Master of Science / Department of Agronomy / Jay Ham / The exchange of water, carbon, and energy between grasslands and the atmosphere is an important biogeochemical pathway affecting ecosystem productivity and sustainability. The eddy covariance (EC) technique directly measures this mass and energy exchange. However, questions remain regarding the accuracy of EC-derived H[subscript]2O and CO[subscript]2 fluxes in landscapes with irregular topography and variable vegetation. These concerns stem from the "energy balance (EB) closure problem" (i.e., measured energy in does not equal measured energy out). My main objectives were to examine EB closure at two topographical positions within an annually burned tallgrass prairie watershed and to examine the effect of landscape position and woody encroachment on carbon and water exchanges. In tallgrass prairie, 14 km south of Manhattan, KS, USA, EC towers were deployed at three sites in 2007 and 2008. One upland and lowland tower were within an annually burned watershed dominated by C[subscript]4 grasses. Another lowland tower was deployed in a separate quadrennial-burned watershed where significant woody vegetation occupied the tower's sampling area. All towers measured EB components (net radiation, R[subscript]n; soil heat flux, G; sensible heat flux, H; and latent heat flux, [lambda]E). In the annually burned watershed, landscape position had little effect on G, H, and R[subscript]n with differences [less than] 2% between sites. However lowland [lambda]E was 8% higher, owing to larger plant biomass/leaf area and greater soil moisture. Energy balance closure (i.e., [[lambda]E + H] / [R[subscript]n - G]) was 0.87 and 0.90 at the upland and lowland sites, respectively. A nearby large-aperture scintillometer provided good validation of EC-derived H in 2007. Data suggested that underestimates of [lambda]E may have accounted for the closure problem; sample calculations showed that increasing [lambda]E by 17% would have resulted in near prefect closure. Data from this study suggests that EB closure does not strongly correlate with topographical position; however these data raise questions regarding accuracy of the [lambda]E term. Mass exchange analysis shows that the prairie carbon cycle is highly dependent on burning. The lowland and upland annually burned sites saw carbon gains of 281 to 444 g C m[superscript]-[superscript]2 yr[superscript]-[superscript]1 before burning with the shrub lowland showing the least (e.g. 159 and 172 g C m[superscript]-[superscript]2 yr[superscript]-[superscript]1). After the prescribed burn, the upland and lowland sites remained slight carbon sinks (68 to 191 g C m[superscript]-[superscript]2 yr[superscript]-[superscript]1), whereas the unburned shrub site was a carbon sink in 2007 (159 g C m[superscript]-[superscript]2 yr[superscript]-[superscript]1, because no carbon loss was incurred via burning) and a large carbon source in 2008 when it was burned the following year (336 g C m[superscript]-[superscript]2 yr[superscript]-[superscript]1 loss). Evapotranspiration (ET) was highest at the shrub lowland where greater soil moisture and abundance of deep-rooted C[subscript]3 shrub vegetation allowed greater uptake and loss of water.

