E-pos ons


Master's degrees completed

Die volgende is 'n lys van meestersgraadverhandelings wat onlangs in hierdie Departement voltooi is. Die lys is gekoppel aan die Powerpoint-aanbiedings wat tydens die publieke verweer van hierdie verhandelings gebruik is.

Mr DS Agenbag 2008: "Longitudinal Handling Characteristics of a Tailless Gull-Wing Aircraft"

A handling quality investigation was performed on the swept gull-wing configuration. The swept gull-wing configuration is tailless and has a wing with a transition in the sweep and dihedral angle. An example of this type of aircraft is the Exulans. This aircraft is currently under development at the University of Pretoria. The handling quality study was focused on pitch axis dynamics. The Exulans is a research testbed that will be used to investigate the swept gull-wing configuration and its special controls by means of full-scale flight testing. Variable wing sweep, twisting elevons and winglets will be investigated as means of control. These control devices are configured in such a way as to have minimum impact on the performance of the aircraft. The handling qualities of the swept gull-wing configuration have to be acceptable while using these different control strategies.

The study was launched to investigate whether a gull-wing configuration aircraft will have satisfactory handling qualities at CG-positions associated with the most favourable aerodynamic performance. There is an aerodynamic performance gain in designing an aircraft so that the CG falls on the so-called 'E-point'. The E-point is the centre of pressure for an elliptical circulation distribution. An elliptical circulation distribution is associated with the highest Oswald efficiency for an aircraft.

Time domain simulation techniques and frequency domain analysis techniques were used to analyze the handling qualities of the gull-wing configuration. The C-star criterion was used to analyze handling qualities with time domain simulation data as input. Comparative time domain simulations were performed between the Exulans and other aircraft to compare handling qualities. Eigenvalue analysis was used together with the thumbprint criteria to investigate inherent gull-wing airframe dynamics. The Shomber-Gertsen and Military Specification 8785 criteria were also used for the same purpose. The Neal-Smith method was used to investigate the effect of control authority on handling qualities and the effect of a pilot. The Monnich and Dalldorff criterion was used to evaluate gust-handling qualities. An analysis chart by Fremaux and Vairo was used to evaluate the tumbling susceptibility of the gull-wing configuration.

The pitch handling quality investigation shows sufficient promise that the swept gull-wing configuration will have acceptable handling qualities with the CG placed at positions associated with optimized aerodynamic performance. Analysis showed that the swept gull-wing configuration is potentially prone to tumbling. With low static margins, the configuration should exhibit improved handling qualities in gusty conditions when compared to existing tailless aircraft.

It is recommended that a lateral handling quality study be performed before full-scale flight testing commences on the Exulans. In addition, the possibility of wingtip stall must be investigated for the case of the swept gull-wing configuration.

Supervisors: Prof NJ Theron and Mr RJ Huyssen.

Mr K Wang 2007: "Condition monitoring electrical machines using Vold-Kalman filter order tracking"

Conventional rotating machine vibration monitoring techniques are based on the assumption that changes in the measured structural response are caused by deterioration in the condition of the rotating machine. However, due to changing rotational speed, the measured signal may be non-stationary and difficult to interpret. For this reason, the order-tracking technique was introduced. One of main advantages of order tracking over traditional vibration monitoring techniques lies in its ability to clearly identify non-stationary vibration data, and to a large extent exclude the influences from varying rotational speed. This work investigates the application of the Vold-Kalman Filtered Order Tracking (VKF-OT) on electrical machinery.

Simulated single-degree-of-freedom and two-degree-of-freedom rotor models were established, and the application of the VKF-OT technique on these simulated models was explored. Because most of the current research draws significantly on an understanding of the VKF-OT theory, it was also necessary to review and summarize the current status of VKF-OT theory from previous work, as well as explore the procedures for selection of its filter bandwidth when dealing with real data. An experimental set-up for monitoring an electrical alternator was also constructed. Real experimental data were subsequently used to compare the advantages and disadvantages of the three popular order-tracking techniques. The unique time domain advantage of VKF-OT was implemented, using crest factor and kurtosis values as indicators of the fault condition of the machine. This gave encouraging results.

Mr B Eggers 2007: "Dragline Gear Monitoring under Fluctuating Conditions"

The aim of this study is to apply computed order tracking with subsequent rotation domain averaging and statistical analysis to typical mining environments. Computed order tracking is a fault detection method that is unaffected by varying speed conditions often found in industry and has been proven effective in laboratory conditions.

