store this heat energy and dissipate it as soon as possible. This work is carried out in a area where there is very limited understanding. Commercial CFD code FLUENT was used for carrying out the simulations with the rotor rotating in still air. Only one passage and blade were simulated as all the rotor passages were identical. Uniform temperatures were used on the rotor to simulate the braking condition. Sixteen different blade angle sets were simulated and the range of blade angles having the best aero-thermal performance were identified using mass flow rate, rate of heat dissipation and temperature uniformity as performance metrics. The effect of rotational speed and rotor temperature (corresponding to various braking conditions) on the aero-thermal performance was evaluated. The rotor speed and temperature were observed to have significant effect on the rotor performance. The number of blades in the ventilated disc brake rotor was also varied and was observed to have an impact on the aero-thermal performance of the disc brake rotor. Detailed design changes like inlet chamfer, blade leading edge rounding, and variable thickness blade and passage aspect ratio were incorporated. All these changes did have an effect on the aero-thermal performance of the disc brake rotor. The inlet chamfer and the leading edge rounding improved both the rate of heat transfer and the temperature uniformity. The variable thickness blade and the lower aspect ratio passage improved the temperature uniformity of the rotor.

This thesis presents the study of transonic cavity flows using CFD. The main focus of the thesis is on the turbulence modelling and simulation of cavity flows. The thesis aims to show the range of applicability of turbulence modelling for cavity flows. Aspects of cavity flow control are also addressed. The cavity models a weapons bay with a length-to-depth (L/D) ratio of 5 and length-to-width (L/W) ratio of 1. The flow is set to transonic speeds (M=0.85) and the Reynolds number based on the cavity length is in the turbulent regime (Re=6.783 million). At these flow conditions, very high noise levels are encountered inside the cavity combined with intense acoustic and turbulent interactions. Unsteady Reynolds-Averaged Navier-Stokes (URANS) was initially applied and the effectiveness of various two-equation turbulence models such as the Wilcox k - ω and the Menter's Baseline k - ω and SST turbulence models was assessed. Computations were first performed with the 2D clean cavity to minimise the computational cost, where the 2D cavity was a reasonable representation of the full 3D cavity with doors-on. Results demonstrated that linear statistical turbulence models generally gave reasonable qualitative predictions of the cavity flow-field but on coarse grids only. The amplitudes of the noise levels and frequencies were however less well predicted and the level of agreement deteriorated with grid refinement for the L/D=5 cavity. Nonetheless, out of the models employed, the SST model proved to be the most robust and provided the best agreement with experimental pressure and PIV measurements. The velocity distributions and the turbulent and acoustic spectra at the cavity floor were also analysed and compared with experiments (where possible) and in doing so the influence of turbulent processes in the cavity highlighted. With the higher acoustic frequencies and the broadband noise less well predicted with linear statistical two-equation turbulence models, attention was diverted towards simulation methods such as Large-Eddy Simulation (LES) and Detached-Eddy Simulation (DES). Numerical results for the 3D L/D=5 cavity with a width-to-depth ratio (W/D) of 1 in both doors-on and doors-off configurations were compared with experiments. Even for coarse grid simulations, better agreement was found between the LES/DES results and experimental pressure and PIV measurements for various grid levels and time-steps than URANS.

In this current work it is presented the two-dimensional axisymmetrical air flow simulations under laminar regime in a conical diffuser using Computer Fluid Dynamics (CFD). The use of diffusers in laminar flow can be seen in micro-pumps applications, especially for micro-electronics cooling. The objective is to analyze the static pressure recovery coefficient (Cp) for Reynolds 64, varying the diffuser expansion angle and the diffuser exit/entrance area ratio (A2/A1 = 1.5 and 2.0) for the cases with and without tail pipe. The diffuser geometric configurations and the Cp formulation are based in the current ESDU 73024 (Engineering Sciences Data Unit) publication. The partial differential equations system (Continuity and Navier-Stoke) was solved using a computer program based in the numerical Finite Element method. For diffusers without tail pipe (A2/A1 = 1.5 and 2.0), the Cp values under turbulent flow are higher than Cp values under laminar flow for the same expansion angles. For diffuser with tail pipe (A2/A1 = 1.5 and 2.0), the Cp values under turbulent flow are higher than Cp values under laminar flow up to 18 diffuser expansion angle. Above 18, the Cp for turbulent and laminar flow follow a similar trend. For diffuser (A2/A1 = 1.5 and 2.0) under turbulent flow with and without tail pipe, the Cp results were similar up to 10 diffuser expansion angle. Above 10, the diffusers with tail pipe presented Cp results higher than diffusers without tail pipe. The same occurs for diffuser (A2/A1 = 1.5 and 2.0) under laminar flow with and without tail pipe. Therefore, the finite element method showed a good agreement to solve this kind of problem and the results are important once static pressure recovery coefficient data for laminar flow is scarce in the literature.

Actuellement, les énergies renouvelables semblent prendre au jour le jour plus de poids dans les politiques énergétiques actuelles. Entre les multiples formes, l’énergie éolienne, ayant une grande présence de nos jours, est celle qui présente des prévisions de majeure croissance lors des prochaines décennies. Un outil très efficace pour la prédiction de la ressource éolienne sont les simulations CFD. Cette technique permet de résoudre les équations qui gouvernent le mouvement d’écoulement en tenant compte des effets de recirculation et de séparation. Evidemment, les résultats d’une simulation CFD pour la prédiction de la ressource éolienne dépendent de la modélisation que l’on effectue mais aussi du maillage. Le maillage du domaine d’étude doit être le résultat d’une fonction optimisée tenant en compte deux paramètres: la qualité des résultats et les coûts de simulation. Visant cette fonction, deux analyses de sensibilité de maillage à travers une batterie de cas. Les paramètres géométriques visé sont la résolution des différentes zones d’études, l’extension des zones, hauteur du domaine, etc. L’objectif principal est de recommander un guide de paramètres de maillage aux usagers de CFDWind1.0 lors de l’étude de l’écoulement du vent en CFD sur terrain complexe puis offshore, tout en ayant atteint la convergence de maillage. Les résultats obtenus lors du maillage en terrain complexe sont très satisfaisant alors que lors de l’étude de l’écoulement en sillage, la modélisation doit être améliorée. Plusieurs voies d’amélioration sont proposées en visant des futures études.

Reliable thermal modelling approaches are crucial to transformer thermal design and operation. The highest temperature in the winding, usually referred to as the hot-spot temperature, is of the greatest interest because the insulation paper at the hot-spot undergoes the severest thermal ageing, and determines the life expectancy of the transformer insulation. Therefore, the primary objective of transformer thermal design is to control the hot-spot temperature rise over the ambient temperature within certain limit. For liquid-immersed power transformers, the hot-spot temperature rise over the ambient temperature is controlled by the winding geometry, power loss distribution, liquid flow rate and liquid properties. In order to obtain universally applicable thermal modelling results, dimensional analysis is adopted in this PhD thesis to guide computational fluid dynamics (CFD) simulations for disc-type transformer windings in steady state and their experimental verification. The modelling work is split into two parts on oil forced and directed (OD) cooling modes and oil natural (ON) cooling modes. COMSOL software is used for the CFD simulation work For OD cooling modes, volumetric oil flow proportion in each horizontal cooling duct (Pfi) and pressure drop coefficient over the winding (Cpd) are found mainly controlled by the Reynolds number at the winding pass inlet (Re) and the ratio of horizontal duct height to vertical duct width. The correlations for Pfi and Cpd with the dimensionless controlling parameters are derived from CFD parametric sweeps and verified by experimental tests. The effects of different liquid types on the flow distribution and pressure drop are investigated using the correlations derived. Reverse flows at the bottom part of winding passes are shown by both CFD simulations and experimental measurements. The hot-spot factor, H, is interpreted as a dimensionless temperature at the hot-spot and the effects of operational conditions e.g. ambient temperature and loading level on H are analysed. For ON cooling modes, the flow is driven by buoyancy forces and hot-streak dynamics play a vital role in determining fluid flow and temperature distributions. The dimensionless liquid flow and temperature distributions and H are all found to be controlled by Re, Pr and Gr/Re2. An optimal design and operational regime in terms of obtaining the minimum H, is identified from CFD parametric sweeps, where the effects of buoyancy forces are balanced by the effects of inertial forces. Reverse flows are found at the top part of winding passes, opposite to the OD results. The total liquid flow rates of different liquids for the same winding geometry with the same power loss distribution in an ON cooling mode are determined and with these determined total liquid flow rates, the effects of different liquids on fluid flow and temperature distributions are investigated by CFD simulations. The CFD modelling work on disc-type transformer windings in steady state present in this PhD thesis is based on the dimensional analyses on the fluid flow and heat transfer in the windings. Therefore, the results obtained are universally applicable and of the simplest form as well. In addition, the dimensional analyses have provided insight into how the flow and temperature distribution patterns are controlled by the dimensionless controlling parameters, regardless of the transformer operational conditions and the coolant liquid types used.

