Electronic Theses and Dissertations (PhDs)

Permanent URI for this collectionhttps://hdl.handle.net/10539/37973

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    Bodies of cunicularity in supersonic flow
    (University of the Witwatersrand, Johannesburg, 2024) Myburgh, Sabrina Gabrielle; Law , Craig
    Prior studies outlined the success of using randomised High Porosity Cellular Material cyl- inders both numerically and experimentally, however showed limited applicability to real-life aerodynamics applications. The primary effect of supersonic flow over porous media is to attenuate the bow wave by reducing the angle of incidence and redistributing the flow field, significantly reducing wave drag. The aim of this work was to investigate the drag-reduction effect of organised porosity within conic bodies of revolution on the supersonic flow field. Various organised porous cone-cylinders were developed to investigate the effect of the porous structure on the flow field, including shock waves, momentum changes and flow structures in and around the por- ous body. Several conic profiles were investigated. A numerical and experimental investigation was carried out in steady, supersonic flow at Mach 3.5 (Re = 3.9 × 10−5). The conic models had a porosity of ≈ 60%. The blow down supersonic wind tunnel facility at the University of the Witwatersrand was used for the exper- imentation, where schlieren photographs were captured along with drag force measurements of the solid and porous baseline cones. Numerical CFD simulations were carried out for a wider range of porous configurations using ANSYS Fluent R22.2. The numerical, experi- mental and literature Cd and flow visualisation were used to validate the numerical method, showing good agreement across data sets. Certain cunicular arrangements effectively reduced the drag in some shapes, while others worsened the drag. The greatest drag reduction of 40% was achieved in the cunicular BS4 Ellipsoid. The change in drag was associated with three sources, namely wave drag, jet v interaction, and momentum addition by the jet. These changes resulted from the flow inter- actions occurring within the inlet, internal and venting structures. Changes in drag depended on several factors, including vent geometry, expansion ratio, exit angle, inlet geometry and the crossflow condition, to name a few. Wave drag reduction was dependent on shock at- tenuation and reducing the wave angle, which was generally ensured by the conic surface pores absorbing energy from the bow wave. The jet interaction and momentum recovery were dependent on vent expansion ratio, exit angle and crossflow condition. The jet interaction had the greatest increase on the drag of the system, however, the effects of this were mitigated by using low vent angles with correct expansion ratios to maximise momentum recovery from the jet. The effect of the combined system was specific to the particular case, where the solid cone shape greatly influenced the overall performance. The effect on drag reduction was cumulative, sensitive and highly dependent on the individual case. The cunicular system is a viable drag reduction mechanism in conic bodies, however careful balancing of the porous inflow, outflow, bow wave and venting conditions is required in order to achieve significant drag reduction.
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    Development of an enterprise engineering strategy execution framework
    (University of the Witwatersrand, Johannesburg, 2024) Mudavanhu, Thabani B.; Emwanu, Bruno
    Even with a myriad of implementation models from consulting firms and academia – the success rate of Strategy Execution (SE) remains low. Extant literature in strategy execution exposes inadequacies of theoretical foundations for the assertions outlined in current SE models. In addition to this, where there is some explanation, the theory is inconsistent, and discipline biased which limits development of general application theory. Further to this ‘enterprises’ as the housing (the system of interest) from which a strategy is launched rarely receive the level of attention and rigor that technical systems do. Against this background, this study sought to understand how the success rate of SE can be improved through the application of Enterprise Engineering (EE) principles and practices. The premise being that the challenges in implementing strategies in any organisation are too an extent related to the design of the enterprise. Consequently, the common challenges attributed to the failure in SE can be linked to the enterprise design and as such organisations can, to an extent be designed to influence SE. Considering the complexity / greyness of the study area and limited literature in the relatively new discipline of EE, specifically enterprise ontology theory (the theoretical lens of this study), a structured literature review was used as the basis for a Delphi study. A two (2) round Delphi study was conducted with experts in the field to determine and validate the critical dimensions in Strategy Execution. Thirty-one (31) and twenty (25) experts participated in Delphi Round 1 and 2 respectively. The experts came from four regions of the world and were largely academics, board members and executive leaders and practitioners many of them tasked with either overseeing or leading strategy execution. The study revealed that there is a significant relationship between the design of an enterprise at the deepest level (the ontological layer) and the seven (7) aggregated themes that were synthesised in this study and are linked to constraining successful SE – (a) the strategy itself, (b) leadership, (c) people (the team); (d) effective communication; (e) organisational capabilities (f) organisational enablers and (g) organisational culture. The study proposed a generic enterprise engineering-based strategy execution (EEbSE) framework anchored in the deepest layer of an organisation, the ontological layer - the level where companies transact (cooperate and enter into agreements). Consequently, the study confirmed the proposition that ‘organisations can, to some degree, be designed or re-engineered for strategy execution’. This study demonstrates how EE can be useful in aiding Successful SE. An example of a key take-away include the need to check for execution readiness at the ontological layer and v eliminating any construction flaws (errors) that will later reflect as ‘common’ challenges. For example, lack of commitment [people issues] and lack of an implementation [culture issues] are flaws associated with SE that can be traced and re-engineered at the ontological level. This study adds to on-going work to confront the SE challenge and demonstrates the relevancy and pervasiveness of the application of EE.
