1834 research outputs found
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Developing techniques for the structure determination of liquid fuels and the control of their physical properties
No full textGreen economy postdoctoral fellowships induction workshop 2016
Investigating the build-up of brightest cluster galaxies over half of cosmic time
No full textCentral galaxies in galaxy clusters are special. These galaxies referred to in the literature as brightest
cluster galaxies (BCGs), are the most luminous (or massive) galaxies in their host clusters and are
located at the bottom of the clusters' gravitational potential wells. The stellar mass growth of
these massive galaxies is a hotly debated topic. The general picture emerging from simulations
suggests that star formation dominates the early stellar mass assembly of the BCGs (at z ≥ 2),
while dry mergers are the dominant mechanism through which these massive galaxies increase their
stellar masses at later times (z ≤ 1). The simulations however predict a much higher mass growth
for the BCGs than is observed. Observational studies on the other hand report conflicting results
regarding the stellar mass growth of BCGs since z ≤ 1. Some studies find that the BCGs have not
experienced any mass growth since z ~ 1, while others measure significant growth over the same
redshift range. In this thesis we investigate the importance of major mergers in the stellar mass
build-up of BCGs between 0.08 ≤ z ≤ 0.50. We do this using two independent methods where we
respectively measure the: (i) total and (ii) merger-inferred stellar mass growth of the BCGs. In
the first method we simply investigate how the average stellar masses of the BCGs change as a
function of redshift. In this analysis we account for the relation between the BCG's stellar mass and
the host cluster's halo mass in selecting the BCG sample. In the second method we use close pair
statistics as a diagnostic of mergers. Thereafter the close pair fraction is used as a proxy for the
BCG merger fraction in order to estimate how much stellar mass growth the BCGs will experience
due to mergers. We find that major mergers will contribute 24±14% on average towards the stellar
mass growth of a present day BCG since z = 0.32. With regards to the total stellar mass growth of
the BCGs, we find that it is 0+15%-3% between 0.08 ≤ z ≤ 0:35. The upper limit on this result depends
sensitively on the correction applied for the SDSS photometry of the brightest, largest (lowest
redshift) galaxies. The total mass growth is lower than the merger-inferred growth, although they
are consistent within the uncertainties. We find that it is important to take the growth of the
clusters' halo masses into account, otherwise one runs the risk of obtaining the non-physical result
that BCGs are somehow losing stellar mass towards the present day (i.e. shrinking). We further
find that the stellar mass growth of BCGs appears to be independent of the range of parent halo
masses probed (providing these clusters are descendants/progenitors of each other). We also find
that major mergers are the dominant contributor to the intracluster light stellar mass build-up at
low redshifts. Together the total and merger-inferred stellar mass growth results imply that mergers
are the dominant mechanism through which a BCG's stellar mass can grow at low redshift.National Research Foundatio
Household economies of low-income, African female-headed households in Khayelitsha: intergenerational support, negotiation and conflict
No full textLow-income, African female-headed households represent a large segment of households in South Africa. Despite this, little is known about how financial and non-financial resources are provided, controlled and used within these households. Less is known about how these dynamics shape intergenerational relationships and positions of power within female-headed households. This thesis aims to contribute to a better understanding of these issues by examining how the household economies of fourteen low-income, African female-headed households in Khayelitsha operated on a day-to-day basis. It also sought to understand how two generations of household members experienced these practices. Through the collection and analysis of qualitative data, this thesis shows that the households were sites of support as household members relied on each other for various forms of support. However, many of the female household heads bore the greatest responsibility for the physical and financial maintenance of their households. Furthermore, the findings build upon existing understandings of low-income, multi-generational households as also being sites of negotiation and contestation. The unequal burden of care experienced by the older women and the patterns of support provided by other household members was often the outcome of intergenerational negotiation. The participants' experiences of these dynamics shed light on the shifting positions of power within their households. The older women struggled to maintain their authority and negotiate for financial and practical assistance from their younger household members. As a result, the provision of support and perceptions about their interpersonal relationships were framed by experiences of intergenerational conflict and feelings of ambivalence. The findings highlight experiences of multi-generational family life and inequality in a context where feelings of obligation, broader socio-economic conditions and the nature of state support may constrain how the participants were able to provide support and handle instances of intergenerational conflict.National Research Foundatio