However, in the controlled environment of a laboratory it is difficult to test the robustness of the order-tracking procedure. The need thus exists to adjust the order-tracking procedure so that it will be effective in the mining environment. The procedure needs to be adjusted to function with a two-pulse-per-revolution speed input. The drag gear aboard a dragline rotates in two directions. This gives the unique opportunity to observe the performance of the order tracking method in a bi-directional rotating environment, allowing relationships between the results of each operating direction to be investigated.

A monitoring station was set up aboard a dragline and data was captured twice daily for a period spanning one year. The data captured consisted of accelerometer and proximity sensor data. The key on the shaft triggers the proximity sensors, allowing speed and direction to be measured. The rudimentary measured speed is interpolated using various documented speed interpolation techniques and by a newly-developed speed interpolation technique. The interpolated speed is then used to complete the order-tracking procedure that re-samples the vibration data with reference to the speed.

The results indicate that computed order tracking can be successfully implemented in typical mining environments. Furthermore, there is a distinct relationship between vibration data taken in both rotational directions: one direction provides a better indication of incipient failure. It is thus important not to choose a direction randomly when monitoring rotation machinery of this kind.

Study leaders: Professor PS Heyns and Dr C Stander

Mr C Visser  2007  "Modelling Heat and Mass Flow through Packed Pebble Beds: A Heterogeneous Volume-Averaged Approach"

This work details modelling buoyancy-driven viscous flow and heat transfer through heterogeneous, saturated, packed pebble beds via a set of volume-averaged conservation equations in which local thermal disequilibrium is accounted for. The latter refers to the two phases considered viz. solid and fluid, differing in temperature. This is effected by describing each phase with its own governing equation.  Further to the aforementioned, the governing equation set is written in terms of intrinsic volume-averaged material properties that are fully variant with respect to temperature. The heterogeneous solid phase is described with a porosity field varying from 0.39 to 0.99. The intent of the stated upper bound is to explicitly model typical packed-bed near-wall phenomena such as wall-channeling and pebble-wall heat transfer as true to reality as possible, while maintaining scientific rigour.  The set of coupled non-linear partial differential equations is solved via a locally preconditioned artificial compressibility method, where spatial discretisation is effected with a compact finite volume edge-based discretisation method.  The latter is done in the interest of accuracy.  Stabilisation is effected via JST scalar-valued artificial dissipation.  This is the first instance in which an artificial compressibility algorithm is applied to modelling heat and fluid flow through heterogeneous porous materials.  As a result of the aforementioned, calculation of the acoustic velocities, stabilization scaling factors and allowable time-step sizes were revised. The developed technology is demonstrated by application to the modelling of SANA-test cases, i.e. natural convective flow inside a heated porous axisymmetric cavity.  Predicted results are shown to be within 12% of experimental measurements in all cases, while having an average deviation of only 3%.

 Supervisors: Dr AG Malan and Prof Josua Meyer

Mr J J Gouws 2007: "Combining a One-Dimensional Empirical and Network Solver with Computational Fluid Dynamics to Investigate Possible Modifications to a Commercial Gas Turbine Combustor"

Gas turbine combustion chambers were traditionally designed through trial and error, which was unfortunately a time-consuming and expensive process. The development of computers, however, contributed a great deal to the development of combustion chambers, enabling one to model such
systems more accurately in less time. Traditionally, preliminary combustor designs were conducted with the use of one-dimensional codes to assist in the prediction of flow distributions and pressure losses across the combustion chamber, mainly due to their rapid execution times and ease of use. The results are generally used as boundary conditions in three-dimensional models to predict the internal flow field of the combustor. More recent studies solve the entire flow field from prediffuser to combustor exit. This approach is, however, a computationally expensive procedure and can only be used if adequate computer resources are available.

The purpose of this study is two-fold; (1) to develop a one-dimensional incompressible code, incorporating an empirical-based combustion model, to assist a one-dimensional network solver in predicting flow- and temperature distributions, as well as pressure losses. This is done due
to the lack of a combustion model in the network solver that was used. An incompressible solution of flow splits, pressure losses, and temperature distributions is also obtained and compared with the
compressible solution obtained by the network solver; (2) to utilize the data, obtained from the network solver, as boundary conditions to a three-dimensional numerical model to investigate possible modifications to the dome wall of a standard T56 combustion chamber. A numerical base
case model is validated against experimental exit temperature data, and based upon that comparison, the remaining numerical models are compared with the numerical base case. The effect of the modification on the dome wall temperature is therefore apparent when the modified numerical model
is compared with the numerical base case.

