Stellenbosch University - Scopus Publications
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- Item250kW transformer-less voltage dip compensator(1999) Visser A.J.; Mouton H.du T.Voltage dips cause serious problems for sensitive equipment in industrial applications. The possibility of applying a transformer-less dip compensator is investigated as a cost-effective solution to overcome this hazard. This is achieved by means of a power electronic multi-level converter placed in series with the supply.
- Item9th biennial scientific meeting of the International Association for Craniofacial Identification 24-29 July 2000, Washington DC.(2001) Phillips P.[No abstract available]
- ItemA comparison between measured leakage current and surface conductivity during salt fog tests(2004) Schwardt W.H.; Holtzhausen J.P.; Vosloo W.L.This paper deals with an Insulator Pollution Monitoring Relay (IPMR), designed to measure and monitor pollution on high voltage insulators. The IPMR determines the pollution severity by measuring the surface conductivity on the test insulator. The paper reports on how the IPMR was used to measure surface conductivity on the test insulator while a salt fog test was performed on typical power line insulators. The leakage currents were measured on the test insulators and the data was captured together with the surface conductivity values. The results discuss whether the IPMR is capable of the early detection of an instantaneous pollution event. The results are also utilised to compare the relationship between measured surface conductivity and leakage current values thus predicting flashover.
- ItemA comparison of mesh termination schemes for 2D scattering analysis using the finite element method(2007) Young A.; Davidson D.B.When the finite element method is used for scattering problems, the outer boundary of the mesh has to be terminated appropriately so that it seems transparent to any outgoing waves. Various solutions to this problem have been developed and here three such methods are discussed and compared. These are absorbing boundary conditions, finite element boundary integral hybrid method and infinite elements. A basic two-dimensional scattering problem is used to compare these methods. ©2007 IEEE.
- ItemA comparison of Multi-criteria evaluation methods for RSFQ circuit optimization(2011) Ortmann F.G.; Van Der Merwe A.; Gerber H.R.; Fourie C.J.Research has been done on the metaheuristic optimization of RSFQ circuits, but these are typically limited to single criteria (e.g., yield). The importance of using multiple criteria and the relevance of each of the criteria is discussed. The application of data envelopment analysis (DEA) and an interactive normalization technique as multi-criteria evaluation methods for the optimization of RSFQ circuits are investigated. We find that these evaluation methods each have clear advantages and disadvantages over each other. The normalization technique requires the analysis designer to have a good understanding of circuit design to be accurate, while theDEAmethod may yield many results that are ranked equally efficient. © 2010 IEEE.
- ItemA computational fluid dynamics model of a spinning pipe gas lens(2010) Mafusire C.; Forbes A.; Snedden G.When a metal horizontal pipe is heated and spun along its axis, a graded refractive index distribution is generated which is can be used as a lens, thus its name, the spinning pipe gas lens (SPGL). Experimental results showed that though increase in rotation speed and/or temperature resulted in a stronger lens and removed distortions due to gravity, it also increased the size of higher order aberrations resulting in an increase in the beam quality factor (M 2). A computational fluid dynamics (CFD) model was prepared to simulate the aerodynamics that show how it operates and, in the process shed some light on the optical results. The results of the model consist of velocity profiles and the resultant density data and profiles. At rest the cross-sectional density profile has a vertical symmetry due to gravity but becomes rotationally symmetric with a higher value of density at the core as rotation speed increases. The longitudinal density distribution is shown to be parabolic towards the ends but is fairly uniform at the centre. The velocity profiles show that this centre is the possible source of higher order aberrations which are responsible for the deterioration of beam quality. © 2010 SPIE.
- ItemA control system analysis for a potential small geostationary satellite for South Africa(2007) Thopil G.A.; Steyn W.H.The availability of an indigenous communications satellite helps in driving down communication and entertainment costs of services obtained through such a satellite. The design and implementation of a small GEO satellite will also act as a stepping stone for missions of greater complexity and stringent specifications [2]. This paper presented is based on a simulation study with real world and worst case scenarios modelled as accurately as possible keeping in mind the trade-off between computational burden and accuracy. An overall control system analysis comprising of the Attitude Control and Determination system is performed, keeping actuator failure in mind. An analytical Orbit Control evaluation is also performed which gives an estimate of the propellant requirement for an average GEO lifetime of 10 years. ©2007 IEEE.
