Mariepskop (direct translation: “Marieps hill”) forms part of the northernmost edge of the Drakensberg Mountain range in the east of South Africa, and is known for its complex topography associated with meso-scale atmospheric circulation, and therefore its numerous climatic zones. As a result the mountain hosts a high degree of biodiversity. The peak of Mariepskop lies at approximately 1900m Above Mean Sea Level (AMSL), which is higher than the surrounding escarpment to the east bordering the Highveld. Its foothills also extend well into the Lowveld at about 700m AMSL. Mariepskop is therefore ideal for studying airflow exchange between the industrialized Highveld and the Lowveld with its diversity of natural resources. It is also ideal for detecting global warming signals on altitudinal gradients extending from the Lowveld to altitudes above the Highveld escarpment. In this study, long-term National Centre for Atmospheric Research / National Centre for Environmental Prediction (NCAR/NCEP) reanalysis wind data at two atmospheric pressure levels (850hPa and 700hPa), as well as reanalysis near-surface temperature data, were obtained for the Mariepskop region for the austral summer (and winter) seasons. The data was used to force a Computational Fluid Dynamics (CFD) model (also known as STAR-CCM+) across its lateral boundaries with the dominant synoptic flow in order to generate mesoscale simulation output over the complex terrain of Mariepskop. Wind speed and direction modelled results were then correlated to observations measured by three weather stations on Mariepskop. Modelled wind flow results for the summer simulation were also validated against aerial photographs in order to infer whether the model could accurately capture areas with high rainfall, which are related to denser vegetation. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Geography, Geoinformatics and Meteorology / unrestricted

Les cellules souches mésenchymateuses (CSM) interviennent de plus en plus dans le domaine de la médecine régénérative, notamment pour traiter des maladies aujourd’hui difficilement curables avec les moyens actuels. Deux verrous scientifiques limitent pourtant leur utilisation et leur commercialisation. D’une part, de grandes quantités de cellules sont nécessaires pour répondre à la forte demande médicale. D’autre part, les cellules étant elles-mêmes le médicament final, délivré chez le patient, leur qualité doit être préservée (phénotype souche, capacité de différenciation). La mise en culture de ces cellules, sur des microporteurs, en bioréacteur agité, semble répondre à ces enjeux. Cependant, une connaissance plus précise de l’impact, sur la réponse physiologique des cellules, des technologies utilisées et de l’hydrodynamique générée est nécessaire pour améliorer les lois d’extrapolation des bioréacteurs de culture de CSM. Dans ce contexte, des travaux ont été mis en œuvre pour étudier l’influence du mode d’agitation (orbital ou mécanique) sur l’attachement, l’expansion et le détachement de CSM issues de la gelée de Wharton (GW-CSM) de cordons ombilicaux, sur des microporteurs de différentes compositions. Pour contribuer à la quantification de l’expansion cellulaire, une méthode de comptage automatique in situ a été développée pour estimer le nombre de cellules par microporteur, ainsi que leur répartition, sans avoir à procéder à leur détachement. Des microporteurs commerciaux ont ensuite pu être comparés à des microporteurs synthétisés dans un laboratoire partenaire, en termes d’attachement et expansion cellulaire, ainsi que de facilité de détachement. En parallèle de ces travaux, l’impact de la conception du mobile d’agitation, en bioréacteur mécaniquement agité, sur la mise en suspension de microporteurs a été analysé. A l’issue de cette étude, une analyse dimensionnelle et des simulations CFD ont été mises en place et deux modèles reliant la fréquence minimale de juste mise en suspension (Njs) avec la géométrie du mobile d’agitation (forme, taille, position dans la cuve) et les propriétés matérielles des particules et de la phase liquide ont été proposés. Une stratégie d’optimisation des paramètres géométriques d’un mobile en minibioréacteur, dédié à la culture de CSM sur microporteurs, a été mise en place, à partir de paramètres caractérisant les contraintes hydromécaniques perçues par la phase solide, judicieusement choisis et intégrés lors des simulations CFD. Selon un plan d’expérience, et les résultats extraits des simulations, des surfaces de réponse ont été construites et une optimisation multi-objective a été réalisée afin de déterminer la géométrie minimisant les contraintes perçues par les particules, et donc par les cellules adhérées. Des cultures de GW-CSM en minibioréacteurs équipés de différents mobiles ont finalement été validées, avec une comparaison préliminaire de l’impact de ces géométries sur l’expansion cellulaire / Mesenchymal stem cells (MSC) are becoming increasingly involved in the regenerative medicine field, particularly to treat diseases that are not effectively curable with the current therapies. Two scientific barriers are nevertheless responsible for MSC use and commercialization limitations. On one side, large amounts of cells are needed to reach the high cell dose requirements. On the other side, cells being the final product themselves, directly injected into the patient, their quality have to be controlled (stem cell phenotype, differentiation capability). MSC cultivation on microcarriers in a stirred bioreactor seems to meet these challenges. However, a precise knowledge about the impact of the technologies and the hydrodynamics generated, on the physiological cell response, is necessary to improve the scale-up of MSC cultures in bioreactors. In this context, present work is dedicated to the study of the impact of the agitation mode (orbital or mechanical) on the cell