The twin-arginine protein transport (Tat) system is one of two general protein transport systems in the cytoplasmic membranes of bacteria, and is conserved in the thylakoid membranes of plants. The Tat system transports fully folded proteins of varying sizes across the membrane. This feat is achieved by the use of just three conserved integral membrane proteins, TatA, TatB and TatC. Passenger proteins are designated for transport by the system by the use of an N-terminal extension, termed a signal peptide which contains an invariant twin arginine motif. The TatC component of the transport system is responsible for recognising the signal peptide, as well as binding to TatB and the TatA components. TatC therefore emerges as the functional and organisational ‘core’ of the Tat system. There is a wealth of functional data relating to the TatC protein, but no high resolution structural information on TatC was available to interpret this data. In order to gain an understanding of the Tat system at the molecular level, a structure was required of TatC. Preliminary expression screening using green fluorescent protein had previously identified the TatC protein from the thermophilic bacterium Aquifex aeolicus as being amenable for structural work. This study purified the A. aeolicus TatC construct in a variety of detergents, used native mass spectrometry and light scattering techniques to assay the sample homogeneity and set up extensive crystallisation trials. Following optimisation of a selenomethionine minimal media growth protocol, crystals grown from selenomethionine-substituted TatC purified in lauryl maltose neopentyl glycol diffracted to 3.5Å resolution, and allowed the structure of TatC to be solved by single wavelength anomalous dispersion. Analysis of the novel structure of TatC, in combination with an isothermal titration calorimetric binding assay, gave information on how TatC binds to signal peptides. Further structural analysis combined with additional experimental data from collaborators allowed a model to be proposed for how TatC binds to the two additional integral membrane components of the system, TatB and TatA, as part of the transport mechanism. Subsequent work was undertaken in this study to attempt to gain a co-crystal structure of TatC with the signal peptide. Construct and detergent screening was carried out in order to aim towards high resolution structural characterisation of the TatBC complex.

This thesis examines the effects of budget deficits on the current account imbalance and inflation in African countries. The aims of this thesis are; first, to use higher frequency data. Most studies in African countries use annual data; by contrast we use quarterly data. Second, to examine the dynamic interaction between fiscal deficits and current account imbalances using VAR models. Third, to explore the long-run relationship between the twin deficits, using the autoregressive distributed lag (ARDL) of Pesaran, Shin and Smith (2001). Fourth, to assess the long-run relationship between the twin deficits using the threshold autoregressive models of Hansen and Seo (2002). Fifth, to model inflation as being non-linearly related to fiscal deficits using the asymmetric cointegration approach of Enders and Siklos (2001). The second chapter discusses the theoretical framework and review of the empirical literature on twin deficits and fiscal deficits and inflation. We find much evidence in support of the twin deficits hypothesis that increase in government deficits leads to increase in the current account deficits. There is little empirical study on the Ricardian equivalence hypothesis. From the twin deficits literature, we observe that where the twin deficits hypothesis holds there is a high degree of openness and also countries operates a flexible exchange rate. The empirical literature on fiscal deficits and inflation suggests that fiscal deficits are inflationary in high inflation economies and developing countries, but not in low inflation and developed countries. The third chapter examines the time series properties of the series using the Augmented Dickey-Fuller test, the Phillip-Perron test and the Lee and Strazicich (2003) two-break unit root test. Results for the unit root test reveals that majority of the series are significant in their first differences. By contrast applying the LM two structural break test shows that the majority of the series are significant around two structural breaks. The fourth chapter analyses the twin deficits hypothesis using a VAR model. Results show that a positive government deficit shock increases the current account deficit in Botswana, Egypt, Ethiopia, Ghana, Morocco, South Africa and Tanzania while the current account improves in response to a positive government deficit shock in Cameroon and Uganda. Also in response to a positive government deficit shock, the current account remains constant in Kenya, Nigeria and Tunisia. The fifth chapter examine the long run relationship between the twin deficits hypothesis accounting for structural breaks using the Autoregressive Distributed Lags (ARDL) model. Results show that the fiscal deficit in the twelve African countries has long run impact on the current account deficit. The sixth chapter examines the relationship between fiscal deficit and current account deficit using the bi-variate threshold cointegration model of Hansen and Seo (2002) for nine countries where the fiscal deficits and current account deficits were significant at first differences. We find evidence of a positive cointegrating relationship between the current account and the fiscal balances for Botswana, Cameroon, Egypt, Morocco, Nigeria and Tanzania; and a negative cointegrating relationship in Ethiopia, Kenya and Uganda. The seventh chapter examines the long-run relationship between fiscal deficits and inflation in eleven African countries using the TAR and M-TAR models of Enders and Siklos (2001). Results show that fiscal deficits and inflation are asymmetry in Botswana, Egypt, Ethiopia, Ghana, Kenya, Morocco and Tanzania. This thesis centres on the twin deficits and fiscal deficits and inflation in African countries. Conclusions from the empirical chapters indicate that large fiscal deficits is the cause of current account deficits, and that fiscal deficits are inflationary. This study further suggests that African countries should spend their resources on projects that will accelerate the level of growth and development.

