Browsing by Department "Department of Mechanical Engineering"

Now showing 1 - 20 of 680
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    Open Access
    25cc HCCI engine fueled with Diethyl Ether
    (2009) Lemberger, Ian; Floweday, Gareth; Yates, Andrew
    This research forms part of an ongoing HCCI study at the SASOL Advanced Fuels Laboratory to investigate and understand engine configuration and fuel chemistry effects on combustion in HCCI engines. This project continues from a previous project where a small Progress Aero Works (PAW) 6.5cc high speed model "diesel" aeroplane engine was found to operate in HCCI mode with surprising ease and flexibility. A 25cc, four-stroke, single cylinder Honda GX25 engine, possessing 2-valves with an overhead cam and separate oil sump lubrication system was used. This research aimed to provide insight with respect to which engine characteristics such as size, heat transfer, speed and fuel blending effects, play the primary role in operational differences between the Honda GX25, conventional HCCI engines and the remarkable operational flexibility of the PAW engine.
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    A CFD framework for aeroelastic gust load calculations
    (2018) Man, William Liw Tat; Malan, Arnaud
    A Computational Fluid Dynamics (CFD) framework for the simulation of the aeroelastic response of aircraft flying under gust loading was developed. The multiphysics, Finite Volume, VertexCentered code Elementaltextsuperscript{textregistered} was employed and calculations were performed for the transonic flow regime. In the structural domain, the fuselage was treated as rigid and the wing was considered for aeroelastic calculations. The latter was represented by a beam stick model using Timoshenko beam theory in Elementaltextsuperscript{textregistered}'s structural module. The case under consideration was the NASA Common Research Model (CRM) flying at Ma = 0.86 with a 30 ft gust applied over the aircraft. Key contributions of this work included implementation of a computationally efficient gust model as well as the development of a fluidstructure interface. The latter was to transfer forces from a deforming wing skin to the wing-beam in a conservative manner while reflecting the resulting displacements on the wing surface. An interface library was developed for this purpose and 3rd order accurate Bezier curves used to recover a smooth deformed wing. The various sub-components of the aeroelastic model were rigorously validated. Following this, the developed framework was applied to the CRM under gust load conditions.
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    A commentary on the intellectual health of the nation
    (2007) Vaughan, Christopher L; Reddy, B Dayanand; Noakes, Timothy D; Moran, V C
    The record of high-quality research at South African universities is not as impressive as we may have thought, according to some international rankings. Whatever we might think of these assessments, we have to take them seriously. We suggest ways in which our universities and other institutions of higher learning might raise the level of their game.
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    A complex transdisciplinary exploration of South African climate mitigation policy
    (2019) Tyler, Emily; Cohen, Brett
    The research journey reflected in this thesis emerged from fifteen years of practice of (predominantly South African) climate mitigation policy from 2001 - 2016; from a dissatisfaction with the pace and depth of progress, and a realisation that the South African climate mitigation policy community of practice approaches what we do in a particular way. Guided by a complex transdisciplinary methodology, in this thesis I explore this realm of 'approach’, asking whether it is consequential for the decarbonisation policy agenda in South Africa, and considering complexity thinking as an alternative. A four-part research question acts as the central attractor to this exploration: 1. What is the current dominant approach to SA CM policy? The thesis starts by articulating the 'dominant approach’ of the SA CM community of practice (CoP) observed during the research and building on my experience in the field. I reveal this approach as being influenced by the perspective of the international climate mitigation policy process, and the 'hegemonic worldview’ - using Capra’s (1974) term as a heuristic to convey the set of assumptions and beliefs dominant in the cultural values and form of scientific knowledge that holds power. A normative undercurrent and an environmental perspective that discounts the social realm further shape the dominant approach, an approach that has particular implications for how the SA CM CoP engages with its key policy concepts of transformative change and development. 2. What does the dominant approach illuminate and what does it obscure about the policy issue? I find in the thesis that the dominant approach illuminates aspects of the climate mitigation policy issue: the greenhouse gas constraint; its macro and sectoral scale and temporal implications; technology and finance mitigation options; how various policy instruments work; with a focus on data. However the dominant approach also actively obscures other aspects: the implications of the complex, systemic and long-term aspects of the SA CM policy issue for policymaking; how policy implementation happens; the roles of power, values, culture and behaviour in transformative change; and how to engage perspectives and contestation. 3. How can a complexity approach contribute towards revealing SA CM policy more fully? The thesis then turns to complexity and complex systems thinking to explore how a view from complexity opens up these important but currently obscured spaces for climate mitigation policymaking. The SA CM policy issue is described in terms of complex systems, and a complexity view is offered of: the relationship between the SA CM policy issue and policymaking, the 'mitigation- development complex’, power patterns relevant to SA CM policy, the SA CM policy objectives, and deliberate transformative change. Building on this view, complex SA CM policymaking is described as a journey, reflecting a shift in focus away from content, plans and evidence towards principles, process and emergent strategies; a re-ordering of policy priorities and leverage points, all premised on the complexity observation that top-down control of a complex social system is impossible. A set of policymaking initiatives arising from this complexity approach is offered, including the establishment of a permanent stakeholder engagement platform, a sense-making function, a dedicated strategic and political policy capacity, and a complexification of CM policy instruments and research practice. 