Browsing by Author "Chetty, Naven."

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A study on the atmospheric and environmental impacts of aerosol, cloud and precipitation interaction.
(2022) Yakubu, Abdulaziz Tunde.; Chetty, Naven.
Understanding the mechanisms and processes of aerosol-cloud-precipitation interactions (ACPI) is essential in the determination of the specific role of aerosols in modulating extreme weather events and climate change in the long run. Atmospheric aerosols are mainly of various types and are emitted from differing sources. Considering they commonly exist in the heterogeneous forms in most environments, they significantly influence the incoming solar energy and the general perturbation of the clouds depending on their constituents. Thus, a systemic identification and characterisation of these particles are essential for proper representation in climate models. To better understand the process of climate change, this research explores the climate diversity of South Africa to examine aerosol sources and types concerning the atmospheric aerosol suspension over the region and their role in clouds and precipitation formation. The study further provided answers to the cause of extreme precipitation events, including drought and occasional flooding experienced over the region. Also, an insightful explanation of the process of ACPI is provided in the context of climate change. Furthermore, the research found that the effective radiative forcing (RF) over South Africa as monitored in Cape Town and Pretoria is negative (i.e., cooling effect) and provided an analysis of the cause. Similarly, the validation of some satellite datasets from MISR (Multiangle Imaging Spectroradiometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) instruments against AERONET (Aerosol Robotic Network) is conducted over the region. Although a significant level of agreement is observed for the two instruments, intense improvements are needed, especially regarding measurements over water surfaces. Finally, the study demonstrated the proficiency of effective rainfall prediction from satellite instrument cloud datasets using machine learning algorithms.
Biochemical-physical mechanisms of light-tissue interactions.
(2021) Buthelezi, Musawenkosi Doctor.; Chetty, Naven.; Adeleye, Oluwabamise.
Optical tissue phantom samples simulating the optical properties of the human prostates and brain tissues were fabricated. The experimental set-up was designed to be cost-effective but reliable, allowing for convenience in its usage and replication, making it ideal for biomedical optical measurements. Gel agar was the base material, and aluminum oxide (Al 2 O3 ) with black ink was employed as the scatter and absorber, respectively. The latter were mixed in various amounts into the gel agar to simulate the desired phantom tissues. Argon red laser and He-Ne green laser light, with wavelengths of 630 nm and 532 nm were incident on varying thicknesses of the phantom samples. The transmitted and incident light powers were measured to determine the scattering and absorption coefficients, from which the attenuation coefficients, penetration depth, and optical albedo were estimated. The optical penetration depths were found to be 0.30 for brain and 0.15 for prostate tissue phantoms. The fabricated tissues successfully mimicked the brain and prostate tissues, with µ a = 0.69 cm−1and µ a = 0.24 cm−1 absorption coefficients as well as 𝜇𝜇𝑠𝑠 = 1.73 cm−1 and µ s = 5.48 cm−1 scattering coefficients at 532 nm and 630 nm wavelengths, respectively. The optical albedo for brain phantom was found to be a = 0.71 and a = 0.96 for prostate phantom tissue. The results verify the reliability of the experimental technique and suitability of the fabricated tissues for use in biomedical, going forward, thus allowing for future work without the need for experimentally complex and expensive setups.
Computational study of the structure and optical properties of perovskite solar cells materials.
(2021) Kheralla, Adam Abdallah.; Chetty, Naven.
Design and fabrication of tissue-like phantoms for use in biomedical imaging.
(2022) Ntombela, Lindokuhle Charles.; Chetty, Naven.; Adeleye, Bamise.