The cost and effort of modelling existing bridges from point clouds currently outweighs the perceived benefits of the resulting model. The time required for generating a geometric Bridge Information Model, a holistic data model which has recently become known as a "Digital Twin", of an existing bridge from Point Cloud Data is roughly ten times greater than laser scanning it. There is a pressing need to automate this process. This is particularly true for the highway infrastructure sector because Bridge Digital Twin Generation is an efficient means for documenting bridge condition data. Based on a two-year inspection cycle, there is a need for at least 315,000 bridge inspections per annum across the United States and the United Kingdom. This explains why there is a huge market demand for less labour-intensive bridge documentation techniques that can efficiently boost bridge management productivity. Previous research has achieved the automatic generation of surface primitives combined with rule-based classification to create labelled cuboids and cylinders from point clouds. While existing methods work well in synthetic datasets or simplified cases, they encounter huge challenges when dealing with real-world bridge point clouds, which are often unevenly distributed and suffer from occlusions. In addition, real bridge topology is much more complicated than idealized cases. Real bridge geometries are defined with curved horizontal alignments, and varying vertical elevations and cross-sections. These characteristics increase the modelling difficulties, which is why none of the existing methods can handle reliably. The objective of this PhD research is to devise, implement, and benchmark a novel framework that can reasonably generate labelled geometric object models of constructed bridges comprising concrete elements in an established data format (i.e. Industry Foundation Classes). This objective is achieved by answering the following research questions: (1) how to effectively detect reinforced concrete bridge components in Point Cloud Data? And (2) how to effectively fit 3D solid models in the format of Industry Foundation Classes to the detected point clusters? The proposed framework employs bridge engineering knowledge that mimics the intelligence of human modellers to detect and model reinforced concrete bridge objects in point clouds. This framework directly extracts structural bridge components and then models them without generating low-level shape primitives. Experimental results suggest that the proposed framework can perform quickly and reliably with complex and incomplete real-world bridge point clouds encounter occlusions and unevenly distributed points. The results of experiments on ten real-world bridge point clouds indicate that the framework achieves an overall micro-average detection F1-score of 98.4%, an average modelling accuracy of (C2C) ̅_Auto 7.05 cm, and the average modelling time of merely 37.8 seconds. Compared to the laborious and time-consuming manual practice, the proposed framework can realize a direct time-savings of 95.8%. This is the first framework of its kind to achieve such high and reliable performance of geometric digital twin generation of existing bridges. Contributions. This PhD research provides the unprecedented ability to rapidly model geometric bridge concrete elements, based on quantitative measurements. This is a huge leap over the current practice of Bridge Digital Twin Generation, which performs this operation manually. The presented research activities will create the foundations for generating meaningful digital twins of existing bridges that can be used over the whole lifecycle of a bridge. As a result, the knowledge created in this PhD research will enable the future development of novel, automated applications for real-time condition assessment and retrofit engineering.

