1834 research outputs found
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Functional responses can unify invasion ecology
No full textWe contend that invasion ecology requires
a universal, measurable trait of species and their
interactions with resources that predicts key elements
of invasibility and ecological impact; here, we advocate
that functional responses can help achieve this
across taxonomic and trophic groups, among habitats
and contexts, and can hence help unify disparate
research interests in invasion ecology
Multiobjective in-core fuel management optimisation for nuclear research reactors
No full textThe efficiency and effectiveness of fuel usage in a typical nuclear reactor is influenced by the
specific arrangement of available fuel assemblies in the reactor core positions. This arrangement of assemblies is referred to as a fuel reload configuration and usually has to be determined anew for each operational cycle of a reactor. Very often, multiple objectives are pursued simultaneously
when designing a reload configuration, especially in the context of nuclear research reactors. In the multiobjective in-core fuel management optimization (MICFMO) problem, the aim is to identify a Pareto optimal set of compromise or trade-off reload configurations. Such a set may then be
presented to a decision maker (i.e. a nuclear reactor operator) for consideration so as to select a preferred configuration.
In the first part of this dissertation, a secularization-based methodology for MICFMO is pro- posed in order to address several shortcomings associated with the popular weighting method often employed in the literature for solving the MICFMO problem. The proposed methodology has been
implemented in a reactor simulation code, called the OSCAR-4 system. In order to demonstrate its practical applicability, the methodology is applied to solve several MICFMO problem instances in the context of two research reactors.
In the second part of the dissertation, an extensive investigation is conducted into the suitability of several multiobjective optimization algorithms for solving the constrained MICFMO problem. The computation time required to perform the investigation is reduced through the usage of
several artificial neural networks constructed in the dissertation for objective and constraint function evaluations. Eight multiobjective metaheuristics are compared in the context of a test suite of several MICFMO problem instances, based on the SAFARI-1 research reactor in South Africa.
The investigation reveals that the NSGA-II, the P-ACO algorithm and the MOOCEM are generally the
best-performing metaheuristics across the problem instances in the test suite, while the MOVNS algorithm also performs well in the context of bi-objective problem instances. As part of this investigation, a multiplicative penalty function (MPF) constraint handling technique is also proposed and compared to an existing constraint handling technique, called constrained-domination.
The comparison reveals that the MPF technique is a competitive alternative to constrained-domination.
In an attempt to raise the level of generality at which MICFMO may be performed and potentially improve the quality of optimization results, a multiobjective hyperheuristic, called the AMALGAM
method, is also considered in this dissertation. This hyperheuristic incorporates multiple metaheuristic sub-algorithms simultaneously for optimization. Testing reveals that the AMALGAM method yields superior results in the majority of problem instances in the test suite, thus
achieving the dual goal of raising the level of generality and of yielding improved optimization results. The method has also been implemented in the OSCAR-4 system and is applied to solve several MICFMO case study problem instances, based on two research reactors, in order to demonstrate its
practical applicability.
Finally, in the third part of this dissertation, a conceptual framework is proposed for an optimization-based personal decision support system, dedicated to MICFM. This framework may serve as the basis for developing a computerized tool to aid nuclear reactor operators in designing suitable reload configurations.National Research Foundatio
The responsiveness of public transport systems to the development of urban and economic nodes in Johannesburg
No full textOver the last decade national, provincial and local governments within South Africa have paid considerable attention to the benefits and need for an effective and reliable public transport system to meet the social challenges facing South African cities. The development of public transport systems is viewed as a tool to alleviate poverty and unemployment, as they provide access to opportunities and services previously out of reach. In Johannesburg the Spatial disparities created during the apartheid era continue to divide the city along the lines of class and race, and serve to marginalise low income groups located on the periphery, far from economic centres. Over the last 10 years urban and economic growth in the west of Johannesburg has occurred at a rapid rate giving rise to numerous informal settlements and government subsidised homes. These developments occurred without the development of public transport systems isolating low income residents. This study seeks to examine the extent to which public transportation systems are responding to the growth of new urban and economic nodes in Johannesburg in a manner that supports the interests and needs of the city’s low income residents.
