Master's Degrees Completed 2013
A Strydom, 2013. "Controllable suspension design using Magnetorheological Fluid"
The purpose of this study is to mitigate the compromise between ride comfort and handling of a small single seat off-road vehicle known as a Baja. This has been achieved by semi-active control of the suspension system containing controllable magnetorheological (MR) dampers and passive hydro-pneumatic spring-damper units.
MR fluid is a viscous fluid whose rheological properties depend on the strength of the magnetic field surrounding the fluid, and typically consists of iron particles suspended in silicone oil. When a magnetic field is applied to the fluid, the iron particles become aligned and change the effective viscosity of the fluid. The use of MR fluid in dampers provides variable damping that can be changed quickly by controlling the intensity of the magnetic field around the fluid. Various benefits associated with the use of MR dampers have led to their widespread implementation in automotive engineering.
Many studies on conventional vehicles in the existing literature have demonstrated the conflicting suspension requirements for favourable ride comfort and handling. Generally, soft springs with low damping are ideal for improved ride comfort, while stiff springs with high damping are required for enhanced handling. This has resulted in the development of passive suspension systems that provide either an enhanced ride quality or good drivability, often targeting one at the expense of the other.
The test vehicle used for this study is distinct in many ways with multiple characteristics that are not commonly observed in the existing literature. For instance, the absence of a differential in the test vehicle driveline causes drivability issues that are aggravated by increased damping.
The majority of existing MR damper models in the literature are developed for uniform excitation and re-characterisation of model parameters is required for changes in input conditions. Although recursive models are more accurate and applicable to a wider range of input conditions, these models require measured force feedback which may not always be available due to limitations such as packaging constraints. These constraints required the development of alternative MR damper models that can be used to prescribe the current input to the damper.
In this study parametric, nonparametric and recursive MR damper models have been developed and evaluated in terms of accuracy, invertibility and applicability to random excitation. The MR damper is used in parallel with passive damping as a certain amount of passive damping is always present in suspension systems due to friction and elastomeric parts.
Most of the existing studies on suspension systems have been performed using linear two degree of freedom vehicle models that are constrained to specific conditions. Usually these models are implemented without an indication of the ability of these models to accurately represent the vehicles that these studies are intended for.
For this study, a nonlinear, three-dimensional, 12 degrees of freedom vehicle model has been developed to represent the test vehicle. This model is validated against experimental results for ride comfort and handling. The MR damper models are combined with the model of the test vehicle, and used in ride comfort and handling simulations at various levels of passive damping and control gains in order to assess the potential impact of suspension control on the ride quality and drivability of the test vehicle.
Simulation results show that lower passive damping levels can significantly improve the ride comfort as well as the handling characteristics of the test vehicle. Furthermore, it is observed that additional improvements that may be obtained by the implementation of continuous damping control may not be justifiable due to the associated cost and complexity.
Supervisor: Prof. P.S. Els
Co-Supervisor: Dr. S. Kaul
H A Hamersma, 2013."Longitudinal vehicle dynamics control for improved vehicle safety"
An autonomous vehicle is a vehicle that is capable of navigating and driving with no human intervention whatsoever through the utilization of various sensors and positioning systems. The possible applications of autonomous vehicles are widespread, ranging from the aerospace industry to the mining and military sectors where the exposure of human operators to the operating conditions is hazardous to their health and safety. Automobile accidents have become the leading cause of death in certain segments of the world population. Removing the human driver from the decision-making process through automation may result in significantly safer highways. Although full autonomy may be the ultimate goal, there is huge scope for systems that aid the driver in decision making or systems that take over from the driver under conditions where the human driver fails.
The aim of the longitudinal control system to be implemented on the Land Rover test vehicle in this study is to improve the vehicle’s safety by controlling the vehicle’s longitudinal behaviour. A common problem with sports-utility-vehicles is the low rollover threshold, due to a high centre of gravity. Rather than modifying the vehicle to increase the rollover threshold, the aim of the control system presented here is to prevent the vehicle from exceeding speeds that would cause the vehicle to reach its rollover threshold.
In order to develop a control system that autonomously controls the longitudinal degree of freedom, a model of the test vehicle (a 1997 Land Rover Defender 110 Wagon) was developed in MSC.ADAMS/View and validated experimentally. The model accurately captures the response of the test vehicle to supply forces as generated by the engine and demand forces applied through drag, braking and engine braking. Furthermore, the model has been validated experimentally to provide reliable simulation results for lateral and vertical dynamics.
The control system was developed by generating a reference speed that the vehicle must track. This reference speed was formulated by taking into account the vehicle’s limits due to lateral acceleration, combined lateral and longitudinal acceleration and the vehicle’s performance capabilities. The control system generates the desired throttle pedal position, hydraulic pressure in the brake lines, clutch position and gear selection as output. The MSC.ADAMS\View model of the test vehicle was used to evaluate the performance of the control system on various racetracks of which the GPS coordinates were available. The simulation results indicate that the control system performs as expected.