- ItemA distributed scheme for value-based bandwidth reconfiguration(2009) Arvidsson A.; Gobel J.; Krzesinski A.; Taylor P.This paper presents a scheme for reallocating bandwidth in path-oriented transport networks. At random time points, bandwidth is allocated to those paths that (possibly temporarily) value it most highly. The scheme acts according to local rules and without centralised control. The proposed scheme is thus distributed and scalable. Previous studies have shown that bandwidth reallocation, together with the provision of appropriate amounts of spare capacity on certain links, can be used to rapidly deploy and capacitate equivalent recovery routes in the event of network equipment failure. The purpose of this study is to determine if the same reallocation mechanism can also deal effectively with repeated, small scale random traffic fluctuations and with time varying traffics. We present a simulation model of a 30-node 46-link network which we use to evaluate the efficacy of the bandwidth reallocation scheme. The simulation study shows that bandwidth reconfiguration can substantially reduce the connection loss probabilities. © 2009 Springer Berlin Heidelberg.
- ItemA flying capacitor multilevel inverter with bootstrap-powered MOSFET gate drive circuits(2002) Molepo S.A.; Mouton H.D.T.The problem of non-technical power loss due to theft of electricity experienced in developing countries is of great concern. This is done mainly by tapping of the overhead AC reticulation networks and bypassing of metering units. Applying DC reticulation at higher voltage levels using the existing AC reticulation networks is one approach to solving this problem. This paper aims at investigating the development of a cost-effective 3.5 kVA flying capacitor multilevel inverter (FCMLI) for DC reticulation. The development of a low-cost bootstrap power supply for multilevel inverter gate drive circuits is also discussed.
- ItemA GPS based time-stamping and scheduling system for wide area power system measurements(2002) Van As M.T.S.; Vermeulen H.J.This paper describes a synchronising and scheduling system for wide area synchronised data acquisition applications and fault location in power systems, using Global Positioning System (GPS) technology. The system hardware features a commercial GPS receiver, serial communications link for a host computer, and a dedicated programmable real time clock with programmable trigger features. Details are given of the design and implementation of these features using Programmable Logic Array (PLA) technology.
- ItemA graph-theoretic proof of the non-existence of self-orthogonal Latin squares of order 6(2011) Burger A.P.; Kidd M.P.; Van Vuuren J.H.The non-existence of a pair of mutually orthogonal Latin squares of order six is a well-known result in the theory of combinatorial designs. It was conjectured by Euler in 1782 and was first proved by Tarry in 1900 by means of an exhaustive enumeration of equivalence classes of Latin squares of order six. Various further proofs have since been given, but these proofs generally require extensive prior subject knowledge in order to follow them, or are 'blind' proofs in the sense that most of the work is done by computer or by exhaustive enumeration. In this paper we present a graph-theoretic proof of a somewhat weaker result, namely the non-existence of self-orthogonal Latin squares of order six, by introducing the concept of a self-orthogonal Latin square graph. The advantage of this proof is that it is easily verifiable and accessible to discrete mathematicians not intimately familiar with the theory of combinatorial designs. The proof also does not require any significant prior knowledge of graph theory. © 2009 Elsevier B.V. All rights reserved.
- ItemA hardware emulator testbed for software defined radio(2002) Witkowsky J.; Van Rooyen G.-J.In software defined radio (SDR), most of the traditionally analogue signal processing, synthesis and analysis functions of a communications system are performed digitally. However, critical hardware system components (such as analogue-digital-analogue conversion) still remain, making the full system design and testing more cumbersome. This paper presents an introductory overview on SDR and work-in-progress concerning the background and design considerations for a basic SDR hardware emulator. Hardware emulation allows fast and low-cost evaluation of possible hardware architectures. An object-orientated software design approach is taken, and the technical issues surrounding the development of hardware models are considered. Finally, a proposed system software architecture is presented.