A second empirical code was developed to design the geometry of axial straight vane swirlers with different swirl angles. To maintain overall engine efficiency, the pressure loss that was determined from the network analysis, of the base case model, is used during the design of the different swirlers. The pressure loss across the modified combustion chamber will therefore remain similar to that of the original design. Hence, to maintain a constant pressure loss across the modified combustion chambers, the network solver is used to determine how many existing hole features should be closed for the pressure loss to remain similar. The hole features are closed, virtually, in such a manner as not to influence the equivalence ratio in each zone significantly, therefore maintaining combustion performance similar to that of the original design. Although the equivalence ratios in each combustion zone will be more or less unaffected, the addition of a swirler will influence the emission levels obtained from the system due to enhanced air-fuel mixing.

A purely numerical parametric analysis was conducted to investigate the influence of different swirler geometries on the dome wall temperature while maintaining an acceptable exit temperature distribution. The data is compared against the data obtained from an experimentally-validated
base case model. The investigation concerns the replacement of the existing splash-cooling devices on the dome wall with that of a single swirler. A number of swirler parameters such as blade angle, mass flow rate, and a number of blades were varied during the study, investigating its influence on the dome wall temperature distribution.

Results showed that the swirlers with approximately the same mass flow as the existing splash-cooling devices had almost no impact on the dome wall temperatures, but maintained the exit temperature profile. An investigation of swirlers with an increased mass flow rate was also done
and results showed that these swirlers had a better impact on the dome wall temperatures. However, due to the increased mass flow rate, stable combustion is not guaranteed since the air/fuel ratio in the primary combustion zone was altered.

The conclusion that was drawn from the study, was that by simply adding an axial air swirler might reduce high-temperature gradients on the dome but will not guarantee stable combustion during off-design operating conditions. Therefore, a complete new hole layout design might be
necessary to ensure good combustion performance across a wide operating range.

Supervisor: Mr M Morris 

Mr Luke Ronne:2007  "Design considerations and analysis of a bioreactor for application in a bio artificial liver support system".

Acute Liver Failure (ALF) is a devastating ailment with a high mortality rate and limited treatment alternatives. This study presents a methodology for the design and development of a bio-artificial bioreactor to be used in a bio-artificial liver support system. The system will ultimately be used either to bridge a patient to orthotopic liver transplant (OLT), the only current cure for end-stage ALF, or spontaneous recovery. Methods to optimize and visualize the flow and related mass transfer in the BR are presented. The use of magnetic resonance imaging (MRI), scanning electron microscope (SEM) and simple testing methodology is applied with emphasis on modeling the flow in the BR. The bioreactor (BR) used in the Bio-Artificial Liver Support System (BALSS), currently undergoing animal trials at the University of Pretoria, was modelled and simulated for the flow conditions in the device. Two different perfusion steps were modelled, including the seeding of hepatocyte cells and later the clinical perfusion step. It was found that the BR was not optimal with dead spots and regions of retarded flow. This would restrict the effective transport of nutrients and oxygen to the cells. The different perfusion rates for the seeding and clinical perfusion steps allowed for different velocity contours with cells seeing inconsistent flow patterns and mass transfer gradients. An optimized BR design is suggested and simulated, that effectively reduces the areas of retarded flow (dead spots) and increases the flow speed uniformly through the BR to an order of magnitude similar to that found in the sinusoidal range. The scaffolding volume was also decreased to allow a larger local cell density promoting cell-cell interaction. Finally a summarized design table for the design of a hepatic BR is presented.

Study Leader: Mr A J van Wyk

Mr Zhi Cheng Lai : 2007 "Finite element analysis of electrostatic coupled systems using geometrically nonlinear mixed assumed stress finite elements."

The micro-electromechanical systems (MEMS) industry has grown rapidly over the past few years. MEMS devices are widely used in various fields such as aerospace, microelectronics and the automobile industry. Increasing priority, largely driven by market demands, is
being given to the development and research of MEMS.

