Modelling the wake effect generated by wind turbines is an essential part for calcu- lating a wind farm’s expected energy production. Operating wind turbines disturb the flow of the wind, which results in decreased production of downwind turbines. The N. O. Jensen model is an industry standard wake model that assumes a linear expansion of the downstream wake. The only adjustable parameter in the model is the wake decay constant (WDC), which has traditionally been derived semi em- pirically from terrain surface roughness. However, the WDC defines the expansion rate of the generated wake, and therefore can be linked to the ambient turbulence intensity (TI): high ambient turbulence leads to a faster decay of the generated wake, and therefore to lower wake losses, and vice-versa. Since the influence of the roughness on the ambient turbulence intensity is expected to be less significant at higher heights, these roughness-based WDC values are rather uncertain for the hub heights employed nowadays. The following study presents the results of a comparison between observed and mod- elled wake losses based on different WDC values. To investigate how a change in height affects the wake modelling, two wake scenarios occurring between two tur- bine sets with different hub heights are selected from an operational wind farm. By modelling the wakes using roughness as well as turbulence intensity-based WDCs, conclusions can be drawn on how the predictive capability of the N.O. Jensen model depends on the selection of a suitable WDC value. Finally it is concluded that the goodness of fit between modelled and observed wake losses shows a clear dependency on the wind speed/power production inter- val. At higher wind speeds, the TI-based WDC resulted in a better accuracy of the modelled wake losses as compared to the roughness-based WDC, while for lower wind speeds the N. O. Jensen model performed most accurately when using WDC = 0.075. However, for the investigated cases the overall accuracy of the modelled wake appears to be higher when choosing WDC = 0.075 instead of a TI-based WDC.

No description available.

Doctorat en philosophie et lettres / info:eu-repo/semantics/nonPublished

Abstract In this thesis nuclear magnetic resonance (NMR) and magneto-optical rotation (MOR) parameters are investigated for water, paying special attention to the effect of solvation from gaseous to liquid phase. Nuclear magnetic shielding and quadrupole coupling tensors of NMR spectroscopy are studied for gaseous and liquid water. Liquid state is modelled by a 32-molecule Car-Parrinello molecular dynamics simulation, followed by property calculations for the central molecules in clusters cut out from the simulation trajectory. Gaseous state is similarly represented by a one-molecule simulation. Gas-to-liquid shifts for shielding constants obtained this way are in good agreement with experiments. To get insight into the local environment and its effect on the properties the clusters are divided into groups of distinct local features, namely the number of hydrogen bonds. The analysis shows in detail how the NMR tensors evolve as the environment changes gradually from the gas to liquid upon increasing the number of hydrogen bonds to the molecule of interest. The study sheds light on the usefulness of NMR experiments in investigating the local coordination of liquid water. To go a bit further, the above mentioned NMR parameters along with the spin-spin coupling constant are examined for water dimer in various geometries to have insight into solvation and hydrogen bonding phenomena from bottom to top. Characteristic changes in the properties are monitored as the geometry of the dimer is systematically varied from very close encounter of the monomers to distances and orientations where hydrogen bonding between monomers ceases to exist. No rapid changes during the hydrogen bond breaking are observed indicating that the hydrogen bonding is a continuous phenomenon rather than an on-off situation. However, for analysis purposes we provide an NMR-based hydrogen bond definition, expressed geometrically, based on the behaviour of the NMR properties as a function of dimer geometry. Our definition closely resembles widely used definitions and thus reinforces their validity. Magneto-optical rotation parameters, the nuclear spin optical rotation (NSOR) and the Verdet constant, are computed for gaseous and liquid water, in the same manner as the NMR properties above. Recent pioneering experiments including NSOR for hydrogen nuclei in liquid water and liquid xenon have demonstrated that this technique has a potential to be a useful new probe of molecular structure. We reproduce computationally, applying a first-principles theory developed recently in the group, the experimental NSOR for hydrogen nuclei in liquid water, and predict hydrogen NSOR in gaseous water along with the oxygen NSOR in liquid and gaseous water. NSOR is an emerging experimental technique that needs interplay between theory and computation for validation, steering and insight.

Cette thèse s'intéresse aux constantes d'Hermite généralisées associées au groupe linéaire adèlique. A l'image de la théorie de Voronoï classique, on y définit deux propriétés, la perfection et l'eutaxie qui caractérisent les maxima locaux de l'invariant d'Hermite. Des inégalités et liens connus dans le cas classique sont étendus au cas général et fournissent la valeur de la constante dans certains cas. Par une théorie des designs définie pour la variété drapeau et semblable à celle des designs sphériques et grassmaniens, on fournit également de nombreux exemples d'objets atteignant l'extrémum. / This thesis studies generalised Hermite constants associated with the adelic general linear group. Like for the classical Voronoi theory, we define two properties, perfection and eutaxy, which characterise the local maxima of the Hermite invariant. Upper bounds and links known in the classical case are extended to the general case and provide the value of the constant in some cases. Through a theory of designs defined for the flag variety and similar to spherical or grassmanian design, we give also many examples of objects reaching locally the extremum.