4. What is the usefulness of this inquiry to the SA CM Community of Practice (CoP)? Finally, the usefulness of the inquiry to the SA CM CoP is assessed. I conclude that 'approach’ is consequential to our work, and that reflecting on our approach can reveal how it might be constraining us and support our explicit consideration of alternatives. The complexity exploration is useful in two ways. First, it offers the set of practical initiatives referred to above for the SA CM CoP’s consideration as SA CM policy is advanced. Second, it offers an alternative underpinning for approaching SA CM policymaking based on rigorous science, aligned with both the complex, systemic nature of the SA CM policy issue and with the increasing complexification and pace of change of the twenty first century. Whilst the gap between the hegemonic worldview and its organisational and physical manifestations and those of a complexity approach is significant - perhaps sufficiently so as to undermine the immediate usefulness of this aspect of the research to most members of the SA CM CoP - a complexity view of transformation as non-linear and episodic proves encouraging. The research journey traverses the territories of practice and academia, the specifics of South Africa and the breadth of global environmental and societal change, disciplines, perspectives, paradigms and worldviews. Essentially, the research comprises a heuristic move which calls attention to the relevance of policy approach in increasing the pace and depth of climate mitigation action in a development context. As required by a transdisciplinary inquiry this contribution - which lies in the realm of knowledge - has both the societal usefulness described above and academic relevance. In the academic realm the thesis opens a new, multi-disciplinary research agenda around 'approach’ at the intersection of climate mitigation, energy, public policy and development studies. By scoping out a complexity interpretation of the mitigation policy issue in a development context, the research contributes to both the climate mitigation and complexity fields, and to thinking on issues of sustainable development. Finally, the thesis provides a rare example of transdisciplinary research and method in climate mitigation and energy studies. It is my hope that these transdisciplinary and reflexivity inroads will some day become paths well trodden.
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    A critical review of the educational philosophies underpinning science and engineering foundation programmes
    (2008) Kloot, B; Case, J M; Marshall, D
    More than a decade prior to the official dismantling of apartheid in South Africa, a number of universities launched foundation programmes to assist disadvantaged students. This article focuses on science and engineering foundation programmes, locating them within their political and institutional context and then tracing the evolution of their educational philosophy. But foundation programmes only represent one strategy for dealing with educational disadvantage. It is therefore compared to an alternative model explored in the early 1990s which emphasised the 'infusion' of academic development principles into the mainstream. This provides a backdrop for considering the educational effectiveness of the foundation programmes that have recently proliferated as a result of the Department of Education's latest funding strategy.
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    A Higher-Order VOF Interface Reconstruction Scheme for Non-Orthogonal Structured Grids - with Application to Surface Tension Modelling
    (2021) Ilangakoon, Niran A; Malan, Arnaud G
    The volume-of-fluid (VOF) method [24] is widely used to track the interface for the purpose of simulating liquid-gas interfacial flows numerically. The key strength of VOF is its mass conserving property. However, interface reconstruction is required when geometric properties such as curvature need to be accurately computed. For surface tension modelling in particular, computing the interface curvature accurately is crucial to avoiding so-called spurious or parasitic currents. Of the existing VOF-based schemes, the height-function (HF) method [10, 16, 18, 42, 46, 53] allows accurate interface representation on Cartesian grids. No work has hitherto been done to extend the HF philosophy to non-orthogonal structured grids. To this end, this work proposes a higher-order accurate VOF interface reconstruction method for non-orthogonal structured grids. Higher-order in the context of this work denotes up to 4 th-order. The scheme generalises the interface reconstruction component of the HF method. Columns of control volumes that straddle the interface are identified, and piecewise-linear interface constructions (PLIC) are computed in a volume-conservative manner in each column. To ensure efficiency, this procedure is executed by a novel sweep-plane algorithm based on the convex decomposition of the control volumes in each column. The PLIC representation of the interface is then smoothed by iteratively refining the PLIC facet normals. Rapid convergence of the latter is achieved via a novel spring-based acceleration procedure. The interface is then reconstructed by fitting higher-order polynomial curves/surfaces to local stencils of PLIC facets in a least squares manner [29]. Volume conservation is optimised for at the central column. The accuracy of the interface reconstruction procedure is evaluated via grid convergence studies in terms of volume conservation and curvature errors. The scheme is shown to achieve arbitrary-order accuracy on Cartesian grids and up to fourth-order accuracy on non-orthogonal structured grids. The curvature computation scheme is finally applied in a balanced-force continuum-surface-force (CSF) [4] surface tension scheme for variable-density flows on nonorthogonal structured grids in 2D. Up to fourth-order accuracy is demonstrated for the Laplace pressure jump in the simulation of a 2D stationary bubble with a high liquid-gas density ratio. A significant reduction in parasitic currents is demonstrated. Lastly, second-order accuracy is achieved when computing the frequency of a 2D inviscid oscillating droplet in zero gravity. The above tools were implemented and evaluated using the Elemental®multi-physics code and using a vertex-centred finite volume framework. For the purpose of VOF advection the algebraic CICSAM scheme (available in Elemental®) was employed.