Les agrosystèmes représentent une importante part des terres émergées (plus d'un tiers de la surface au sol en Europe) et sont au cœur de nombreuses problématiques de développement durable. Ils sont consommateur d'eau et produisent des gaz à effet de serre (GES) qui contribuent aux changements climatiques en cours, ceux-ci ayant en retour des impacts encore difficiles à prévoir sur le fonctionnement et la gestion des agrosystèmes. L'étude des cycles biogéochimiques au sein des agrosystèmes est donc fondamentale. Le principal objectif de ces travaux de thèse a été d'étudier le fonctionnement carboné et hydrique des agrosystèmes à partir du suivi de la végétation (phénologie, biomasse, surface foliaire) et de mesures micro météorologiques d'échanges de matière (CO2, H2O) et d'énergie à l'interface entre le système sol/couvert et l'atmosphère sur deux parcelles agricoles expérimentales situées au Sud Ouest de Toulouse. L'ensemble de ces travaux a permis de montrer que la mesure des flux par la méthode des fluctuations turbulentes (EC) permet de quantifier les bilans de carbone et d'eau des agrosystèmes et d'étudier certains des processus physiques et écophysiologiques à l'origine des différents flux. A partir de ces mesures, une analyse des flux et bilans d'eau et de carbone a été effectuée. Une méthode de calcul des écobilans intégrant les émissions de GES liées aux pratiques culturales a été établie pour nos sites et appliqués aux parcelles expérimentales de cultures du réseau Européen CarboEurope-IP, représentant un panel important de cultures et de pratiques culturales. Les mesures annuelles de flux net de CO2 à l'interface sol/couvert et atmosphère ont montré que les agrosystèmes se comportent le plus souvent comme des puits atmosphériques de carbone. Cependant, la prise en compte des imports de carbone (fertilisation organique et semences) et des exports de carbone au moment de la récolte en plus des mesures de flux net vertical au dessus de la parcelle a permis de montrer que le bilan de carbone des cultures correspond rarement à un puits et que le plus souvent il est une source de carbone pour l'atmosphère (127 ± 243 g C m-2 an-1 pour 41 années-sites du réseau CarboEurope-IP). En moyenne, pour ces 41 années-sites étudiés, le flux net vertical de CO2 représentait 37 % du bilan de carbone (soit 88 % des entrées de carbone dans la parcelle), les apports de carbone sous forme de fertilisation organique et de semences représentaient 5 % du bilan (soit 12 % des entrées de carbone) et les exports de carbone au moment de la récolte représentaient 58 % du bilan. Les émissions de GES liées aux pratiques culturales représentaient en moyenne seulement 7.6 % du bilan de GES. Ces résultats montrent qu'il est donc fondamental de considérer les flux biosphériques de CO2 (qui représentent 88 % des entrées de carbone) dans le bilan annuel de GES de la parcelle sans quoi ce bilan serait très fortement surestimé. L'efficience de l'utilisation de l'eau (WUE) a été abordée à travers des points de vue agronomiques (production de biomasse par quantité d'eau évapotranspirée) et environnementaux (production nette de carbone de l'écosystème par quantité d'eau évapotranspirée). Ces approches pouvaient parfois produire des résultats contradictoires dans le cas de cultures avec de fortes exportations de biomasse (cas du maïs utilisé pour l'ensilage par exemple) et doivent donc être précautionneusement pris en compte dans l'optique d'une gestion durable des agrosystèmes. Finalement les mesures de flux ont permis 1) de tester une première version du modèle ICASTICS qui permettra de mieux comprendre les processus et de simuler les différentes composantes des flux nets d'eau et de carbone des agrosystèmes pour une gamme importante de cultures, de modes de gestions et de conditions climatiques. 2) de faire évoluer le modèle SAFY pour qu'il puisse calculer des flux et bilans d'eau et de carbone à des échelles supérieures a celles de la parcelle, en utilisant des données de télédétection, dans une perspective de gestion des ressources.

The South African government’s integrated resource plan for electricity IRP2010 states that the country plans to have an additional 9.6 GW of nuclear power on the national electricity grid by 2030. In support of this, the NRF-funded SARChI Research Chair in Nuclear Engineering within the School of Mechanical and Nuclear Engineering at the North-West University recently initiated research studies focused on Light Water Reactor (LWR) systems. These studies inter alia involve coupled neutronic and thermal hydraulic analyses of selected LWR systems. This study focuses on the steady state neutronic analysis of the European Pressurised Reactor (EPR) using Monte-Carlo N-Particle (MCNP5). The neutronic model will in due course be coupled to a thermal hydraulic model forming part of a broader study of the system. The Monte Carlo neutron transport code MCNP5 has been widely used since the 1950s for analysis of existing and future reactor systems due to its ability to simulate complex fuel assemblies without making any significant approximations. The primary aim of the study was to develop an input model for a representative fresh fuel assembly of the US EPR reactor core from which the fluxes and fission power of the reactor can be obtained. There after a 3D model of full EPR core developed by the school of mechanical and nuclear engineering based on findings of this work is also tested. The results are compared to those in the US EPR Final Safety Analysis Report. Agreement in major core operational parameters including the keff eigenvalue, axial and radial power profiles and control rod worth are evaluated, from which consistency of the model and results will be confirmed. Further convergence of the model within a reasonable time is assessed. / MSc (Engineering Sciences in Nuclear Engineering), North-West University, Potchefstroom Campus, 2014