attachment, expansion and detachment on various microcarrier types, in the case of MSC derived from the Wharton’s jelly (WJ-MSC) of umbilical cords. To quantify more precisely cell distribution and expansion on microcarriers, an automatic and in situ counting method was developed, which need no detachment step. This allowed the identification of commercial microcarriers suitable for WJ-MSC cultures, which were then compared to home-made microcarriers, synthesized by a partner laboratory, in terms of cell attachment and expansion, and detachment efficiency. In parallel to these works, the impact of the impeller design on the microcarrier suspension in stirred tank bioreactors was investigated. Based on a dimensional analysis and CFD simulations, it resulted in the establishment of two models relating the minimal agitation rate to ensure all particle suspension (Njs) with the impeller geometrical characteristics (design, size, off-bottom clearance) and the material properties of both the solid and the liquid phases. CFD models validation allowed then to develop a strategy to optimize the geometrical configuration of an impeller, dedicated to MSC cultures on microcarriers in a minibioreactor. Parameters characterizing the hydromechanical stress encountered by the solid phase were wisely chosen and integrated into CFD simulations. Based on a design of experiments, and the hydrodynamics data recovered from simulations, response surfaces were built and a multiobjective optimization was achieved in order to determine the geometry minimizing the particle stress, and also by adhered cells. WJ-MSC cultures in minibioreactors equipped with impellers displaying various geometries were finally validated, with a preliminary comparison of the impact of these geometries on the cell expansion

In the wind resource and wind turbine suitability industry Computational Fluid Dynamics has gained widespread use to model the airflow at proposed wind farm locations. These models typically focus on the neutrally stratified surface layer and ignore physical process such as buoyancy and the Coriolis force. These physical processes are integral to the accurate description of the atmospheric boundary layer and reductions in uncertainties of turbine suitability and power production calculations can be achieved if these processes are included. The present work focuses on atmospheric flows in which atmospheric stability and the Coriolis force are included. The study uses Monin-Obukhov Similarity Theory to analyse time series data output from a proposed wind farm location to determine the prevalence and impact of stability at the location. The output provides the necessary site data required for the CFD model as well as stability-dependent wind profiles from measurements. The results show non-neutral stratification to be the dominant condition onsite with impactful windfield changes between stability conditions. The wind flows considered in this work are classified as high Reynolds number flows and are based on numerical solutions of the Reynolds-Averaged Navier-Stokes equations. A two-equation closure method for turbulence based on the k __ turbulence model is utilized. Modifications are introduced to standard CFD model equations to account for the impact of atmospheric stability and ground roughness effects. The modifications are introduced by User Defined Functions that describe the profiles, source terms and wall functions required for the ABL CFD model. Two MOST models and two wall-function methods are investigated. The modifications are successfully validated using the horizontal homogeneity test in which the modifications are proved to be in equilibrium by the model�s ability to maintain inlet profiles of velocity and turbulence in an empty domain. The ABL model is applied to the complex terrain of the proposed wind farm location used in the data analysis study. The inputs required for the stability modifications are generated using the available measured data. Mesoscale data are used to describe the inlet boundary conditions. The model is successfully validated by cross prediction of the stabilitydependent wind velocity profiles between the two onsite masts. The advantage of the developed model is the applicability into standard wind industry loading and power production calculations using outputs from typical onsite measurement campaigns. The model is tuning-free and the site-specific modifications are input directly into the developed User Defined Functions. In summary, the results show that the implemented modifications and developed methods are applicable and reproduce the main wind flow characteristics in neutral and non-neutral flows over complex wind farm terrains. In additions, the developed method reduce modelling uncertainties compared against models and measurements that neglect non-neutral stratification. / Dissertation (MEng)--University of Pretoria, 2018. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Turbines encounter blade failures due to fatigue and creep. It has been shown in the literature that the primary cause of steam turbine blade failures worldwide can be ascribed to fatigue in low pressure (LP) turbine blades. The failure and damage to these blades can lead to catastrophic consequences. Some utilities use empirical methods to determine the forces experienced by turbine blades but desire more accurate methods. The inaccurate prediction of high-cycle fatigue (HCF), thermal durability and stage performance is introduced when one does not consider blade row interaction. Blade row interactions can, however, be accounted for by means of computational fluid dynamics (CFD). Furthermore, modern high- fidelity CFD tools would be able to contribute greatly in predicting the forces experienced by turbine blades. Numerical tools such as CFD and nite element analysis (FEA) can greatly contribute to the estimation of the remaining