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    A kinetic study of the oligomerization of propene, butene and various hexenes over solid phosphoric acid
    (1987) McClean, Deaghlan Martin; O'Connor, Cyril T; Kojima, Masami
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    A Mechano-Chemical Computational model of Deep Vein Thrombosis
    (2021) Jimoh-Taiwo, Qudus Boluwatife; Ngoepe, Malebogo
    Deep Vein Thrombosis (DVT) is the formation of a blood clot in a vein, usually in the body's lower extremities. If untreated, DVT can lead to pulmonary embolism (PE), heart attack and/or stroke, which can be fatal. According to literature, DVT affects 0.2% of people in developed countries and about 0.3%-1% in developing countries. In the past, various computational models of DVT were developed. Most models account for either the mechanical factors or biochemical factors involved with DVT. Developing a model that accounts for both factors will improve our understanding of the coagulation process. This study developed a three-dimensional DVT computational model in idealized and realistic common femoral vein (CFV) geometries. The model considers the biochemical reactions between thrombin and fibrinogen, pulsatile blood flow, and clot growth within the vessel. The model was validated using a simplified experimental setup with flow, thrombin, and fibrinogen. Computational fluid dynamics (CFD) simulations were carried out using the ANSYS modelling suite. The Navier-Stokes equations were solved to determine the fluid flow. Based on a clinical dataset of pulsatile blood flow, the laminar flow of blood with a Poiseuille velocity profile was applied at the inlet. Darcy's law was used to account for porosity changes in the clot, with the clot represented by zones with lower porosities. The transport equations were used for changes in the concentration of the biochemical protein species. Thrombin was released into the bloodstream from an injury zone on the wall of the vein. The Michaelis-Menten equation was used to represent the conversion of thrombin and fibrinogen to fibrin, the final product of the coagulation process. The computational model solves the blood flow pattern proximally, locally, and distally to clot formation at the injury zone. The model also predicts the size of the clot and the rate of clot growth. The model was first developed in a two-dimensional geometry. This model was used to investigate clot formation under different cases comparing how introducing thrombin as a flux value differs from specifying it as a fixed concentration. It was confirmed that to apply the flux condition, the thrombin concentration needs to be divided by a factor derived by multiplying the area of the injury zone and the time step size. The same model was then used to conduct a parametric study to determine the effects of varying parameters such as inlet velocity, vein diameter, and peak thrombin concentration on the size and shape of clot formed. Peak thrombin concentration was the key factor driving the initiation and propagation of clot in the vein. The model was then extended to an idealized three-dimensional geometry. This computational model was validated using results from an experimental clot growth study. The experiment comprised a steady flow of fibrinogen in a cylindrical pipe, with an injection of thrombin into the flow at the injury site, resulting in fibrin formation. A qualitative comparison was then made between the experimental clot and the clot formed in silico. Although quantitative measurements were not made, there were similarities in the shapes and sizes of the clots. The validated computational model was used to compare clot formation under steady and pulsatile flow conditions. Realistic clot growth was observed and compared to the steady flow condition. It was found that a larger clot formed under pulsatile conditions. Clot formation with the presence of valve activity was also investigated. The effect of opening and closing of the valves was achieved by varying the blood flow diameter at the inlet instead of modelling the valves as solid walls and accounting for the leaflet movement by solving the governing equations for the fluid-solid interaction (FSI), as used in existing models. The model was then applied to a patient-specific geometry. Realistic clot growth was achieved using this model, and the clot was compared to a clot formed in vivo, as depicted in the original imaging scan. The model helps us better understand the clot growth process in the femoral vein on a patient-specific level. It also shows that the presence of venous valves increases the size of clot formed compared to steady flow. However, the high strain rate present makes the clot formed smaller than in standard pulsatile flow cases.
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    A methodology for implementing a water balance of ESKOM power stations using the online condition monitoring software EtaPRO
    (2021) Sewlall, Preetha; Fuls, Wim
    Eskom produces approximately 90% of the electricity used in South Africa of which approximately 90.8% is from fossil fuel power plants. The process of electricity generation requires a significant quantity of raw water; therefore, Eskom is considered a strategic water user in South Africa. Water management is a growing focus area due to the increase in water usage and requires continuous improvement. Water management has been identified as an area lagging behind on the advanced analytics initiatives in Eskom. Excel based tools were used for the development of water balance models and water performance calculations in Eskom. This was attributed to the user-friendly functionality and availability to all users. However, the Excel tool posed challenges in allowing for standardisation and validation of calculations, tracking of model changes, continuous trending and storage of data as well as structured graphical user interfaces for screens and dashboard developments. There was therefore a need to develop a methodology on how to structure a water balance model for coal-fired power plants with standard calculation templates that allowed for customisation by each power plant within Eskom. It was required that the water balance model be implemented on a performance and monitoring tool allowing for comparison of power plant targets to actual online data in real time, enhancing the monitoring capabilities. It should have the ability to generate real time water performance data creating an opportunity for improved water management across the generation fleet. The approach adopted in this dissertation was to learn from existing Eskom Excel water balance tools and develop a standard mathematical model in the form of EtaPRO calculation templates. These templates are be structured such that they function as process components to develop water balances at power plants. The mathematical verification of the Excel calculations were to be conducted using Mathcad. The access to real time data, performance monitoring capabilities and availability at all Eskom Power plants, led to the selection of EtaPRO as the modelling platform. The research conducted led to the development of a methodology for setting up a water balance model for a wet-cooled coal-fired power plant. Calculation templates developed into EtaPRO were validated against the Mathcad mathematical model. The results included a well-documented mathematical model of a water balance in Mathcad and the development of 19 calculation templates that perform the function of standard process components. In addition to calculation templates, multiple Non Volatile (NV) records were created to allow the power plants to capture and track permanent data inputs. NV records also allow for creation of case studies, improving the process monitoring capabilities. A water balance model for a selected power plant was simulated in EtaPRO using the developed calculation templates and user defined formulae. Test screens and dashboards were created to illustrate how the calculation templates and water balance framework would be used to develop a typical water balance model and monitoring system. In conclusion, it is possible to develop process models within the EtaPRO software from well-defined mathematical models to address the performance monitoring concerns on water systems within Eskom.