Data assimilation into sea ice models designed for climate studies has started about 15 years ago. In most of the studies conducted so far, it is assumed that the improvement brought by the assimilation is straightforward. However, some studies suggest this might not be true. In order to elucidate this question and to find an appropriate way to further assimilate sea ice concentration and velocity observations into large-scale sea ice models, we analyze here results from a number of twin experiments (i.e. experiments in which the assimilated data are model outputs) carried out with first a simplified model of the Arctic sea ice pack then with NEMO-LIM2, a primitive equation ocean general circulation model coupled to LIM (Louvain-la-Neuve sea ice model). Our objective is to determine to what degree the assimilation of ice velocity and/or concentration data improves the global performance of the model and, more specifically, reduces the error in the computed ice thickness. A simple scheme is used, and outputs from a control run and from perturbed experiments without and with data assimilation are thoroughly compared. Our results indicate that, under certain conditions depending on the assimilation weights and the type of model error, the assimilation of ice velocity data enhances the model performance. The assimilation of ice concentration data also helps in improving the model results, but it has to be handled with care because of the strong link between ice concentration and ice thickness. Therefore, we show that one should conserve the ice thickness (not the ice volume) when ice concentration data are assimilated into the model. We also demonstrate that one should assimilate sea ice concentration and velocity data simultaneously. Finally, we give some concrete keys in order to choose which observational data set to assimilate.

The schools located in many township and rural areas of South Africa struggle to provide a high standard of education because many of the inhabitants of these areas are very poor and there is thus a lack of the funding required for the necessary educational equipment. The Afri Twin project, started by Jayne Martin from the UK, addresses this problem. The project is collaboration between a British school, a South African town/city school, and a school from a poor township or rural community in South Africa. This article reports on a research project that made use of a qualitative case study approach, to focus on the collaboration between Wellacre Technology College, Ferrum High School, and Siyalungelwa High School as part of the Afri Twin project. The objectives of the research project are to determine the benefits of the Afri Twin project for the schools involved; to investigate the unique interaction between Wellacre, Ferrum, and Siyalungelwa; and to determine the cultural enrichment of learners and the multi-cultural interaction between the three schools. Data was collected through focus group interviews, in-depth interviews, and questionnaires. Data analysis led to the emergence of four major themes with categories and subcategories. The conclusion underlined the cultural tolerance that is established through the Afri Twin project, as well as the importance of financial support for promoting an improved learning environment. / Thesis (M.A. (Psychology))--North-West University, Potchefstroom Campus, 2009.

Data assimilation into sea ice models designed for climate studies has started about 15 years ago. In most of the studies conducted so far, it is assumed that the improvement brought by the assimilation is straightforward. However, some studies suggest this might not be true. In order to elucidate this question and to find an appropriate way to further assimilate sea ice concentration and velocity observations into large-scale sea ice models, we analyze here results from a number of twin experiments (i.e. experiments in which the assimilated data are model outputs) carried out with first a simplified model of the Arctic sea ice pack then with NEMO-LIM2, a primitive equation ocean general circulation model coupled to LIM (Louvain-la-Neuve sea ice model). Our objective is to determine to what degree the assimilation of ice velocity and/or concentration data improves the global performance of the model and, more specifically, reduces the error in the computed ice thickness. A simple scheme is used, and outputs from a control run and from perturbed experiments without and with data assimilation are thoroughly compared. Our results indicate that, under certain conditions depending on the assimilation weights and the type of model error, the assimilation of ice velocity data enhances the model performance. The assimilation of ice concentration data also helps in improving the model results, but it has to be handled with care because of the strong link between ice concentration and ice thickness. Therefore, we show that one should conserve the ice thickness (not the ice volume) when ice concentration data are assimilated into the model. We also demonstrate that one should assimilate sea ice concentration and velocity data simultaneously. Finally, we give some concrete keys in order to choose which observational data set to assimilate.