This project is concerned with CFD simulations of jets issued from elliptical nozzles. The investigated jet flow in this project is turbulent flow emanating from microscopic nozzles into a combustion chamber. Jet flows are very common in engineering, medical and environmental applications and are for instance used in fuel injection systems, spray painting and drying. Jet flow devices are also very common in applications such as cutting, hydraulic drilling, cooling and heating. A better understanding of the flow phenomenons in jet flows are required in order to make these devices function and perform in a more efficient way. The performance of diesel engines is strongly affected by the fuel spray, atomization and in turn the mixing process. This depends ultimately on the dimensions and geometry of the nozzle. The purpose of this project was therefore to investigate different elliptical nozzle geometries which also was compared to a conventional circular nozzle. Three dimensional simulations have been performed to investigate flow quantities in the turbulent Reynold’s Averaged Navier Stokes and Large Eddy Simulation models in a single phase flow. Simulation of a two-phase flow with the Large Eddy Simulation model was also performed to investigate the inception and development of cavitation. The Volume of Fluid approach was used to describe the twophase flow and Rayleigh-Plesset equation to solve bubble dynamics.The mathematical models regarding those in single phase flow have been solved in the CFD software ANSYS FLUENT, while those in two-phase flow have been solved in the open source C++ toolbox OpenFOAM 2.0.0.

To maximise the amount of energy extracted from wind turbines, the rotor diameter has increased, reaching values of 160m in some cases. Large scale wind turbines are working at high Reynolds numbers and a wide range of flow conditions, with virtually incompressible flow present at the root and mildly compressible near the blade tips, where the Mach numbers can reach locally 0.48 for the largest wind turbines employed to date. In traditional aerodynamics, most CFD methods were designed to cope with high Mach number flows and consequently solve the compressible Navier-Stokes equations. This is the case of the Helicopter Multi-Block (HMB2) CFD method from Liverpool University. The present PhD thesis aims to provide an all-Mach-number capability to the HMB2 method, by implementing modified Roe schemes to account for low-Mach flows. For 2D cases, the modified Roe schemes showed great improvement in the convergence and the quality of the solution, when compared with the Original Roe and Osher schemes, and the Low-Mach Roe scheme showed the best performance. With the low-Mach capability included in the compressible solver, both MEXICO and NREL Annex XX experiments were simulated. A detailed analysis of the velocity field behind the MEXICO rotor was performed, where the low-Mach scheme (LM-Roe) showed less sensitivity on the grid size than the Osher scheme. Accurate prediction of wind turbine wake breakdown is also important for the performance analysis of the turbines and their optimal positioning within tightly-spaced wind farms. Using a fine mesh able to preserve the vortices up to 8R downstream the MEXICO rotor plane, the instabilities on the wake leading to vortex pairing were captured. FFTs of the axial velocity component enabled to identify the main harmonics in the wake. In the stable region, the wake was a perfect spiral and the main frequency was the bladepassing one. An approximate exponential growth was then observed and in the region where instabilities were present, higher frequencies dominated, leading an oscillatory pattern. Simple wake models were also investigated and a combination between a kinematic model to account for the wake initial expansion and a field model to account for the far wake decay was proposed, showing good agreement with the CFD solution. With the correct set of constants, it was proved that this simple model can be used to approximate the behaviour of wind turbine wakes with minimal computational cost. Another consequence of the increased size of wind turbines is that their stiffness lowers and aeroelasticity therefore plays an important role, since the blades can suffer great deformations. To account for the blade deformations, a tightly coupled CFD-CSD method was employed to analyse the MEXICO and NREL Annex XX wind turbines. For the latter, the tower and nacelle were considered as stiff bodies and the blades were allowed to deform. As a result of the aeroelastic calculations, the blades showed deformation in bending (towards the tower). The maximum deflections were present after the blades had passed in front of the tower, and maximum amplitudes of 0.59%R, at 20m/s were observed.

The CFD simulation of cooling piston compressor Stream, manufactured by Emerson company, is the topic of this diploma thesis. Analysis of moving parts (piston, valves) and refrigerant or physical settings for simulation were based on experimental data provided by Emerson. The goal of the thesis is to test opportunities of Star-CCM+ in simulating the flow inside the compressor. In the end there will be a comparison of experimental data and results from the simulation. The thesis also contains a theoretical background of piston compressors and phenomenon following the operating compressor.

The aim of this thesis is to design an investment strategy focused on tool called CFD. The first theoretical part contains basic information about financial derivates and explains basic principles of trading on stock market with contracts for difference. Chapter of the investment analysis explains the methods and rules used in trading. The practical part presents the results of my trading CFDs and evaluated my proposed strategy along with the benefits of work.

In this study, the thermal management system of a notebook computer is investigated by using a commercial finite volume Computational Fluid Dynamics (CFD) software. After taking the computer apart, all dimensions are measured and all major components are modeled as accurately as possible. Heat dissipation values and necessary characteristics of the components are obtained from the manufacturer'
s specifications. The different heat dissipation paths that are utilized in the design are investigated. Two active fans and aluminum heat dissipation plates as well as the heat pipe system are modeled according to their specifications. The first and second order discretization schemes as well as two different mesh densities are investigated as modeling choices. Under different operating powers, adequacy of the existing thermal management system is observed. Average and maximum temperatures of the internal components are reported in the form of tables. Thermal resistance networks for five different operating conditions are obtained from the analysis of the CFD simulation results. Temperature distributions on the top surface of the chassis where the keyboard and touchpad are located are investigated considering the user comfort.

The increasing use of Intelligent Transport System (ITS) can enable very close vehicle spacings which generally results in a net drag reduction for the resulting convoys. The majority of vehicle development has, to date, been for vehicles in isolation, thus the study of interaction effects is becoming increasingly important. The main objective of this research is to investigate the use of Computational Fluid Dynamics (CFD) for understanding convoy aerodynamics and to further the understanding of airflow interaction between vehicles via CFD. In this study, time-averaged characteristics of a simplified, generic passenger vehicle, called the Ahmed car model, after Ahmed et.al (1984) is investigated computationally using the available commercial CFD code, Fluent version 6.1.22. Three different platoon combinations were analysed for the current study which includes a two, three and six model platoons for various rear end configurations of the Ahmed model geometry. Experiments were conducted in RMIT University Industrial Wind Tunnel for analysing the effects of drafting on drag coefficients using two different scales of Ahmed car models. This is an extension to the previous study performed on two 100% scales of Ahmed models (Vino and Watkins, 2004) and the results for both the current and previous experiments were compared using CFD. The CFD proved to be a useful technique since its results compared reasonably well for both the current and the previous experiments on drafting, using Ahmed models of identical (30°) rear slant configurations. However, near critical rear slant angles (~30°) for isolated vehicles some discrepancies were noted. The reasonable validation of experimental results enabled the study to be extended further computationally using CFD, to analyse the effects of inter-vehicle spacing on a platoon of 3 and 6 models for various rear end configurations (between 0° and 40°), in an attempt to provide useful information on vehicle-wake interaction for the Future Generation Intelligent Transport System (FGITS). Critical gaps were identified via CFD for the case of a two, three and six model platoons and the simulations clearly exposed the reasons for these critical gaps. At extremely close proximity, the models experienced more pressure recovery at their rear vertical base, which reduced the drag coefficient. Surprisingly, at some of the close vehicle spacings, the drag coefficients reached values that were higher than that of a vehicle in isolation. This was found due to the high momentum flow impingement to the fore body of the model and was similar to results found in physical experiments. Thus the current CFD analysis revealed that rear slant angle of the model and the inter-vehicle spacing greatly influences the wake structures and ultimately the vehicles aerodynamic drag coefficients in platoons. Even though the current CFD model (Realizable k-B turbulence model) predicted the basic flow structures such as the C-pillar vortices from the rear slant and 2D horse shoe vortices in the model's vertical rear base, the separation bubble on the rear slant that supplies energy to the strong C-pillar vortices was not replicated accurately, which is evidenced from the flow structure analysis. Hence it is recommended for further work, that the study should be extended using the Reynold's stress models or the Large Eddy Simulation (LES) turbulence models for flow structure observation and analysing vortex interactions between the models.