Multi-physics-coupled fields are often present in MEMS, which makes the modelling and analysis of these devices difficult and sometimes costly. Mr Lai studied a fundamental phenomena in MEMS, namely the coupling between electrostatic and mechanical fields. The following issues are addressed:

1. In order to capture the system configuration accurately, with relatively little computational effort, a geometric non-linear mixed assumed stress element is developed. It is shown that the accuracy of the developed element is comparable to that of the Q8-element, for practical problems containing singularities in the solution.

2.Selected algorithms for solving highly non-linear coupled systems are evaluated. It is concluded that Newton's algorithm performed the best. The computational cost of Newton's method is significantly reduced by using analytical gradients when computing gradients.

3.Finally, a practical engineering MEMS problem is studied. The developed geometric nonlinear mixed assumed stress element is used to model the structural part of a fixed-fixed beam that experiences large axial stress due to an applied electrostatic force.

Study leaders: Professors S Kok and A A Groenwold

Mr M Herzog : 2007 "Machine and Component Residual Life Estimation through the Application of Neural Networks"

Analysis of reliability data plays an important role in the maintenance decision-making process. The accurate estimation of residual life in components and systems can be a great asset when planning the preventive replacement of components on machines. Artificial intelligence is a field that has rapidly developed over the last 20 years and practical applications have been found in many diverse areas. The use of such methods in the maintenance field have, however, not yet been fully explored.

This work investigates the use of artificial neural networks as an alternative to the various regression models, such as the proportional hazards model, for this purpose, using two types of data sets.

The advantage of using neural networks for predicting residual life was clearly illustrated when comparing their performance to the results achieved through the use of the traditional statistical methods. The potential of using neural networks for residual life prediction was therefore illustrated in both cases.

Study leaders: Professors PS Heyns and T Marwala

Mr E van Rooyen : Time-fractional Analysis of Flow Patterns during Refrigerant Condensation"

Mr Van Rooyen developed a novel technique with which to objectively predict the prevailing flow pattern during two-phase flow (specifically during refrigerant condensation in a horizontally smooth tube). The technique relies on edge detection and analysis of high-speed videography images of the prevailing two-phase flow pattern. A probabilistic time fraction factor is developed from this analysis, which design engineers can use to correct correlations for heat transfer coefficients and pressure drops. This technique has not been demonstrated before in the available technical literature. This new technique has been validated with spectral analyses of pressure drop and void fraction data during two-phase flow, and complements and enhances the widely-used Thome flow pattern prediction map.
Supervisor: Prof Leon Liebenberg
Co-supervisor: Prof Josua Meyer

M Christians : Flow Pattern-based Heat Transfer and Pressure-drop Correlations for Condensing Refrigerants in Smooth Tubes

Mr Christians developed flow-pattern based heat transfer and pressure-drop correlations during refrigerant condensation in horizontally smooth tubes, employing the time-frequency corrections developed by the previous candidate. The new correlations are of a generalized nature and capture the physics of the prevailing flow phenomena, specifically in the stochastic intermittent flow regime, to such an extent that data are correlated to within 10% accuracy that exceeds the accepted norm in the international heat transfer engineering community by nearly 15%. The candidate also employed a new laminar-turbulent flow Wilson Plot technique to deduce the heat transfer coefficients of an inner tube (of a co-axial heat exchanger), based on temperature measurements on the outer tube. This has never before been reported in the open literature. The candidate also performed a rigorous experimental uncertainly analyses of void fraction and momentum-pressure drop, both quantities of which play critical roles in the determination of accurate pressure drop and heat transfer correlations. 

Supervisor: Prof Leon Liebenberg
Co-supervisor: Prof Josua Meyer

Mr de Beer PG: "Continuous Cast Width Prediction Using a Data Mining Approach"

In modern times continuous casting is the preferred way to convert molten steel into solid forms to enable further processing. At Columbus Stainless the continuous casting machine casts slabs of constant thickness with varying width. One important aspect of the continuously cast strand that must be controlled, is the strand width. The strand width exiting from the casting machine, has a direct influence on the product yield which in turn influences the profitability of the company. In general, the strand width control on the austentic and ferritic type steels achieved is excellent with the exception of the 12% chrome non-stabilised ferritic steel. This steel type exhibited different strand width changes when a sequence of different heats was cast. The strand width changes corresponded to the different heats in the sequence. Each heat has a unique chemistry and a relationship between the austenite and ferrite fraction at high temperature and the resulting strand-width change was explained by Siyasiya. The relationship between the heat composition and width change has in the past resulted in the development of a model that enabled the prediction of the expected width change of a specific heat before it is cast to enable preventative action to be taken. This model has been implemented as an on-line prediction model in the production environment with very encouraging results. This study was initiated because it was uncertain if the implemented model was the most accurate for this application. This study is concerned with the development of more models based on different techniques in an attempt to implement a more accurate model. The data mining techniques used include statistical regression, decision trees and fuzzy logic. The results indicated that the existing model was the most accurate and it could not be improved upon.