Finally, the control system was implemented on the test vehicle and the performance was evaluated by conducting field tests in the form of a severe double lane change manoeuvre. The results of the field tests indicated that the control system limited the acceleration vector of the vehicle’s centre of gravity to prescribed limits, as predicted by the simulation results.
Supervisor: Prof P.S. Els
MD Stocks, 2013. "Geometric optimisation of heat transfer in complex geometries using Newtonian and non-Newtonian fluids"
The continual advance in manufacturing processes has resulted in significantly more compact, high performance, devices. Consequently, heat extraction has become the limiting factor, and of primary concern. Therefore, a substantial amount of research has been done regarding high efficiency micro heat exchangers, employing novel working fluids.
This dissertation numerically investigated the thermal behaviour of microchannel elements cooled by Newtonian and non-Newtonian fluids, with the objective of maximising thermal conductance subject to constraints. This was done, firstly, for a two-dimensional simple microchannel, and secondly, for a three-dimensional complex microchannel. A numerical model was used to solve the governing equations relating to the flow and temperature fields for both cases. The geometric configuration of each cooling channel was optimised for Newtonian and non-Newtonian fluids, at a fixed inlet velocity and heat transfer rate. In addition, the effect of porosity on thermal conductance was investigated.
Geometric optimisation was employed to the simple and complex microchannels, whereby an optimal geometric ratio (height versus length) was found to maximise thermal conductance. Moreover, analysis indicated that the bifurcation point of the complex microchannel could be manipulated to achieve a higher thermal conductance.
In both cases, it was found that the non-Newtonian fluid characteristics resulted in a significant variation in thermal conductance as inlet velocity was increased. The characteristics of a dilatant fluid greatly reduced thermal conductance on account of shear-thickening on the boundary surface. In contrast, a pseudoplastic fluid showed increased thermal conductance.
A comparison of the simple and complex microchannel showed an improved thermal conductance resulting from greater flow access to the conductive area, achieved by the complex microchannel.
Therefore, it could be concluded that a complex microchannel, in combination with a pseudoplastic working fluid, substantially increased the thermal conductance and efficiency, as opposed to a conventional methodology.
Keywords: Non-Newtonian fluid; Thermal conductance; Geometric optimisation; Microchannel; Complex geometry
Supervisors: Prof T Bello-Ochende and Prof JP Meyer
PENI Junior YEKOLADIO, 2013."Thermodynamic Optimization of Sustainable Energy System: Application to the optimal design of heat exchangers for geothermal power systems"
The present work addresses the thermodynamic optimization of small binary-cycle geothermal power plants. The optimization process and entropy generation minimization analysis were performed to minimize the overall exergy loss of the power plant, and the irreversibilities associated with heat transfer and fluid friction caused by the system components. The effect of the geothermal resource temperature to impact on the cycle power output was studied, and it was found that the maximum cycle power output increases exponentially with the geothermal resource temperature. In addition, an optimal turbine inlet temperature was determined, and observed to increase almost linearly with the increase in the geothermal heat source. Furthermore, a coaxial geothermal heat exchanger was modeled and sized for minimum pumping power and maximum extracted heat energy. The geofluid circulation flow rate was also optimized, subject to a nearly linear increase in geothermal gradient. In both limits of the fully turbulent and laminar fully-developed flows, a nearly identical diameter ratio of the coaxial pipes was determined irrespective of the flow regime, whereas the optimal geofluid mass flow rate increased exponentially with the Reynolds number. Several organic Rankine Cycles were also considered as part of the study. The basic types of the ORCs were observed to yield maximum cycle power output. The addition of an IHE and/or an OFOH improved significantly the effectiveness of the conversion of the available geothermal energy into useful work, and increased the thermal efficiency of the geothermal power plant. Therefore, the regenerative ORCs were preferred for high-grade geothermal heat. In addition, a performance analysis of several organic fluids was conducted under saturation temperature and subcritical pressure operating conditions of the turbine. Organic fluids with higher boiling point temperature, such as n-pentane, were recommended for the basic type of ORCs, whereas those with lower vapour specific heat capacity, such as butane, were more suitable for the regenerative ORCs.
Keywords: Geothermal energy, Organic Rankine Cycles, Optimization, Exergy analysis, Entropy Generation Minimization analysis, binary cycle, Enhanced Geothermal System.
