2019

F. Feldesi, 2019. "FULL-FIELD CONTACT PATCH STRAIN MEASUREMENT ON THE INNER SURFACE OF AGRICULTURAL TYRES USING STEREOVISION CAMERAS"

The forces generated in the tyre contact-patch are important for vehicle dynamics analysis. The tyre contact patch is not directly visible due to the terrain. Measuring the strain in the contact patch region may give insight into the forces generated by the tyre as it deforms. Strain measurement in the contact patch is often limited to discrete points, using strain gauges or other novel techniques which limits data capture to once per revolution. In this study stereovision cameras are used to capture unique features in the pattern painted on the tyres inner surface. An in-tyre mechanically stabilized camera system allows the contact patch to be captured continuously and the stereovision cameras allow for full field measurement of the tyre inner surface. In post processing the features are tracked and triangulated to form point-clouds for each time step. Point-clouds are compared to determine the strain of common points in two directions. The system is applied to an agricultural tyre with large tread-blocks. The wheel is instrumented to measure pressure and forces. The tyre is tested statically in a series of tyre tests where the lateral, longitudinal and vertical displacement is controlled. The strain measured in the tyre contact patch region is compared to the forces measured at the wheel centre. It is noticed that as the measured forces increases so too does the magnitudes of the strains. Unique patterns are found in the contact patch strain for each test type. These patterns could be used to identify the type of forces experienced by the wheel while the strain magnitude could give an indication of the magnitude of the forces. Future work could allow for strain measurement in the contact patch as the tyre rolls over deformable terrain where displacement is not easily controlled.

Supervisor: Dr T.R. Botha

Co-Supervisor: Prof P.S. Els

A. M. Torr, 2019. "HEAT TRANSFER AUGMENTATION IN A RECTANGULAR CHANNEL BY THE USE OF POROUS SINUSOIDAL SCREEN INSERTS"

Most regions in Southern Africa, South Africa in particular receives an abundancy of solar energy when compared to other regions across the globe. The most economical way to harness this energy is to design and implement economical heat exchangers with air as the working fluid. The performance of heat exchangers can be enhanced by implementing passive techniques such as coated or roughened surfaces, extended surfaces, displaced insert devices, flow swirl devices and coiled tubes among others. The addition of inserts in heat exchangers can increase the overall efficiency of the heat exchanger by increasing the convective heat transfer at the wall by generating local turbulence and mixing. The porous insert aims to the increase the rate of heat transfer by either increasing the convective heat transfer coefficient, the heat transfer area or both. The porous inserts act in such a way to repeatedly grow and destroy thin boundary layers on the surface providing a higher heat transfer coefficient. The porous inserts are lightweight, increase the structural integrity of the heat exchanger walls and can be retrofitted to the heat exchanger. By measuring the effects on the convection heat transfer rate, pressure drop, and turbulence caused by a wavy screen insert in a rectangular channel with air as the working fluid will help determine the thermal performance of the wavy screen insert. By independently investigating the effects of geometrical properties of the wavy screen insert, such as the porosity and periodicity of the insert, the size and shape of the insert can be determined to increase the thermal performance and reduce the size of the compact heat exchanger. This research aims to increase the thermal effectiveness of the heat exchanger channels while minimizing the increase in pressure penalty in the same channels. Increased thermal effectiveness directly affects the convection heat transfer, compact size, initial cost of the heat exchanger and how a small pressure penalty affects the operating cost of the heat exchanger. This research aims to investigate the effects of porosity and periodicity of the sinusoidal screen insert in a rectangular channel on the convection heat transfer rate and pressure drop across the channel to improve the thermal performance of the heat exchanger employed in solar panels, energy recovery systems, electronic chips, and machine components. To investigate the thermal performance of sinusoidal screen inserts in flat plate heat exchangers an experimental test facility is constructed which is capable of replicating the conditions of the heat exchanger. Six inserts are fabricated with varying wavelength and porosity. The height or amplitude of the sinusoidal waveform remains constant at 14 mm for all the inserts. The wavelength, λ is investigated at 12 mm, 16 mm and 20 mm. The effects of the porosity of the insert, ζ is investigated for at 48 % and 68 % porosity. The inserts are placed inside a 203 x 14 mm rectangular test section and wall temperature and pressure measurements along the test section are recorded over a range of 400 < Re < 35 000. The study shows that the use of sinusoidal inserts in flat plate heat exchangers can increase the thermal performance over the range of 400 ≤ Re ≤ 3000. Above Re = 3000 the increased pressure penalty decreases the performance of the heat exchanger.

Supervisor: Dr. G. I. Mahmood

Co-Supervisor: Prof. J. P. Meyer

J.-P. Theron, 2019. "Practical implementation of a trajectory planning algorithm for an autonomous UAV"

A near real-time optimal trajectory planning system for UAVs is presented and tested in a series of low altitude obstacle avoidance planning scenarios. 

The system uses the differential flatness based Inverse Dynamics Trajectory Optimisation approach with a simplified 3-DoF quaternion point-mass inverse aircraft dynamic model.  It was tasked with planning a trajectory that minimised the total amount of fuel required to fly the trajectory subject to the UAV's dynamic and performance constraints and avoiding obstacles along the trajectory.