- ItemA high energy-resolution zero degree facility for (p,p′) and (p,t) reactions(2011) Neveling R.; Fujita H.; Smit F.D.; Adachi T.; Berg G.P.A.; Buthelezi E.Z.; Carter J.; Conradie J.L.; Couder M.; Fearick R.W.; Fortsch S.V.; Fourie D.; Fujita Y.; Gorres J.; Hatanaka K.; Heilmann A.M.; Mira J.P.; Murray S.H.T.; Von Neumann-Cosel P.; O'Brien S.; Papka P.; Poltoratska I.; Richter A.; Sideras-Haddad E.; Swartz J.A.; Tamii A.; Usman I.T.; Van Zyl J.J.
- ItemA landscape with objects: Private game reserves, game lodges and the 'New Safari'(2010) Green L.This paper investigates the proliferation of high-end private game reserves in Africa as the manifestation of an emerging economic but also symbolic and emotional investment in the value of nature. Increasing concern for 'the environment' along with anxiety about the future of individual species has given the game reserve a special place in the contemporary global imagination. Through analysis of some of the ephemeral texts through which these fantasies and anxieties about nature are articulated - an advertisement, some publicity material and a design monograph - this paper explores a number of different objects which serve to mediate the relationship between the public and nature. Both the 4×4 whose increasing popularity seems to parallel that of the game reserve, and the carefully designed and decorated game lodges offer examples of the complex way in which die experience of nature needs to be mediated by objects in order to confirm nature as a site for the rediscovery of authenticity. Around the game lodge, I argue, there emerges a new discursive mode for articulating Africa, and South Africa's relationship with the world, a novel aesthetic known as the 'New Safari'. This design category goes beyond design to construct a moral and experiential fantasy in which conservation not as a practice but rather as an imagined value makes possible a disturbing reconceptualisation of the relation between the national and the global. © 2010 Taylor & Francis.
- ItemA mechanism for zinc toxicity in neuroblastoma cells(2004) Daniels W.M.U.; Hendricks J.; Salie R.; Van Rensburg S.J.Zinc is an important component of proteins essential for normal functioning of the brain. However, it has been shown in vitro that this metal, at elevated levels, can be toxic to cells leading to their death. We investigated possible mechanisms of cell death caused by zinc: firstly, generation of reactive oxygen species, and secondly, the activation of the MAP-kinase pathway. Cell viability was assessed by means of the methyl-thiazolyl tetrazolium salt (MTT) assay and confirmed by tetramethylrhodamine methyl ester (TMRM) staining. We measured the phosphorylation status of Erk and p38 as indicators of MAP-kinase activity, using Western Blot techniques. A time curve was established when neuroblastoma (N2α) cells were exposed to 100 μM of zinc for 4, 12, and 24 h. Zinc caused a significant reduction in cell viability as early as 4 h, and indirectly stimulated the accumulation of reactive oxygen species as determined by 2.7 dichlorodihydrofluorescein diacetate (DCDHF) staining and confocal microscopy. Investigation of the MAP-kinase pathway indicated that Erk was downregulated, while p38 was stimulated. Our results therefore led us to conclude that in vitro, zinc toxicity involved the generation of reactive oxygen species and the activation of the MAP-kinase pathway.
- ItemA method to determine the performance characteristics of cooling tower spray zones(2010) Reuter H.C.R.; Viljoen D.J.; Kroger D.G.Cooling tower spray zones play an important role in cooling tower performance. Ideally they must distribute the cooling water uniformly onto the fill and must produce small drops at minimal pressure head to maximise heat and mass transfer in the spray zone with minimal pumping power. Limited thermal performance characteristic data is found in literature for cooling tower spray zones, since it is virtually impossible to measure spray zone performance accurately. In this paper, the method used to model the performance of cooling tower spray zones and results obtained for a medium pressure swirl nozzle are presented. Water flow distribution and drop size distribution tests are conducted on cooling tower spray nozzles to investigate the effects of varying different operating parameters, such as air and water flow rates, and installation parameters, such as nozzle height, nozzle spacing and direction of spray, on performance. The suitability of superimposing single nozzle flow distribution data to obtain the water distribution for a grid of equally spaced nozzles with variable nozzle spacing is investigated. Furthermore, a single nozzle simulation model is developed and used to model single spray nozzles. The single nozzle model and the superposition model are subsequently used to obtain initial drop conditions to model the spray zone using the commercial CFD package FLUENT®. The proposed modelling approach allows for the evaluation and performance prediction of existing and new nozzle design configurations. Correlations are presented for the Merkel number and loss coefficient for the downspray nozzle investigated. © 2010 by ASME.