The continuous need for tissue-like samples to understand biological systems and the development of new diagnostic and therapeutic applications has led to the adoption of tissue models using potential materials. This work presents a low-cost method for manufacturing PVAslime glue-based phantoms to replicate diseased and healthy biological tissues’ optical, mechanical, and structural properties. The deformable phantoms with complex geometries are vital to model tissues’ anatomic shapes and chemical composition. Absorption and scattering properties were set by adding black India ink and aluminium oxide (Al2O3) particles in varying quantities to obtain slime phantom tissues with optical properties of the brain, malignant brain tumour, lung carcinoma, and post-menopausal uterus. The phantom properties were characterized and validated using a He-Ne laser emitting at 532 nm and 630 nm wavelengths propagated through various thicknesses of the fabricated phantom. The incident and transmitted intensity were measured to determine the absorption coefficient (a) and scattering coefficient (s). Furthermore, the effective attenuation coefficient (eff ) and penetration depth () were deduced from the reduced scattering coefficient (0s) and the anisotropy factor (g) obtained through the scattering phase function and Wolfram Mathematica. The anisotropy factor demonstrated a forward scatter, typical of strongly scattering media as real tissues. Such geometrically and optically realistic phantoms would function as effective tools for developing techniques in diagnostic and therapeutic applications such as laser ablation and PDT cancer treatment.
Development and performance evaluation of a prototype bio-optical sensor for in-water applications.
(2012) Ramkilowan, Arshath.; Chetty, Naven.; Lysko, M. D.
No abstract available.
Development of a mathematical model to enable optimal efficiency of the indabuko lithium-ion battery.
(2020) Mphaka, Johannes.; Chetty, Naven.; Maphanga, Regina R.
Cathode materials are the foremost primary challenge for the vast scale application of lithium-ion batteries in electric vehicles and the stockpiles of power. Foreseeing the properties of cathode materials is one of the central issues in energy storage. In the recent past, density functional theory (DFT) calculations aimed at materials property predictions offered the best trade-off between computational cost and accuracy compared to experiments. However, these calculations are still excessive and costly, limiting the acceleration of new materials discovery. Now the results from different computational materials science codes are made available in databases, which permit quick inquiry and screening of various materials by their properties. Such gigantic materials databases allow a dominant data-driven methodology in materials discovery, which should quicken advancements in the field. This study was aimed at applying machine learning algorithms on existing computations to make precise predictions of physical properties. Thus, the dissertation primary goal was build best ML models that are capable of predicting DFT calculated properties such as, formation energy, energy band-gap and classify materials as stable or unstable based on their thermodynamic stability. It was established that the algorithms only require the chemical formula as input when predicting materials properties. The theoretical aspect of this work describes the current machine learning algorithms and presents "descriptors"-representations of materials in a dataset that plays a significant role in prediction accuracy. Also, the dissertation examined how various descriptors and algorithms influence learning model. The Catboost Regressor was found to be the best algorithm for determining all the properties that were selected in this study. Results indicated that with appropriate descriptors and ML algorithms it is feasible to foresee formation energy with coefficient of determination (R2) of 0.95, mean absolute error (MAE) of 0.11 eV and classify materials into stable and unstable with 86% of accuracy and area under the ROC Curve (AUC) of 89%. Lastly, we build a web application that allow users to predict material properties easily.
Development of an integrated model and system to enable optimal efficiency of the HartRAO LLR signal path.
(2017) Ndlovu, Sphumelele Colin.; Combrinck, Ludwig.; Akombelwa, Mulemwa.; Chetty, Naven.
