Diffusion-weighted magnetic resonance imaging (dMRI) is a powerful tool for thecharacterisation of neural tissue microstructural features. The role of neural projectioncurvature on the diffusion signal was recently studied for three temporal regimes of the diffusion pulse sequence in search for a description of the different decay trends in the orientationally-averaged diffusion signal reported in in vivo human studies.This work experimentally investigates the effects of neural projection curvedness in one of these regimes, namely the short diffusion time regime. Multi-shell diffusion MRI acquisitions on fixed rat spinal cord were performed using a custom number of diffusion gradient directions on a vertical bore pre-clinical MRI scanner capable of generating 3000 mT/m. Diffusion was probed in three different q-values ranges [450, 970], [600, 1400] and [1500, 1750] mm-1 using diffusion pulse durations of 1.4,2 and 2.5ms, respectively. Noise correction was performed on the diffusion data and the orientationally-averaged signal was computed for each shell using a weighted mean. The signal from selected regions in the sample was then fitted to a power law. Results show that gray matter areas exhibit a signal reduction with variable decay trends in the range of diffusion sensitivity values used here. This suggests that gray matter microstructure features are pictured by the orientationally-averaged signal in the high diffusion sensitivity regime and, as theoretically suggested, neurite curvature might play a role in characterizing the signal decay. These preliminary results may prove useful in the development of models for the interpretation of the diffusion signal and the design of acquisition strategies that aim to study the high diffusion sensitivity regime.

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Yes / In this paper, an improved Shiono-Knight model (SKM) has been proposed to calculate the rectangular compound open channel flows by considering a Multi-Zonal (MZ) approach in modelling turbulence and secondary flows across lateral flow direction. This is an effort to represent natural flows with compound shape more closely. The proposed model improves the estimation of secondary flow by original SKM model to increase the accuracy of depthaveraged velocity profile solution formed within the transitional region between different sections (i.e. between main-channel and floodplain) of compound channel. This proposed MZ model works by sectioning intermediate zones between floodplain and main-channel for running computation in order to improve the modelling accuracy. The modelling results have been validated using the experimental data by national UK Flood Channel Facility (FCF). It has been proven to work reasonably well to model secondary flows within the investigated compound channel flow cases and hence produce better representation to their flow lateral velocity profile.

Fuel cells are a clean, high-efficiency source for power generation. This innovative technology is going to penetrate all aspects in our life, from utility distributed power, transportation applications, down to power sources for portable devices such as laptop computer and cell phones. To enable the usage of fuel cell, developing power converters dedicated for fuel cells becomes imminent. Currently, the full-bridge converter is the dominating topology in high power dc/dc applications. Although multiphase converters have been proposed, most of them are dealing with high input-voltage systems, and their device characteristic is not suitable for a low voltage source such as a fuel cell. For a high power fuel cell system, high voltage conversion ratios and high input currents are the major obstacles to achieving high-efficiency power conversions. This dissertation proposes a novel 3-phase 6-leg dc/dc power converter with transformer isolation to overcome these obstacles. Major features of the proposed converter include: (1) Increase converter power rating by paralleling phases, not by paralleling multiple devices; (2) Double the output voltage by transformer delta-wye connection, thus lowering the turns-ratio; (3) Reduce the size of output filter and input dc bus capacitor with interleaved control; (4) Achieve Zero-Voltage Zero-Current Switching (ZVZCS) over a wide load range without auxiliary circuitry. High conversion efficiency above 96% is verified with different measurement approaches in experiments. This dissertation also presents the power stage and control design for the proposed converter. Control design guideline is provided and the design result is confirmed with both simulation and hardware experiments. When using the fuel cell for stationary utility power applications, a low-frequency ripple interaction was identified among fuel cell, dc/dc converter and dc/ac inverter. This low frequency ripple tends to not only damage the fuel cell, but also reduce the source capability. This dissertation also investigates the mechanism of ripple current propagation and exploits the solutions. A linearized ac model is derived and used to explain the ripple propagation. An active ripple reduction technique by the use of the current loop control is proposed. This active current loop control does not add extra converters or expensive energy storage components. Rather, it allows a reduction in capacitance because the ripple current flowing into the capacitor is substantially reduced, and less capacitance can be used while maintaining a clean dc bus voltage. The design process and guideline for the proposed control is suggested, and the effectiveness of this active control is validated by both simulation and experimental results. / Ph. D.