The study investigates the cases of the Gautrain and Rea Vaya. The study evaluates the systems through an analysis of the state’s public transportation plans and policies and the challenges and successes of each system, taking into account what are the state’s current and future public transport plans and how are they responding to urban and economic growth in the west of Johannesburg.
Through the use of spatial data, the study examines the spatial and economic trends in Johannesburg, the location of the urban poor and the effects of changes in commuter patterns. The study takes into account current and future urban and economic trends and examines the way in which public transport systems can make a positive impact upon the urban poor. This evaluation is done through an analysis of international literature and best practices that can be used in the development of public transport systems that are responsive, effective and reliable. Key respondents were interviewed to examine the impediments facing the development of responsive
public transport systems and how this affects low income commuters in Johannesburg.
Through the use of numerous forms of data including maps, policy documents and key respondent interviews, study reveals that future transport developments see the Gautrain and the Rea Vaya expanding to the west of Johannesburg. These developments will only occur in the late future, leaving low income residents without a formal means of public transport for many years to come. Low income residents within these regions lack the means to access opportunities and services. Research into the phenomenon reveals that in order to address the challenges faced by the urban poor in the west a more dynamic and integrated approach is required in the development of public transport systems in Johannesburg. These findings provide a critical understanding the development of public transport systems and the impediments that restrict the responsiveness of public transportation development in Johannesburg.National Research Foundatio
Characterization of spirocerca lupi in wild and domestic canids
No full textSpirocerca lupi is a parasitic nematode that causes spirocercosis predominantly in domestic dogs. This disease is often fatal if not detected early before malignant transformation occurs. The nematodes may also cause serious aortic lesions which may weaken arteries and cause fatal haemorrhaging. There is currently no cure for the disease. Coprophagous dung beetles serve as intermediate hosts for the parasite. Lizards and birds serve as paratenic hosts when they consume dung beetles containing infective larvae.
Spirocerca lupi may pose a threat to wild carnivores due to the encroachment of humans with domestic dog pets near wildlife borders. There is very little information available regarding the spread of S. lupi among wild carnivores. In this study nematodes were collected from black backed jackals as well as several domestic dogs around the country. Corresponding haplotypes of cox1 were identified in the black backed jackals and domestic dogs. This is a clear indication of gene flow of S. lupi between these two species. Phylogenetic analysis indicates that S. lupi from South Africa may be a cryptic species. Further analysis is still required to confirm this as only a small section of the cox1 gene was used during the analysis. Further investigation with a larger section of the cox1 gene as well as other genes may be required to further classify S. lupi in South Africa as well as other countries around the world.
To further investigate the role of S. lupi infection among wild carnivores a faecal DNA extraction method was designed to screen these carnivores. Many challenges were encountered since S. lupi egg shedding is irregular and faecal samples from various wild carnivores were not always of the correct consistency to be analysed. The eggs of S. lupi were also found to be very resilient and did not yield easily to disruption. Other nematodes were identified with these methods indicating that it is a potential option for other nematodes but not for S. lupi. In addition to these challenges, the methods tested were found to be time consuming and costly without reliable repeatability.