For the simulation of control surface buzz accurate prediction of the shock location and the chock strength is essential and this is currently achieved using Euler and RANS based CFD analysis. To calculate the motion of the control surface only the flap rotation mode needs to be modelled. In the current work the CFD solver is coupled with a modal based FEM solver. The multi-level hierarchical blending transformation methodology is applied for the aeroelastic analyses of complex geometries. The methodology is used for the treatment of blended control surfaces and the effect of the blending on the aero-structural response is measured. Forced clap oscillations of a Supersonic Transport (SST) configuration are simulated and the dynamic deformation of the wing and the unsteady pressure due to the forced oscillations are validated against experiments. Transonic buzz on a trailing edge flap is investigated on the Supersonic Transport configuration using the RANS and the Euler equations. Characteristics associated with buzz instability are reproduced computationally and the effect of the flap on the wing flutter is measured. Finally, aeroelastic simulations are performed on the Hawk aircraft. The combat flap configuration of the Hawk aircraft is investigated using CFD and the effect of the flap on wing flutter is assessed. The aeroelastic response of the rudder at supersonic freestream Mach numbers is studied. The importance of aerodynamic interference on the aeroelastic behaviour is assessed.

Dry powder inhalers (DPIs) are widely used for the therapy of respiratory and pulmonary diseases. In this study, a coupled discrete element method and computational fluid dynamics (DEM-CFD) is employed to investigate the micromechanics of carrier-based DPIs. The effects of van der Waals forces and electrostatic forces on the mixing process, and the influences of air flow and particle-wall impact on the dispersion process are examined. For the mixing of carrier and active pharmaceutical ingredient (API) particles in a vibrating container, it is found that vibration conditions affect the mixing performance. While there is an optimal mixing condition to maximise the number of API particles attaching to the carrier (i.e. contact number) for van der Waals cases, the contact number decreases with increasing vibration velocity amplitude and frequency for electrostatic force cases. It is also revealed that van der Waals forces (short range) and electrostatic forces (long range) result in different mixing behaviours. For the air flow induced and impact induced dispersion, it is found that the dispersion performance improves with increasing air velocity, impact velocity and impact angle, and reduces with increasing work of adhesion. The dispersion performance can be approximated using the cumulative Weibull distribution function governed by the ratio of air drag force to adhesive force or the ratio of impact energy to adhesion energy.

This thesis reports numerical analyses of powder flow, mixing and segregation behavior during die filling in a vacuum and in air using an Eulerian-Lagrangian model, which employs a Discrete Element Method (DEM) for the particles and Computational Fluid Dynamics (CFD) for the air with a two-way air-particle interaction coupling term. The effects of air and particle properties (size, density, size distribution, cohesion etc.) on powder flow are explored. The results are in a good agreement with experimental observations. Powder flow is characterized in terms of a dimensionless mass flow rate and a critical filling velocity. When air is present, the powder flow characteristics depend on the particle size and density and can be classified into an air-sensitive regime and an air-inert regime. It is found that the difference in particle size and/or density can cause segregation during die filling. Therefore, parametric studies are undertaken to examine the effects of some factors, such as particle size or density ratio, shoe speed, initial mass fraction of fine particles, initial height of powder bed and cohesion. Suction filling with a movable punch is also simulated. It has been shown that the utilization of suction can significantly improve the powder flow rate and reduce the density-induced segregation.

Thesis (MEng)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: The capability of computational fluid dynamics (CFD), in particular the FLUENT ™ commercial software suite, to predict wind loadings on heliostats has been investigated. If CFD proves useful in this area then the overall development costs of heliostats and concentrating solar thermal power plants could be reduced. Due to the largest loading on the heliostat originating from wind loads, by using CFD to determine these loads it could be possible to ensure heliostats are not overdesigned. This thesis contains a first study within the Solar Thermal Energy Research Group (STERG) at Stellenbosch University into the use of CFD for determining heliostat wind loads. The relevant theoretical background concerning the turbulence models used in this study, namely, the RNG k-ε, Realisable k-ε and SST k-ω turbulence models is reiterated. The „standard‟ k-ε model and the large eddy simulation (LES) approach, due to their relevance to bluff body flows, are also revisited. Some analysis is also provided around each model to gain insight as to the role of respective modelling sensitivities and their advantages. Previous work done in the area of heliostat wind studies is reviewed. The geometric considerations when dealing with heliostats leads onto the discussion concerning the requirement of modelling boundary layer profiles. Hence some background is provided on boundary layer modelling techniques. Further insight is drawn from more general previous bluff body CFD reported in the literature, from which observations and recommendations regarding the use of variations of the k-ε turbulence model can be inferred. The simulation procedure from geometry creation to results obtained for the flow over a vertical flat plate is reported. This investigation led to the conclusion that the Realisable k-ε should be used for the heliostat simulations on account of its accurate drag prediction under steady state flow conditions. It was also found that for transient simulations for heliostat like geometries, the SST k-ω model appears most suitable. The Realisable k-ε model is then used to model the flow about a heliostat using the same procedures as for the flat plate; both with flat and boundary layer inlet profiles. The overall conclusions drawn from this work are that the Realisable k-ε would not be suitable for predicting wind loads used in the final design of heliostats although it may be used with flat velocity and turbulence profiles to compare differences between early heliostat designs. The conclusion that the Realisable k-ε model should not be used to predict the flow field in the vicinity of a heliostat is also reached. It is recommended that further work should be carried out by using more advanced modelling techniques, such as the LES, to determine wind loads on heliostats. Furthermore, additional studies focused on accurately reproducing the velocity and turbulence profiles should be done. Lastly a larger set of data containing the orientations mentioned in literature should be generated using the methods contained within this study.
AFRIKAANSE OPSOMMING: Die vermoë van Numeriese Vloei Meganika (NVM), spesifiek die van die FLUENT ™ kommersiële sagtewarepakket, om die windlaste op heliostate te voorspel was ondersoek. As daar gevind word dat NVM wel betekinsvolle resultate kan lewer, kan dit die totale ontwikkelingskoste van heliostate en gekonsentreerdesonkragstasies verlaag. Wind plaas die grootste las op heliostate, dus deur gebruik te maak van NVM om die windlaste op heliostate te voorspel, kan dit gebruik word om te verseker dat heliostate nie oorontwerp word nie. Hierdie tesis bevat „n eerste studie binne die Sontermiese Energie Navorsings Groep aan die Universiteit van Stellenbosch, wat die gebruik van NVM om windlaste op heliostate te voorspel ondersoek. Alle relevante teoretiese agtergrond wat turbulensiemodelle aanbetref, naamlik die RNG k-ε, Realiseerbare k-ε en SST k-ω turbulensiemodelle, word bespreek. Hulle relevansie tot stompligaamvloei toegestaan, word die „standaard‟ k-ε model en die groot werwel simulasie (GWS) benaderings ook bespreek. Elke model word bespreek om die leser insig te gee in dié model se sensitiwiteite en voordele. Vorige studies wat betrekking het tot die studie van heliostate en wind word bespreek. Die geometrie van heliostate lei tot „n bespreking oor die noodsaklikheid vir „n model vir die grenslaagprofiel, dus word grenslaagmodelleringstegnieke bespreek. Verdere insig word verkry van vorige NVM studies uit die literatuur met meer algemene stomp liggame, wat waarnemings en voorstelle vir die gebruik van die k-ε turbulensiemodel en variante verskaf. Die simulasieproses, vanaf geometrieskepping tot die resultate vir die vloei oor 'n vertikale vlak, word bespreek. Hierdie ondersoek het tot die gevolgtrekking gelei dat die realiseerbare k-ε model gebruik moet word vir die heliostaat simulasies, as gevolg van die akkurate sleurvoorspellings onder bestendigetoestande. Daar was ook gevind dat vir heliostaatagtige liggame onder oorgangskondisies, die SST k-ω model mees geskik sal wees. Die Realiseerbare k-ε model word dan gebruik om die vloei om 'n heliostaat te modelleer deur gebruik te maak van dieselfde proses wat gebruik word om vloei oor 'n plat plaat te analiseer: albei met plat en grenslaaginlaatprofiele. Die gevolgtrekkings van hierdie studie is dat die Realiseerbare k-ε model nie gebruik kan word tydens die finale ontwerpfase om die windlaste op 'n heliostaat te voorspel nie. Dit kan wel gebruik word met plat snelheids- en turbulensieprofile om die versikille tussen vroeë heliostaatkonsepte te vergelyk. Daar was ook bepaal dat die Realiseerbare k-ε model nie gebruik moet word om die vloeiveld om 'n heliostaat te voorspel nie. Daar word voorgestel dat verdere studies in hierdie vakgebied met meer gevorderde modelleringstegnieke aangepak word. Dit word aanbeveel dat verdere werk uitgevoer moet word deur die gebruik van meer gevorderde modellering tegnieke, soos GWS, om die wind kragte op heliostats te bepaal. Verder, studies wat akkurate snelheid en turbulensieprofiele produseer sal nog bygelas moet word. Laastens 'n groter stel data met oriëntasies soos wat in die literatuur beskryf word, moet deur middel van die metodes van dié studie gegenereer word.