Self-excited systems are open loop unstable plants having a nonlinearity that prevents an exponentially increasing time response. The resulting limit cycle is induced by any slight disturbance that causes the response of the system to grow to the saturation level of the nonlinearity. Because there is no external disturbance, control of these self-excited systems requires that the open loop system dynamics are altered so that any unstable open loop poles are stabilized in the closed loop. This work examines a variety of adaptive control approaches for controlling a thermoacoustic instability, a physical self-excited system. Initially, a static feedback controller loopshaping design and associated system identification method is presented. This design approach is shown to effectively stabilize an unstable Rijke tube combustor while preventing the creation of additional controller induced instabilities. The loopshaping design method is then used in conjunction with a trained artificial neural network to demonstrate stabilizing control in the presence of changing plant dynamics over a wide variety of operating conditions. However, because the ANN is designed specifically for a single combustor/actuator arrangement, its limited portability is a distinct disadvantage. Filtered-X least mean squares (LMS) adaptive feedback control approaches are examined when applied to both stable and unstable plants. An identification method for approximating the relevant plant dynamics to be modeled is proposed and shown to effectively stabilize the self-excited system in simulations and experiments. The adaptive feedback controller is further analyzed for robust performance when applied to the stable, disturbance rejection control problem. It is shown that robust stability cannot be guaranteed because arbitrarily small errors in the plant model can generate gradient divergence and unstable feedback loops. Finally, a time-averaged-gradient (TAG) algorithm is investigated for use in controlling self-excited systems such as the thermoacoustic instability. The TAG algorithm is shown to be very effective in stabilizing the unstable dynamics using a variety of controller parameterizations, without the need for plant estimation information from the system to be controlled. / Ph. D.

The thesis deals with the flow in pipe bends and radial turbines geometries that are commonly found in an Internal Combustion Engine (ICE). The development phase of internal combustion engines relies more and more on simulations as an important complement to experiments. This is partly because of the reduction in development cost and the shortening of the development time. This is one of the reasons for the need of more accurate and predictive simulations. By using more complex computational methods the accuracy and predictive capabilities are increased. The disadvantage of using more sophisticated tools is that the computational time is increasing, making such tools less attractive for standard design purposes. Hence, one of the goals of the work has been to contribute to assess and improve the predictive capability of the simpler methods used by the industry. By comparing results from experiments, Reynolds Averaged Navier-Stokes (RANS) computations, and Large Eddy Simulations (LES) the accuracy of the different computational methods can be established. The advantages of using LES over RANS for the flows under consideration stems from the unsteadiness of the flow in the engine manifold. When such unsteadiness overlaps the natural turbulence the model lacks a rational foundation. The thesis considers the effect of the cyclic flow on the chosen numerical models. The LES calculations have proven to be able to predict the mean field and the fluctuations very well when compared to the experimental data. Also the effects of pulsatile exhaust flow on the performance of the turbine of a turbocharging system is assessed. Both steady and pulsating inlet conditions are considered for the turbine case, where the latter is a more realistic representation of the real flow situation inside the exhaust manifold and turbine. The results have been analysed using different methods: single point Fast Fourier Transforms (FFT), probe line means and statistics, area and volume based Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). / Denna avhandling behandlar flödet i rörkrökar och radiella turbiner som vanligtvis återfinns i en förbränningsmotor. Utvecklingsfasen av förbränningsmotorer bygger mer och mer på att simuleringar är ett viktigt komplement till experiment. Detta beror delvis på minskade utvecklingskostnader men även på kortare utevklningstider. Detta är en av anledningarna till att man behöver mer exakta och prediktiva simuleringsmetoder. Genom att använda mer komplexa beräkningsmetoder så kan både nogrannheten och prediktiviteten öka. Nackdelen med att använda mer sofistikerade metoder är att beräkningstiden ökar, vilket medför att sådana verktyg är mindre attraktiva för standardiserade design ändamål. Härav, ett av målen med projektet har varit att bidra med att bedöma och förbättra de enklare metodernas prediktionsförmåga som används utav industrin. Genom att jämföra resultat från experiment, Reynolds Averaged Navier-Stokes (RANS) och Large Eddy Simulations (LES) så kan nogrannheten hos de olika simuleringsmetoderna fastställas. Fördelarna med att använda LES istället för RANS när det gäller de undersökta flödena kommer ifrån det instationära flödet i grenröret. När denna instationäritet överlappar den naturligt förekommande turbulensen så saknar modellen en rationell grund. Denna avhandling behandlar effekten av de cykliska flöderna på de valda numeriska modellerna. LES beräkningarna har bevisats kunna förutsäga medelfältet och fluktuationerna väldigt väl när man jämför med experimentell data. Effekterna som den pulserande avgasströmning har på turboladdarens turbin prestanda har också kunnat fastställas. Både konstant och pulserande inlopps randvillkor har används för turbinfallet, där det senare är ett mer realistiskt representation av den riktiga strömningsbilden innuti avgasgrenröret och turbinen. Resultaten har analyserats på flera olika sätt: snabba Fourier transformer (FFT) i enskilda punkter, medelvärden och statistik på problinjer, area och volumsbaserade metoder så som Proper Orthogonal Decomposition (POD) samt Dynamic Mode Decomposition (DMD). / <p>QC 20140919</p>

La demande croissante pour alléger les moteurs d’avions et diminuer les émissions polluantes de la propulsion aéronautique réclame à rendre plus compact le système de compression des moteurs, qui représente environ 40%-50% de la masse totale. Or, à taux de compression global égal, la réduction du nombre d’étage exige d’un point de vue aérodynamique une augmentation de la charge des aubes de compresseur par étage. La charge d’aube est aujourd’hui limitée car elle induit différents mécanismes de pertes tridimensionnelles très pénalisant. L’un des plus importants est le décollement de coin qui se forme à la jonction entre l’extrados de l’aube et le moyeu ou le carter. Bien que des travaux existent sur les mécanismes et paramètres intervenant dans le décollement de coin, il est encore difficile de proposer une méthode de contrôle efficace. Cela est principalement dû à deux raisons : (i) le manque de compréhension fine des mécanismes physiques, (ii) l’utilisation pour la conception de modèles de turbulence classiques de type RANS (Reynolds-averaged Navier-Stokes) qui ne sont pas capables de prédire précisément le décollement de coin, car ils ne peuvent pas décrire correctement les mécanismes de transport turbulent. Des simulations de type RANS et LES (large-eddy simulation = simulation des grandes échelles) sont présentées dans cette thèse sur une configuration de grille d’aubes de compresseur, et comparées avec les données expérimentales obtenues au LMFA (issues de travaux séparés). L’approche RANS surestime globalement le décollement de coin. Une amélioration significative est obtenue par la méthode LES, en particulier pour le coefficient de pression statique sur l’aube et les pertes de pression totale. Ces résultats montrent que la zone de décollement de coin, qui est la source principale des pertes, génère des tourbillons de grande échelle associés à de forts niveaux d’énergie. Les histogrammes bimodaux de la vitesse tangentielle qui ont été observés expérimentalement semblent confirmés par les résultats LES. En ce qui concerne les amplitudes des fluctuations de vitesse tangentielle, les résultats expérimentaux et ceux de la LES mettent en évidence deux