In this work, the peroxide-initiated degradation of polypropylene (PP) in co-rotating intermeshing twin-screw extruders (COITSEs) is analyzed by means of numerical simulations. This reactive extrusion (REX) operation is simulated by implementing (i) a one-dimensional and (ii) a three-dimensional (3D) modeling approach. In the case of the 1D modeling, a REX mathematical model previously developed and implemented as a computer code is used for the evaluation of two scale-up rules for COITSEs of various sizes. The first scale-up rule which is proposed in this work is based on the concept of thermal time introduced by Nauman (1977), and the second one is based on specific energy consumption (SEC) requirements. The processing parameters used in testing the previously referred to scale-up approaches are the mass throughput, the screw rotating speed, and the peroxide concentration, whereas the extruder screw configuration and the barrel temperature profiles are kept constant. The results for the simulated operating conditions show that when the REX operation is scaled-up under constant thermal time, very good agreement is obtained between the weight-average molecular weight (Mw) and poly-dispersity index (PDI) from the larger extruders and the values of these parameters corresponding to the reference extruder. For the constant SEC approach, on the other hand, more significant variations are observed for both of the aforementioned parameters. In the case of the implemented constant thermal time procedure, a further analysis of the effect of the mass throughput and screw speed of the reference device on the scaled-up operation is performed. It is observed that when the lower mass throughput is implemented for the smaller extruder keeping a constant screw speed, the predicted residence times of extrusion for the larger extruders are lower, in general terms, than those corresponding to the reference device, and a converse situation occurs for the higher implemented value of the mass throughput. Also, in general terms, the higher increase of the reaction temperature on the scaled-up operation corresponds to the lower mass throughputs and higher screw speeds specified for the reference extruder. For the 3D modeling approach, two different case studies are analyzed by means of a commercial FEM software package. The REX simulations are performed under the assumption of steady-state conditions using the concept of a moving relative system (MRS). To complement the information obtained from the MRS calculations, simulations for selected conditions (for non-reactive cases) are performed considering the more realistic transient-state (TS) flow conditions. The TS flow conditions are associated to the time periodicity of the flow field inside the conveying elements of COITSEs. In the first case study, the peroxide-initiated degradation of PP is simulated in fully-filled screw elements of two different size COITSEs in order to evaluate scale-up implications of the REX operation. In the second case, the reacting flow is simulated for a conventional conveying screw element and a conveying screw element having a special design and corresponding to the same extruder size. For both of the analyzed cases, the effects of the initial peroxide concentration and mass throughput on the final Mw and PDI of the degraded resin are studied. The effect of the processing conditions is discussed in terms of the residence time distribution (RTD), the temperature of reaction, and the distributive mixing capabilities of the REX system. When analyzing the scale-up case, it is found that for the implemented processing conditions, the final Mws and PDIs are very close to each other in both of the analyzed flow geometries when the specified flow is close to that corresponding to the maximum conveying capabilities of the screw elements. For more restrictive flow conditions, the final Mws and PDIs are lower in the case of the screw element of the larger extruder. It is found that the distributive mixing ability of the reactive flow is mainly related to the specified mass throughput and almost independent of the specified peroxide concentration for a particular extruder size. For the analyzed screw elements, the conveying element corresponding to the small size extruder shows a slightly better distributive mixing performance. For this same case study, a further evaluation of the proposed scale-up criterion under constant thermal time confirms the trend of the results observed for the 1D simulations. In the second case study, the special type of screw element consists of screws rotating at different speeds which have different cross sections. In this case, the outer and inner diameters of both the special and the conventional type of screw elements are specified to be the same. As in the previous case study, the distributive mixing capabilities appear to be independent of the specified peroxide concentrations but dependent on the mass flow rate. It is speculated from the simulation results, from both the transient- as well as the steady-state flow conditions, that the screw element with the special design would yield lower final values of the PDI and Mw. Also, this screw element appears to have improved distributive mixing capabilities as well as a wider RTD.

The discovery of a solar twin in the open star cluster M67 (Önehag et al. 2011) implies a near-solar chemical composition for the cluster. This study uses high-resolution spectroscopic data of five main-sequence stars in M67 to analyze their abundance of a few key elements and compare results to the solar-twin composition and the composition of field twins (Melendez et al. 2009). The derived composition was also compared to predictions of stellar-structure models including the effects of  element diffusion. (Richard, private communication).  It is found that all analyzed elements are, to varying degree, less abundant in the five main sequence stars than in the solar twin. With the possible exception of iron, all derived abundances also fall clearly below the diffusion predictions.