The evaluation of how Computational Fluid Dynamics (CFD) package may be incorporated into a conceptual design method is performed. The repeatability of the CFD solution as well as the accuracy of the calculated aerodynamic coefficients and pressure distributions was also evaluated on two different wing-body models. The overall run times of three different mesh densities was also evaluated to investigate if the mesh density could be reduced enough so that the computational stage of the CFD cycle may become affordable to use in the conceptual design stage. A farfield method was derived and used in this analysis to calculate the lift and drag coefficients. The CFD solutions were also compared with two methods currently used in conceptual design - the vortex lattice based program Vorview and ACSYNT. The unstructured Euler based CFD package FELISA was used in this study.
Master of Science

In the current work the focus is on natural ventilation occurring in buildings. The first step is to discuss the potential of numerical models applied to determine building performance and air flow as part of a mixed mode building control scheme with respect to a test case. A dynamic simulation software (TRNSYS) is used to estimate the annual energy demand. An optimization program (GenOpt) changes iteratively the parameters regulating the airflow within the building model in order to minimize the whole year energy use. Elements which are considered in the analysis are outdoor climatic conditions and elements representing the building use, such as internal gains. The numerical results of this analysis have shown how a proper analysis of natural ventilation phenomena occurring in small premises can lead to energy saving while thermal comfort is not compromised. This has been tested in various Italian climates, by means of tuning different parameters handling the natural ventilation in a mixed mode office room. The natural ventilation to be effective has to be carefully designed and it may not just occur. On the other hand in large enclosures such as atria or churches, where much more complicated phenomena occur in the indoor air flow, the natural ventilation is not so straightforward to analyse. This fact pushes the application of numerical methods capable of higher resolution in order to catch the features of the streams, with the aim of achieving good levels of thermal comfort and indoor air quality. In particular, in the last years a growing attention has been paid on the analysis of building airflow, mostly due to the diffuse interest in reducing energy losses and optimizing the efficiency of heating systems. A nontrivial technical problem is the heating of churches, because nowadays they are used both for religious services and as cultural places. Such problem is still open and no solution has been so far found. In the course of the last century, with the installation of new heating systems, an increase in damage and the decay of valuable interior decoration have been noticed. Moreover in this kind of environments, because of the remarkable heights and the presence of great windows, relevant natural and mixed convection flows may show up and, on the other hand, stratification phenomena may occur, with hot air stagnation far from occupied zones. This can cause people discomfort or energy waste. Therefore it is not possible to design heating plants only to maximize energy efficiency, but the heating systems have to accomplish the best compromise between preservation of cultural property, economy, energy and comfort. For handling all these conflicting requirements at once, simplified or macroscopic models, which describe the real system only with a small number of temperature, pressure and flow rate values, are no more enough, but microclimate field flow models, based on computational fluid dynamics methods (CFD), provide a powerful and versatile tool to obtain a more reliable prediction of the air movement and temperature distribution within the built environment. The case of St. Marien’s church has been investigated to prove the utility of this kind of analysis. On the basis of experimental data collected during winter 2003-2004, a thermal model of St. Marien church has been produced and tuned. The results provided by this model have been used in this work to perform several simulations on St. Marien church with the commercial CFD software FLUENT, in order to find out a computational model which could be a good compromise between simplicity on geometrical representation, saving on computational resources, accuracy and reliability of the solution. Once the model was set up, it has been validated against some temperature values recorded during the monitoring period. Simulations have highlighted a shortcut of the flow from the inlet to the outlet, giving the reason why energy model of the church tuned on the experimental data are overestimating energy consumption. This would not have been possible with a macroscopic analysis, showing the need for large enclosure to carry on both the analyses together. This promising result pushed to test a more direct way to couple energy models and CFD models to predict energy consumptions in buildings or buildings components involving remarkable ventilation phenomena. It came out a method to estimate ventilated components performance on annual bases in a sensible amount of time. The method was applied to a ventilated roof. In this case the considered vented roof showed to increase energy saving with respect to the typical roof layout in climates with high solar radiation levels during a wide period of the year.
Nel lavoro svolto in questa tesi l'attenzione è rivolta ai fenomeni di ventilazione naturale che avvengono negli edifici. Il primo studio ha riguardato il potenziale della modellazione numerica relativamente alle prestazioni dell'edificio come parte di uno schema di controllo della ventilazione mista in riferimento a un caso test. A tal fine è stato utilizzato un software di simulazione dinamica (TRNSYS) per valutare il fabbisogno annuale di energia accoppiato a un programma di ottimizzazione (GenOpt) in grado di modificare i parametri di interesse in maniera iterativa; i risultati dell’analisi hanno permesso di individuare la combinazione di parametri per cui il consumo energetico è minimo su base annuale. Sono state considerate nell'analisi le condizioni climatiche e gli elementi rappresentativi del tipo di utilizzo dell'edificio come i carichi interni. Dai risultati numerici di questa analisi è stato possibile mostrare come un’analisi dettagliata della ventilazione naturale all’interno di ambienti di piccole dimensioni possa portare a un risparmio di energia senza compromettere il comfort termico. Questo è stato provato per diverse regioni climatiche, tarando i diversi parametri che gestiscono la ventilazione naturale nella stanza adibita a ufficio soggetta a ventilazione mista. Il lavoro ha dimostrato come la ventilazione naturale per essere efficiente debba essere pianificata a priori sia per definire le aperture sia per stabilirne la gestione. D'altro canto, in ambienti interni di grandi dimensioni come atri o chiese, dove avvengono fenomeni molto più complessi nei campi di moto dell'aria, la ventilazione naturale non è così semplice da analizzare. Questo fatto spinge all'adozione di metodi caratterizzati da una maggiore risoluzione al fine di meglio definire le caratteristiche del deflusso, quando si studiando intende studiare il comfort termico e la qualità dell'aria. In particolare, negli ultimi anni, c'è stata una crescente attenzione riguardo allo studio dei campi di moto dell'aria all’interno degli edifici, dovuto al diffuso interesse nel ridurre le perdite di energia e a incrementare l'efficienza dei sistemi di riscaldamento. Un problema tecnico non banale riguarda il riscaldamento delle chiese, dal momento che queste oggi sono sempre più utilizzate sia per funzioni religiose che come centri culturali. Tale questione è tuttora aperta, non avendo ancora trovato una soluzione definitiva. Nel corso dell'ultimo secolo, a seguito dell'installazione dei sistemi di riscaldamento, si è manifestato un aumento di danni delle opere d’arte e delle preziose decorazioni all’interno delle chiese storiche. Inoltre in ambienti di questo tipo, a causa dell’accentuato sviluppo verticale e della presenza di grandi finestrature, si possono verificare importanti fenomeni di convezione naturale o fenomeni di stratificazione in cui l'aria calda tende a ristagnare in regioni lontane dalla zona occupata. Le conseguenze possono essere discomfort termico o spreco di energia. Pertanto non è possibile progettare impianti di riscaldamento secondo metodologie standard, quanto dal momento che gli impianti di riscaldamento devono realizzare il miglior compromesso fra conservazione dei beni culturali, costi di esercizio e di manutenzione, risparmio energetico e comfort. Per gestire queste esigenze spesso contrastanti, i modelli macroscopici o ingegneristici che descrivono il sistema reale con un numero ridotto di valori di temperatura, pressione e portata di massa, non sono molto spesso adeguati, mentre la fluidodinamica numerica è uno strumento potente e versatile per ottenere una previsione più affidabile del moto dell'aria e dei campi termici che avvengono negli edifici. Dopo aver illustrato il problema del riscaldamento delle chiese storiche e i principi della CFD, si è condotta un'analisi dettagliata della chiesa di St.Marien a Wismar per dimostrare l'utilità di questi metodi per questo tipo di applicazioni. Sulla base dei dati sperimentali raccolti durante l'inverno 2003-2004, è stato realizzato e tarato un modello energetico dinamico della chiesa. I risultati forniti hanno permesso di stimare le condizioni al contorno per una serie di simulazioni della chiesa di St.Marien basate sul codice commerciale FLUENT, per identificare un modello numerico che potesse essere un buon compromesso fra semplicità del dominio spaziale di calcolo, risparmio di risorse di calcolo, accuratezza e affidabilità della soluzione. Una volta realizzato, il modello è stato validato con alcuni valori di temperatura registrati durante il periodo di monitoraggio. Le simulazioni hanno evidenziato la presenza un cortocircuito in corrispondenza a un fan-coil installato a pavimento. Questo non sarebbe stato possibile con un'analisi basata su modelli semplificati, indicando la necessità, per i grandi ambienti, di portare avanti insieme sia le analisi macroscopiche che quelle di dettaglio con metodi CFD. Questo risultato ha spinto a provare in modo più stretto di accoppiare modelli energetici e CFD al fine di predire le prestazioni energetiche degli edifici. E’ stato quindi prodotto un metodo per stimare le prestazioni di componenti ventilati dell'involucro edilizio su base annuale. Il metodo è stato testato su un tetto ventilato. Si è quindi potuto verificare il miglior comportamento energetico del tetto ventilato rispetto a una equivalente copertura tradizionale.