The South African government’s integrated resource plan for electricity IRP2010 states that the country plans to have an additional 9.6 GW of nuclear power on the national electricity grid by 2030. In support of this, the NRF-funded SARChI Research Chair in Nuclear Engineering within the School of Mechanical and Nuclear Engineering at the North-West University recently initiated research studies focused on Light Water Reactor (LWR) systems. These studies inter alia involve coupled neutronic and thermal hydraulic analyses of selected LWR systems. This study focuses on the steady state neutronic analysis of the European Pressurised Reactor (EPR) using Monte-Carlo N-Particle (MCNP5). The neutronic model will in due course be coupled to a thermal hydraulic model forming part of a broader study of the system. The Monte Carlo neutron transport code MCNP5 has been widely used since the 1950s for analysis of existing and future reactor systems due to its ability to simulate complex fuel assemblies without making any significant approximations. The primary aim of the study was to develop an input model for a representative fresh fuel assembly of the US EPR reactor core from which the fluxes and fission power of the reactor can be obtained. There after a 3D model of full EPR core developed by the school of mechanical and nuclear engineering based on findings of this work is also tested. The results are compared to those in the US EPR Final Safety Analysis Report. Agreement in major core operational parameters including the keff eigenvalue, axial and radial power profiles and control rod worth are evaluated, from which consistency of the model and results will be confirmed. Further convergence of the model within a reasonable time is assessed. / MSc (Engineering Sciences in Nuclear Engineering), North-West University, Potchefstroom Campus, 2014

Le glioblastome multiforme (GBM) est l'une des tumeurs des plus létales qui soient. En dépit du traitement optimal actuellement disponible, la survie médiane des patients atteints d'un GBM n'atteint que 14,6 mois et n'a pu être améliorée de façon significative au cours des dernières décennies. La barrière hématoencéphalique endigue l'entrée de la majorité des xénobiotiques au système nerveux central et handicape sérieusement l'efficacité de la chimiothérapie. Une panoplie de stratagèmes fut développée afin de contourner cet obstacle majeur et l'emploi de liposomes comme véhicules recèle un grand potentiel. Nous avons donc entrepris l'élaboration d'une formulation liposomale dédiée à cette fin. Une formulation de base, inspirée de la littérature, a d'abord été modifiée de façon à produire plusieurs formulations dérivées. Le criblage de ces dernières a permis d'identifier une formulation candidate ainsi que des propriétés favorables à la lipofection des lignées cellulaires gliales F98 et U-118 MG. L'internalisation cellulaire des liposomes et la libération cytosolique de leur chargement hydrophile ont été évaluées quantitativement en cytométrie de flux. La formulation cationique, sensible au pH et dépourvue de polyéthylène glycol (PEG) s'est avérée la plus performante. Chez les deux lignées cellulaires, les liposomes de cette formulation vedette ont accédé au milieu intracellulaire entre 4 et 6 h, et y ont libéré leur cargaison sur plus de 24 h. Le balayage de cellules F98 et U-118 MG lipofectées en microscopie confocale a confirmé la libération intracellulaire du contenu des liposomes à 6 h, et a dévoilé un patron d'internalisation typique à l'endocytose. Enfin, cette formulation liposomale vedette s'est avérée très peu cytotoxique et aucun effet cytostatique n'a été remarqué chez ces deux lignées. La performance inférieure de la formulation de base et des autres dérivés indique qu'une réduction de la fluidité membranaire, l'inclusion de polymères PEG ainsi que l'absence combinée d'une charge cationique et de la sensibilité au pH ont des conséquences délétères sur la capacité de lipofection des liposomes. Ces résultats soulignent avec emphase l'importance d'adapter la composition lipidique des liposomes au type cellulaire ciblé et corroborent le potentiel des liposomes cationiques et sensibles au pH pour l'acheminement intracellulaire de xénobiotiques. Des études in vivo permettront d'établir le potentiel de la formulation liposomale vedette dans la thérapie des GBM.