Case studies were conducted into the numerical behaviour of the objective and constraint functions.  With the goal being to gain insight as to the numerical reality that the numerical optimisation algorithms would have to contend with.

A total of 38 optimisation algorithms and strategies were also tested in conjunction with the inverse model over the course of 4 experiments on a simulation computer.  The first 3 experiments were plain trajectory planning experiments where the system was tasked with planning an optimal trajectory in 3 different environments that were increasingly environmentally more complex.  At the heart of these experiments were the fact that the numerical optimisation algorithm would start the planning process with a random initial guess.

A good dataset of performance results were generated, which was then used to compare the different optimisation algorithms.  Out of the pool of 38 optimisation algorithms, 3 algorithms were identified that had a good balance between calculation speed and numerical robustness.  It was also concluded that these 3 algorithms had the potential to be viable for near real-time implementation on UAVs.

In the fourth experiment, a replanning scenario was emulated where the system had to use the result of a previous planning cycle to start the numerical optimisation process.  It was found system had increased performance across the board of optimisation algorithms and thus it was concluded that where possible designers had to use a replanning methodology.

Ultimately it was concluded that the project had met all of its objectives and that the project had produced an optimal trajectory planning system that could potentially be viable for near real-time use on-board UAVs.

Supervisor: Prof L.Dala

Co-Supervisor: Mr P. Barrier 

L. Brits, 2019. "Feasibility of Vibration-based Damage Detection for Pinned Turbine Blades​​​"

Turbine blades are subjected to various damage mechanisms with fatigue as the primary contributor. During operation, damage accumulates in the form of crack initiation and propagation. This may lead to catastrophic failure, which is cause for concern in terms of availability and safety of the turbine. To optimize the maintenance schedule and to provide operational flexibility of the turbine, the state of health of the blades is monitored. This is usually accomplished through non-destructive testing (NDT) during outages. Conventional NDT techniques for in-situ inspection of turbine blade and disk assemblies is difficult and often ineffective, due to limited access to areas of concern, as well as the complex geometries of blade roots. Off-site inspection can be costly if the blades are still assembled in the turbine disk since the process of removing and reinstalling these blades is critical and labour-intensive, increasing the turbine downtime and overall costs.

These problems could potentially be overcome by employing inspection techniques that offer the prospect of assessing obstructed areas through monitoring the global dynamic characteristics of the structure, which provide relatively easily interpretable data. With global inspection methods, a degree of measurement sensitivity is forfeited but the potential to detect more severe damage, without prior knowledge of the precise damage area location, exists.

In this dissertation, the feasibility of a vibration-based structural damage identification technique that could be usable in support of conventional NDT to detect cracks in pinned turbine blades during off-line in-situ inspection, is evaluated. The investigation was limited to considering uninstalled single blades only, and thus off-site inspection of this component is regarded above the turbine disk assembly. This is clearly a simplified case and does not address the critical case from a practical perspective of having a large number of blades mounted onto a disk with pins, which is really the circumstance under which the technique could become useful. This study must thus be considered as a first step towards addressing the real practical problem. In this simplified problem, the following questions are answered: is it possible to detect damage in an unconstrained and isolated blade using vibration response, and if so, can different damage scenarios be identified? The proposed vibration-based damage detection method entails a multi-class support vector machine classification procedure in which the natural frequencies are employed as the discriminatory feature for damage detection and identification of different single-location damage scenarios.

The natural frequencies were acquired from accurate experimental modal analysis of freely supported individual pinned turbine blades through impact testing. To confirm and predict the expected behaviour of the blades, a healthy numerical model was built and validated whereafter defects and damage were introduced. This includes geometrical variability at the root, observed in the procured blades, and the anticipated worst-case single-location damage at the most probable locations near or on the root, obtained from literature and discussions with experts in the industry. Artificial damage, i.e. a uniform 1mm notch, was introduced in the root at the upper pinhole on the leading edge pressure side; and just above the root at the aerofoil base on the trailing- and the leading edge. To establish the discriminative quality of the modal property natural frequency, it was necessary to determine its sensitivity to geometrical variability and damage. It was also required to establish the damage-specific behaviour or damage trend in the experimental data of these damage scenarios to conclude their distinctiveness. This analysis was extended to outlining the feature quality by exploring the separability of class clusters for the healthy and damage scenario(s).

The feasibility of the proposed method is assessed using experimental data through simple hypothesis testing regarding the detection and identification of both geometrical variability in healthy blades, and damage. It was found that healthy blades are very similar, as geometrical variability cannot be detected. This is because the distributions of natural frequencies fall within a range about a mean value in an ambiguous cluster. In contrast to this, the damage scenarios were found to be distinct, and thus discernible from the healthy blades. These classes formed discrete clusters, each with a similar distribution than the healthy blades. The conclusion of the feasibility study serves as proof of concept.

Supervisor: Prof P.S. Heyns

Co-Supervisor: Dr. H.M. Inglis