Research Articles (Mechanical and Mechatronic Engineering)

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An algorithm for fast optimal Latin hypercube design of experiments
(Wiley-Blackwell, 2010-04) Viana, Felipe A. C.; Venter, Gerhard; Balabanov, Vladimir
This paper presents the translational propagation algorithm, a new method for obtaining optimal or near optimal Latin hypercube designs (LHDs) without using formal optimization. The procedure requires minimal computational effort with results virtually provided in real time. The algorithm exploits patterns of point locations for optimal LHDs based on the ɸp criterion (a variation of the maximum distance criterion). Small building blocks, consisting of one or more points each, are used to recreate these patterns by simple translation in the hyperspace. Monte Carlo simulations were used to evaluate the performance of the new algorithm for different design configurations where both the dimensionality and the point density were studied. The proposed algorithm was also compared against three formal optimization approaches (namely random search, genetic algorithm, and enhanced stochastic evolutionary algorithm). It was found that (i) the distribution of the ɸp values tends to lower values as the dimensionality is increased and (ii) the proposed translational propagation algorithm represents a computationally attractive strategy to obtain near optimum LHDs up to medium dimensions.
Combined effect of buoyancy force and Navier slip on MHD flow of a nanofluid over a convectively heated vertical porous plate
(Hindawi, 2013) Mutuku-Njane, Winifred Nduku; Makinde, Oluwole Daniel
We examine the effect of magnetic field on boundary layer flow of an incompressible electrically conducting water-based nanofluids past a convectively heated vertical porous plate with Navier slip boundary condition. A suitable similarity transformation is employed to reduce the governing partial differential equations into nonlinear ordinary differential equations, which are solved numerically by employing fourth-order Runge-Kutta with a shooting technique. Three different water-based nanofluids containing copper (Cu), aluminium oxide (Al2O3), and titanium dioxide (TiO2) are taken into consideration. Graphical results are presented and discussed quantitatively with respect to the influence of pertinent parameters, such as solid volume fraction of nanoparticles (), magnetic field parameter (Ha), buoyancy effect (Gr), Eckert number (Ec), suction/injection parameter (), Biot number (Bi), and slip parameter (β), on the dimensionless velocity, temperature, skin friction coefficient, and heat transfer rate.
Computational fluid dynamic modelling of a stealthy air inlet
(Science Africa for the South African Institution of Mechanical Engineers, 2004) Thiart, G. D.; Marais, A. L.
The aerodynamic performance of a stealthy air inlet design was evaluated using commercial computational fluid dynamics software. The simulations showed that the aerodynamic performance of this type of inlet is adversely affected by the constraints of a stealthy design. Flow separation ahead of the inlet capture plane was identified as one of the major problem areas. It is demonstrated that by modifying the base configuration to alleviate the flow separation problem, it is possible to improve the aerodynamic performance of the inlet. The two designs are compared quantitatively in terms of the kinetic energy correction factor at the engine face, which is a measure of the uniformity of the flow into the engine; significant reduction of this factor is seen to be possible, albeit only in theory.
Constrained particle swarm optimization using a bi-objective formulation
(Springer Verlag, 2010) Venter, Gerhard; Haftka, R. T.
This paper introduces an approach for dealing with constraints when using particle swarm optimization. The constrained, single objective optimization problem is converted into an unconstrained, bi-objective optimization problem that is solved using a multi-objective implementation of the particle swarm optimization algorithm. A specialized bi-objective particle swarm optimization algorithm is presented and an engineering example problem is used to illustrate the performance of the algorithm. An additional set of 13 test problems from the literature is used to further validate the performance of the newly proposed algorithm. For the example problems considered here, the proposed algorithm produced promising results, indicating that it is an approach that deserves further consideration. The newly proposed algorithm provides performance similar to that of a tuned penalty function approach, without having to tune any penalty parameters.
Cooling tower performance : a critical evaluation of the Merkel assumptions
(South African Institution of Mechanical Engineering, 2004) Kloppers, J. C.; Kroger, D. G.
The simpltfying assamptions made by Merkel ure critically evaluated by comparing the Merkel analysis to the more rigoroas Poppe analysis of cooling tower performnnce. It is shown that the uccaracy of the Merkel method can be greatly improved, under certain cooling tower operating conditions, to predict cooling tower performance within very close tolerance of the performance predicted by the Poppe method, It is shown under which tower operating conditions the thermal tower performance, according to the Merkel method, is likely to diffe, from the performance predicted by the Poppe method.