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    Effects of Ni-Mo binder and laser surface engineering of NbC based cutting inserts during face-milling of automotive grey cast iron
    (University of the Witwatersrand, Johannesburg, 2024) Rabothata, Mahlatse Solomon; Genga, Rodney; Polese, Claudia
    The main aim of this study was to design, develop and produce NbC-Ni cermet based cutting inserts as potential substitutes for conventional WC-Co based inserts for the face-milling machining of automotive grey cast iron (a-GCI), an alloy that plays a critical role in the automotive manufacturing industry. For this purpose, rapid pulsed electric current sintering (PECS), additions of Mo as a partial binder replacement and TiC and TiC 7 N3 as secondary hard phases, and femtosecond laser surface modification (LSM) technique were used in an effort to enhance the NbC-Ni based cutting inserts’ machining capabilities during face-milling of a- GCI. All the sintered samples achieved relative densities of 97% and above, irrespective of the sintering process. Adding Mo, TiC and TiC 7 N3 to the NbC-12Ni (wt%) composition refined the NbC grain size in PECS samples, enhancing hardness and wear resistance. Mechanical impact shock and wear resistance of inserts were further improved via femtosecond LSM, creating pyramid (P) LSM and shark skin (S) LSM based micro-patterns on the surface of the cutting inserts. Face milling machining tests of a-GCI were performed at 200-400 m/min cutting speed (𝑉𝑉𝑐𝑐) and 0.25-1.0 mm depth of cut (ap ). The inserts’ cutting-edge wear and failure were evaluated after every pass using optical microscopy and analysed via high angular annular dark field (HAADF)-scanning electron transmission microscopy (STEM). Machining performance was assessed by technique for order of preference by similarity to ideal solution (TOPSIS) based model using insert tool life (𝐼𝐼𝑙𝑙), specific cutting energy (𝑈𝑈𝑐𝑐) and maximum resultant cutting forces (Fmax ) as criteria and including surface roughness (Ra) during finishing operations. The pyramid LSM PECS NbC-10TiC-12N[Ni/Mo] (wt%) (R2MS-P1) insert was the top performer during semi-finishing, with 20 min 𝐼𝐼𝑙𝑙 , 22 J/mm 3 𝑈𝑈𝑐𝑐 and 1087 N Fmax , obtaining an overall preference score (𝑂𝑂𝑖𝑖) of 0.953. The best inserts during finishing 2 were the blank (B) (i.e. unmodified cutting edge) PECS NbC-10TiC 7 N3 -12Ni (wt.%) (TCN1S-B3) and LPS WC-10TiC-10[Co/Mo] (wt.%) (T1ML-B3) inserts with both inserts obtaining 𝑂𝑂𝑖𝑖s of 0.826, respectively. In general, additions of Mo, TiC, TiC7 N3 , PECS and LSM improved hardness and abrasion wear resistance, resulting in enhanced performance of NbC-Ni based cutting inserts during machining.