Dynamic stall deeply affects the response of helicopter rotor blades, making its modeling accuracy very important. Two commonly used dynamic stall models were implemented in a comprehensive code, validated, and contrasted to provide improved analysis accuracy and versatility. Next, computational fluid dynamics and computational structural dynamics loose coupling methodologies are reviewed, and a general tight coupling approach was implemented and tested. The tightly coupled computational fluid dynamics and computational structural dynamics methodology is then used to assess the stability characteristics of complex rotorcraft problems. An aeroelastic analysis of rotors must include an assessment of potential instabilities and the determination of damping ratios for all modes of interest. If the governing equations of motion of a system can be formulated as linear, ordinary differential equations with constant coefficients, classical stability evaluation methodologies based on the characteristic exponents of the system can rapidly and accurately provide the system's stability characteristics. For systems described by linear, ordinary differential equations with periodic coefficients, Floquet's theory is the preferred approach. While these methods provide excellent results for simplified linear models with a moderate number of degrees of freedom, they become quickly unwieldy as the number of degrees of freedom increases. Therefore, to accurately analyze rotorcraft aeroelastic periodic systems, a fully nonlinear, coupled simulation tool is used to determine the response of the system to perturbations about an equilibrium configuration and determine the presence of instabilities and damping ratios. The stability analysis is undertaken using an algorithm based on a Partial Floquet approach that has been successfully applied with computational structural dynamics tools on rotors and wind turbines. The stability analysis approach is computationally inexpensive and consists of post processing aeroelastic data, which can be used with any aeroelastic rotorcraft code or with experimental data.

Abstract— The object of this paper is to investigate the influence of wave-current interaction on a tidal turbine. An experiment at Norwegian Marine Technology Research Institute (MARINTEK) has been carried out, and a CFD analysis has been performed in order to enhance the understanding of the wave induced loads on the tidal turbine. These loads are known to be the governing forces and it is therefore of great importance to predict them accurately. The CFD results are found to be trustworthy with calculated values close to experimental data. In addition to the wave-induced forces, the wake characteristics and wave influence on the wake are investigated. Results from Blade Element Moment Theory (BEM) are also compared to validate the accuracy of this method. CFD is a powerful tool if used properly, but it is computationally expensive, especially when dealing with complex geometry like a tidal turbine. A high performance computer (HPC) has been used to carry out the transient CFD wave-current simulations in order to obtain reliable results within reasonable time.

Thesis (PhD)--Stellenbosch University, 2014.
ENGLISH ABSTRACT: A Computational Fluid Dynamics (CFD) model was developed for the prediction of flotation rate constants in a stirred flotation tank and validated against experimental data. The model incorporated local, time-varying values of the turbulent flow field into an existing kinetic flotation model based on the Generalised Sutherland Equation to predict the overall flotation rate constant. Simulations were performed for the flotation of various minerals at different operational conditions and the predictions were compared with experimental data. It was found that the CFD-based model yielded improvements in the prediction of flotation rate constant for a range of hydrophobicities, agitation speeds and gas flow rates compared with existing methodologies, which use volume-averaged empirical expressions for flow variables. Moreover, comparing to the available CFD alternatives for the flotation modelling this approach eliminates the need for solving an extra partial differential equation resulting in a more computationally economic model. The model was developed in three stages. In the first, a single-phase model was used to establish the requirements for successful modelling of the velocity components and turbulent properties of water inside flotation tanks. Also, a novel use of the Grid Convergence Index for this application was carried out, which allowed determination of the maximum achievable reduction in numerical uncertainties through systematic grid refinement and adaptation. All subsequent simulations were performed at the optimal discretization level determined in this manner. It was found that the Moving Reference Frames (MRF) method was adequate for representation of the impeller movement when the rotational zone was located close to the impeller, using a time step advance of between 10◦ and 15◦ of impeller rotation. Comparison of the different turbulence models for the single-phase modelling revealed that the standard k-e and Large Eddy Simulation turbulence models both performed equally well and that the computational requirement was lower for the standard k-e model, making it the method of choice. Validation of the methodology was done by comparison with experimental data for two different stirred tanks including an unbaffled mixer and a fully baffled standard Rushton turbine tank. The validation against experimental data showed that the model was capable of predicting the flow pattern, turbulent properties and the generation of trailing vortices. The second stage of modelling used an Eulerian-Eulerian formulation for gasliquid modelling of gas-sparged fully baffled vessels (2.25 l, 10 l and 50 l) using a Rushton turbine. It was determined that the minimum model uncertainty resulting from simulation of the sparger was achieved using a disk sparger with a diameter equal to 40% of the impeller diameter. The only significant interfacial force was found to be the drag force, and this was included in the multiphase methodology. A parametric study on the available formulations for the drag coefficient was performed which showed that the effect of turbulence on the air bubbles can accurately be represented using the proposed model of Lane (Lane, 2006). Validation of the methodology was conducted by comparison of the available experimental gas holdup measurements with the numerical predictions for three different scales of Rushton turbine tanks. The results verified that the application of the designed sparger in conjunction with Lane drag coefficient can yield accurate predictions of the gas-liquid flow inside the flotation tank with the error percentage less than 6%, 13%, and 23% for laboratory, pilot and industrial scale Rushton turbine tanks, respectively. The last stage of this study broadened the Eulerian-Eulerian framework to predict the flotation rate constant. The spatially and temporally varying flow variables were incorporated into an established fundamental flotation model due to Pyke (Pyke, 2004) based on the Generalized Sutherland equation for the flotation rate constant. The computation of the efficiency of the flotation sub-processes also incorporated the turbulent fluctuating flow characteristics. Values of the flotation rate constants were computed and volume-weight averaged for validation against available experimental data. The numerical predictions of the flotation rate constants for quartz particles for a range of particle diameters showed improvements in the predictions when compared with values determined from existing methodologies which use spatially uniform values for the important hydrodynamic variables as obtained from empirical correlations. Further validations of the developed CFD-kinetic model were carried out for the prediction of the flotation rate constants of quartz and galena floating under different hydrophobicities, agitation speeds and gas flow rates. The good agreement between the numerical predictions and experimental data (less than 12% error) confirmed that the new model can be used for the flotation modelling, design and optimization. Considering the limited number of CFD studies for flotation modelling, the main contribution of this work is that it provides a validated and optimised numerical methodology that predicts the flotation macro response (i.e., flotation rate constant) by integrating the significance of the hydrodynamic flow features into the flotation micro-processes. This approach also provides a more economical model when it is compared to the available CFD models for the flotation process. Such an approach opens the possibility of extracting maximum advantage from the computed parameters of the flow field in developing more effective flotation devices.
AFRIKAANSE OPSOMMING: 'n Wye verskeidenheid van industriële toepassings gebruik meganies geroerde tenks vir doeleindes soos die meng van verskillende vloeistowwe, verspreiding van 'n afsonderlike fase in 'n deurlopende vloeistoffase en die skeiding van verskillende komponente in ‘n tenk. Die hoofdoel van die tesis is om ‘n numeriese model te ontwikkel vir ʼn flotteringstenk. Die kompleksiteit van die vloei (drie-dimensioneel, veelvuldige fases en volledig turbulent) maak die voorspelling van die werksverrigting van die flottasieproses moeilik. Konvensioneel word empiriese korrelasies gebruik vir modellering, ontwerp en die optimalisering van die flotteringstenks. In die huidige studie word ‘Computational Fluid Dynamics’ (CFD) egter gebruik vir die modellerings doel, aangesien dit ‘n alternatief bied vir empiriese vergelykings deurdat dit volledig inligting verskaf aangaande die gedrag van vloei in die tenk. Die model is ontwikkel in drie agtereenvolgende stadiums. Dit begin met ‘n strategie vir enkelfase modellering in die tenk, vorder dan na ‘n gas-vloeistof CFD model en brei dan die tweede stap uit om ‘n CFD model te skep vir die skeidingsproses deur flottering. ‘n Enkelfase model, gebaseer op die kontinuïteits- en momentumvergelykings, dien as basis vir die flottasie model. Die ‘Multiple Reference Frames’ (MRF) metode word gebruik om die rotasie van die stuwer na te boots, terwyl die dimensies van die rotasie-sone gekies is om die gepaardgaande onsekerhede, insluitend die model- en numeriese foute veroorsaak deur die dimensies van die roterende sones, te verminder. Die turbulensie model studie het getoon dat die standaard k-e turbulensie model redelike akkuraatheid kon lewer in die numeriese voorspellings en die resultate verskil in gemiddeld net minder as 15% van die eksperimentele lesings, terwyl die rekenaartyd min genoeg was om die simulasies op 'n persoonlike rekenaar uit te voer. Verder het die ‘Grid Convergence Index’ (GCI) metode die inherente onsekerhede in die numeriese voorspellings gerapporteer en gewys dat die onderskatting van die turbulensie wat algemeen plaasvind reggestel kan word deur van ‘Large Eddie’ (LES) of ‘Direct Numerical Simulations’ (DNS) gebruik te maak. Die metode wat ontwikkel is, is op twee tipes geroerde tenks getoets, naamlik 'n onafgeskorte menger en 'n standaard Rushton turbine tenk. Die numeriese resultate is teen eksperimentele data gevalideer en het gewys dat die model in staat is om die vloeipatrone, turbulensie einskappe en die vorming van agterblywende vortekse te voorspel. Die CFD resultate het getoon dat die vloeipatroon twee simmetriese rotasies siklusse bo en onder die roterende sone vorm, terwyl die vlak van die ooreenkoms tussen die numeriese voorspellings van die turbulente eienskappe en die eksperimentele lesings met minder as 25% verskil. As die tweede stap van hierdie navorsing is 'n Eulerian-Eulerian struktuur ontwikkel vir die gas-vloeistof modellering binne 'n standaard Rushton turbine flotteringstenk. Soos vir die enkelfase modellering is die Reynolds spanningstensor opgelos deur die standaard k-e turbulensie model, terwyl die lugborrels ingevoer/versamel is in/van die tenk deurmiddel van bron/sink terme. Verskeie ‘sparger’ rangskikkings is in die tenk geïmplementeer om die onsekerheid in die model weens die metode van luginspuiting te verminder. Verder is verskillende korrelasies vir die sleursyfer vergelyk vir laminêre en turbulente vloei in die tenk. Daar is gevind dat die skyf ‘sparger’, met 'n deursnee gelykstaande aan 40% van die stuwer deursnee, in samewerking met die voorgestelde model van Lane (Lane, 2006) vir die bepaalde sleursyfer die naaste ooreenkoms met die eksperimentele metings lewer (met 'n gemiddelde verskil van minder as 25%). 'n Vergelykende studie is ook uitgevoer om die gevolge van die gas vloeitempo en roerspoed vir drie verskillende geroerde tenks met volumes van 2.5 l, 10 l en 50 l te ondersoek. Die resultate van hierdie afdeling bevestig dat die CFD metode in staat was om die gas-vloeistof vloei in die flotteringstenk korrek te voorspel. Die veelvuldigefase model wat ontwikkel is, is uitgebrei vir flottasie modellering. Dit behels die integrasie van die CFD resultate met die fundamentele flottasie model van Pyke (Pyke, 2004) vir die flotteringstempo konstant. Die CFD model is toegerus met Pyke se model deur aanvullende gebruiker gedefinieerde funksies. Die CFD-kinetiese model is geëvalueer vir die flottering van kwartsdeeltjies en die resultate het die geloofwaardigheid van die model bevestig, aangesien die gemiddelde verskil tussen die numeriese voorspellings vir die flotteringstempo konstante en die eksperimentele data minder as 5% was. Die resultate is ook vergelyk met die analitiese berekeninge van Newell en daar is bevind dat die model vergelykbare voorspellings van die flotteringtempo konstantes lewer, met die ‘root mean square deviations’ (RMSD) gelyk of minder as die RMSD waardes vir die analitiese berekeninge. Verdere ondersoeke van die CFD-kinetiese model bestaan uit 'n parametriese studie wat die gevolge van die roertempo, gas vloeitempo en die oppervlak hidrofobisiteit op die flottering van kwarts- en galenietdeeltjies bestudeer. Die aanvaarbare ooreenkoms tussen die numeriese voorspellings en eksperimentele data (oor die algemeen minder as 12% fout) bewys dat die nuwe model gebruik kan word vir flotterings modellering en optimalisering.