The Lunar Laser Ranger (LLR) system under development at the Hartebeesthoek Radio Astronomy Observatory (Hartford) in South Africa is being built to accurately measure the Earth-Moon distance (at 1 cm level) through the use of short laser pulses, a single photon detection system, an accurate timing system and other sophisticated components. This LLR system is unique in Africa and indeed in the entire Southern Hemisphere. The system utilizes a 1 m diameter optical telescope, which was donated to the project by the Observatoire de la Côte d’Azur of France. In this work, the author discusses the development of an integrated model that will be utilized to obtain optimal efficiency of the HartRAO-LLR system. The model is used to estimate the expected number of returned photons by considering a number of parameters which affects the laser beam pulses as they traverse the atmosphere from the LLR telescope to the Moon and back to the telescope. Factors such as the apparent Earth-Moon range, atmospheric extinction, laser beam characteristics, optical path efficiencies and others, affect the estimated (predicted by software) and actual (measured) number of returned photons for the HartRAO-LLR station. The estimated average signal return rate (which is dependent on a number of factors) of the HartRAO-LLR ranges between 0 to 12 photons per minute, which is in agreement with the available data from five globally distributed LLR stations. It also correlated with the estimated returns that were obtained using least squares parameter estimations. They were in agreement by an average difference of 0.00272. Our estimated signal returns are strongly affected by two-way atmospheric extinction (atmospheric and cirrus cloud transmissions), variations in the laser beam incident angle on the retroreflectors located on the Moon as well as the varying Earth-Moon range. A new parameter, named lunar reflectivity ranging between 0 and 1, was introduced in the link budget equation to consider the effects of Moon Phases on the returned photons. Modelling the returned number of photons and comparing these to the actual number received leads to an understanding of the effects of numerous variables on the total laser path efficiency. Total system efficiency can be improved as well, as particular atmospheric conditions will not allow LLR to be successful on certain days. For these days, the system can be utilized for other purposes such as maintenance or satellite laser ranging.
Digital control of light.
(2019) Majola, Nombuso.; Chetty, Naven.; Dudley, Angela.; Forbes, Andrew.
The objective of this research was to describe innovative ways in which digital holography can be applied in controlling laser light. The ability to control and manipulate a laser beam has become an extremely desirable feature since it enables improvement in the efficiency and quality of a number of applications. Methods of controlling light make use of optical components to change the properties of a light beam according to the function of that optical element; therefore, a particular arrange- ment of optical elements in a system controls light in a certain way. Technological advancements in the field of optics have developed a versatile device called a spatial light modulator (SLM), which is a novel instrument that employs computer gener- ated holographic patterns (or phase masks) to modulate the amplitude and /or phase of a laser beam and it can therefore perform the function of a number of optical elements. This research presents novel optical set-ups based on the phase-only liquid crystal spatial light modulator (LC-SLM) for generating, controlling and exploring different laser beam pat- terns. The thesis has three main sections, the first one is Holographic beam shaping, where a Gaussian beam was reshaped using an SLM to produce Vortex, Bessel or Laguerre-Gaussian beams. These beams were found to agree with theoretically generated beams. Secondly, we produce o -axis laser beams by constructing coherent superpositions of Gaussian and vortex modes and then use two measurement techniques, peak intensity ratio and modal decomposition technique, to obtain the constituent components of these fields. Finally, we investigate the propagation dynamics of Vortex and Laguerre-Gaussian beams by using a SLM to digitally propagate these beams in free space, and then perform mea- surements on the far field intensity pattern. The results show that the Laguerre-Gaussian beam suffers less spreading and beam distortion compared to the vortex beam in free space propagation.
Experimental measurement of the fluctuations of a laser beam due to thermal turbulence.
(2013) Ndlovu, Sphumelele Colin.; Chetty, Naven.
In this work, we considered and developed a new method to detect and quantify the fluctuations of a laser beam due to thermal turbulence. The new method consisted of a single laser beam propagating in air and passing through a point diffraction interferometer (PDI). Stable interferograms were thus formed by diffraction of light at the PDI pinhole. Such interferograms underwent phase shifts due to the application of simulated thermal turbulence on the propagating laser beam. These phase shifts were then used to obtain atmospheric turbulence parameters such as the atmospheric turbulence strength, temperature near the propagating beam and the scintillation index. Chapter 1 of this thesis is an introduction and discussion of the theory on the propagation of laser beams in air. Gaussian beam propagation, turbulence detectors, Rytov’s theory and Kolmogorov’s theory of turbulence is also discussed in detail. artefact descriptive experimental procedure is then provided. This chapter focuses on the behavior of a laser beam propagating under the conditions of weak turbulence and relates the Rytov weak fluctuations to the Kolmogorov spectrum since the Rytov variance can be exactly equal to the scintillation index under the conditions of weak turbulence. Two unpublished scientific papers were submitted for publication to the Canadian Journal of Physics and Europian Journal of Remote Sensing. Chapter 2 consist of paper 1 which is based on the development of the experiment and it describes the apparatus in detail as well as it explains the experimental procedure. The preliminary results presented in paper 1 showed that a PDI can produce stable interferograms that can be used to extract the atmospheric turbulence parameters and thus, the PDI method can be used for atmospheric detection and ranging. In chapter 3, we discussed and analysed the experimental results, where the phase shifts were used to estimate the temperature that caused the purturbations on the interferograms. In chapter 4, we concluded about the use of a PDI as a remote sensing technique.