There is a global need for sustainable, renewable and clean energy sources. Microbial production of hydrogen from renewable carbon sources, biorefinery compounds such as succinic acid or from food and drinks industry waste meets all these criteria. Although it has been studied for several decades, there is still no large scale bio-hydrogen production because the rate and yield of hydrogen production are not high enough to render the process economical. The dependency of biological hydrogen production of incipient light energy is also an important factor affecting economics. In order to improve the prospects of biohydrogen as a renewable and sustainable energy alternative, the genetic and process engineering approaches should be helped and targeted by metabolic engineering tools such as metabolic flux balance analysis. The overall aim of this research was the development of computational metabolic flux balance analysis for the study of growth and hydrogen production in Rhodobacter sphaeroides. The research reported in this thesis had two approaches; experimental and computational. Batch culture experiments for growth and hydrogen production by Rhodobacter sphaeroides were performed with either malate or succinate as carbon source and with glutamate as the nitrogen source. Other conditions investigated included; i) aerobic and anaerobic growth, ii) light and dark fermentation for growth, and iii) continuous light and cycled light/dark conditions for hydrogen production. The best growth was obtained with succinate under anaerobic photoheterotrophic conditions with the maximum specific growth rate of 0.0467 h– 1, which was accompanied with the maximum specific hydrogen production rate of 1.249 mmol(gDW.h)– 1. The range of the photon flux used was 5.457 - 0.080 mmol(gDW.h)– 1. The metabolic flux balance model involved 218 reactions and 176 metabolites. As expected the optimised specific rates of growth and hydrogen production were higher than those of the experimental values. The best prediction was for hydrogen production on succinate with computed specific hydrogen production rates in the range of 2.314 - 1.322 mmol(gDW.h)– 1. Sensitivity analyses indicated that the specific growth rate was affected by the nitrogen source uptake rate under aerobic dark condition whereas the flux of protein formation had the largest effect on the specific growth rate under anaerobic light condition.

Σημαντικό ρόλο στις βιομηχανικές εφαρμογές πλάσματος παίζει η ροή των ιόντων. Ωστόσο, συχνά συναντάται το πρόβλημα της εναπόθεσης μονωτικών υμενίων καθιστώντας τους ηλεκτροστατικούς καθετήρες μη λειτουργικούς για τη μέτρηση της ροής αυτής. Ένας εναλλακτικός καθετήρας, ικανός να λειτουργήσει σε αυτές τις συνθήκες, προτάθηκε από τους N. S. J. Braithwaite et al. [1]. Στην παρούσα εργασία παρουσιάζονται, πρώτον, η θεωρία λειτουργίας του καθετήρα αυτού, δεύτερον η κατασκευή του και τρίτον η ανάπτυξη τού συστήματος για την οδήγησή του καθώς και την καταγραφή και την επεξεργασία των σημάτων του. Τέλος, γίνεται πειραματική εξακρίβωση της σωστής λειτουργίας της συσκευής, σε διάταξη επαγωγικού πλάσματος ραδιοσυχνοτήτων (13.56 MHz). / The ion flux holds a major role in the industrial applications of plasma. Often, though, the problem of insulative film deposition is encountered, making the electrostatic probes non functional for the measurement of the ion flux. An alternative probe, capable of functioning under these circumstances has been proposed by N. S. J. Braithwaite et al. [1]. In this paper there are presented, firstly, the theory under which this probe functions, secondly the implementation and thirdly the development of a system for biasing and also data acquisition and processing the signals obtained by the probe. Finally, an experimental identification of the good operation of the device is done, in RF inductively couple plasma (13.56 MHz).