pics sur certains profils perpendiculaires aux parois. Enfin, grâce à l’approche LES, les bilans de l’énergie cinétique turbulente sont calculés et analysés. Ils décrivent l’équilibre entre les termes de production, de dissipation et de transport. Une des perspectives de cette analyse est d’aider à améliorer la modélisation de la turbulence en approche RANS. / The increasing demand to reduce the mass of aircraft jet engines and emissions of aircraft propulsion requires to make the compression system of engines more compact, since this component accounts for about 40%-50% of the total mass. However, at a given overall pressure ratio, decreasing the number of stages will raise the compressor blade loading per stage. The blade loading is extremely restricted by different three-dimensional flow loss mechanisms. One of them is the corner separation that forms between the blade suction side and the hub or shroud. Although some works previously investigated the mechanisms and the parameters of corner separation, it is still difficult to propose an effective control method of the corner separation. That is mainly due to two reasons: (i) the lack of knowledge of the physical mechanisms, (ii) the nowadays classical RANS (Reynolds-averaged Navier-Stokes) turbulence models are not capable to accurately predict the corner separation, since they cannot correctly describe the turbulent transport mechanisms. RANS (Reynolds-averaged Navier-Stokes) and LES (large-eddy simulation) simulations are here presented on a compressor cascade configuration, in comparison with experimental data obtained at LMFA (from separate works). The RANS approach globally over-estimates the corner separation, whereas a significant improvement is achieved with the LES, especially for the blade surface static pressure coefficient and the total pressure losses. The corner separation region, which is the main source of the total pressure losses, is shown to generate large-scale energy-containing eddies. The bimodal histograms of the streamwise velocity that were observed experimentally seem to be confirmed by the LES results. Concerning the streamwise velocity fluctuations (RMS), both the experiment and the LES show some profiles with two peaks. Finally, thanks to the LES approach, the turbulent kinetic energy budget, which represents the balance between the production, dissipation and transport terms, are computed and analyzed. This may help the improvement of RANS turbulence modeling.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: 138La is a very long-lived and low abundant p-isotope. Most p-nuclei with Z > 54 are thought to be produced through photodisintegration of s- and r-process seed nuclei. However, this p-process cannot satisfactorily explain the observed abundance of 138La, and more exotic processes, such as ve + 138Ba → 138La + e− have to be considered. This v-process can reproduce the observed solar abundance of 138La, but the significance of the p-process cannot be ruled out due to very high uncertainties in its predicted reaction rates. These errors have been discussed to be mainly due to the unavailability of the experimental nuclear level densities and radiative strength functions of 138,139La, which are critical ingredients for astrophysical reaction rate calculations based on the Hauser-Feshbach approach. Thus, nuclear physics measurements are necessary to place the nuclear properties on a strong footing, in order to make statements regarding the importance of p- and v-processes. In this research project the experimental nuclear level densities and radiative strength functions of 138,139La were measured below the neutron thresholds. From this new experimental data, the Maxwellian averaged cross sections for the 137La(n, y) and 138La(n, y) reactions, at the p-process temperature of 2.5⇥109 K, were computed with the TALYS code. Using these reaction rates the nucleosynthesis calculations in the O/Ne-rich layers of Type II supernovae were performed. The results imply that the standard p-process still under-produces 138La, which puts the v-process on a very strong footing as the main production process for 138La. / AFRIKAANSE OPSOMMING: 138La is ’n p-isotoop met ’n baie lang halfleeftyd. Daar word tans vermoed dat p-nukiede met Z > 54 geproduseer word deur