Experimental verification of the turbulent effects on laser beam propagation in air.
(2014) Augustine, Shivan Michael.; Chetty, Naven.
Abstract available in PDF file.
High-resolution geophysical imaging and characterization of severe landslides vicinities at South-Eastern Nigeria for uni-vario-seasonal degradation monitoring and civil engineering remediation.
(2021) Chikwelu, Edward Emenike.; Chetty, Naven.
This research work used high-resolution geophysical methods to explore the near surface of severe gully-erosion sites. The erosion that mesmerized the environment as landslides is the major landscaping challenge facing Anambra Basin in the South Eastern Nigeria. The geophysical techniques used include electrical resistivity tomography (ERT), induced polarization (IP), vertical electrical sounding (VES), and geotechnical analysis to study the subsurface conditions of severe gullies. Principally Wenner, Schlumberger, and dipole-dipole are the geophysical tomography technique used during the survey, depending on the peculiarity of each selected site. Geotechnical data analysis was used to confirm the results of vertical electrical sounding at a specific location, with the aid of resistivity formations. The geodynamics of the sites as related to rocks’ susceptibility to failure, and the mechanisms of slope failure was investigated, and foundation depths of the immediate surroundings of the Nanka gully were studied using geotechnical data. The surrounding and the subsurface of the eroded portions were monitored through imaging and analysis across the basin to measure hydrological contribution to the gully risks and other prevailing factors. The geoelectrical data was acquired with the ABEM Terrameter SAS 4000 and the ABEM LUND ES464 electrode selection system (using resistivity method) and processed with the RES2DINV software to produce 2D subsurface images. The VES resistivity curve matching was developed by a partial curve matching technique; and interpreted by Minitab 18 software to produce subsurface contour maps. In addition, the subsurface contour maps were qualitatively analysed with mapped surface geology and information on current geological typical rocks. In the geotechnical analysis, Spangler and Handy sampling techniques were used to collect eighteen (18) samples from dug gully walls, and the laboratory tests were to ascertain the soil properties index. The results of the models indicate that the study area is mainly clayey and sandstone formations that exhibited low resistivity values corresponding to the shale layers and groundwater zones. Many features that may lead to slope failure are present in the study areas, such as fractures, boulders, weak zone, and saturated zone. The results also showed that the soils in the study areas are friable hence easily washed off whenever there is storm water runoff from the surface, making the landslide active over the years despite every protective precaution put in place. In conclusion, this research work has identified lithologies, structural deformations, and distinguishing clayey zones from water-saturated zones, proving that the geophysical technique is the most successful tool in the landslide investigation.
An investigation into the radioprotective potential of Costus afer and Drymaria cordata extracts on whole-body irradiated mice.
(2021) Akomolafe, Idowu Richard.; Chetty, Naven.
