A three-dimensional, parallel, anisotropic, block-based, adaptive mesh refinement (AMR) algorithm is proposed and described for the solution of fluid flows on body-fitted, multi-block, hexahedral meshes. Refinement and de-refinement in any grid block computational direction, or combination of directions, allows the mesh to rapidly adapt to anisotropic flow features such as shocks, boundary layers, or flame fronts, common to complex flow physics. Anisotropic refinements and an efficient and highly scalable parallel implementation lead to a potential for significant reduction in computational cost as compared to a more typical isotropic approach. Unstructured root-block topology allows for greater flexibility in the treatment of complex geometries. The AMR algorithm is coupled with an upwind finite-volume scheme for the solution of the Euler equations governing inviscid, compressible, gaseous flow. Steady-state and time varying, three-dimensional, flow problems are investigated for various geometries, including the cubed-sphere mesh.

The present work is a result of collaboration between the LMFA (Laboratoire de Mécanique des Fluides et d'Acoustique, Ecole Centrale de Lyon - France), Snecma and the Cerfacs. It aims at studying the flow in the 3.5-stages high-speed axial compressor CREATE (Compresseur de Recherche pour l'Etude des effets Aérodynamique et TEchnologique - rotation speed: 11543 RPM, Rotor 1 tip speed: 313 m/s), designed and built by Snecma and investigated at LMFA on a 2-MW test rig. Steady measurements, as well as laser velocimetry, fast-response wall static and total pressure measurements have been used to experimentally investigate the flow. The analysis focuses on two main aspects: the study of the flow at stable operating points, with a special interest on the rotor-stator interactions, and the study of the instabilities arising in the machine at low mass flow rates.The description of the unsteady flow field at stable operating points is done through measurements of wall-static pressure, total pressure and velocity, but also total temperature, entropy and angle of the fluid. It is shown that the complexity and unsteadiness of the flow in a multistage compressor strongly increases in the rear part of the machine, because of the interactions between steady and rotating rows. Therefore, a modal analysis method developed at LMFA and based on the decomposition of Tyler and Sofrin is presented to analyze these interactions. It is first applied to the pressure measurements, in order to extract the contributions of each row. It shows that all the complex pressure interactions in CREATE can be reduced to three main types of interactions. The decomposition method is then applied to the entropy field extracted from URANS CFD calculations performed by the Cerfacs, in order to evaluate the impact of the interactions on the performance of the machine in term of production of losses.The last part of this work is devoted to the analysis of the instabilities arising in CREATE at low mass flows. It shows that rotating pressure waves appear at stable operating points, and increase in amplitude when going towards the surge line, until reaching a critical size provoking the onset a full span stall cell bringing the machine to surge within a few rotor revolutions. The study of these pressure waves, and the understanding of their true nature is achieved through the experimental results and the use of some analytical models. A precise description of the surge transient through wall-static pressure measurements above the rotors is also provided, as well as a description of a complete surge cycle. An anti-surge control system based on the detection of the amplitude of the pressure waves is finally proposed.

A three-dimensional, parallel, anisotropic, block-based, adaptive mesh refinement (AMR) algorithm is proposed and described for the solution of fluid flows on body-fitted, multi-block, hexahedral meshes. Refinement and de-refinement in any grid block computational direction, or combination of directions, allows the mesh to rapidly adapt to anisotropic flow features such as shocks, boundary layers, or flame fronts, common to complex flow physics. Anisotropic refinements and an efficient and highly scalable parallel implementation lead to a potential for significant reduction in computational cost as compared to a more typical isotropic approach. Unstructured root-block topology allows for greater flexibility in the treatment of complex geometries. The AMR algorithm is coupled with an upwind finite-volume scheme for the solution of the Euler equations governing inviscid, compressible, gaseous flow. Steady-state and time varying, three-dimensional, flow problems are investigated for various geometries, including the cubed-sphere mesh.

Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: The use of air-cooled steam condensers (ACSCs) is the preferred cooling method in the chemical and power industry due to stringent environmental and water use regulations. The performance of ACSCs is however highly dependent on the influence of windy conditions. Research has shown that the presence of wind reduces the performance of ACSCs. It has been found that cross-winds (wind perpendicular to the longest side of the ACSC) cause distorted inlet flow conditions, particularly at the upstream peripheral fans near the symmetry plane of the ACSC. These fans are subjected to what is referred to as '2-D' wind conditions, which are characterised by flow separation on the upstream edge of the fan inlets. Experimental investigations into inlet flow distortion have simulated these conditions by varying the fan platform height. Low platform heights resulted in higher levels of inlet flow distortion, as also found to exist with high cross-wind speeds. This investigation determines the performance of various fan configurations (representative of configurations used in the South- African power industry) subjected to distorted inlet flow conditions through experimental and numerical investigations. The similarity between platform height and cross-wind effects is also investigated and a correlation between system volumetric effectiveness, platform height and cross-wind velocity is found. / AFRIKAANSE OPSOMMING: Die gebruik van lugverkoelde stoom kondensors (LVSK's) word verkies as 'n verkoelingsmetode in die chemiese- en kragvoorsieningsindustrie as gevolg van streng omgewings- en waterverbruiksregulasies. Die werkverrigting van LVSK's word egter grootliks beïnvloed deur die teenwoordigheid van wind. Navorsing het gewys dat die teenwoordigheid van wind die werkverrigting van LVSK's verminder. Daar was gevind dat kruiswinde (wind loodreg tot die langste sy van die LVSK) versteurde inlaat vloeitoestande veroorsaak, veral by waaiers wat aan die stroomop kant van die LVSK naby die simmetrievlak geleë is. Hierdie waaiers word blootgestel aan na wat verwys word as '2-D' windtoestande wat gekenmerk word deur vloeiwegbreking wat plaasvind by die stroomop rand van die waaierinlate. Eksperimentele ondersoeke van inlaat vloeiversteurings het hierdie toestande gesimuleer deur die waaier platformhoogte te verstel. Lae platform hoogtes het gelei tot hoër vlakke van inlaat vloeiversteuring, soortgelyk aan wat gevind word met hoë kruiswindsnelhede. Hierdie ondersoek gebruik numeriese en eksperimentele metodes om die werkverrigting van verskeie waaierkon gurasies (verteenwoordigend van kon- gurasies wat gebruik word in die Suid-Afrikaanse kragvoorsieningsindustrie) wat blootgestel word aan versteurde inlaat vloeitoestande te bepaal. Die ooreenkoms tussen platformhoogte en kruiswind e ekte word ook ondersoek en 'n korrelasie tussen die sisteem volumetriese e ektiwiteit, platformhoogte en kruiswindsnelheid word bepaal.

The aerospace industry has been concerned with propeller noise levels for years. This interest is two-fold: government regulation and comfort in cabin. This report attempts to create a simulation mechanism needed to evaluate the far-field noise generation levels. However, in order to do that, the tandem cylinder case was evaluated first as a validation step before the SR-7A propeller case was performed. Both cases use STAR-CCM+, a commercial software, to perform the simulations.

In this study, The Beach Dewatering System, a relatively recent technology to combat beach erosion, which is proposed as a practical alternative to more traditional shoreline stabilization methods, is investigated and an informative overview on the genesis, development and recent use of this technique is provided. On the basis of the link existing between the elevation of beach groundwater and erosional or accretionary trends at the beach face, a numerical model that simulates groundwater flow in a coastal aquifer under beach drainage is presented. In this model, the seaward boundary of the domain is considered to be tidally fluctuating in a large scale to represent the occurrence of seepage face significantly. The unsteady groundwater flow equation is solved numerically using the method of finite differences. The results clearly showed that the water table being lowered caused the reduction of the seepage face which is the main aim of Beach Dewatering projects. The positional design parameters, i.e. horizontal and vertical location of the drain, are also investigated by utilizing an efficiency index. It is observed that the system efficiency decreased as the drain is shifted landward. The results also indicated that, the efficiency slightly increased with the vertical drain elevation.

Transmissions are an essential part of a vehicle powertrain. An optimally designed powertrain can result in energy savings, reduced environmental impact and increased comfort and reliability. Along with other components of the powertrain, efficiency is also a major concern in the design of transmissions. The churning power losses associated with the motion of gears through the oil represent a significant portion of the total power losses in a transmission and therefore need to be estimated. A lack of reliable empirical models for the prediction of these losses has led to the emergence of CFD (Computational Fluid Dynamics) as a means to (i) predict these losses and (ii) promote a deeper understanding of the physical phenomena responsible for theselosses in order to improve existing models. The commercial CFD solver STAR-CCM+ is used to investigate the oil distribution and the churning power losses inside two gearbox configurations namely an FZG (Technical Institute for the Study of Gears and Drive Mechanisms) gearbox and a planetary gearbox. A comparison of two motion handling techniques in STARCCM+ namely MRF (Moving Reference Frame) and RBM (Rigid Body Motion) models is made in terms of the accuracy of results and the computational requirements using the FZG gearbox. A sensitivity analysis on how the size of gap between the meshing gear teeth affects the flow and the computational requirements is also done using the FZG gearbox. Different modelling alternatives are investigated for the planetary gearbox and the best choices have been determined. The numerical simulations are solved in an unsteady framework where the VOF (Volume Of Fluid) multiphase model is used to track the interface between the immiscible phases. The overset meshing technique has been used to reconfigure the mesh at each time step. The results from the CFD simulations are presented and discussed in terms of the modelling choices made and their effect on the accuracy of the results. The MRF method is a cheaper alternative compared to the RBM model however, the former model does not accurately simulate the transient start-up and instead provides just a regime solution of the unsteady problem. As expected, the accuracy of the results suffers from having a large gap between the meshing gear teeth. The use of compressible ideal gas model for the air phase with a pressure boundary condition gives the optimum performance for the planetary gearbox. The outcomes can be used toeffectively study transmission flows using CFD and thereby improve the design of future transmissions for improved efficiency.