Les besoins de la microélectronique pour les condensateurs de type DRAM sont résumés dans la feuille de route ITRS (International Technology Roadmap for Semiconductors). Pour descendre en dessous du noeud technologique 22 nm, des performances électriques telles qu'une épaisseur d'oxyde équivalent (EOT) < 0.5 nm et un niveau de courant de fuite < 1.10-7 A/cm² à 0.8 V sont nécessaires. Ces performances sont difficiles à atteindre si l'on considère des oxydes standards largement utilisés tels que le SiO2, le Si3N4 ou l'Al2O3. Le dioxyde de Titane constitue un matériau diélectrique de choix pour ce type d'application si l'on considère sa forte constante diélectrique, la plus haute des oxydes binaires. Selon les conditions de croissance de la couche de TiO2, celle-ci peut se présenter sous forme amorphe ou posséder une structure cristalline appelé phase anatase ou phase rutile. Cette dernière présente une très forte constante diélectrique (90 à 170 selon l'orientation de la maille cristalline) et en fait un atout indéniable pour le développement de condensateur DRAM. Toutefois, cette phase rutile est aussi à l'origine d'un fort courant de fuite mesuré à partir des structures Métal - Isolant - Métal (MIM) associées. De ce fait, il est primordial de savoir contrôler ces courants de fuite tout en gardant la forte valeur de constante diélectrique de la phase rutile. Dans ce travail, nous proposons de travailler sur la croissance des couches minces de TiO2 intégrées dans des structures MIM et déposées sur des substrats différents tels que des électrodes de RuO2/Ru ou de Pt. La technique de dépôt employée pour les couches minces de TiO2 est la technique ALD pour son contrôle très précis de l'épaisseur déposée et sa souplesse d'utilisation pour ce type d'applications. Les propriétés physico-chimiques des couches de TiO2 et l'influence du substrat sur ces propriétés sont analysées. Des compositions différentes de diélectriques sont élaborées au moyen de la technique de dépôt par ALD et notamment des couches minces de TiO2 dopés à l'aluminium. Les propriétés électriques de ces couches sont étudiées afin de déterminer les performances électriques des structures MIM associées en termes de courant de fuite et de densité capacitive. / The requirements for future dynamic random access memory (DRAM) capacitors are summarized in the International Technology Roadmap for Semiconductors. For sub-22 nm node, performances like equivalent oxide thickness (EOT) < 0.5 nm and leakage current density < 1.10-7 A/cm² at 0.8 V are required but are difficult to meet. Titanium dioxide (TiO2) is an attractive dielectric material for such application regarding its high dielectric constant (k). Depending on its growth conditions, TiO2 can be prepared in amorphous, anatase or rutile phase. From the structural point of view, it is generally preferred that TiO2 remains amorphous throughout a complete technological process to minimize leakage transport along grain boundaries. However, the rutile phase exhibits very high dielectric constant ranging from 90 to 170, depending on the lattice orientation. Due to this high dielectric constant, TiO2 rutile phase is considered as a promising material for capacitors in future generations of Dynamic Random Access Memories (DRAMs). A key issue is how to control the high leakage current of rutile phase while keeping the highest dielectric constant in order to get the best electrical performances. In this work, we investigate the growth of high dielectric constant rutile TiO2 films in Metal - Insulator - Metal (MIM) structures deposited on different substrates such as RuO2/Ru or Pt electrodes using ALD (Atomic Layer Deposition). A study of physico-chemical properties of TiO2 layer and influence of bottom electrodes on TiO2's crystalline structure is proposed. Different compositions of dielectrics are processed using flexibility of ALD deposition technique, including Al-doped TiO2 layers and pure TiO2 layers. Electrical properties in terms of leakage current or capacitance density of MIM structures embedding that kind of dielectrics and comparison between these MIM structures in terms of electrical performances is proposed in order to determine the best dielectric film composition to meet the requirements for next generation of DRAM capacitors.

Please read the abstract in the section 00front of this document / Thesis (PhD)--University of Pretoria, 2008. / Materials Science and Metallurgical Engineering / PhD / Unrestricted