Supervisor: Prof T.Bello-Ochende
Co-supervisor: Prof J P Meyer
Warren Van Zyl,2013. "SINGLE-PHASE CONVECTIVE HEAT TRANSFER AND PRESSURE DROP COEFFICIENTS IN CONCENTRIC ANNULI"
Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on their convective heat transfer capabilities. Linear and non-linear regression models exist for determining the heat transfer coefficients, however, these are complex and time consuming, and require much experimental data in order to obtain accurate solutions. A large dataset of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593 and 0.712 with respective hydraulic diameters of 17.01 mm, 13.84 mm, 10.88 mm and 7.71 mm was gathered. Mean Nusselt numbers were determined using the modified Wilson plot method, a non-linear regression scheme and the logarithmic mean temperature difference method. These three methods presented disagreements with existing correlations based on local wall temperatures. The local Nusselt numbers were determined using the logarithmic mean temperature difference method. Local wall temperature measurements were made using a novel method which minimized obstructions within the annulus. Friction factors were calculated directly from measured pressure drops across the annuli. Both heated and cooled horizontal annuli in fully turbulent flow with Reynolds numbers based on the hydraulic diameter varying from 10 000 to 45 000 with water as the working medium were investigated.
Supervisor: Dr J Dirker
Co-supervisor: Prof J P Meyer
Timothy Prinsloo, 2013. "Damage Detection Methodology for Composite UAV Wings using Modal Analysis and Probabilistic Concepts"
Monitoring of structural integrity is critical in manyfields today, and particularly so in the civil, mechanical and aerospaceengineering industries. In the aerospace industry, appreciably sized and almostexclusively composite UAVs share the airspace with other aircraft. Such composite structuresalso pose numerous uncertainties to structural health monitoring and analysistechniques. This necessitates research into a methodology for practicaland effective structural health monitoring techniques.
Thiswork presents a methodology for structural health monitoring and particularlydelamination detection in composite wing structures. The approach usesexperimental modal analysis with due consideration for the probabilisticeffects of random variations in material and geometrical properties, for thepurpose of a general and non wing-specific damage detection technique.
Alarge number of composite material coupons were tested to determine statisticaldistributions of 2D orthotropic material properties, using an optical imagecorrelation system to reduce the expense of testing. Uncertainties in the winggeometry arising from manufacturing variances were taken into consideration. Thematerial properties of the foam spar and resin beadings were considered isotropicand deterministic. A finite element model of the wing was subsequently improvedusing a scanning laser vibrometer to conduct detailed experimental modalanalyses of five wings, and a multi-model updating approach based on frequencyand mode shape information was used to update selected sensitive materialproperties. Significant improvement was accomplished.
Usingthe probabilistic material property database, a confidence region wasestablished for wing mode shapes through a Monte Carlo procedure. It was shownthat delamination effects are capable of perturbing the dynamic mode shapesbeyond the confidence regions implied by the material uncertainties. Thisprovides a basis for further development of a structural health monitoringmethodology for composite structures, taking due account of the manyuncertainties in the structure.
Supervisor: Professor P.S. Heyns
Patrick M.K. Sambayi, 2013. "DRILL WEAR MONITORING USING INSTANTANEOUS ANGULAR SPEED"
Various researchers have investigated the use of conventional measurements (vibration, acoustic emission, force, and torque measurements) to assess drill tool condition on drilling machines. Although it has been successfully established that such measurements could in principle be used, it however not yet found its way into industry as standard practice, and drill bits are still often replaced non-optimally. The major problems are related to the lack of system-made sensors for drilling, the poor signal-to-noise ratio resulting in sophisticated signal processing and the inconvenience of using drill based strain or conventional sensors on production machines. To overcome the above problems, some researchers recommend advanced and sophisticated signal processing, while others advise that effort should be put into the intelligent and/or innovative use of available sensor technology by developing some form of instrumented tool for drill operations.
This dissertation considers the use of these conventional technologies monitoring parameters against the use of the instantaneous angular speed (IAS) of the spindle for monitoring the drill condition. It is shown that an angular speed approach can generate diagnostic information similar to the conventional measurements, without some of the instrumented sensor complications. No advanced signal processing techniques are required as the delay associated with the processing is often not beneficial in automated drilling monitoring. Satisfactory results were obtained using common time domain signal processing methods. The curve trends have illustrated that the drill bits wear rapidly at the end of its life except for small drills which in general fail by fracture. A diagnosis of the drill wear based on a simple regression analysis was done and the results are promising.
Keywords: Wear monitoring; Drilling; Vibration; Torque; Angular speed; Regression analysis
Supervisor: Professor P.S. Heyns
Vishaal Bhana, 2013."Online damage detection on shafts using torsional and undersampling measurement techniques"
The presence of cracks in rotors is one of the most dangerous defects of rotating machinery. This can lead to catastrophic failure of the shaft and long out-of-service periods. The occurrence of a crack in a rotating shaft introduces changes in flexibilities which alters the dynamics during operation. This research deals with detecting damage in rotors by means of constantly monitoring the variation in the rotor’s dynamics during normal operating conditions. This project entails a computer finite element section as well as an experimental investigation.