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    A methodology for integrated thermofluid modelling of radiant superheaters in steady state and transient operations
    (2019) Gwebu, Excellent Zibhekele; Rousseau, Pieter G.; Malan, Arnaud G.; Jestin, Louis M.
    Critical components in coal-fired power plants such as final superheater heat exchangers experience severe conditions associated with high metal temperatures and high temperature gradients during base load and transient operations. Such adverse conditions could significantly reduce the life span of the components, especially due to the requirement of greater plant flexibility that is an essential part of the global power system transformation. Integrated thermofluid process models can be employed to obtain a better understanding of the relationship between the operational conditions and the metal temperatures. Thus, a methodology was developed to model radiant superheater heat exchangers in steady state and transient operations. The methodology is based on a network approach which entails solving the transient one-dimensional forms of the conservation equations for mass, energy and momentum. The model building blocks account for the convective thermal resistance on the steam side, the conductive thermal resistances of the tube wall and scaling or fouling on the tube walls, as well as the convective and radiative thermal resistances and direct radiation on the flue gas side. The model captures the physical layout of the tube passes in a tubesheet via the arrangement of the network building blocks. It is also possible to connect tubesheets together across the width of the boiler as per the arrangement in a real plant. The modelling methodology was first used to develop a process model of a convective cross-flow primary superheater heat exchanger with complex flow arrangement. The dual-tube 12-pass superheater was discretized along the flue gas flow path as well as along the steam flow path. The model was qualitatively validated using real plant data from literature and for reference purposes also systematically compared to conventional lumped parameter models. The ability of the model to analyse the effect of ramp rate during load changes on the tube metal temperature was demonstrated, as well as the ability to determine the maldistribution of flow and temperature on the steam and flue gas sides. The methodology was also applied to model a U-shaped radiant superheater heat exchanger. Due to the challenges associated with obtaining comprehensive real plant data in an industrial setting, a validation methodology was proposed that is based on a combination of plant design C-schedules and a boiler mass and energy balance, as well as limited plant measurements. The consistent comparisons with C-schedule data provide evidence of the validity of the model, which was further demonstrated via the comparisons with real plant data. The model allows prediction of the steam mass flow and temperature distribution going into the outlet stub headers as well as the main outlet headers for different inlet flow and temperature distributions on the steam and flue gas sides. These results were compared to detail real-plant measurements of the outlet header temperatures. The model also allows prediction of the metal temperatures along the length of the tubes which cannot readily be measured in the plant. The model was applied to demonstrate the impact of different operational conditions on the tube metal temperatures. Such integrated process models can be employed to study complex thermofluid process phenomena that may occur during intermittent, transient and low load operation of power plants. In addition, such models could be useful for predictive and preventative maintenance as well as online condition monitoring.
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    A methodology to investigate the cause of quenching in once-through tower type power plant boilers
    (2020) De Klerk, Gary; Rousseau, Pieter; Jestin, Louis
    Due to the penetration of variable renewable energy (VRE) sources, conventional coal fired power plants need to operate with greater flexibility via two-shifting or low load operation whilst remaining reliable and conserving the lifetime of components. Thick sectioned components are prone to thermal fatigue cracking as a result of through-wall temperature gradients during start up and shutdown. These temperature gradients can be significantly amplified during quenching when components at high temperature are unintentionally exposed to colder liquid or steam. Such quench events are known to occur during two-shift operation of a large once-through coal fired tower type boiler, which is the subject of this study. The purpose of this study is to develop and demonstrate a methodology to determine the root cause of quenching in a once-through tower type boiler and provide information that can be used to predict the impact on thick-walled components by estimating the through-wall temperature gradients. The first modelling element in the methodology is a simplified transient heat transfer model for investigating condensation of steam in the superheater. The model is presented and verified by comparison with real plant data. The second element is a liquid tracking model that approximates the liquid level in the superheater as a function of time to predict the location and magnitude of through-wall temperature gradients. The complex geometry of the superheater was divided into a number of control volumes and a dynamic thermo-fluid process model was developed to solve the transient conservation of mass and energy equations for each volume using a semi-implicit time wise integration scheme. The liquid tracking model was verified by comparison with a similar model constructed in Flownex and also by comparison with plant data. Varying levels of discretisation were applied to a particular quench event and the results are presented. The third modelling element is a two-dimensional transient pipe wall conduction model that is used at selected localities to evaluate the temperature gradients within the pipe wall. The temperature gradients and internal heat flux were verified by temperature measurements from the outer surface of a main steam pipe undergoing quenching. The stresses associated with the temperature gradients were also briefly considered. The real plant quenching problem is analysed in detail and found to be caused by liquid overflow from the separators. A particular plant configuration creates a previously unidentified siphon of water from the separating and collecting vessel system into the superheater. This situation is not recognised by plant operators and thus persists for some time and causes flooding of the superheater. Analysis of the resultant through-wall temperature gradients show that quenching causes significant stresses which can be avoided. By understanding the causes and preventing the occurrence of quenching, the life of thick-walled high temperature components can be conserved.