This thesis describes the development of grid generation and numerical methods for predicting the flow in variable geometry, positive displacement screw machines. It has been shown, from a review of available literature, that the two main approaches available to generate deforming grids for the CFD analysis of 3D transient flow in screw machines are algebraic and differential. Grids that maintain the cell count and connectivity, during solution, provide the highest accuracy and customised grid generation tools have the capability to accommodate large mesh deformations. For the analysis of screw rotors with a variable lead or varying profile, these techniques are suitable but are required to be developed further with new procedures that can define the three dimensional variation of geometry of the rotors onto the computational grid. An algebraic grid generation method was used for deforming grid generation of variable lead and varying profile rotors. Functions were developed for correlating a specified lead variation along the rotor axis with the grid spacing. These can be used to build a continuously variable lead with linear, quadratic or higher order functions. For variable profile rotors, a novel approach has been developed for three dimensional grid structuring. This can be used to specify a continuously variable rotor profile, a variable lead, and both internal and external rotor engagement, thus making it possible to generate rotor domains with conical and variable lead geometries. New grid distribution techniques were developed to distribute boundary points on the rotors from the fixed points on the rack and the casing. These can refine the grid in the region of interlobe leakage gaps between the rotors, produce a one to one connected interface between them and improve the cell quality. Inflation layers were applied and tested for mesh refinement near the rotor boundaries. Case studies have been presented to validate the proposed grid generation techniques and the results have been compared with experimental data. Simulated results agreed well with measured data and highlighted the conditions where deviations are highest. Results with variable geometry rotors showed that they achieve steeper internal pressure rise and a larger discharge port area could be used. With variable lead rotors the volumetric efficiency could be improved by reducing the sealing line length in the high pressure zone. Calculations with inflation layers showed that local velocities were better predicted but there was no substantial influence on the integral performance parameters.

The object of this work is to apply CFD simulation in the description of the spray burning. As a case study, a pressure swirl injector, characterized and tested by NIST, has been chosen, which atomize liquid kerosene in an atmosphere of gaseous oxygen. The chamber dimensions allow a complete evaporation, avoiding the impact of drops on the circular wall. Swirl-axisymmetric domain and steady state permit to include combustion, a complex process, without requiring of high computational resources. Continuous phase is treated with an Eulerian reference, while fuel drops are tracked following the Lagrangian formulation. Chemical kinetics is reduced to the concept of mixture fraction. This assumption avoids the solution of too many transport equations for all involved species. In the first simulation, the inlet boundary of the continuous phase is obtained from the numerical solution of a fully developed flow transporting the oxidant gas. Then, four cases are proposed and solved, changing the turbulence intensity and swirl velocity on the inlet boundary, each parameter with two different values. Finally, results for the axial velocity, streamlines, drops trajectories, temperature, distribution and total production of selected species are analyzed and compared with other related studies.

Oil and gas platforms, refineries and chemical plants need to burn off the excess gas resulting from pressure variations during production. The failure of flare tips, sometimes with short lifetimes, has been a major cause for concern in the oil and gas industry for many years. The aim of this study was to evaluate and improve the performance of flare tips. The study has been approached from two perspectives: (i) material requirements, identifying the most suitable alloys for use in flare tips, and (ii) design optimisation, aimed at the development of a flare tip that minimises interaction with flame, therefore giving lower operating temperatures and longer lifetimes. The thesis also includes an Infra-Red (IR) thermal imaging study to establish flare tip temperature profiles during flaring. Examination of failed flare tips has provided evidence of intergranular oxidation and stress corrosion cracking as possible failure mechanisms. A study of the effect of thermal shock on the oxidation resistance of alloys 800H and 625, currently used in flare tips, is presented. The embrittlement of alloy 625 in the range 650 °C to 800 °C has also been investigated. Thermal imaging of three flares in operation has indicated metal temperatures of up to 1000 °C, above levels that can be sustained by alloys currently in use. A Finite Element model of stress distributions based on the temperature profiles has been developed. It was concluded that flare tip lifetimes would be limited by a combination of creep and fatigue of the support brackets, and by plastic deformation at the top of the windshield. The model successfully predicted the failure of two flare tips and lead to a timely replacement, resulting in significant financial savings and the prevention of catastrophic failure. Commercial Computational Fluid Dynamics software, that solves the Navier-Stokes equation combined with a combustion model, has been used to assess the effect of gas flow rates and wind conditions on combustion behaviour and the resulting operating temperatures of flare tips. The model has been validated with data obtained from thermal imaging studies and shows reasonable agreement, especially at low gas flow rates. As a result, a procedure has been developed to calculate flare tip temperature profiles (via CFD) and mechanical integrity (via FE stress analysis) of flare tips, and thus assess suitability of any flare tip design prior to manufacture and installation.