Vegetation is commonly modelled as emergent arrays of rigid, circular cylinders. However, the drag coefficient (CD) of real stems or trunks is closer to that of cylinders with a square cross-section. In this thesis, vegetation has been idealised as square cylinders in laboratory experiments with a turbulence intensity of the order of 10% which is similar to that of typical river flows. These cylinders may also represent other obstacles such as architectural structures. This research has determined CD of an isolated cylinder and cylinder pairs as a function of position as well as the average drag coefficient (CDv) of larger arrays. A strain gauge was used to measure CD whilst CDv was computed with a momentum balance which was validated by strain gauge measurements for a regularly spaced array. The velocity and turbulence intensity surrounding a pair of cylinders arranged one behind the other with respect to mean flow (in tandem) were also measured with an Acoustic Doppler Velocimeter. The isolated cylinder CD was found to be 2.11 in close agreement with other researchers. Under fixed flow conditions CD for a cylinder in a pair was found to be as low as -0.40 and as high as 3.46 depending on their relative positioning. For arrays, CDv was influenced more by the distribution of cylinders than the flow conditions over the range of conditions tested. Mean values of CDv for each array were found to be between 1.52 and 3.06. This new insight therefore suggests that CDv for vegetation in bulk may actually be much higher than the typical value of 1 which is often assumed to apply in practice. If little other information is available, a crude estimate of CDv = 2 would be reasonable for many practical applications. The validity of a 2D realizable k-epsilon turbulence model for predicting the flow around square cylinders was evaluated. The model was successful in predicting CD for an isolated cylinder. In this regard the model performed as well as Large Eddy Simulations by other authors with a significant increase in computational efficiency. However, the numerical model underestimates CD of downstream cylinders in tandem pairs and overestimates velocities in their wake. This suggests it may be necessary to expand the model to three-dimensions when attempting to simulate the flow around two or more bluff obstacles with sharp edges.

Les Stations de Transfert de l’Énergie par Pompage (STEP) munies de turbines-pompes réversibles de type Francis (TP) permettent de stocker et de restituer de grandes quantités d’énergie avec des rendements très élevés. Celles-ci apparaissent comme un moyen viable d’assurer la réactivité et la stabilité du réseau électrique vis-à-vis de l’augmentation croissante des sources d’énergie renouvelables intermittentes. Pour répondre aux nouveaux besoins en régulation du réseau électrique, la technologie actuelle des STEP doit être adaptée. Accroître la réactivité requiert d’optimiser les procédures de démarrage et d’arrêt des machines. Dans les quadrants turbine, turbine-frein et pompe inverse, les TP de haute chute ont des courbes caractéristiques présentant la forme d’un « S ». Cette forme particulière peut engendrer des coups de béliers lors des phases d’arrêts d’urgences, exposant les conduites à de sévères surpressions et dépressions. De plus, pour ces points de fonctionnement associés au « S » les écoulements sont fortement instationnaires et induisent des fluctuations de pression responsables de chargements dynamiques sur les parties mécaniques. Les objectifs de ce travail sont la modélisation et la compréhension des phénomènes hydrauliques complexes liés au « S ». Des simulations numériques instationnaires sont réalisées en utilisant le modèle de turbulence SAS-SST. Moins couteux que les modèles LES, ce modèle permet de résoudre d’une partie du spectre turbulent et ainsi de prendre en compte les principaux effets instationnaires. Trois configurations de turbine-pompe de même vitesse spécifique (nq=40) sont étudiées. Une seule (grande) ouverture de directrices est retenue pour chaque configuration. Les points de fonctionnement considérés couvrent une large gamme de conditions d’opération, allant du fonctionnement en régime continu (rendement élevé) jusqu’au débit nul, en passant par le point d’emballement. Les résultats des calculs sont comparés aux mesures expérimentales. La bonne corrélation entre valeurs numériques et expérimentales valide la pertinence du modèle numérique. Les analyses des performances de la machine et des fluctuations de pression permettent d’identifier les régions de l’écoulement associées aux principales instabilités. Enfin, les visualisations de l’écoulement couplées à une étude des mécanismes de dissipation de l’énergie mettent en évidence les principaux phénomènes à l’origine de la forme en « S » des courbes caractéristiques. / Pumped Storage Plants (PSP) using reversible Francis pump-turbines can store large amounts of energy with high efficiency. They therefore appear as a cost-effective tool to provide stability to the energy production network against the intermittency of renewable energy sources. Nevertheless, start-up and shutdown procedures still need to be improved to increase the reactivity of the PSP. Reversible high head pump-turbines have characteristic curves that exhibit an S-Shape in the turbine, turbine-brake and reverse pump quadrants. This S-Shape may be responsible for surge transient phenomena in the case of an emergency shutdown (for large guide vane opening). Moreover, for operating point in the S-Shape region, the flow is highly unsteady and leads to a high level of pressure fluctuations and strong dynamic loadings on the mechanical parts. The objective of the current work is to perform a comprehensive study of the complex hydraulic phenomena linked with the S-Shape. Unsteady numerical computations are carried out using the turbulence model SAS-SST. Such a model can resolve part of the turbulent spectrum while maintaining affordable computational cost. It therefore offers an interesting alternative to more expensive LES computations. Three different configurations of pump-turbine with the same specific speed (nq=40) are investigated. Several operating conditions from optimal efficiency point to zero discharge condition for a given large guide vane opening are studied. Numerical results show good agreement with the experimental data. Accuracy of the numerical model is thus assessed. The investigations of the global performances of the pump-turbine and the pressure pulsations help to identify the region of the flow which are associated with the main instabilities. Finally, flow visualizations linked with the analysis of the mechanisms of energy dissipation reveal the major flow phenomena at the origin of the S-Shape.