In this work, the peroxide-initiated degradation of polypropylene (PP) in co-rotating intermeshing twin-screw extruders (COITSEs) is analyzed by means of numerical simulations. This reactive extrusion (REX) operation is simulated by implementing (i) a one-dimensional and (ii) a three-dimensional (3D) modeling approach. In the case of the 1D modeling, a REX mathematical model previously developed and implemented as a computer code is used for the evaluation of two scale-up rules for COITSEs of various sizes. The first scale-up rule which is proposed in this work is based on the concept of thermal time introduced by Nauman (1977), and the second one is based on specific energy consumption (SEC) requirements. The processing parameters used in testing the previously referred to scale-up approaches are the mass throughput, the screw rotating speed, and the peroxide concentration, whereas the extruder screw configuration and the barrel temperature profiles are kept constant. The results for the simulated operating conditions show that when the REX operation is scaled-up under constant thermal time, very good agreement is obtained between the weight-average molecular weight (Mw) and poly-dispersity index (PDI) from the larger extruders and the values of these parameters corresponding to the reference extruder. For the constant SEC approach, on the other hand, more significant variations are observed for both of the aforementioned parameters. In the case of the implemented constant thermal time procedure, a further analysis of the effect of the mass throughput and screw speed of the reference device on the scaled-up operation is performed. It is observed that when the lower mass throughput is implemented for the smaller extruder keeping a constant screw speed, the predicted residence times of extrusion for the larger extruders are lower, in general terms, than those corresponding to the reference device, and a converse situation occurs for the higher implemented value of the mass throughput. Also, in general terms, the higher increase of the reaction temperature on the scaled-up operation corresponds to the lower mass throughputs and higher screw speeds specified for the reference extruder. For the 3D modeling approach, two different case studies are analyzed by means of a commercial FEM software package. The REX simulations are performed under the assumption of steady-state conditions using the concept of a moving relative system (MRS). To complement the information obtained from the MRS calculations, simulations for selected conditions (for non-reactive cases) are performed considering the more realistic transient-state (TS) flow conditions. The TS flow conditions are associated to the time periodicity of the flow field inside the conveying elements of COITSEs. In the first case study, the peroxide-initiated degradation of PP is simulated in fully-filled screw elements of two different size COITSEs in order to evaluate scale-up implications of the REX operation. In the second case, the reacting flow is simulated for a conventional conveying screw element and a conveying screw element having a special design and corresponding to the same extruder size. For both of the analyzed cases, the effects of the initial peroxide concentration and mass throughput on the final Mw and PDI of the degraded resin are studied. The effect of the processing conditions is discussed in terms of the residence time distribution (RTD), the temperature of reaction, and the distributive mixing capabilities of the REX system. When analyzing the scale-up case, it is found that for the implemented processing conditions, the final Mws and PDIs are very close to each other in both of the analyzed flow geometries when the specified flow is close to that corresponding to the maximum conveying capabilities of the screw elements. For more restrictive flow conditions, the final Mws and PDIs are lower in the case of the screw element of the larger extruder. It is found that the distributive mixing ability of the reactive flow is mainly related to the specified mass throughput and almost independent of the specified peroxide concentration for a particular extruder size. For the analyzed screw elements, the conveying element corresponding to the small size extruder shows a slightly better distributive mixing performance. For this same case study, a further evaluation of the proposed scale-up criterion under constant thermal time confirms the trend of the results observed for the 1D simulations. In the second case study, the special type of screw element consists of screws rotating at different speeds which have different cross sections. In this case, the outer and inner diameters of both the special and the conventional type of screw elements are specified to be the same. As in the previous case study, the distributive mixing capabilities appear to be independent of the specified peroxide concentrations but dependent on the mass flow rate. It is speculated from the simulation results, from both the transient- as well as the steady-state flow conditions, that the screw element with the special design would yield lower final values of the PDI and Mw. Also, this screw element appears to have improved distributive mixing capabilities as well as a wider RTD.

A twin-buck converter with zero-voltage-transition (ZVT) is proposed in this thesis. The converter comprises two identical buck conversion units connected in parallel by an interleaved inductor. The ZVT is accomplished by the resonating the currents between the interleaved inductor and the parasitic capacitances of the power MOSFETs. The circuit efficiency can be further improved by introducing synchronous rectification to reduce the condition loss on the diodes. The detailed circuit analysis and operation characteristics are provided. A laboratory circuit rated at 300 W is designed and tested. Experimental results show that the switching losses can be effectively reduced by smoothly transiting the currents of the active power switches.