Assessing the impact of inlet flow distortion in turbomachinery is desired early in the design cycle. This thesis introduces and validates the use of methods based on the Proper Orthogonal Decomposition (POD) to analyze clean and 1/rev static pressure distortion simulation results at design and near stall operating condition. The value of POD comes in its ability to efficiently extract both quantitative and qualitative information about dominant spatial flow structures as well as information about temporal fluctuations in flow properties. Observation of the modes allowed qualitative identification of shock waves as well as quantification of their location and range of motion. Modal coefficients revealed the location of the passage shock at a given angular location. Distortion amplification and attenuation between rotors was also identified. A relationship was identified between how distortion manifests itself based on downstream conditions. POD provides an efficient means for extracting the most meaningful information from large CFD simulation data. Static pressure and axial velocity were analyzed to explore the flow physics of 3 rotors of a compressor with a distorted inlet. Based on the results of the analysis of static pressure using the POD modes, it was concluded that there was a decreased range of motion in passage shock oscillation. Analysis of axial velocity POD modes revealed the presence of a separated region on the low pressure surface of the blade which was most dynamic in rotor 1. The thickness of this structure decreased in the near stall operating condition. The general conclusion is made that as the fan approaches stall the apparent effects of distortion are lessened which leads to less variation in the operating condition. This is due to the change in operating condition placing the fan at a different position on the speedline such that distortion effects are less pronounced. POD modes of entropy flux were used to identify three distinct levels of entropy flux in the blade row passage. The separated region was the region with the highest entropy due to the irreversibilities associated with separation.

Orientadores: Rubens Maciel Filho, Andre Luiz Jardini Munhoz
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Quimica
Made available in DSpace on 2018-08-14T19:20:20Z (GMT). No. of bitstreams: 1 Bineli_AulusRobertoRomao_M.pdf: 24103427 bytes, checksum: e0dc4ef614d43eab9b12a55f01124378 (MD5) Previous issue date: 2009
Resumo: Em tratamentos térmicos de têmpera há uma grande dificuldade em entender os diferentes perfis de resfriamento que ocorrem na superfície e no interior dos materiais, e que definem o controle da estrutura formada e das propriedades finais desejadas. A formação de diferentes tipos de estruturas no mesmo material pode ocorrer devido ao resfriamento não uniforme provocado pelas condições fluidodinâmicas do tanque e do fluido refrigerante, os quais determinam as taxas de resfriamento e o valor do coeficiente de transferência de calor. Além disso, há muito pouco na literatura sobre os critérios para a construção de tanques de têmpera. Portanto este trabalho investiga por meio da Fluidodinâmica Computacional (CFD), utilizando o software ANSYS CFX® 11, duas configurações de um sistema de agitação submerso em tanque de têmpera e o impacto das condições fluidodinâmicas e das propriedades físicas do fluido sobre a uniformidade do resfriamento e no coeficiente de transferência de calor na interface do bloco de aço. Como conseqüência as simulações permitem a verificação de alternativas de como o processo pode ser melhorado a partir deste tipo de análise. O processo físico estudado consiste no resfriamento de um bloco de aço nas dimensões 2,3m x 1,2m x 0,86m imerso em tanque com água de dimensões 8,7m x 2,8m x 4,0 m com um sistema de agitação de jato submerso distribuídos em vários bicos reguladores de saída de água. Foram realizadas duas simulações, a primeira envolvendo o sistema de agitação localizado sob o bloco. Na segunda, entretanto, foi acrescentado um sistema de agitação localizada nas laterais do material na tentativa de homogeneizar o fluxo do fluido entorno do bloco, consequentemente sobre a uniformidade do resfriamento. Os resultados deste trabalho indicam que o sistema foi suscetível a variação das propriedades físicas do fluido e do fluxo sobre o material o que levou a grandes variações na curva de resfriamento para o primeiro caso. Contudo, a implementação do sistema lateral de agitação promoveu uma melhora significativa na uniformidade da têmpera, além disso, o modelo foi capaz de predizer as curvas de resfriamento, os coeficientes de transferência de calor na interface do material, e os fluxos do fluido no tanque. A análise discutida fornece informações de como o software pode melhorar o controle do processo de resfriamento por estudos sobre a uniformidade da têmpera, o que pode auxiliar os engenheiros na concepção e desenvolvimento de novos projetos de tanque levando-se em consideração a forma e o tipo do sistema de agitação, bem como a geometria do tanque e do material, e o fluido utilizado no processo. Esta abordagem pode produzir melhorias significativas na qualidade do material enquanto simultaneamente prevê condições para redução de distorções do material durante o tratamento térmico.
Abstract: In the quenching heat treatment is a great difficulty to understand the different cooling profiles occurring at the surface and subsurface of the material, that define the structure formed and the final properties desired. The formation of different types of structures in the material can occurs due to uneven cooling caused by fluid dynamic conditions of the tank, which determine the cooling rates and the heat transfer coefficient. Moreover, there is very little literature concerning the criteria for the construction of quenching tanks. Therefore in this work was analyzed by means of Computational Fluid Dynamics (CFD), two configurations of submerged agitation system and the impact of fluid dynamic conditions and the physical properties of the fluid on the cooling uniformity and the heat transfer coefficient at the interface of the steel block. The simulations performed allow the verification of alternatives of how the process can be improved from this type of analysis. The physical process studied consist in the cooling of a steel block with dimensions 2.3m x 1.2m x 0.86m immersed in water tank with dimensions 8.7m x 2.8m x 4.0m with submerged agitation system. There were two simulations, the first involving the agitation system located under the block. In the second, however, was added agitation system located next the sides of the material in an attempt to homogenize the fluid flow around the block, consequently on the uniformity of cooling. The results indicate that the system was susceptible to variations in the fluid properties and fluid flow on the material which led to large variations in the cooling curve for the first case. The implementation of the sideway agitation system led to a significant improvement in uniformity of quenching, in addition, the model was able to predict the cooling curves, the heat transfer coefficient at the interface of the material, and fluid flow in the tank. The analysis provides information about how software can improve the control of the cooling process by studies of quench uniformity, which can help engineers in the design and development of new tank taking into account the type of agitation system, tank geometry and material, and the fluid used in the process. This approach can produce significant improvements in the quality of the material while simultaneously provide conditions to reduce distortions in the material during heat treating.
Mestrado
Mestre em Engenharia Química