Creep damage assessment of ex-service 12% Cr power plant steel using digital image correlation and quantitative microstructural evaluation
(MDPI, 2019) Van Rooyen, Melody; Becker, Thorsten; Westraadt, Johan; Marx, Geneveve
ENGLISH ABSTRACT: The lifetime of steam pipelines in long-term operation in coal-fired power plants are limited due to material damage that resulted from creep exposure. In the present study, the authors comparatively assess the damage of ex-service 12% Cr piping steel with varying degrees of exposure while using accelerated creep tests that employ digital image correlation (DIC) as well as microstructural investigation that is based on electron microscopy. The DIC technique, which allows multiple creep curves to be measured at temperatures ranging from 550–600 °C from a single specimen, revealed higher Zener–Hollomon parameters for a high damage material with a high void density when compared to a material with lower damage and lower void density. Both of the material states showed similar hardness values, subgrain sizes, and boundary character, despite the difference in void densities. Slightly higher inter-particle spacing of MX precipitates results in a lower threshold stress of 79 MPa for the high damage steel when compared to 97 MPa for the low damage material. Besides large Laves phase particles (>0.2 µm) that are found in the higher damaged materials that result in solid solution depletion, the most prominent microstructural damage indicator was a lower density of M₂₃C₆ precipitates. Therefore, the observations indicate that the Zener–Hollomon parameter and M₂₃C₆ particles are good damage assessment indicators between the most extreme damage states and they predict a lower damage level for a medium void density material.
Data for indirect load case estimation of ice-induced moments from shaft line torque measurements
(Elsevier, 2018) De Waal, R. J. O.; Bekker, A.; Heyns, P. S.
ENGLISH ABSTRACT: During ice navigation, blade measurements of ice-induced moments on ship propellers, are challenged by the harsh operating environment. To overcome this problem, shaft line measurements are performed inboard, and the required propeller loads are subsequently estimated using a dynamic model and the solution of an inverse problem. The inverse problem is mathematically ill-posed and requires the determination of the ice-induced moment on the propeller blades from shaft line measurements. Full-scale torsional response data is presented as calculated from indirect strain measurements on the shaft line of a polar supply and research vessel. The vessel operated on a 68-day voyage between Cape Town and Antarctica and spent almost 11 days in sea ice with observed concentrations above 90% and a maximum thickness of 3 m. Data for five ice-induced load cases are presented, including the shaft torque from indirect measurements and the estimated ice-induced moment, which is obtained by solving an ill-posed inverse problem. The ice-induced moments on the propeller are obtained by approximating the drive-train as a viscously damped, elastic lumped mass model. The ice-induced moment is then determined through existing approaches to solving the ill-conditioned inverse problem. The lumped mass model is presented along with algorithms to solve the inverse problem, including truncated singular value decomposition, truncated generalized singular value decomposition and Tikhonov׳s method. The resulting time series data for the inversely calculated ice-induced moments is published to provide industry with load cases for ice-going propulsion design.
Deflection of Ag-atoms in a magnetic field
(2011) Kheswa, Bonginkosi Vincent; Dobson, Robert Thomas
ENGLISH ABSTRACT: An apparatus that generates and deflect silver-109 atoms with an inhomogeneous and homogeneous magnetic field was constructed. The experimental results found using this apparatus were used to make a recommendation, to the idea of removing silver-110 atoms from the helium fluid of Pebble Bed Modular Reactor (PBMR) with an inhomogeneous magnetic field based on the Stern-Gerlach principle. It is shown that experimental results corresponded well with the theoretical predictions. The apparatus was also used for a practical for a course for mechanical engineers in modern physics.
Design analysis methods for Stirling engines
(Energy Research Centre, University of Cape Town, 2008) Snyman, H.; Harms, T. M.; Strauss, J. M.
Worldwide attempts are being made to increase the use of our renewable energy sources as well as to use our current fossil fuel energy sources more efficiently. Waste heat recovery forms a substantial part of the latter and is the focus of this project. Stirling technology finds application in both the renewable energy sector and in waste heat recovery. Investigating the applicability of Stirling engines in the above-mentioned fields is relevant to develop more efficient external combustion units as well as to utilize our renewable energy sources. Developing a design analysis and synthesis tool capable of optimizing Stirling powered units forms the main objective of this project. The methodology followed to achieve this, involved the application of three different methods of analysis, namely the method of Schmidt, the adiabatic analysis and the simple analysis based on a five volume approach. The Schmidt analysis is used to obtain the internal engine pressure which is a required input for the adiabatic analysis while the simple analysis introduces pumping losses and regenerator inefficiencies. These methodologies are discussed briefly in this paper. Experimental verification of the analytical data was carried out on a Heinrici Stirling engine and both the analytical data and the experimental data are presented here. Shortcomings of these methods of analysis are highlighted and an alternative approach to solve particular shortcomings is presented.