Study leader: Prof Ken Craig

Mr R Bester :2007 "The Ride Comfort Versus Handling Decision for Off-Road Vehicles"

The two main factors contributing to the most appropriate choice between a soft (soft spring and low damping) and a hard (stiff spring and high damping) suspension setting are the terrain profile and the driver's actions. Terrain classification methods are proposed and tested with measured data from vehicle tests performed on known terrain types ranging from rough and uncomfortable to smooth with high-speed maneuvering. Good results are obtained from the terrain classification methods. Five terrains were accurately identified from over an hour's worth of vehicle testing.

Handling manoeuvre happen unexpectedly, often to avoid an accident. The decision to switch to a hard suspension setting to reduce body roll angle should be made quickly with the accupants' safety as the priority. Methods were investigated that will determine when to switch the suspension to the handling mode based on the kinematics of the vehicle. The switching strategies proposed have the potential, with little refinement, to make the ride versus handling decision correctly.

 Supervisors: Dr PS Els and Dr PE Uys

Mr CJ van Eeden:2007 "A Study to Investigate The Steering Relationship Between The First and Second Axles for an 6x6 Off-Road Military Vehicle"

The steering arrangement of a 6x6 military vehicle was investigated, with the aim to determine if a variable steering ratio between the first and second steering axle of the vehicle will make an improvement in the steady and unsteady state handling of the vehicle. Low-speed maneuvering was evaluated, comparing the vehicle steering geometry with Ackerman geometry. For steady-state handling, a bicycle model was developed, and constant radius simulations at various track radii, vehicle speeds and steering ratios (ratio between the first and second steering axle) was performed. For unsteady dynamic simulations, a mathematical model was developed that included a simple driver model to steer the vehicle through a single lane change, again at various speeds and steering ratios. The vehicle was instrumented, and actual constant radii tests, as well as single-lane change tests were performed. The measurements enabled the comparison of simulated and measured results. Although basic mathematical models were used, acceptable correlation was obtained for both steady and unsteady dynamic behaviour. The results indicated that for a this specific vehicle geometry, where the centre of mass is above the second axle, no marked improvement will be obtained by implementing a variable-ratio steering system. The mathematical model was changed to simulate a vehicle with longer wheelbase and different centre of mass. With the new geometry, theoretical slip angles (and therefore tyre wear) reductions were more noticeable. It was concluded that a variable ratio system between front and second axle would not be an economically viable improvement for this vehicle, since the improvement achieved will not warrant the additional cost and complexity added to the vehicle.

Supervisor: Dr PS Els

Mr A Grove : 2007  "Development of a Finite Element Based Nominal Stress Extraction Procedure for Fatigue Analysis of Welded Structures"

This study entails the fatigue analysis of a complex plate-like structure subjected to random loading. The stress and fatigue life assessment is performed by means of experimental strain gauge measurements, finite element analysis and a quasi-static fatigue assessment procedure known as the Fatigue Equivalent Static Load (FESL) methodology. Firstly, the integrity of shell elements for accurately capturing the stiffness properties and stress distribution in the vicinity of welds is investigated. Extensive numerical investigations into the stress concentration characteristics of various T-piece and stiffener configurations are done, resulting in a nominal stress extraction procedure. The integrity of the procedure is investigated as well as its versatility and dependence on parameters such as plate thickness and loading mode. Furthermore, the stress response of the structure to random dynamic loading is investigated and validated in terms of its suitability for assessment by the FESL methodology. Nominal stress and hot-spot stress fatigue life predictions are made, based on measurements as well as the FESL procedure. The viability and integrity of the FESL methodology is critically assessed after which the actual fatigue life of the structure under particular loading conditions is determined for comparison.

Study leaders: Mr F van Tonder and Prof PS Heyns 

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Geredigeer deur Barbara HuyssenEdit