Les instabilités de combustion induites par le couplage combustion-acoustique se produisent dans de nombreux systèmes industriels et domestiques tels que les chaudières, les turbines à gaz et les moteurs de fusée. Ces instabilités se traduisent par des fluctuations de pression et un dégagement de chaleur qui peuvent provoquer une défaillance mécanique ou des dégâts désastreux dans certains cas extrêmes. Ces phénomènes ont été largement étudiés par le passé, et les mécanismes responsables du couplage ont déjà été identifiés. Cependant, il apparaît que la plupart des systèmes se comportent différemment lors du démarrage à froid ou en régime permanent. Le couplage entre la température des parois et les instabilités de combustion reste encore méconnu et n’a pas été étudié en détail jusqu’à présent. Dans le cadre de ces travaux de thèse, on s’intéresse à ce mécanisme. Ces travaux présentent une étude expérimentale des instabilités de combustion pour une flamme laminaire de pré-mélange stabilisée sur un brûleur à fente. Pour certaines conditions de fonctionnement, le système présente un mode instable autour du mode de Helmholtz du brûleur. Il est démontré que l’instabilité peut être contrôlée, et même supprimée, en changeant uniquement la température de la surface du brûleur. Une analyse de stabilité linéaire peut être mise en œuvre afin d’identifier les paramètres jouant un rôle dans les mécanismes d’instabilité, et il est possible de modéliser analytiquement les phénomènes observés expérimentalement. Des études expérimentales détaillées de différents processus élémentaires impliqués dans le couplage thermo-acoustique ont été menées pour évaluer la sensibilité de ces paramètres à la température de la paroi. Enfin un modèle théorique du couplage entre le transfert de chaleur instationnaire à la paroi et la fluctuation du pied de flamme a été proposé. Par ailleurs, d’autres mesures expérimentales ont permis de comprendre les mécanismes physiques responsables de la dépendance de la réponse de la flamme à la température de paroi. / Combustion instabilities, induced by the resonant coupling of acoustics and combustion occur in many practical systems such as domestic boilers, gas turbine and rocket engines. They produce pressure and heat release fluctuations that in some extreme cases can provoke mechanical failure or catastrophic damage. These phenomena have been extensively studied in the past, and the basic driving and coupling mechanisms have already been identified. However, it is well known that most systems behave differently at cold start and in the permanent regime and the coupling between the temperature of the solid material and combustion instabilities still remains unclear. The aim of this thesis is to study this mechanism. This work presents an experimental investigation of combustion instabilities for a laminar premixed flame stabilized on a slot burner with controlled wall temperature. For certain operating conditions, the system exhibits a combustion instability locked on the Helmholtz mode of the burner. It is shown that this instability can be controlled and even suppressed by changing solely the temperature of the burner rim. A linear stability analysis is used to identify the parameters playing a role in the resonant coupling and retrieves the features observed experimentally. Detailed experimental studies of the different elementary processes involved in the thermo-acoustic coupling are used to evaluate the sensitivity of these parameters to the wall temperature. Finally a theoretical model of unsteady heat transfer from the flame root to the burner-rim and detailed experimental measurements permit to establish the physical mechanism for the temperature dependance on the flame response.