A new proposed constitutional methodology for effecting transformation in the South African law of delict
No full textThe problem identified in this thesis is the haphazard methodology that features in constitutional application to the South African common law of delict. Conventionally, the Constitution is said to apply either 'directly', 'indirectly' or 'not at all' to common-law problems. How judges are supposed to choose between those three possibilities is unclear. In light of a number of recent delict cases, the author observes that the uncertainty, coupled with a conservative political commitment to 'contemporary common-law purism', has resulted in judges preferring the option of 'constitutional avoidance' instead of the direct- and indirect-application models. On the other hand, the author also notes with concern the radically alternative approach of 'constitutional over-excitement' where judges overlook established common-law rules and instead turn to a pure application of constitutional principles. The author argues that these competing approaches tend to valorise one source of law at the expense of an integrated reading of various sources. This invariably leads to an uncritical acceptance of either the common law or the Constitution. The author contends that the competing problems of constitutional avoidance and over-excitement in the law of delict could be addressed by turning to the transformative method of adjudicative subsidiarity that places the Constitution's commitment to human rights at the centre of common-law disputes (thus rejecting a blind veneration of the common law) while being humbly mindful of the limits of constitutionalism. The author finally evaluates judicial performance in a broader spectrum of delict cases since democratisation that further demonstrates the need for the application of adjudicative subsidiarity.National Research Foundatio
Electric organ discharges of South African Marcusenius species (Teleostei: Mormyridae) and their effectiveness as indicators of local species diversity
No full textRecent morphological and genetic studies have revealed two new species of snoutfish in South Africa, Marcusenius caudisquamatus
and M. krameri, which had been confused with M. pongolensis, the South African bulldog fish. All known
mormyriform fish are nocturnal and emit electric organ discharges (EODs) for communication that are characteristic for
their species. This paper examines whether or not the EODs of these three closely-related South African species can be
differentiated from each other. An EOD pulse of a bulldog fish consists of a head-positive phase P, followed by a headnegative
phase N of short duration. We measured and compared six variables of the EOD pulse waveform for South African
samples for the three species from different locations using MANOVA, ANOVA and Discriminant Analysis, with
M. devosi from Kenya as an outgroup. The EOD waveforms, normalized to the same P-phase amplitude, varied significantly
from each other in four variables, most strongly in the amplitude of the N phase and the duration of the P phase. In
two species, M. devosi and M. krameri, there was no evidence of difference between sexes, in contrast to M. pongolensis
and M. caudisquamatus whose male pulses were of longer duration. M. devosi and M. krameri were statistically significantly
independent of each other and of any other group studied. By contrast, the M. pongolensis specimens from different
locations showed a high degree of variability amongst each other, including significant separation, and overlap with M.
caudisquamatus
Characterisation of HIV-1 envelope features of breakthrough infections from the CAPRISA 004 microbicide trial
No full textThe CAPRISA 004 trial demonstrated the safety and a 39% efficacy of a 1% tenofovir (TFV) gel for the prevention of HIV-1 acquisition in young African women. It was subsequently shown that women assigned to the TFV arm who became infected had higher viral loads, slower anti-HIV-1 antibody avidity maturation, and higher Gag-specific IFN-γ+ CD4+ T cell responses; although replication capacity, as measured by Gag-Pro recombinant viruses, did not differ between arms. We thus aimed to investigate if there were differences in Envelope function, or TFV susceptibility, which may be selected for during transmission in those who became infected despite being assigned to the TFV arm. Viruses from 39 out of 48 recently HIV-1 infected individuals from the trial (matched on time post-infection and the presence of protective HLAs) were isolated. Isolate env genes were sequenced using a single genome amplification approach and were compared to plasma sequences from the same time-point. To evaluate phenotypic characteristics of env, inhibition assays were performed using the following inhibitors: tenofovir, maraviroc, T20, PSC-RANTES and anti-CD4 antibody clone SK3. In addition, envs for 19 participants were cloned and used to generate pseudoviruses which were evaluated for entry efficiency. Viral isolates were identical or very similar to viruses in circulation in vivo; however had a lower diversity, indicating that they were representative of in vivo virus but did not reflect the entire quasispecies in plasma. The TFV arm viruses were not more resistant to TFV