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    A modelling methodology to quantify the impact of plant anomalies on ID fan capacity in coal fired power plants
    (2020) Khobo, Rendani Yaw-Boateng Sean; Rousseau, Pieter; Gosai, Priyesh
    In South Africa, nearly 80 % of electricity is generated from coal fired power plants. Due to the complexity of the interconnected systems that make up a typical power plant, analysis of the root causes of load losses is not a straightforward process. This often leads to losses incorrectly being ascribed to the Induced Draught (ID) fan, where detection occurs, while the problem actually originates elsewhere in the plant. The focus of this study was to develop and demonstrate a modelling methodology to quantify the effects of major plant anomalies on the capacity of ID fans in coal fired power plants. The ensuing model calculates the operating point of the ID fan that is a result of anomalies experienced elsewhere in the plant. This model can be applied in conjunction with performance test data as part of a root cause analysis procedure. The model has three main sections that are integrated to determine the ID fan operating point. The first section is a water/steam cycle model that was pre-configured in VirtualPlantTM. The steam plant model was verified via energy balance calculations and validated against original heat balance diagrams. The second is a draught group model developed using FlownexSETM. This onedimensional network is a simplification of the flue gas side of the five main draught group components, from the furnace inlet to the chimney exit, characterising only the aggregate heat transfer and pressure loss in the system. The designated ID fan model is based on the original fan performance curves. The third section is a Boiler Mass and Energy Balance (BMEB) specifically created for this purpose to: (1) translate the VirtualPlant results for the steam cycle into applicable boundary conditions for the Flownex draught group model; and (2) to calculate the fluid properties applicable to the draught group based on the coal characteristics and combustion process. The integrated modelling methodology was applied to a 600 MW class coal fired power plant to investigate the impact of six major anomalies that are typically encountered. These are: changes in coal quality; increased boiler flue gas exit temperatures; air ingress into the boiler; air heater inleakage to the flue gas stream; feed water heaters out-of-service; and condenser backpressure degradation. It was inter alia found that a low calorific value (CV) coal of 14 MJ/kg compared to a typical 17 MJ/kg reduced the fan's capacity by 2.1 %. Also, having both HP FWH out of service decreased the fan's capacity by 16.2 %.
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    A Non-linear Visco-elastic Model for Dynamic Finite Element Simulation of Bovine Cortical Bone
    (2021) Blignaut, Caitlyn; Ismail, Ernesto; Cloete, Trevor
    Modelling and simulation of the human body during an impact situation such as a car accident, can lead to better designed safety features on vehicles. In order to achieve this, investigation into the material properties and the creation of a numerical model of cortical bone is needed. One approach to creating a material model of cortical bone suitable for these situations is to describe the material model as visco-elastic, as reported by Shim et al. [1], Bekker et al. [2] and Cloete et al. [3]. The work by Shim et al. and Bekker et al. developed three-dimensional models, but do not accurately capture the transition in behaviour in the intermediate strain rate region, while Cloete et al. developed a phenomenological model which captures the intermediate strain rate behaviour in one dimension. This work aims to verify and extend these models. The intermediate strain rate regime (1 s−1 to 100 s−1 ) is of particular interest because it is a key characteristic of the behaviour of cortical bone and several studies have been conducted to gather experimental data in this region [3, 4, 5, 6]. The behaviour can be captured using non-linear viscoelastic models. This dissertation focuses on the development and implementation of a material model of cortical bone based on non-linear visco-elastic models to capture the intermediate strain rate regime behaviour. The material model was developed using uni-axial test results from cortical bone. The model by Cloete et al. has been improved and extended, and issues of local and global strain rate with regards to the viscosity have been clarified. A hereditary integral approach was taken in the analysis and implementation of discrete models and was found to be consistent with mathematical models. The model developed was extended to three dimensions in a manner similar to that of Shim et al. and Bekker et al. for implementation in commercial finite element software (LS-Dyna and Abaqus).