The design and testing of a secure surface tension driven water pump
(2015) Fraser, J.; Dobson, Robert Thomas
ENGLISH ABSTRACT: Water pumping is essential for life as we know it. The drive for alternative energies together with the need for water pumping has inspired the construction of a surface tension driven water pump (STDWP). The goal of this study was to construct and test a passive water pump inspired by tree water transport mechanisms. Here various experimental tests were done to determine the STDWP’s performance and behaviour under various environmental conditions and pumping heads. Theoretical mass transfer relations were used to model the evaporative rate from the “leaf” surface and compared to the experimentally measured results. It was found that the developed pump could pump water at a rate of nearly 400 mL/hr.m2, reaching pumping heads of 1.8 m, with a maximum functional lifespan of 13 days. The STDWP’s water collection efficiency was found to be 98% on average. The mechanistic causes of pump failure are addressed with recommendations for the STDWP improvement.
The development of a 1D-transient simulation model of a CO2 refrigeration system
(2016) Garces de Gois, M.; Dobson, Robert Thomas
ENGLISH ABSTRACT: Refrigeration is a highly essential part of modern day living. The drive for alternative cleaner technologies has renewed interest in designing refrigeration systems using CO2 as refrigerant. The goal of this project was to develop a numerical transient simulation model of a CO2 refrigeration cycle containing a capillary tube as expansion device. This simulation model can then be used to design CO2 refrigerators and provide insight into their operation. In this paper, the transient conservation equations are developed into forms that can be solved on a computer program. An algorithm for solving compressible flow equations is discussed. Lastly the use of the real gas equation of state for CO2 from Span and Wagner (1996) is discussed and methods are developed to calculate single phase and two-phase properties. It was found that the simulation model predicted evaporator temperature and phase-change processes with reasonable accuracy.
Development of a vibration-absorbing handle for rock drills
(Southern African Institute of Mining and Metallurgy, 2002) Strydom, J. P. D.; Heyns, P. S.; Van Niekerk, J. L.
The transmission of excessive vibration to the human body can cause physical stress that may result in crippling pain or permanent disability. Rock drills are known to transmit high levels of vibration that are difficult to attenuate through conventional approaches to vibration isolation. However, it can be shown that once the ISO frequency weighting has been applied, most of the vibratory effect of the rock drillï¿½s motion is concentrated at the drillï¿½s operating frequency. This observation opens the way for utilizing a concept known as vibration absorption, aimed at attenuating the rock drillï¿½s vibration. In essence this entails using the inertia of a secondary vibratory system to counteract the motion of the handle. This article documents an investigation into the use of such an absorber for attenuating rock drill vibration and demonstrates the feasibility of such an approach. A mathematical model of such an absorbing handle was developed and used for design studies. Based on these investigations an experimental prototype model was manufactured and tested. A significant reduction in vibration levels could be achieved.
Developoment of a magnetic intra-uterine manipulator
(South African Institution of Mechanical Engineering, 2013) Doll, S.; Scheffer, C.; Du Toit, G.
This paper introduces the concept of a novel magnetic intra-uterine manipulator, intended to overcome conventional medical devices’ shortcomings, and enabling non-invasive uterine manipulation during surgery. However, analyses have shown that the magnetic manipulator is unable to compete in terms of the range of motion of the existing devices. A limited anterior sagittal rotation range of 60° was observed in a magnetic manipulator compared to a range of 140° for conventional devices. Despite these limitations, use of a magnetic manipulator could eliminate the need for an additional medical assistant during surgery; it is also reusable and thus also more economical. The second goal of the research was to investigate which type of setup would be most successful in effective uterine manipulation. Through concept analysis, a cart-onarch system was deemed most effective. To lift an effective load of 1 N over an air gap of 150 mm, rareearth N38 neodymium (NdFeBr) magnets showed the most promise as magnetic actuators for the manipulator. Finite Element Analysis (FEA) simulations of the magnetic set-up were validated experimentally and produced an acceptable Mean Absolute Error (MAE) of 0.15 N.