than those in the placebo arm. A comparison of variable loop characteristics, distance to a consensus representative of viruses circulating in the region, and sensitivity to inhibitors or entry efficiencies of the viruses, also found no difference in genotypic nor phenotypic properties between study arms. When assessing the impact of viral phenotype on markers of disease progression, it was found that sensitivity to inhibitors did not contribute to VL or CD4+ count in this cohort. To evaluate envelope in isolation of the rest of the genome, pseudoviruses were generated from 11 participants. We found that PSV entry efficiency did not correlate with VL at isolation, 3 months post-infection and set-point, or with CD4+ counts at set-point. However, pseudovirus inhibitor sensitivities were significantly different to those of isolates for the inhibitors T20, anti-CD4 antibody SK3 and PSC-RANTES. Overall, the isolate env genotypic and phenotypic characteristics investigated in this study did not differ between trial arms. Interestingly, pseudoviruses showed significant differences in their sensitivity to entry inhibitors when compared to their corresponding isolate, highlighting the importance of caution when interpreting data from in vitro studies, and motivates for further evaluation of in vitro models.National Research Foundatio
A cooperative prediction based approach to spectrum management in cognitive radio networks
No full textOne of the problems facing wireless network planners is a perceived scarcity of spectrum. A technology that addresses this problem is cognitive radio (CR). A critical function of a CR network is spectrum sensing (SS). A secondary user (SU) in a CR network will perform SS to gather information about the radio environment within which it wishes to operate and then make decisions based on that information. While SS by individual SUs is very useful it has been found in the literature that a cooperative approach, where SUs share their individual results, may provide more accurate information about the radio environment. It has also been shown that it is beneficial for SUs to be able to make proactive decisions about spectrum resource allocation. To be able to make these proactive decisions, a SU will need to be able to make predictions about the future behaviour of other users of the same spectrum.
This research project was divided into two parts. Firstly, a measurement campaign was performed to characterise spectrum scarcity in the South African context. Detailed information, about the occupancy of various commercially utilised South African frequency bands, was collected from spectrum measurement campaigns carried at the Hatfield campus of the University of Pretoria and at Pinmill Farm in Johannesburg. These bands included the television broadcast and mobile cellular bands. On average, the television broadcast bands were found to be underutilised highlighting the existence of a number of opportunities for television white space devices. However, the mobile cellular bands were found to be much more heavily occupied, particularly for the bands around 900 MHz, suggesting that mobile operators are currently in need of additional spectrum resources.
The second part of this thesis followed a more theoretical approach and was based on the need for proactive decision making in CR networks. A single SU prediction method, of relatively cheap computational complexity, was proposed and tested under various traffic conditions. The premise that collaboration between SUs may improve the accuracy of single SU traffic predictions was then explored. Pre-fusion and post-fusion approaches to cooperative prediction were compared with the single SU prediction scenario. The prediction error for the cooperative approaches was found to be lower than for the single SU case, especially for the pre-fusion scenario. For example, for a signal-to-noise ratio of 8 dB and individual forecast probability of 0.9, the pre-fusion prediction error was found to be approximately 2% compared with 26% for single SU prediction error. The cost of this improvement, however, was added algorithm complexity.
It was then demonstrated that primary user traffic prediction could be used to improve the energy consumption associated with cooperative SS in a CR network. Combined with an optimal scheduling algorithm, this approach was shown to prolong the lifetime of a group of twenty cooperating SUs by 21.2 time samples for a uniformly distributed group of SUs when predictions were made ten time samples into the future.National Research Foundatio
New developments
No full textThe Grants Management and Systems Administration (GMSA) Directorate hosted its 7th annual Research Administrators Workshop (RAW) from 4 to 6 October 2016 in Polokwane. The workshop aimed to bring together key stakeholders from universities and science councils across South Africa, including international delegates, to share experiences and best practices in research administration and management
NRF internal audit: grant management & institutional visits
No full textThe Grants Management and Systems Administration (GMSA) Directorate hosted its 7th annual Research Administrators Workshop (RAW) from 4 to 6 October 2016 in Polokwane. The workshop aimed to bring together key stakeholders from universities and science councils across South Africa, including international delegates, to share experiences and best practices in research administration and management