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    A numerical eigenmode analysis of the progressive buckling of thin-walled tubes
    (2005) Fuggle, Timothy Mark; Nurick, G. N
    [page 76 missing]Structural impact is concerned with the behaviour of structures subject to static and dynamic loads which produce inelastic deformations. This field is of particular interest for the crash-worthiness design of aeroplanes, motor cars, buses, trains, ships and marine structures, where the survivability of passengers is at risk, as well as for the energy absorbing characteristics of various other components such as the petrochemical industry for safety devices. Progressive buckling of thin-walled tubes has already been used very successfully to absorb the kinetic energy during impact between motor vehicles, trains, etc. A great deal of research has been performed in the field of energy absorption devices and in particular for thin-walled square and cylindrical tubes being axially crushed. Many rigorous and time-consuming experimental models have been carried out in order to gain a better understanding of the different collapse modes of thin-walled square and cylindrical tubes. The ability of numerical models to predict the failure modes and forces associated with the collapse of the tubes accurately has meant that extensive time and expense required for experimental methods can be eliminated.
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    A physics-informed neural network modelling methodology to analyse integrated thermofluid systems
    (2024) Laugksch, Kristina Karli; Rousseau, Pieter; Laubscher, Ryno
    Physics-informed neural networks (PINNs) were developed to overcome the limitations of acquiring large training datasets that are commonly encountered when using purely data-driven machine learning methods. This study explores a PINN modelling methodology to analyse steady-state integrated thermofluid systems based on the mass, energy, and momentum balance equations, combined with the relevant component characteristics and fluid property relationships. The PINN methodology is applied to three thermofluid systems that encapsulate important phenomena typically encountered in integrated thermofluid systems modelling, namely: (i) a heat exchanger network with two different fluid streams and components linked in series and parallel; (ii) a recuperated closed Brayton cycle containing various turbomachines and heat exchangers, and (iii) a simplified boiler consisting of a furnace and radiative convective superheater. The predictions of the three PINN models were compared to benchmark solutions generated via conventional, physics-based thermofluid process models. The largest average relative error across all three models is only 0.93%, indicating that the PINN methodology can successfully be implemented to generate accurate solutions using the non-dimensionalised forms of the balance equations. Furthermore, it was shown that the trained PINN models provided a significant increase in inference speed compared to the conventional process models. The PINN modelling methodology was then extended to develop a surrogate model for the heat exchanger network. An additional surrogate model was developed for comparison using a data-driven multilayer perceptron (MLP) neural network. The MLP surrogate model was able to interpolate accurately. However, its predictive performance declined when making predictions for samples that fell outside of the range of training data. Despite various refinements, the PINN surrogate model could only be trained successfully for datasets that contained up to five unique samples. This limitation could not be resolved within the scope of the present study and should be investigated further. The accuracy of the PINN surrogate model degraded significantly when used to extrapolate beyond the training envelope. Due to the constraint on the number of training samples, it is impossible to draw a general conclusion regarding the extrapolation ability of the PINN concept. In spite of its current limitations, the significant increase in computational speed offered by the PINN modelling methodology when used to analyse integrated thermofluid systems suggests that this is a promising modelling technique that should be explored further.
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    A Precision Experiment for the Deformation and Fracture of Blast Loaded Plates
    (2022) Cloete, Trevor John; Nurick, Gerald N
    The purpose of this thesis is to present a novel laboratory scale precision test [1] apparatus, developed specifically to study of the deformation and fracture of blast loaded plates. A review of published laboratory scale blast testing showed that classical ballistic pendulum techniques satisfy several precision testing criteria. However, specific aspects required improvement. Therefore, an instrumented ballistic pendulum has been developed, incorporating a central support, in the form of a Hopkinson bar, for use with a novel peripherally clamped centrally supported (PCCS) plate specimen, which allows the reaction force history at the inner boundary to be captured. Hence, in addition to the traditional impulse vs deflection data, this approach allows details such as the total deformation duration or the exact timing and magnitude of fracture processes to be captured. Furthermore, when used with a peripherally clamped annular (PCA) plate specimen, this approach allows the blast load pressure history to be captured. A literature review found several classical closed form solutions for the large deflection of impulsively loaded peripherally clamped solid (PCS) circular plates. However, several crucial aspects of the solutions were contradictory and/or incomplete. To address this a generalized energy method for modelling impulsively loaded axisymmetric plates was developed which subsumes previous solutions and facilitated novel analytical solutions that provide a theoretical framework for interpreting the experimental data. Extensive experimental results reported in this thesis provide a rich set of novel data for code validation. While the PCCS and PCA plate configurations require a refined definition of the observed failure modes, the transitions between the modes and the fracture behaviour are studied in great detail. The data is analysed using dimensionless impulse expressions obtained from the analytical solutions in this thesis. In addition to accurate deflection predictions, these solutions also provide improved deflection duration predictions using a novel two phase solutions that can also accommodate finite load duration. It is concluded that the instrumented ballistic pendulum, incorporating a central support in the form of a Hopkinson bar, in conjunction with the novel PCCS and PCA plate configurations and analytical solutions, satisfies all the requirement of a precision test.