fotodisintegrasie van sen r-proses saadnukliede. Nogtans verklaar hierdie p-proses die waargenome natuurlike voorkoms van 138La nie behoorlik nie, en meer eksotiese prosesse soos byvoorbeeld ve+ 138Ba → 138La + e− moet in aanmerking geneem word. Hierdie v-proses kan die waargenome natuurlike voorkoms van 138La verklaar, maar die belangrikheid van die p-proses kan nie afgewys word nie weens die onsekerheid in die voorspelde reaksie snelheid. Sodanige onsekerhede word bespreek en word hoofsaaklik toegeskryf aan die gebrek aan eksperimentele vlakdigthede en stralings sterkefunksies van die kerne 138,139La, wat van kritiese belang is vir berekeninge van astrofisiese reaksie snelhede gebaseer op die Hauser-Feshbach benadering. Kernfisiese metings is derhalwe noodsaaklik om die eienskappe van kerne op ’n stewige grondslag te plaas sodat uitlatings gemaak kan word omtrent die belangrikheid van p- en v-prosesse. In hierdie esperimentele navorsingsprojek is die kern vlakdigthede en stralings sterkefunksies van 138,139La onder die neutron reaksiedrumpels gemeet. Die nuwe gemete data maak dit moontlik om die Maxwell-gemiddelde kansvlakke vir die 137La(n, y) en 138La(n, y) reaksies by ’n p-proses temperatuur van 2.5 x 109 K met die TALYS program te bereken. Hierdie reaksie snelhede is daarna gebruik om berekeninge van elementvorming in die O/Ne-ryke lae van Tipe-II supernovas te maak. Die resultate wys uit dat die stadaard p-proses nie genoegsame 138La produseer nie, wat derhalwe die v-proses op ’n baie stewige grondslag plaas as die hoof produksie proses vir 138La.

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The hydrodynamic characteristics of three typical ship-like hull sections, in different motions, are numerically investigated using FLUENT, 2009. These simple shapes, namely a v-bottom (triangle) hull, a at-bottom (square) hull and a round-bottom (semi-circle) hull, are investigated in uncoupled heave, sway and roll. The problem is described in two dimensions. A combination of numerical methods and models, found in literature, are used to conduct this investigation. Hull characterisation is achieved through the use of hull mass and damping coe cients. These numerically determined coe cients are compared to experimental work conducted by Vugts (1968). A good correlation between the numerical and experimental results exists for the heave and sway cases. By normalising the coe cients, different hulls are comparable to one another. The numerical models used are validated and veri ed. Roll motion remains largely unsolved for very large angles of roll (in excess of 11°). Different uid ow phenomena occurring around the hull sections have varying degrees of in uence on the motions of a hull. It is found that not one of the turbulence models investigated can be employed to globally solve each type of hull-motion case. Also, forced oscillations in computational simulations require considerably more computational time than free-decay oscillating hull simulations. / AFRIKAANSE OPSOMMING: Die hidrodinamiese karakteristieke van verskillende skeepsrompvorms, in verskeie bewegingswieë, is numeries ondersoek met behulp van FLUENT, 2009. Drie eenvoudige vorms ('n v-bodem (driehoek), plat-bodem (reghoek) en rondebodem (semi-sirkel) romp) is onderskeidelik ondersoek in opwieg, dwarswieg en rol. Die probleem is twee-dimensioneel. Daar is gebruik gemaak van 'n kombinasie van numeriese metodes en modelle, uit die literatuur, om die ondersoek uit te voer. Die rompe is gekarakteriseer met behulp van massa- en dempingskoëffi siënte. Hierdie numeries bepaalde koë ffisiënte is vergelyk met die eksperimentele werk van Vugts (1968). Daar bestaan 'n goeie korrelasie tussen die numeriese en eksperimentele resultate vir die opwieg en dwarswieg gevalle. Die koë ffisiënte is genormaliseer om die verskeie rompvorms te vergelyk. Die numeriese modelle is geverifi eer en valideer. Rolbewegings is onopgelos vir groot rolhoeke (groter as 11°). Die mate waartoe die romp se beweging beïnvloed word deur die verskillende vloei verskynsels wat om die rompe ontstaan, verskil. Daar is bevind dat geen van die turbulensie modelle gebruik kan word om alle skeepsbeweging-gevalle op te los nie. Gedwonge-ossilasie numeriese simulasies benodig meer berekeningstyd as vrye-verval ossilasie gevalle.