The need for effective and non-toxic radioprotectors has shifted researchers' attention to plants and natural products as an alternative to synthetic compounds. This study investigated the radioprotective potential of Costus afer (CAE) and Drymaria cordata (DC) extracts on mice's survival, haematological and histopathological parameters following X-ray irradiation. One hundred and fourteen (54 male & 60 female) mice with total body masses between 38-45g and aged between 10-12 weeks old were used for this study. The mice were divided into twelve groups containing six and ten mice, respectively, for experiments CAE and DC. Animals were further sub-divided into irradiated and un-irradiated groups. The animals in both experiments received 250mg/kg extract of CAE and DC by oral gavage for six days and thirteen days, respectively, in addition to feeding and water ad libitum. Exposure of mice to radiation was done at the Radiotherapy and Oncology Department, Grey's Hospital using a linear accelerator. Blood samples were collected at different time intervals for the haematology test. Harvesting of kidney and liver for histopathology examination also occurred. Post-irradiation monitoring then continued for 30 days. Data were analysed by a one-way ANOVA test, followed by Tukey's multiple comparison test. Our findings revealed that the mice irradiated with 3Gy, 4Gy, 6Gy and 8Gy doses of X-ray radiation experienced a significant reduction in their White Blood Cell, Packed Cell Volume, Haemoglobin, Neutrophils, Lymphocytes, Eosinophils, and Platelet counts when compared with the control group in both experiments. In both experiments, CAE and DC extract offered protection against the radiation-induced haematological alterations by elevating all the blood parameters, except red blood cells and monocyte in the CAE treatment groups. In addition, the pre-treatment of mice with DC delayed the onset of mortality, thereby increasing the mice's survival rate. Histopathological changes in the CAE treatment groups' kidney and liver sections revealed no visible lesion in the pre-treated mice. Hepatocytes seem to be within normal histological limits. Although it is evident that the CAE and DC extracts protect against radiation-induced haematological damage and increases survival rate, no significant improvement in the histopathological parameters was recorded. Thus, further research is needed to prove the CAE and DC radioprotective potential on histopathological variables.
Measurement and modelling of the directional scattering of light.
(2015) Griffith, Derek John.; Chetty, Naven.
The quantum nature of light suggests that a photon can interact with matter in two primary ways. Firstly and perhaps more simply, the photon could be absorbed or secondly and more complex, it could be scattered into a new direction of propagation. The scattering process can be thought of as probabilistic, with a statistical distribution of possible new directions of travel with respect to the original. In the case of interaction with a small particle of matter, the probability distribution is referred to as the phase function. In the case of scattering at a surface interface between two bulk materials, the new direction of travel is distributed according to a function called the Bidirectional Scattering Distribution Function (BSDF). The BSDF depends on both the direction of arrival and the direction of scatter (hence bidirectional ), the type of material and the condition of the surface as well as the wavelength of light. This work explores a number of areas related to the BSDF, with special attention to the effects of random light scatter in high performance optical imaging systems such as space telescopes. These demanding imaging applications require optical components manufactured to very high standards with respect to shape, smoothness and cleanliness. This means that random scatter from the surfaces of these optical components must be controlled to very low levels. The measurement of very weak optical surface scatter is therefore a problem of particular interest. An interferometric technique has been proposed here for improving the quality of such measurements. The interference effects produced in the image by this technique were analysed using Nijboer-Zernike diffraction theory, leading to a journal publication in Current Applied Physics.
Measurement of the temperature dependence of the Buckingham effect (electric-field-gradient-induced birefringence) in gases
(2009) Chetty, Naven.
The aim of this research project was to assemble an apparatus to measure the electric quadrupole moments of gas molecules using the technique of electricfield- gradient-induced birefringence, or the Buckingham effect. Comprehensive research by various workers in the field has shown that this technique provides the only direct means of obtaining the quadrupole moment of a molecule. Theory has shown that the most accurate determination of the electric quadrupole moment is through a study of the temperature dependence of the effect. This not only allows for the quadrupole moment to be obtained but also enables the temperature-independent quadrupole hyperpolarisability term to be extracted. Both the quadrupole moment and the hyperpolarisabilty provide valuable information in a variety of applications, including intermolecular forces, electrostatic potentials and non-linear optical phenomena. This thesis fully describes the apparatus used in these measurements, including a description of the custom built oven that allowed for measurements to be performed over a temperature range spanning from 25"C up to 200"C. Results for the quadrupole moments and quadrupole hyperpolarisabilities of carbon dioxide, carbon monoxide, nitrous oxide and hydrogen are presented, together with a quadrupole moment for carbonyl sulphide from room-temperature measurements. Wherever possible, the results of this work are compared to previously published experimental and theoretical data.