The economic reality of home PV systems : matching consumption to generation
(SAIEE, 2018) Roux, J. K.; Harms, T. M.
ENGLISH ABSTRACT: The aim of this paper is to provide an economic viewpoint of the benefit of the installation of a home Photo Voltaic system. An hourly daily consumption and generation profile is created for every calendar month and the cost saving is applied to calculate the IRR and payback period. Different systems options are evaluated as well as feed in tariff options. The study was performed for two sample homes located in South Africa. The result suggests the optimum size for the PV system needs to be matched to the consumption in the peak generation month such that no excess power is generated. Therefore, home owners should not base the value of a system on the generation potential thereof but rather the savings potential matched to the consumption profile.
Effects of defects on mechanical properties in metal additive manufacturing : a review focusing on X-ray tomography insights
(Elsevier, 2019) Du Plessis, Anton; Yadroitsava, Ina; Yadroitsev, Igor
ENGLISH ABSTRACT: X-ray tomography has emerged as a uniquely powerful and non-destructive tool to analyze defects in additive manufacturing. Defects include unintended porosity, rough surfaces and deviations from design, which can have different root causes and can vary significantly among samples. Powder material properties, non-uniform delivery of the powder layer, deformation during manufacturing, deviations from optimal process-parameters caused by changes in the laser beam, the optical components and the scanning system operation, may result in lack of fusion pores, metallurgical pores, keyhole pores, etc. These different types of pores have different typical sizes, shapes and 3D distributions. All types of defects have effects on the mechanical properties of a final part. The use of X-ray tomography to visualize pores in parts (non-destructively) prior to mechanical testing has allowed us to improve our understanding of the effect of this porosity on the mechanical properties of the part (also referred to as “effect of defect”). This can provide the possibility to discriminate critical defects from harmless ones, and thereby build confidence in additivemanufacturing processes. This paper reviews the current state of knowledge with regard to the “effect of defect” in metal additivemanufacturing, and highlights some relevant examples from our recent work.
Electric vehicle sound stimuli data and enhancements
(Elsevier, 2018) Swart, D. J.; Bekker, A.; Bienert, J.
Data for six electric vehicle WOT interior sound measurements and eight enhanced sound signatures are presented. The measurement of electric vehicle interior sound signature data and the enhancement of these stimuli are documented in this data article. The procedures and equipment that were used to record the data, as well as the transposition, harmony and order addition, frequency filtering and modulation enhancement techniques that were applied to these stimuli are explained in detail. The transient frequency content of the 12 sound stimuli is presented in acoustic spectrograms along with the audio files in.mp3 format.
Energy supply in Malawi : options and issues
(Energy Research Centre, 2015) Taulo, John L.; Gondwe, Kenneth Joseph; Sebitosi, Adoniya Ben
Inadequate energy supply is one of the major problems confronting Malawi and limiting its social, economic and industrial development. This paper reviews the current status of energy supply and demand in Malawi; examines the major sources of energy, current exploitation status and their potential contribution to the electricity supply of the country; discusses key issues facing the energy sector; and identifies broad strategies to be implemented to tackle the energy supply challenges. Using secondary data for its critical analysis, the paper also presents modelling of long-term energy demand forecast in the economic sectors of Malawi using the Model for Analysis of Energy Demand (MAED) for a study period from 2008-2030. Three scenarios namely reference (REF), moderate growth (MGS) and accelerated growth (AGS) were formulated to simulate possible future long-term energy demand based on socio-economic and technological development with the base year of 2008. Results from all scenarios suggest an increased energy demand in consuming sectors with biomass being a dominant energy form in household and industry sectors in the study period. Forecast results reveal that energy demand will increase at an annual growth rate of 1.2% and reach 5160 ktoe in 2030 under REF scenario. The growth rates for MGS and AGS are projected at 1.5% each reaching 4639 ktoe and 5974 ktoe in 2030, respectively. The final electricity demand of about 105 ktoe in the base year will grow annually at average rates of 13.8%, 15.3% and 12.6% for REF, AGS and MGS, respectively. Over the study period 2008-2030 the annual electricity per capita will increase from about 111 kWh to 1062, 1418 and 844 kWh for the REF, AGS and MGS, respectively. The final energy intensity will decrease continuously from about 13.71 kWh/US$ in the base year to 3.88 kWh/US$, 2.98 kWh/US$ and 5.27 kWh/US$ for the REF, AGS and MGS, respectively in the year 2030. In conclusion, the paper outlines strategies that could be utilized to ensure adequate supply of modern energy which is a key ingredient for achieving sustainable social and economic growth.