Several adult S. lupi nematodes were tested for endosymbiotic bacteria. Targeting endosymbiotic bacteria with antibiotics could be an alternative treatment for nematode infection. No endosymbiotic bacteria were detected in S. lupi from South Africa with the primers tested. Further analysis may yet reveal some symbiotic relationship but current results do not suggest that is a feasible option for alternative treatment.National Research Foundatio
Food security and the urban informal economy in South Africa: the state of knowledge and perspectives from street-food traders in khayelitsha
No full textAn emerging body of research suggests acute levels of food insecurity in urban informal areas (See Naicker
et al 2015; Crush & Caesar 2014; Rudolph et al 2012; Battersby 2011; Frayne et al 2009; de Wet et al
2008). Simultaneously, available research indicates substantial reliance on the informal economy to
satisfy daily/weekly food needs of urban informal residents (See Crush & Frayne 2011; Battersby 2011;
Frayne et al 2009). Nevertheless, the food security contribution of the informal food economy on the
margins of South Africa’s cities, particularly of street-foods, is poorly conceived and supported. By
systematically reviewing the contemporary literature (2009-2014), this research maps out the present
state of knowledge about the informal food economy and its contribution to food (in)security. It explores
the illusive nature of the ‘invisible crisis’
1 of urban food insecurity and what is known about the role played
by the informal economy in this regard. Building on the results and gaps from the systematic review,
fieldwork findings – in the form of in-depth immersive ‘apprenticeships’ with two street-food traders in
Khayelitsha township – are then presented and discussed. The findings reveal an informal economy that
trades not only in food commodities but also in the provision of other socially invaluable services: of
kinship, reciprocity, tradition and nostalgia. Contrary to mainstream theories, traders were neither forced
into the informal economy, nor do they wish to formalise. Food security literature was found to focus on
techno-scientific food-centric perspectives of the informal economy, which, while valuable, is largely
divorced from the lived realities, means and concerns of workers in the informal food economy. The thesis
concludes that, in the absence of human-centred, trader-centric accounts of the informal food economy,
there is little prospect for appropriate prescriptions and successful interventions.National Research Foundatio
Internal ballistic modelling of solid rocket motors using level set methods for simulating grain burnback
No full textSolid Rocket Motors are propulsive systems providing thrust from products exiting through
a nozzle, obtained by burning a solid fuel (the grain) inside a chamber. Various thrust
profiles can be obtained by different initial grain geometries. The purpose of numerical
modelling, is to predict the thrust curve, given an initial geometry.
A simplified model for Solid Rocket Motor behaviour is developed. The grain geometry is
represented by a signed distance function, and the burning surface geometry is calculated
using a numerical solution of the burning process based on level-set methods.
The gas flux is modelled using a 1-D approximation of the gas dynamics inside the chamber,
following a method proposed by Lamberty.
2D and 3D models for modelling the regression of the grain surface over time are developed.
The evolution of different grain geometries are simulated and the results obtained
are compared to the experimental data available.
The burning surface and port area for some grain geometries is calculated analytically
and the results found are compared with the ones found numerically using 2D model of
the level set method.National Research Foundatio
A demographic history of settler South Africa
No full textEconomic incentives affect demographic outcomes. That is to say, fertility, mortality,
migration and mobility are a result of economic performance, growth and inequality.
While demographic changes may be slow, the long-run effects can be significant. The
Western demographic transition of the late nineteenth and early twentieth centuries
has had a profound effect on the living conditions of people across the world. Instead
of having six or more children, most families today have only two, and life expectancy
in most Western countries doubled in the four or five decades around the turn of the
twentieth century. It is within this broad framework relating to the nature and causes
of demographic transitions that this dissertation is orientated.
How these demographic changes spread across the globe remains an important
question for theoretical and empirical research, with obvious policy implications. It is
therefore surprising that so little is known about the demographic history of South
Africa, the wealthiest African country during the nineteenth and twentieth centuries
and the first to undergo a demographic transition. There is remarkably limited
empirical evidence of what living conditions, social interactions and family formation
might have been like for the inhabitants of eighteenth, nineteenth and early-twentieth
century South Africa. By focusing on the demographic characteristics of European
settlers and their descendants in South Africa, this dissertation begins to provide a
more comprehensive account of South Africa’s demographic history.
The first question addressed in this study investigates the nature and causes of the
settler fertility decline. It aims to provide, for the first time, a thorough descriptive
account of the changing levels of fertility and explores land constraint as a potential
mechanism for the limitation of fertility. To do so, it uses geographic and socioeconomic
differentials in fertility over time, first, in a simple regression analysis
framework and second, in an event-history analysis framework, to allow for a deeper
understanding of the possible mechanisms at work at the individual level.