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy

This thesis first outlines the testing undertaken on a partial core superconducting transformer under open circuit, short circuit, full load and endurance test conditions. During the endurance test, a failure occurred after 1 minute and 35 seconds. During the failure, voltage dipping and rapid liquid nitrogen boil off was observed. This prompted a failure investigation which concluded that the lack of cooling in the windings was the most probable cause to the failure. Full core transformer and superconductor theories are then introduced. A copper winding transformer model, based on a Steinmetz equivalent circuit and a reverse design method, is described. A superconductor loss model which outlines the different types of losses experienced under AC conditions is used to determine the resistance of the windings in the Steinmetz equivalent circuit. This resistance changes with the magnitude of current and the strength of the magnetic field that is present in the gaps between each layer of the windings. An alternative leakage flux model is then presented, where the flux is modelled based on the combination of the reluctance of the core and the air surrounding the windings. Based on these theories, an iterative algorithm to calculate the resistance of the superconductor is developed. A new design of a 15kVA single phase full core superconducting transformer, operating in liquid nitrogen, is presented. The issues with building the superconducting transformer are outlined. First, a copper mockup of the superconducting transformer was designed where the mockup would have the same tape and winding dimensions as the superconducting transformer, which means the same core can be used for two different sets of windings. This led to designing a core that could be easily taken apart as well as reassembled. Construction of the core, the copper windings and the superconductor windings ensued. The process of cutting the core laminations, insulating the copper and superconductor tapes, and making the steel fasteners and terminations are described. The copper mockup and superconducting transformers was then tested under open circuit, short circuit, different load and endurance conditions at both liquid nitrogen and room temperatures. These test results were then compared with the those from two models. The comparison showed a significant inaccuracy in the reactances in the models. This introduced a correction factor into the superconductor model which ii made it more accurate. However, further work is required to explain and quantify the correction factors for the copper transformer model under different load conditions.

The remarkable synchrony between changes in temperature recorded in Greenland ice cores and variations in N isotope records from sedimentary cores recovered from the Arabian Sea and the Eastern Tropical North Pacific (ETNP) has provided evidence for teleconnections between changes in marine denitrification in the tropics and climate variations in the northern high latitudes. Changes in tropical denitrification have been attributed to changes in productivity, changes in the source of intermediate waters and the flux of dissolved oxygen to suboxic zones. Variations in marine denitrification and anammox occurring at intermediate depths in proximity to productive continental margins have had profound effects on the N:P ratio of upwelled waters between stadials and interstadials, and may have indirectly affected carbon sequestration in the ocean by changing the balance of nutrients available to primary productivity. Competitive equilibrium, the changing stoichiometric balance of elements available as nutrients and the shorter residence time of N compared to P are factors that are believed to favour diazotrophs (N2-fixing organisms) during interstadials and shift the competitive advantage to non-N2-fixing ecosystems during stadials. This study presents a very high-resolution analysis of sedimentary nitrogen isotope records, phosphorus concentrations and bulk detrital element concentrations from two cores collected along the Pacific Mexican Margin. The results show that the oxygen minimum zone (OMZ) bathing intermediate waters in ETNP is modulated by the interaction of a Northern Hemisphere climate component with the “leakage” of heavy nitrate believed to derive from the Eastern South Pacific (ESP). This southerly component has a more “Antarctic” timing and is similar to records from the Peru-Chile margin. The sedimentary core recovered from the Mazatlan margin shows a “Greenland” timing of millennial-scale events, with reduced upwelling and reduced primary productivity, a less intense OMZ leading to reduced denitrification and a more southerly position of the mid-tropospheric subtropical ridge during stadials. This would have increased the onshore flow of moist air, ultimately leading to increased precipitation along the western Mexican Margin. Interstadials show a reversal of these conditions. In contrast to the Mazatlan core, the N isotope record from the core recovered from the Gulf of Tehuantepec records an element of “Antarctic” timing superimposed on local, millennial-scale variations in denitrification that are more similar in timing to Greenland temperature changes. In addition, the interpretation of observed variations in detrital elements from the Gulf of Tehuantepec highlights latitudinal displacements of the ITCZ that are consistent with those observed in the Cariaco Basin in Venezuela. Bulk P concentrations from both cores suggest that although phosphorite formation in the ETNP during interstadials is not as widespread as previously thought, the very high accumulation rates in the Gulf of Tehuantepec and Mazatlan Margin lead to total Holocene phosphorus burial rates that are up to 4-5 times higher than had been estimated in previous studies. These observations lead to the argument that the ETNP may play a more important role in regulating global P budgets than was previously thought and call for an improved appreciation of the benthic microbial communities that modulate biomes at tropical latitudes.