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    A reduced order modelling methodology for external cylindrical concentrated solar power central receivers
    (2023) Heydenrych, James; Rousseau, Pieter; Du Sart Colin
    The use of supercritical carbon dioxide (sCO2) power cycles for concentrated solar power (CSP) applications is becoming increasingly attractive since these cycles may offer lower capital costs and increased thermal efficiency. However, there are currently no utility-scale sCO2-CSP tower plants in operation. Therefore, to aid in the design and analysis process, there is a need to develop sufficiently accurate and computationally inexpensive models for such plants. This dissertation presents a reduced order modelling methodology for external cylindrical concentrated solar power central receivers. The methodology is built on a one-dimensional thermofluid network to model the heat transfer through the tube walls, coupled to a fluid flow network of the solar salt flowing inside the tubes. This is combined with a neural network surrogate model to determine the radiative heat flux impinging upon the tube surfaces. The receiver geometry is discretized along the height and around the circumference and each increment is represented by an equivalent thermal resistance network that represents the heat transfer within the tube walls. The heat transfer network parameters are calibrated using a detailed computational fluid dynamics model, which enables the calculation of the maximum tube wall temperatures. The heat transfer network is connected to the fluid flow network that solves the mass, energy, and momentum balance equations to determine the mass flow rates, pressure drops and temperature distributions. The radiative heat flux profile impinging on the receiver is typically calculated for a specific location and specific time of the day using a tool such as SolarPILOT. However, this can be computationally expensive since the central tower is surrounded by thousands of individual heliostats that are all sources of radiative flux, which depends on the position relative to the sun and relative to the receiver, as well as the direct normal irradiation (DNI) at that location and time. To reduce the associated computational expense, a multilayer perceptron (MLP) surrogate model is developed that allows the prediction of the flux profile for a range of plant configurations and atmospheric conditions at a specific location. The application of the methodology is demonstrated via a case study. The methodology may be used in future studies where sCO2-CSP tower plants are investigated, especially those with an interest in the detail design and analysis of the central receiver.
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    Open Access
    A study on the effect of increased heat input on residual stress, microstructure evolution and mechanical properties in Ti6Al4V selective laser melting
    (2021) Motibane, Londiwe Portia; Knutsen, Robert
    The Aeroswift machine is a novel high-speed powder bed fusion machine developed through a collaborative effort between the CSIR, Aerosud and the DSI. Its novelty lies in the substantial increase in build rate achieved through the implementation of a 5kW IPG laser and faster laser scanning speeds employed during processing. It is capable of producing Ti6Al4V low volume, high value and high integrity components required by the aerospace industry. Commercial selective laser melting (SLM) systems are a good benchmark for the type of quality needed in the integrity of aerospace components although they don't always meet them. The biggest difference between commercial systems and the Aeroswift machine is the amount of heat input used to make components based on the laser powers. Heat input is the ratio of the laser power to the scanning speed and it plays a role in the thermal history of a built part, its thermal gradients and therefore its residual stress. Heat input also has a big influence on the microstructure produced which determines the resultant mechanical properties. The focus of this project was to investigate the effect of increased heat input on residual stress, the development of microstructure and mechanical properties of Ti6Al4V specimens produced by the Aeroswift high (400 J/m) heat input system and commercial SLM Solution M280 low (150 J/m) heat input machine. This was to be accomplished by comparing the tested results of Aeroswift built specimens (High Heat Input) to those built by a commercial SLM machine (Low Heat Input). The effect of preheating on these properties was also studied. The low heat input specimens had two sets of test specimens, where one set was built without preheating and the other built at a preheating temperature of 200°C. This was the maximum preheating temperature for the commercial system used in this study. Firstly, the cantilever specimen were used to measure the amount of distortions that processing caused for both systems. The measured spread of the cantilever gave an indication of the amount of distortion caused by each processing condition. Distortion was found to be similar between the high heat specimen and the low heat specimen. Preheating at 200°C also did not give an appreciable difference in the amount of distortion. X-ray Diffraction was used to measure very near surface residual stresses up to a penetration depth of 5 microns. Blocks of 20X20X22 mm3 for each processing condition were used with measurements taken at the top surface center of the blocks. The very near surface stresses were higher with an increase in heat input, where high heat input specimens had average tensile residual stress in excess of 650 MPa while the low heat input specimens had average tensile residual stresses below 400 MPa. The Incremental hole drilling technique was utilised to measure the stresses in the blocks up to a depth of 1 mm from the top surface. Holes were drilled at the top surface center of each block. The stress distribution for both the high heat input specimens and the low heat specimens increased from 0.2 mm to a similar range of 500-600 MPa between 0.3 mm to 0.8 mm depth. Preheating at 200 °C yielded the same amount of stress. The microstructural analysis involved imaging from Optical Microscopy, Scanning Electron Microscopy and Electron Backscattered Diffraction. This combination of techniques confirmed a martensitic microstructure morphology of α' laths within prior β grains for all the specimens. The α' laths were arranged in the form of basket-weaves as well as colonies. The high heat input specimen prior β grains were columnar having grown across several layers in the build direction. For the low heat input specimens both with no preheating and with 200°C preheating, the prior β grains were atypically discontinuous. A hexagonal-titanium phase was identified in all the specimens as the dominant phase, with essentially no presence of the cubic phase. Dog-bone tensile specimens built in the z-direction (build direction) were used to test for static mechanical properties. The Yield Strength and the Ultimate Tensile Strength were above 1000 MPa and 1200 MPa respectively for all specimens. The average elongation of 11.2% in the low heat input specimen with no preheating was significantly higher than the 4.3% achieved by the high heat input specimen. The effect of the observed micro porosity under the microscope is thought to have contributed to this behaviour. Compact tension specimens for fracture toughness and fatigue crack growth rate testing were built in the ZX direction as per ASTM E399-17 labelling. The high heat input specimens had an average fracture toughness of 43 MPa√m compared to the less than 38 MPa√m achieved by the low heat input specimens. The high heat input specimens also had a better crack growth resistance than the low heat input specimens. The low heat input specimens without preheating had better crack initiation resistance. The results show that an increase in heat input does not have a substantial effect on the integrity and quality of parts. In fact, it produces comparable results to commercial SLM processing deployed in this study with respect to the properties studied, with the exception of a lower ductility. This brings about even more confidence on the advantage of high-speed processing. Future work should include testing at other orientations as well as testing higher preheating temperatures.