The flexibility in the region of the crack is different from an uncracked section. A finite element model of a shaft is built and investigated. The damaged model is the same except that the nodes in the location of the crack are not equivalenced in order to represent the crack. A simple constant cross-sectional shaft with semi-circular transverse surface cracks varying in size have been modelled on the Patran finite element software and a normal modes analysis was done using the Nastran solver. The results revealed a change in the natural frequencies due to the variation in the size of the crack.
The experimental investigation involved creating sample shafts with damage positioned in them that would closely resemble what one may find in actual real-life situations and the dynamics during rotation with various torsional loadings are investigated and monitored using three methods. A fibre-optical sensor, Digital image correlation system and telemetry strain gauges were used. Undersampling techniques were used for the DIC system. Results showed that the fibre-optic sensor is by far the most favourable as it is able to detect damage under constant operation. The finite element model was updated by re-modelling the geometry, damage and material properties. The solution of the analysis matched the experimental results closely and model verification was achieved.
Supervisor: Professor P.S Heyns
Sarel Francois van der Westhuizen, 2013."Slow active suspension control for rollover prevention"
Rollover prevention in Sports Utility Vehicles (SUV’s) offers a great challenge in vehicle safety. By reducing the body roll angle of the vehicle the load transfer will increase and thus decrease the lateral force that can be generated by the tires. This decrease in the lateral force can cause the vehicle to slide rather than to roll over. This study presents the possibility of using slow active suspension control to reduce the body roll and thus reduce the rollover propensity of a vehicle fitted with a hydro-pneumatic suspension system. The slow active control is obtained by pumping oil into and draining oil out of each hydro-pneumatic suspension unit individually.
A real gas model for the suspension units as well as for the accumulator that supplies the oil is incorporated in a validated full vehicle Adams model. This model is then used to simulate a double lane change manoeuvre performed by a SUV at 60 km/h and it is shown that a significant improvement in body roll can be obtained with relatively low energy requirements.
The proposed control is successfully implemented on a Land Rover Defender test vehicle. A Proportional-Derivative (PD) controller is used to control on-off solenoid operated valves and the flow is adjusted using the lateral acceleration as a parameter. Experimental results confirm that a significant improvement in body roll is possible.
Supervisor: Prof. Pieter Schalk Els
Mr Kersten Grote, 2013."The inﬂuence of multi-walled carbon nanotubes on single-phase heat transfer and pressure drop characteristics in the transitional ﬂow regime of smooth tubes."
There are in general two diﬀerent types of studies concerning nanoﬂuids. The ﬁrst one concerns itself with the study of the eﬀective thermal conductivity and the other with the study of convective heat transfer enhancement. The study on convective heat transfer enhancement generally incorporates the study on the thermal conductivity. Not many papers have been written on the convective heat transfer enhancement and even fewer concerning the study on multi-walled carbon nanotubes in the transitional ﬂow regime. In this paper the thermal conductivity and viscosity was determined experimentally in order to study the convective heat transfer enhancement of the nanoﬂuids. Multi-walled carbon nanotubes suspended in distilled water ﬂowing through a straight, horizontal tube was investigated experimentally for a Reynolds number range of a 1 000 - 8 000, which included the transitional ﬂow regime. The tube was made out of copper and has an internal diameter of 5.16 mm. Results on the thermal conductivity and viscosity indicated that they increase with nanoparticle concentration. Convective heat transfer experiments were conducted at a constant heat ﬂux of 13 kW/m2 with 0.33%, 75% and 1.0% volume concentrations of multi-walled carbon nanotubes. The nanotubes had an outside diameter of 10 - 20 nm, an inside diameter of 3 - 5 nm and a length of 10 - 30µm. Temperature and pressure drop measurements were taken from which the heat transfer coeﬃcients and friction factors were determined as a function of Reynolds number. The thermal conductivities and viscosities of the nanoﬂuids were also determined experimentally so that the Reynolds and Nusselt numbers could be determined accurately. It was found that heat transfer was enhanced when comparing the data on a Nusselt number as a function of Reynolds number graph but comparing the results on a heat transfer coeﬃcient as a function of average velocity graph the opposite eﬀect was observed. Performance evaluation of the nanoﬂuids showed that the increase in viscosity was four times the increase in the thermal conductivity which resulted in an ineﬃcient nanoﬂuid. However, a study on the performance evaluation criterion showed that operating nanoﬂuids in the transition and turbulent ﬂow regime due to the energy budget being better than that of the distilled water.
Keywords: Nanoﬂuids, multi-walled carbon nanotubes, transition, convective heat transfer, performance evaluation
Supervisor: Prof J P Meyer
Co-supervisor: Dr TJ McKrell (MIT)
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