useful life (RUL) of turbine blades. However, in this estimation process, there are various uncertainties and aspects that affect the estimated RUL. Understanding the sensitivity of the estimated RUL to these various uncertainties and aspects is of great importance if RUL is to be estimated as accurately as possible. In this dissertation, a sensitivity analysis is performed with the purpose of establishing the sensitivity of the estimated RUL of the last stage rotor of an LP steam turbine, to the number of harmonics used in a nonlinear harmonic (NLH) CFD simulation. The sensitivity of the estimated RUL is evaluated in the HCF regime, where the cyclic stresses occur below the yield strength of the turbine blade. A CFD model, FE model, and fatigue model were therefore developed in such a manner that would suffice, regarding the purpose of the sensitivity analysis. The CFD model is validated by comparing the predicted CFD power to that of actual generated power of a dual 100MW LP steam turbine. The sensitivity analysis is performed for 3 operation conditions, and for each operational condition the aerodynamic forces were computed using 1, 2, and 3 harmonics in an NLH simulation. The estimation process considers a weak coupling between the CFD model and FE model. NLH simulations are firstly performed to calculate the unsteady static surface pressure distributions on the last stage rotor. This is followed by the mapping thereof to the FE model, for which a transient structural analysis is performed. Finally, the RUL is estimated by performing a fatigue analysis on the stress history obtained from the transient structural analysis. Based on the results of the sensitivity analysis, the following recommendations were made, from a conservative point of view. Firstly, in general, if the RUL is to be estimated with reasonable accuracy, just using 1 harmonic in an NLH simulation will not be sufficient and 2 harmonics should be used. Secondly, if the RUL has to be estimated with high accuracy, 3 harmonics should be used. / Dissertation (MEng)--University of Pretoria, 2018. / National Research Foundation (NRF) / Mechanical and Aeronautical Engineering / MEng / Unrestricted

This licentiate thesis describes development of modellingtools, experimental physical modelling and numerical modellingto simulate real combustion processes for advanced industrialutility boiler before and after retrofit. The work presents extended study about formation,destruction and control of pollutants, especially NOx, whichoccur during combustion process. The main aim of this work is to improve mixing process incombustion chamber. To do this, the optimization of placementand direction of additional air and fuel nozzles, the physicalmodelling technique is used. By using that method, it ispossible to obtain qualitative information about processes,which occur in the real boiler. The numerical simulationsverify the results from physical modelling, because duringmathematical modelling quantitative informations about flow andmixing patterns, temperature field, species concentration areobtained. Two 3D cases, before and after retrofit, of pulverized fuelfired boiler at 125 MW output thermal power are simulated. Theunstructured mesh technique is also used to discretize theboiler. The number of grid was 427 656 before retrofit and 513362 after retrofit. The comparisons of results of numericalsimulation before and after retrofit are presented. The resultsfrom physical modelling and numerical simulation are alsoshown. Results present that nozzles of additional air and fuel givea considerably better mixing process, uniform temperature fieldand CO2 mass fraction. The whole combustion chamber worksalmost as a "well stirred reactor", while upper part of boilerworks as a "plug flow reactor". Differences between from measured of temperatures andpredicted temperatures are not too big, the maximum differenceis about 100 K. It seems, that calculated temperatures showgood agreement with measurement data. The results illuminate the potential of physical andnumerical modelling methods as promising tools to deal with thecomplicated combustion processes, even for practicalapplication in the industry. <b>Keywords:</b>air staging, fuel staging, boiler, furnace,computational fluid dynamics, numerical simulation, pollutants,physical modeling, pulverized fuel combustion. / NR 20140805

Recently it has become more common in the construction of large dams to reuse diversion tunnels as flood discharge tunnels in the final structure. These tunnels handle large flows with the characteristics of open channel flow. When such large hydrological forces act upon a structure there are several problems to be expected. One of these is the occurrence of cavitation, which could have potential hazardous erosion as a consequence. Cavitation is the formation and collapse of bubbles that create a shockwave strong enough to erode the underlying material. The Baihetan dam is one of the largest hydro power projects in China at present. It has three discharge tunnels that all run the risk of developing cavitation damages. By modelling one of the tunnels using Computational Fluid Dynamics (CFD) it is possible to investigate where in the tunnel structure cavitation is likely to occur. This degree project assesses the risk of cavitation erosion in the Baihetan tunnel using the static pressure distribution, the velocity distribution and modern cavitation theory. Several modifications of the tunnel – including alterations in the gradient and construction parameters – are simulated in order to investigate if changes in the design can mitigate the cavitation problem. None of the analysed modifications completely eliminate the problem and aeration is recommended to counteract the problem. This study indicates where cavitation might be a problem in the Baihetan tunnel and can be used as a basis for further research.