Evaluating the displacement field of paperboardpackages subjected to compression loading using digital image correlation (DIC)
(Elsevier, 2020-09) Fadiji, Tobi; Coetzee, Corne J.; Opara, Umezuruike Linus
Digital image correlation (DIC) is a full-field non-contact optical technique for measuring displacements in experimental testing based on correlating several digital images taken during the test, particularly images before and after deformation. Application of DIC cuts across several fields, particularly in experimental solid mechanics; however, its potential application to paperboard packaging has not been fully explored. To preserve fresh horticultural produce during postharvest handling, it is crucial to understand how the packages deform under mechanical loading. In this study, 3D digital image correlation with two cameras and stereovision was used to determine the full-field displacement of corrugated paperboard packaging subjected to compression loading. Strain fields were derived from the displacement fields. Results obtained from the displacement fields showed the initiation and development of the buckling behaviour of the carton panels. The displacement was observed to be largely heterogeneous. The displacement field in the horizontal direction was smaller compared to that of vertical and out-of-plane directions. In addition, the strain variation increased as load increased, which could be a precursor to material failure. The technique proved to be efficient in providing relevant information on the displacement and strain fields at the surface panels of corrugated paperboard packages used for handling horticultural produce. In addition, it offers prospects for improved mechanical design of fresh produce packaging.
Experimental evaluation of a Ranque-Hilsch vortex tube as a particle separator
(South African Institution of Mechanical Engineering, 2012) Burger, A.; Dobson, R. T.
The Pebble Bed Modular Reactor (PBMR) is a Generation IV graphite-moderated helium cooled nuclear reactor developed in South Africa from the German Arbeitsgemeinschaft Versuchreaktor (AVR). After decommissioning of the AVR plant, radioactive isotopes of silver 44Ag110 as well as graphite particles were found in the primary helium coolant loop of the reactor. Hence the main objective of this work was to evaluate the efficiency of the Ranque-Hilsch vortex tube (RHVT) as a separation device for removing graphite particles from a helium coolant stream. This objective was accomplished by designing and building an experimental test apparatus and measuring the particle separation efficiency of the RHVT under different operating conditions. It was found that the RHVT is a very efficient particle separator, which may, however, not easily be incorporated into the PBMR system.
Experimental investigation and numerical simulation of instabilities in a multi-parallel channel two-phase natural circulation system
(2015) Sangweni, L. S.; Dobson, Robert Thomas
ENGLISH ABSTRACT: In the present study, two-phase natural circulation flow in a multi-parallel channel system is investigated using experimental and numerical modelling. The experimental model consisted, essentially, of four 25 mm diameter and two-meter long vertically orientated transparent polycarbonate pipes connected to a common manifold at the bottom and a relatively large steam drum at the top; three one-meter long electrical resistance heating elements were inserted into the lower end of three of the vertical pipes. Tests were conducted using different combinations of input power and as-defined and so-called open, closed and heat pipe system operating modes. A water-cooled condenser was placed in the upper portion of the steam drum and an expansion tank was connected to the lower manifold. For different power inputs and operating modes twelve temperatures and three flow rates as a function of time were recorded. In this way start-up transients and dynamical oscillatory responses were captured. So-called Type I instability were observed at low power inputs and open system operating modes (system open to the atmosphere). Type II instabilities and flashing instability were observed at medium and high-power excitations for the open system mode of operation. The fluid flow became more stable and less oscillatory at all power excitations for the closed system operation mode (system not open to the atmosphere). For the heat pipe mode of operation so-called geysering, followed by flashing-induced boiling was observed. After boiling had commenced downward single phase flow was invariable noted to occur in the central of the three hearted risers, even when all three heater power inputs were the same. Also, after boing had started a further increase in power input did not necessarily result in an increase in flow rate. The experimental system was discretized into a number of control volumes. The conservation equations, mass, momentum and energy were applied to each control volume and a set of time dependent temperature-coupled finite difference equations simulating the thermal-hydraulic behaviour of the system thus derived. This set of difference equations was solved using an explicit solution method. An encouragingly-good correspondence between the experimental and theoretical simulation model of the temperature and flow rate in the system was obtained.

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