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    Open Access
    A study on the response of a target plate to a foreign object placed at various depths in a cylindrical Charge
    (2023) Hoare, Matthew; Chung, Kim Yuen Steeve; Govender Reuben Ashley
    The threat of Improvised Explosive Devices (IEDs) has grown exponentially in the 21st century, as the methods and means of warfare have adapted to modern threats such as terrorism. IEDs are especially damaging and lethal because they are often randomly embedded with a variety of projectiles that consist of readily available items, such as ball bearings, nails or glass. The versatile nature of IEDs makes it very difficult to conduct a generalised study on their impact. One major challenge in IED research is the wide range of potential IED geometries, sizes, explosion types and embedded object configurations. Understanding the behaviour of a simplified IED, consisting of a blast-driven ball bearings embedded in explosive charges, will provide insights into the mechanics of IEDs and its subsequent interactions with a target with a view to developing better protection from IEDs. This dissertation presents the results of a study investigating the damage caused by a simplified IED which consists of a cylindrical explosive charge that was embedded with a single ball bearing. The influence of the placement of a ball bearing along the axis within a rear-detonated cylindrical charge was studied and the placement effects were evaluated in terms of the impact velocity of the ball bearing and its subsequent damage on a Domex 700 steel (also referred to as Strenx in Europe) target plate. Typical deformation of a structure from an IED is a result from of blast loading (pressure wave) and impact loading from the shrapnel/fragment. In this study the combined blast and impact events of a simplified IED were decoupled into separate events to gain a better understanding of the contributions of the different loading conditions. The target plates were exposed to bare charges to quantify the effects on blast loading events. Impact tests were carried out using a two-stage gas gun to relate impact velocity to the deflection of the target plate. Tests were also carried out with explosive charges embedded with a ball bearing at varying depths to analyse the combined event. For all blast tests, the charge diameter was kept constant. Three different charge masses with varying placements of the ball bearing were investigated. Computational simulations, validated using experimental data, were used to elucidate additional details to gain insight about the momentum transfer during the blast event. The results showed similar critical influence of the placement of the ball bearing relative to the charge for the different charge masses used.
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    Open Access
    A study on the utilisation of small-scale hydropower for rural electrification in Malawi
    (2007) Taulo, John Loti; Bennett, Kevin
    This study explores the potential of utilising small-scale hydropower as an alternative source of rural electrification in Malawi. Approximately 7.5% of the country's population has access to electricity. In rural areas, the electricity access rate is 0.8%. A number of factors such as the cost of infrastructure, dispersed nature of the population, low consumption and poor load factors have prevented the majority of the rural population from getting connected to the national electricity grid. The study seeks to answer a key question such as ''what is the potential of utilising smallscale hydropower to increase the lectrification level and reduce green house emissions in Malawi?" An ex ante study has been conducted in Nkolokosa village, to estimate the energy consumption, income levels and willingness to pay. Using the relationship between number of households in that village and estimated peak demand, a regression model to forecast future electricity demands has been derived. The capital cost of such a rural electrification project, unit cost of generation, and unit cost of electricity to the user are analysed. Emission reductions for green house gases have been estimated. A preliminary economic analysis of the cost of supplying power to the village has been presented. The study results indicate that small-scale hydropower would be a favourable option. The results show that providing 85kWh/year of electricity per household increases the national electrification level by 2.3%. About 230,000 people in rural areas would have access to electricity. A typical 40 kW small-scale hydropower plant would serve about 110 households. This corresponds to about one fifth of total number of households found in a typical village in Malawi. The specific construction cost per kW is MWK 2,216,992.86 (US$15,835.66). The net present value was found to be MWK 4,490,389.08 with an 8% discount rate, and the economic rate of return (EIRR) was 5.98%. The benefit cost ratio was 6.25. The break- even tariff is MWK 44.80 per kWh. The average willingness to pay is MWK 5.58/kWh. The project would deliver 192,337 kWh/year of electricity and result in avoided emissions of 59.62 tons CO2/year. The results show that significant increase in electrification using small-scale hydropower alone is not achievable. However, the evaluation seems quite promising and, with some refined costing at each potential site, small-scale hydropower projects should be implemented.