Les phénylpropanoïdes (PPs) constituent l'une des familles les plus abondantes des métabolites secondaires dans le règne végétal. Ils protègent les plantes contre les stress biotiques et abiotiques. De nos jours, les études portent sur l'utilisation des PPs comme alternatifs aux agents antimicrobiens, antioxydants et anti-inflammatoires de synthèse pour leur incorporation dans la formulation des produits alimentaires et pharmaceutiques. Cependant, l'utilisation de PPs est généralement limitée en raison de leur faible solubilité, stabilité et volatilité. L'objectif de notre travail a été d'encapsuler sept PPs dans des molécules cages, les cyclodextrines (CDs), en vue de développer des systèmes naturels et éco-compatibles ayant des applications potentielles dans les domaines alimentaire et pharmaceutique. Trois axes ont été abordés. Le premier axe a porté sur la préparation et la caractérisation des complexes d'inclusion CD/PP en solution et à l'état solide. Les techniques d'"headspace" couplé à la chromatographie en phase gazeuse (HS-CG), spectroscopie UV-visible, ¹H RMN, (2D) ROESY RMN, FTIR, DSC et de la modélisation moléculaire ont été utilisées comme outils pour la caractérisation des complexes obtenus. Des études de phase de solubilité ont été également réalisées. Le deuxième axe a porté sur l'évaluation de l'effet des CDs sur la photostabilité et la vitesse de libéralisation des PPs. Le dernier axe a été orienté vers l'étude des activités anti-radicalaire, antibactérienne et antifongique des complexes d'inclusion CD/PP. Les résultats montrent que les CDs sont capables d'encapsuler les PPs étudiés, réduire leur volatilité, augmenter leur solubilité et photostabilité ainsi que de générer des systèmes de libération prolongée. De plus, l'encapsulation conserve les propriétés antioxydante, antibactérienne et antifongique des PPS. Les résultats de cette étude suggèrent que les complexes d'inclusion des PPs avec les CDs peuvent être considérés comme outils prometteurs pour l'optimisation des formulations alimentaires et pharmaceutiques. / Phenylpropanoids (PPs) are one of the largest families of plants secondary metabolites. They protect plants against biotic and abiotic stresses. Nowadays, extensive research has been dedicated to PPs aiming their use as natural alternatives to synthetic antimicrobial, antioxidant and anti-inflammatory agents in food and pharmaceutical formulations. However, PPs suffer from a low water solubility, high volatility, high light and thermal sensitivity that limit their further use. This current study aimed to encapsulate seven PPs in host cage molecules, cyclodextrins (CDs), in order to develop natural and biocompatible formulation that may find applications in food and pharmaceutical fields. It focused on three main research axes. The first part dealt with the preparation and the characterization of CD/PP inclusion complexes both in solution and in solid state. Characterizations were performed with Static-Headspace-Gas Chromatography (SH-GC), UV-Visible, ¹H NMR, (2D) ROESY NMR, FTIR, DSC and molecular modeling. These investigations were complemented with phase solubility studies. The second part was devoted to the evaluation of the effect of CDs on the PPs photostability and controlled release. The last part aimed to evaluate the CD/PP inclusion complexes as radical scavengers, antibacterial and antifungal agents. Results showed that CDs could successfully encapsulate PPs, reduce their volatility, enhance their solubility and photostability and generate controlled release system. In addition, encapsulation maintained the antioxydant, antibacterial and antifungal properties of PPs. Thus, the CD/PP inclusion complexes could be considered as a promising tool for formulation optimization.

AC-DC converters find application in every day life as a front-end to DC-DC and DC-AC converters. Active three-phase converters shape the three-phase input current to be sinusoidal and to be in-phase with the input voltage, as well as to provide a steady DC output voltage. This thesis investigates various active three-phase rectifier and control topologies and identifies a rectifier and control topology most suitable for use in converting a variable voltage variable frequency generator output to a DC voltage. In this dissertation, design relations are derived for determining the plant transfer response (for the suitable topology/controller), design equations are derived for designing/choosing the filter components, and guidelines are derived that will assist in choosing the right semi-conductor components and to give an estimation of expected system efficiency. The dissertation investigates the implementation of both analogue and digital control and provides implementation methodologies for both controllers. Expected results are verified by simulation and a build-up prototype. It was shown that the VIENNA rectifier is able to convert a generator type input, with variable input voltage amplitude and variable frequency, to a constant DC-bus voltage whilst controlling the input current to be sinusoidal and in phase with the input voltage. The rectifier was able to maintain a constant DC voltage at the output for input voltages as low as half the rated input voltage and for an equivalent output power of half the rated output power. This suggests that the VIENNA rectifier, controlled as a dual-boost rectifier, is suitable for applications that require power factor corrections and simultaneously operate from a wide input voltage range. / Dissertation (MEng (Electrical Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / MEng / unrestricted

We consider the problem of equalizing data blocks of signals, which have been transmitted over an aeronautical channel using two different modulation schemes. The equalization is performed using the block-processing constant modulus algorithm (CMA), and in order to achieve real-time processing a Fast Fourier Transform (FFT) is used to compute the gradient of this cost function during equalization. The equalizer length is chosen to be five times of the channel length. For the first experiment, we present the result of equalizing a set of measured data, which was modulated and transmitted using the iNET packet structure with SOQPSK modulation. In this case, the CMA equalizer is first initialized using MMSE and the equalizer coefficients are then updated once, using each entire block (iNET packet). In the second experiment, we apply the FFT-based block processing equalizer to received data blocks of QPSK signals, which have been randomly generated and transmitted over an aeronautical channel. A modified constant modulus algorithm and alphabet matched algorithm (CMA + AMA) equalizer is used to recover these data blocks. For this case of QPSK signals, the equalizer performance is evaluated over 500 Monte Carlo runs, using the average symbol error rate (SER).