This is a report of a verification study of the multiphysics simulation framework MOOSE which was preformed at the company Seaborg Technologies. In the process of designing molten salt reactors there is a special need of making credible multiphysics simulations since the fuel is in motion. In this study the incompressible version of Navier-Stokes equations of finite volumes available in the Navier-Stokes module of the MOOSE framework is verified by modelling and simulations of fluid flow and heat transfer in two different systems with available benchmarks. The first system, a thin buoyancy driven molten sodium hydroxide test loop which is verified by a similar model made with the high fidelity CFD software STAR-CCM+ as benchmark. The second system, forced convection of air through a straight pipe with heated walls which is verified by comparisons with an analytical solution. The resulting velocity profiles from simulations of the first system corresponds well with the benchmark but certain conclusions can not be drawn from it since the the transient simulations stops to converge before reaching equilibrium. The results from simulations of the second system corresponds well with the analytical solution and no convergence issues arise. The conclusion from the results is that the incompressible version of Navier-Stokes equations of finite volumes available in the Navier-Stokes module of the MOOSE framework has potential to be used in multiphysics simulations of molten salt reactors but seemingly not in cases of buoyancy driven flows in thin geometries. Two proposals for further work is recommended. The first is that this implementation is applied in a context with forced fluid flow or a context with thicker fluid domain. The second proposal is that the other available abilities of MOOSE such as finite element method and/or the compressible version of the Navier-Stokes equations should be tested.

Les lanceurs ont un impact sur la composition de l'atmosphere, et en particulier sur l'ozone stratospherique. Parmi tous les types de propulsion, les moteurs à propergol solide ont fait l'objet d'une attention particulière car leurs émissions sont responsables d'un appauvrissement significatif d'ozone dans le panache des lanceurs lors des premières heures suivant le lancement. Ce phénomène est principalement dû à la conversion de l'acide chlorhydrique, un composé chimique présent en grandes quantités dans les émissions de ce type de moteur, en chlore actif qui réagit par la suite avec l'ozone dans un cycle catalytique similaire à celui responsable du "trou de la couche d'ozone", cette diminution périodique de l'ozone en Antarctique. Cette conversion se produit dans le panache supersonique, où les hautes températures favorisent une seconde combustion entre certaines espèces chimiques du panache et l'air ambiant. L'objectif de cette étude est d'évaluer la concentration de chlore actif dans le panache d'un moteur à propergol solide en utilisant la technique des Simulations aux Grandes Echelles (SGE). Le gaz est injecté à travers la tuyère d'un moteur et une méthode de couplage entre deux instances du solveur de mécanique des fluides est utilisée pour étendre autant que possible le domaine de calcul derrière la tuyère (jusqu'à l'équivalent de 400 diamètres de sortie de la tuyère). Cette méthodologie est validée par une première SGE sans chimie, en analysant les caractéristiques de l'écoulement supersonique avec co-écoulement obtenu par ce calcul. Ensuite, le chimie mettant en jeu la conversion des espèces chlorées a été étudiée au moyen d'un modèle "hors-ligne" permettant de résoudre une chimie complexe le long de lignes de courant extraites d'un écoulement moyenné dans le temps résultant du calcul précédent (non réactif). Enfin, une SGE multi-espèces est réalisée, incluant un schéma chimique auparavant réduit afin de limiter le coût de calcul. Cette simulation représente une des toutes premières SGE d'un jet supersonique réactif, incluant la tuyère, effectuée sur un domaine de calcul aussi long. En capturant avec précision le mélange du panache avec l'air ambiant ainsi que les interactions entre turbulence et combustion, la technique des simulations aux grandes échelles offre une évaluation des concentrations des espèces chimiques dans le jet d'une precision inédite. Ces résultats peuvent être utilisés pour initialiser des calculs atmosphériques sur de plus larges domaines, afin de modéliser les réactions entre chlore actif et ozone