- ItemA new two-dimensional cfd model to predict the performance of natural draught wet-cooling towers packed with trickle or splash fills(2010) Reuter H.C.R.; Kroger D.G.In the design of a modern natural draught wet-cooling tower, structural and performance characteristics must be considered. Air flow distortions and resistances must be minimised to achieve optimal cooling which requires that the cooling towers must be modelled two-dimensionally and ultimately three-dimensionally to be optimized. It is found that CFD models in literature are limited to counterflow cooling towers packed with film fills which are porous in one direction only and generally have a high pressure drop, as well as purely crossflow cooling towers packed with splash fill, which simplifies the analysis considerably. Many counterflow cooling towers are however packed with trickle and splash fills which have anisotropic flow resistances, which means the fills are porous in all flow directions and thus air flow can be oblique through the fill, particularly near the cooling tower air inlet. This provides a challenge since available fill test facilities and subsequently fill performance characteristics are limited to purely counter- and crossflow configuration. This paper presents a CFD model to predict the performance of natural draught wet-cooling tower with any type of fill configuration, which can be used to investigate the effects of different atmospheric temperature distributions, air inlet and outlet geometries, air inlet heights, variations in radial water loading and fill depth, fill configurations, rain zone drop size distributions, and spray zone performance characteristics on cooling tower performance for optimization purposes. Furthermore the effects of damage or removal of fill in annular sections and boiler flue gas discharge in the centre of the tower can be investigated. The fill performance characteristics for oblique air flow are determined by linear interpolation between counter- and crossflow fill characteristics in terms of the air flow angle. The CFD results are validated by means of corresponding one-dimensional computational model data. © 2010 by ASME.
- ItemA novel detector for high neutron flux measurements(2009) Singo T.D.; Papka P.; Wyngaardt S.M.; Dobson R.T.Measuring alpha particles from a neutron induced break-up reaction with a mass spectrometer can be an excellent tool for detecting neutrons in a high neutron flux environment. Break-up reactions of 6Li and 12C can be used in the detection of slow and fast neutrons, respectively. A high neutron flux detection system that integrates the neutron energy sensitive material and helium mass spectrometer has been developed. The description of the detector configuration is given and it is soon to be tested at iThemba LABS, South Africa. © 2009 American Institute of Physics.
- ItemA novel isolated, compensated Darlington based-drive configuration(Publ by IEEE, Piscataway, NJ, United States, 1992) Enslin J.H.R.; Hartman S.J.B.An isolated version of the compensated base-drive for high power bipolar transistor switches is introduced. A unique pulse transformer is used to drive a high power bipolar transistor by means of a compensated Darlington pair, consisting of an IGBT driving transistor and a high current single bipolar transistor. In high efficiency, high power bridge applications of high current switches, the saturation voltage should be ultra-low, which implies a high power bipolar transistor in deep saturation. This isolated base-drive circuit is described and evaluated in these applications.
- ItemA novel P106L mutation in EPSPS and an unknown mechanism(s) act additively to confer resistance to glyphosate in a South African lolium rigidum population(2011) Kaundun S.S.; Dale R.P.; Zelaya I.A.; Dinelli G.; Marotti I.; McIndoe E.; Cairns A.Glyphosate resistance evolution in weeds is a growing problem in world agriculture. Here, we have investigated the mechanism(s) of glyphosate resistance in a Lolium rigidum population (DAG1) from South Africa. Nucleotide sequencing revealed the existence of at least three EPSPS homologues in the L. rigidum genome and identified a novel proline 106 to leucine substitution (P106L) in 52% DAG1 individuals. This mutation conferred a 1.7-fold resistance increase to glyphosate at the whole plant level. Additionally, a 3.1-fold resistance increase, not linked to metabolism or translocation, was estimated between wild-type P106-DAG1 and P106-STDS sensitive plants. Point accepted mutation analysis suggested that other amino acid substitutions at EPSPS position 106 are likely to be found in nature besides the P106/S/A/T/L point mutations reported to date. This study highlights the importance of minor mechanisms acting additively to confer significant levels of resistance to commercial field rates of glyphosate in weed populations subjected to high selection pressure. © 2011 American Chemical Society.