The second question addressed in this study relates to the gender composition of
offspring as a determinant of future fertility behaviour. While couples in modern
societies have been shown to have gender neutral preferences for their offspring, new research on past populations suggests that a preference for sons over daughters
might have influenced couples’ fertility decision-making behaviour, and potentially
have limited the onset of the fertility decline. An investigation into whether a
preference for sons existed in the settler Cape Colony context, through an eventhistory
analysis of birth-spacing behaviour at high parities, conditional on the
couple’s existing offspring gender-mix, informs the debate on within-marriage birth
control practices in history, as well as the effect of economic development on couples’
fertility behaviour.
Finally, in societies with a large rural majority and a small group of elites, the
prospects for social mobility are said to be limited. However, the liberal theory of
industrialism suggests that social mobility will likely increase as a result of the
process of industrialisation itself, as new occupations replace those held by members
of previous generations. Industrialisation is also expected to result in a shift away
from ascription by birth towards achievement-based mobility. The third question
addressed in this study investigates whether social (occupational) mobility increased
under late nineteenth and early twentieth-century South African industrialism and
whether or not this translated into real improvements in settler living standards.National Research Foundatio
Transient simulation of a sub- to transcritical CO2 refrigeration cycle
No full textThe purpose of this project is to develop a numerical transient simulation model
of the CO2 refrigeration cycle containing a capillary tube as expansion device. A
design procedure for CO2 refrigerators is also developed; which makes use of a
steady-state capillary tube model for acquiring the initial design of the capillary
tube, and uses the transient simulation program to determine the low pressure
side internal volume and validate the complete design of the refrigerator. Using
this design procedure a CO2 refrigerator was designed, built and then tested to
validate the simulation model developed. The project was conducted at the
Department of Mechanical and Mechatronic Engineering at the University of
Stellenbosch.
A literature review on the history of refrigeration, the vapour-compression
refrigeration cycle, the transcritical CO2 refrigeration cycle, refrigerant properties,
CO2 properties, and a comparison of refrigerant performance was done as
background to the use of CO2 as refrigerant. Furthermore, literature review of an
alternative expansion device, the vortex tube, was included, but found not
practical for implementing with CO2 which behaves more like a real gas and not
an ideal gas. Lastly, theory on the development of the one-dimensional CFD
finite volume method in terms of a general variable is shown. This provides
background to the development of discretised conservation equations for the
simulation model.
The transient model developed is capable of predicting transient operation from
stand-still starting conditions through to steady-state conditions. The model
makes use of well-known pipe friction and heat transfer correlations.
Furthermore, it makes use of the real gas equation of state for CO2 from Span
and Wagner (1996). Moreover, the model determines whether the flow is single -
or two-phase and then calculates the appropriate properties. Lastly, it has the
whole refrigerant circuit, from compressor outlet back to compressor inlet, as the
simulation domain.
The simulation results for the pressure and temperature distributions show the
transient behaviour at start-up. The transient and steady-state results also agree
fairly well with the experimental results. The steady-state pressure graphs are as
expected and the constant evaporation temperature is a confirmation that the
model approximates real life operation quite well. In conclusion, the results show
that the simulation model is a useful tool for designing and understanding
capillary tube CO2 refrigeration cycles. Future work involves developing a nonadiabatic
compressor model and models for alternative expansion devices to be
included in the current refrigerator simulation model.National Research Foundatio
Open Access Seminar, Mafikeng Campus, North-West University
No full textNWU Library Mafikeng Campus: NRF OA Statement - Grantees' Research Output Submissions - Presentation. The presentation focused on sharing details around the workflow processes with regard to the archiving, dissemination and harvesting of Grantees’ research outputs in/from the university’s repository as a public good. This requires collaboration of the Research Office and the University library
South African PhD Project. Creating diversity for the future
No full textPresentations SA PhD Project Conference: Free State 25-26 February 201