et de quantifier l'appauvrissement en ozone dans le panache. / Rockets have an impact on the chemical composition of the atmosphere, and particularly on stratospheric ozone. Among all types of propulsion, Solid-Rocket Motors (SRMs) have given rise to concerns since their emissions are responsible for a severe decrease in ozone concentration in the rocket plume during the first hours after a launch. The main source of ozone depletion is due to the conversion of hydrogen chloride, a chemical compound emitted in large quantities by ammonium perchlorate based propellants, into active chlorine compounds, which then react with ozone in a destructive catalytic cycle, similar to those responsible for the Antartic "Ozone hole". This conversion occurs in the hot, supersonic exhaust plume, as part of a strong second combustion between chemical species of the plume and air. The objective of this study is to evaluate the active chlorine concentration in the far-field plume of a solid-rocket motor using large-eddy simulations (LES). The gas is injected through the entire nozzle of the SRM and a local time-stepping method based on coupling multi-instances of the fluid solver is used to extend the computational domain up to 400 nozzle exit diameters downstream of the nozzle exit. The methodology is validated for a non-reactive case by analyzing the flow characteristics of the resulting supersonic co-flowing under-expanded jet. Then the chemistry of chlorine is studied off-line using a complex chemistry solver applied on trajectories extracted from the LES time-averaged flow-field. Finally, the online chemistry is analyzed by means of the multi-species version of the LES solver using a reduced chemical scheme. To the best of our knowledge, this represents one of the first LES of a reactive supersonic jet, including nozzle geometry, performed over such a long computational domain. By capturing the effect of mixing of the exhaust plume with ambient air and the interactions between turbulence and combustion, LES offers an evaluation of chemical species distribution in the SRM plume with an unprecedented accuracy. These results can be used to initialize atmospheric simulations on larger domains, in order to model the chemical reactions between active chlorine and ozone and to quantify the ozone loss in SRM plumes.

ABSTRACT Left Ventricular Assistant Devices (LVAD's) have been routinely used to treat patients with heart failure, and to help bridge patients awaiting heart transplant surgery. A major problem with LVAD's is their tendency to stimulate the formation of blood clots that can cause serious conditions such as strokes, thrombosis, and even death. A study on an alternative aortic bypass for patients with LVAD implants as a mean to reduce the number of thrombi that eventually flow into the carotid arteries by promoting them to flow into the subclavian arteries and descending aorta is presented. The study consists of Computational Fluid Dynamics (CFD) models for standard and alternative aortic bypass L V AD configurations. Results show that thrombi with diameters in the range of 2mm to 5mm have the highest chance of flowing into the carotid arteries from the aortic arch. The CFD study of the alternative aortic bypass implementation shows an increase in the number of thrombi that flow out of the aortic arch to the descending aorta by 4.65% for 0.5mm diameter, 11.63% for 2mm diameter, 37.21 % for 3mm diameter, and 9.3% for 5mm diameter thrombi.

The focus of this thesis is to analyse the flight characteristics of the blended wingbody (BWB) unmanned aerial vehicle (UAV) Green Raven currently being developed by students at the Royal Institute of Technology (KTH) in Stockholm,Sweden. The purpose of evaluating a BWB aircraft is due to its potential increasein fuel efficiency and payload compared to conventional aircrafts which would enable more sustainable flights. The analysis is conducted in ANSYS Fluent 2020R2 where the goals are to extrapolate lift, drag and pitching moment coefficients,aerodynamic efficiency and evaluate stall patterns. The analysis is conducted with free stream velocities from 5 m/s to 40 m/s with5 m/s increments at angles of attack from −4◦ to stall plus 4◦. The result of thisthesis is that an analysis have not been able to be conducted due to a lack ofcomputational power. Thusly, the conclusion to this thesis is that to be able toperform a complete analysis of the Green Raven, a more powerful computer needsto be used.