Accidental releases of flammable hydrocarbons in chemical process industries can trigger severe hazards: explosions, fires, and dispersion of toxic vapour clouds. Explosions and toxic releases may injure people within large damage radius; however, fires are the most common accidental events that may lead to catastrophic consequences in terms of life and property losses. Within this framework, the prediction of the related-fire effects may significantly contribute to identify measures needed to eliminate or mitigate the consequences of accidents in processing environments. Semi-empirical methods can provide rapid estimations of the flame-geometry descriptors as well as estimations of the heat flux received at a given distance from the fire origin. Based on that information, active protection systems and inherent safer design measures (i.e. safety distances between equipment) can be determined to prevent major fire accidents. Nevertheless, these are based on empirical and statistical data, and do not cover the overall characteristics of the fire behaviour. Computational Fluid Dynamics (CFD) modelling can provide more detailed insights of the related fire effects considering additional complexity, such as different geometries and alternative boundary conditions, and representing different fire sizes: from small to large scale fires. Nevertheless, CFD requires detailed input data, expert knowledge on the phenomenon simulated and on the physical models implemented, and demands high computational resources. The use of CFD modelling for technological risk analysis is still incipient, so detailed validation exercises are needed before their use in real applications. This thesis is mainly aimed at assessing the predictive capabilities of different CFD codes (FDS, FLACS-Fire and FireFOAM) when predicting the hazardous effects of hydrocarbon pool fires and jet fires. Specifically, large-scale pool fires of diesel and gasoline (from 1.5 to 6 m-diameter), vertical sonic jet fires of propane (from 0.09 to 0.34 kg/s with orifice diameters of from 10 to 25.5 mm), vertical subsonic jet fires of methane in normal- and sub- atmospheric pressures (from 0.6 to 1 bar with an orifice diameter of 3 mm), and vertical and horizontal subsonic jet fires of propane (from 0.007 to 0.11 kg/s with orifice diameters of from 12.75 to 43.1 mm-diameter) have been modelled in different CFD codes. Prescribing burning rates provide accurate predictions of the pool fire effects with maximum cell sizes of 0.2 m. On the other hand, the cell sizes of sonic and subsonic jet fires should be determined by considering a fire characteristic diameter of 16 and 12, respectively. A minimum number of 400 solid angles is recommended to obtain accurate estimations of the thermal flux. Based on the numerous computational simulations performed, Best Practice Guidelines (BPG) are developed to determine a code as ‘valid’ or not, and to provide guidance on the most suitable modelling settings when performing CFD simulations of accidental hydrocarbon fires. The BPG usefulness is proved through a case study of an oil storage farm located in the Port of Barcelona. Large over-estimations of the heat flux values are found with semi-empirical correlations and thus, the safety measures required would be very conservative and costly. Therefore, CFD modelling is recommended method to perform detailed FHA in chemical and process industries.
Les fuites accidentals d'hidrocarburs inflamables en indústries de processos químics poden desencadenar greus riscos: explosions, incendis i dispersions de núvols de vapor tòxics. Les explosions i les dispersions de gasos poden ferir a persones en un radi de danys més gran; tanmateix, els incendis són els esdeveniments accidentals més habituals que poden causar conseqüències catastròfiques en termes de pèrdues de vida i de propietats. En aquest marc, la predicció dels efectes dels incendis pot contribuir significativament a identificar les mesures necessàries per eliminar o mitigar les conseqüències dels accidents en entorns de processos. Els mètodes semi-empírics poden proporcionar estimacions ràpides de la geometria de la flama així com del flux de calor rebut a una distància determinada de l'origen de l'incendi. A partir d'aquesta informació, es poden implementar sistemes de protecció actius i mesures de disseny inherents (és a dir, distàncies de seguretat entre equips) per evitar grans accidents d'incendis. No obstant, aquestes es basen en dades empíriques i no cobreixen les característiques generals del desenvolupaments dels incendis. El modelatge de dinàmica de fluids computacionals (CFD) pot proporcionar una visió més detallada dels efectes dels incendis ja que tenen en compte la complexitat addicional dels escenaris, com ara geometries i condicions límits diferents, i poden representar diferents mides d'incendis: des de petita fins a gran escala. No obstant, les simulacions CFD requereixen dades d'entrada detallades, coneixements experts sobre el fenomen simulat i sobre els models físics implementats, i exigeixen elevats recursos computacionals. L'ús del modelat CFD per a l'anàlisi del risc tecnològic encara és incipient, i per tant, es necessiten exercicis de validació abans de fomentar la seva aplicació en casos reals. Aquesta tesi està dirigida principalment a avaluar les capacitats predictives de diferents codis CFD (FDS, FLACS-Fire i FireFOAM) alhora de predir els efectes perillosos dels incendis de bassa i de dolls de foc. Concretament, de bassa a gran escala amb dièsel i gasolina (d'1.5 fins a 6 m de diàmetre), dolls de foc verticals sònics amb propà (de 0.09 fins a 0.34 kg/s amb diàmetres d'orificis compresos entre 10 i 25.5 mm), dolls de foc verticals subsònics amb metà a diferents pressions atmosfèriques (des de 0.6 fins a 1 bar amb un diàmetre d'orifici de 3 mm), i dolls de foc verticals i horitzontals subsònics amb propà (de 0.007 fins a 0.11 kg/s amb diàmetres d'orifici compresos entre 12.75 i 43.1 mm) s¿han simulat amb les diferents eines CFD. La prescripció de la velocitat de combustió proporciona prediccions precises dels efectes dels incendis de bassal quan la mida de la cel·la és de 0.2 m com a màxim. D'altra banda, la mida de la cel·la per a simulacions de dolls de foc sònics i subsònics s'ha de determinar tenint en compte un diàmetre característic de l'incendi de 16 i 12, respectivament. Es recomana un número mínim de 400 angles sòlids per obtenir estimacions precises dels fluxos tèrmics. A partir de les nombroses simulacions computacionals realitzades es desenvolupament directrius de bones pràctiques (BPG) per determinar un codi com a 'vàlid' o no, i per proporcionar orientació sobre els paràmetres de modelatge més adequats quan es realitzen simulacions CFD d'incendis accidentals d'hidrocarburs. La utilitat del les BPG es demostra mitjançant un cas d'estudi d'una granja d'emmagatzematge d'hidrocarburs situada al Port de Barcelona. Es troben grans sobreestimacions dels valors del fluxos de calor mitjançant correlacions semi-empíriques. Per tant, es recomana la utilització d'eines CFD per realitzar FHA detallats en indústries químiques i de processos.

In order to understand flow characteristics inside a positive displacement pump, every point in the flow field must be carefully observed. Such observations are difficult, expensive and usually time consuming to achieve with physical testing. During tests one can observe flow characteristics only at the locations where the instrument device is attached, not the whole flow domain. This thesis mainly focuses on the evaluation of design and performance characteristics of a positive displacement triplex diaphragm pump. For this purpose not only numerical investigations but also experimental studies were conducted using a positive displacement pump which is supplied by the pump manufacturer and is available in the fluid mechanics laboratory of Middle East Technical University. The effect of valve characteristics on the pump efficiency such as valve spring stiffness, valve displacement, mass of the check valves, and diaphragm shape are investigated in this thesis by using CFD (Computational Fluid Dynamics) technique. The pump performance is analyzed in terms of its volumetric and hydraulic efficiencies. The effect of the valve closure delay is also discussed. After the CFD and experimental results of the current pump model are compared and it is seen that they are in close agreement with each other, parametric studies are performed in computer environment. From analysis results it is observed that using stiffer springs reduces valve closing time and tend to decrease flow reversal effects. Secondly, using heavier check valves increases valve closing and opening times and also increases the stresses on the components of the pump with the increased pressure drop through discharge valve. As a result of this condition, hydraulic and volumetric efficiency reduce. Thirdly, with the longer valve displacement arrangement, more time is required for opening and closing of the check valves therefore efficiency of the pump reduces.

Although pre-swirlers play a determinant role in the transport of air from stationary parts to rotating holes, knowledge about their actual performance is limited. Therefore, this paper aims to relate how the pre-swirler pressure drop affects the performance of different pre-swirlers in terms of discharge coefficient, adiabatic pre-swirl effectiveness, and swirl ratio. The results are extracted from numerical simulations carried out on three different designs, two guide vanes, and a nozzle. When available, the results are compared to experimental data. The guide vanes have shown similar responses to the pressure drop variations. Their discharge coefficients remain relatively insensitive with an average value of 97%. The swirl ratio range from 0.704 to 1.013 and 0.703 to 1.023 respectively for a pressure drop varying from 3 to 7 bars. The adiabatic pre-swirl effectiveness is of 96% and 94%, respectively, under steady state operation.The nozzle design has shown inferior performance as compared to the guide vane designs. Its discharge coefficient remains around 91% and the swirl ratio varies between 0.678 and 1.121 for a pressure drop ranging from 3 to 10 bars. Under steady state operation, the adiabatic pre-swirl effectiveness is 1.22. The influence of through-flows on the aforementioned parameters was also analyzed. It was observed that the through-flow deteriorates the performance of the pre-swirlers, whether in terms of dimensionless pre-swirl effectiveness, or swirl ratio. The discharge coefficient was however not affected.

The multigrid algorithm is an extremely efficient method of approximating the solution to a given problem. The functions involved in the calculations are all discrete, or discontinuous, and are represented by an array of values taken from equally-spaced points along their range a "grid". The algorithm's efficiency lies in the fact that once an approximate solution to the problem is found its accuracy can be improved using calculations on increasingly sparse grids which require less processing power. In this project the theory behind the multigrid algorithm was studied and a computer program was written which demonstrates the use of this algorithm in solving the problem of natural convection. Stream function-Vorticity approach and the Bossinesq approximation were used in the programs. Also, the same problem was solved using the Multigrid algorithm using Fluent software. The results obtained were matched with the analytical results.

The application of cryocoolers are in various fields of modern day applications for adequate refrigeration at specified temperature with low power input, long lifetime, high reliability and maintenance free operation with minimum vibration and noise, compactness and light weight. The demand of Stirling cryocoolers has increased due to the ineffectiveness of Rankin cooling systems at lower temperatures. With the rise in applications of Stirling cryocoolers, especially in the field of space and military, several simulations of Stirling cryocoolers were developed. In this project, so far a detailed analysis has been carried out on previous developed model with computed lengths of different sections of the cryocooler. The previous models have been developed by solving governing partial differential equations mathematically. The regenerator efficiency had been taken 100% in the previous analysis and with performing Schmidt analyses taking sinusoidal variations of pressure and volume, previous model geometry parameters were calculated. The CFD analysis has been done in this project taking the previous model and a UDF for the movement of the piston has been developed in C++ language. The regenerator parameters have been calculated from the results of the analysis. Simulations are done on Stirling cryocooler with adiabatic conditions at the cold space side and later with a heat load of 0.25 W at cold end and their results were compared. An attempt has also been made in determining the optimum frequency of operation of the proposed model of Stirling cryocooler by comparing the minimum cool down temperature attained by them with different frequencies of operation.