Natural radioactivity level in soil, crops, river sediments, and selected aquatic species in South Africa’s oil-producing areas.
(2020) Ilori, Abiola Olawale.; Chetty, Naven.; Adeleye, Oluwabamise.
Non-linear multivariate analysis of the global solar radiation received across five cities in South Africa.
(2019) Govindasamy, Tamara Rosemary.; Chetty, Naven.
South Africa is considered one of the most developed countries in Africa, however with more than 80% of its electricity being generated from coal, this country is considered one of the highest contributors to greenhouse gas emission throughout the continent. The impacts of this fossil fuel dependency are prominent in the environmental degradation experienced - climate change conditions as well as the current state of emergency faced by the national power utility, ESKOM. While provisions such as load shedding are being made to avoid the country from facing blackout, the consequences of these resolves significantly influence the economy of the country. Although the cost of applicable renewable energy technologies has decreased considerably over the past few years, South Africa continues to lag in the adoption of renewable energy systems in a global comparison. Most applications of potential solar renewable energy systems are currently in the investigation stages, leaving this readily accessible resource's capacity idle. This makes research in solar renewable energy highly significant with regards to progressing the country's uptake of green energy technologies. Our study proposes linear and non-linear analysis of multivariate models for the estimation of global solar radiation (GSR) received across five major cities in South Africa. The significance of this study is to allow for effective GSR estimation in the application of solar technologies, while increasing implementation of these alternatives. Measured quantities such as sunshine duration and solar radiation for certain regions are limited due to the expensive equipment required and maintenance thereof. Local meteorological sources are unable to provide historic data which is complete, as these quantities are scarcely quantified. The dependency of GSR on meteorological variables such as air temperature, relative humidity and relative sunshine duration was evaluated for the period January 2007 - June 2018 to realize estimation models for each of the study sites. The Hargreaves-Samani and Angstrom-Prescott empirical models served as the foundation for our single variable analysis of GSR reliance on each meteorological parameter and their relative variations. Our results have indicated that our proposed multivariate, non-linear equations perform better than the empirical models as well as single variable, linear regression equations. Our suggested models are site-specific and demonstrate a strong correlation to historic GSR values with low, acceptable error indicators. Further to this, we have recognized that second and third-order relationships between H/Ho and multiple meteorological variables provide a more accurate description of GSR for most of the cities under study. This analysis could potentially contribute significantly to the investigation of solar radiation alternatives and photovoltaic (PV) technologies in South Africa. We believe that integration of estimation models within the design and installation stages of PV technologies will be largely beneficial in ensuring their optimum intake. The models discussed in this study verify the reliability and accuracy of GSR estimation through readily accessible meteorological factors in a cost-effective manner.
Optical properties of tissue-like phantoms.
(2016) Singh, Keshav.; Chetty, Naven.
Abstract available in PDF file.
Quantifying the global solar radiation received in Pietermaritzburg, KwaZulu-Natal using a temperature based method (Hargreaves-Samani) to determine the Angstrom coefficients through the clearness index.
(2015) Govindasamy, Tamara Rosemary.; Chetty, Naven.
Abstract available in PDF file.
Radiation dose and cancer risk estimates from tuberculosis infections imaging.
(2018) Adeleye, Oluwabamise.; Chetty, Naven.
A robust air refractometer for accurate compensation of the refractive index of air in everyday use.
(2016) Kruger, Oelof Abraham.; Chetty, Naven.
Abstract available in PDF file.