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Today’s aerospace industry continues to exploit liquid hydrocarbon fossil fuels. Motivated by operational considerations, continued availability and cost, this is likely to be the case for many years, despite the obvious environmental concerns. The interplay of liquid atomisation, spray vaporisation and the combustion process are intricately linked. However, the physical process of fuel injection and its atomisation into tiny droplets prior to combustion remains poorly understood. Because atomisation governs the size of the fuel droplets, and therefore their subsequent evaporation rate, adjusting the injection sequence is of paramount importance and will have far-reaching repercussions on many aspects of the combustion process, for example pollutant formation. In the context of jet engines, kerosene is usually injected in its liquid form via an airblast-type fuel injector. A coflowing high-speed airstream destabilises the liquid fuel, which is thus sprayed into fine droplets into the combustion chamber. The prediction of this phenomenon for various operating conditions relevant to the aeronautical industry requires a deeper understanding of the mechanisms involved in the interaction of the two fluids. A key element in predicting the complex behaviour of spray formation and evolution in jet engines is accurate modelling of fuel atomisation. Atomisation represents one of the key challenges that remains to be undertaken to make predictive computational simulations possible. However, the inherent multi-physics and multi-scale nature of this process limits numerical investigations. Thanks to the steady progress in computer power and Computational Fluid Dynamics (CFD) methods, computational modelling of injection systems emerges as a promising tool that can drive the design of future devices. This research project sets out to investigate the atomisation process in detail, in particular in providing physical insight into the fundamental physics of the phenomenon, in conjunction with an analysis on wetting behaviours and liquid droplet tracking. High-fidelity numerical simulations are performed using a novel in-house state-of-the-art multiphase flow modelling capability, RCLSFoam. The performance of the numerical scheme is demonstrated on typical two-dimensional and three-dimensional benchmark test cases relevant to both multiphase flow modelling and atomisation, and validated against other computational methods. An informed and systematic qualitative assessment of the topological variations of the phase interface during primary atomisation of a liquid film is made through dynamical analysis, while investigating an extensive domain of operating conditions at ambient and aero-engine injection conditions relevant to industry. This analysis demonstrated the influence of shear-driven instabilities on the atomisation process. The shear stress and difference in inertia between liquid and gas are observed to play a significant role in the atomisation process. In addition, the key physical mechanisms and their competing effects have been mapped out in order to predict the evolution of the process according to the operating conditions of the injection system. The proposed cartography gathers four different atomisation mechanisms. In particular, for sufficiently high liquid injection speeds, three-dimensional wave modes were observed to co-exist (the “3-D wave mode” regime). For very low liquid flow rates, accumulated liquid at the atomising edge undergoes deformation by which droplets are generated (the “accumulation” regime). For an increasing gas injection speed and a fixed liquid velocity, the effects of surface tension were observed to result in the generation of streamwise ligaments only, which tend to pair up (the “ligament-merging” regime). Finally, “vortex action” is another observed mechanism by which the liquid film is fragmented. Overall, this research project culminated in (i) the study of dynamic wetting behaviours, with the implementation and validation against experimental data of the Kistler dynamic contact model; and (ii) the demonstration of an algorithm for droplet capture and subsequent post-processing analysis of the droplet characteristics.

<p> Particle filters are being used in a number of state estimation applications because of their capability to effectively solve nonlinear and non-Gaussian problems. However, they have high computational requirements and this becomes even more so in the case of multitarget tracking, where data association is the bottleneck. In order to perform data association and estimation jointly, typically an augmented state vector, whose dimensions depend on the number of targets, is used in particle filters. As the number of targets increases, the corresponding computational load increases exponentially. In this case, parallelization is a possibility for achieving real-time feasibility in large-scale multitarget tracking applications. In this paper, we present an optimization-based scheduling algorithm that minimizes the total computation time for the bus-connected heterogeneous primary-secondary architecture. This scheduler is capable of selecting the optimal number of processors from a large pool of secondary processors and mapping the particles among the selected ones. A new distributed resampling algorithm suitable for parallel computing is also proposed. Furthermore, a less communication intensive parallel implementation of the particle filter without sacrificing tracking accuracy using an efficient load balancing technique, in which optimal particle migration among secondary processors is ensured, is presented. Simulation results demonstrate the tracking effectiveness of the new parallel particle filter and the speedup achieved using parallelization.</p> / Thesis / Master of Applied Science (MASc)

A significant barrier to applying the techniques of machine learning to the domain of object boundary detection is the need to obtain a large database of correctly labeled examples. Inspired by developmental psychology, this paper proposes that boundary detection can be learned from the output of a motion tracking algorithm that separates moving objects from their static surroundings. Motion segmentation solves the database problem by providing cheap, unlimited, labeled training data. A probabilistic model of the textural and shape properties of object boundaries can be trained from this data and then used to efficiently detect boundaries in novel images via loopy belief propagation. / Singapore-MIT Alliance (SMA)

In this study, the phenomenon of two-phase flow was investigated in a square channel. The experiment was performed with stagnant liquid conditions. The gas and liquid dynamics of the bubbly flow were observed in two regions far from the inlet. Air was inserted through a porous media at three superficial gas velocities: 4.6 mm/s, 2.5 mm/s, and 1.4 mm/s. Two techniques were applied in the experiment to measure the bubbly flow: an optical probe and an in-house developed tracking algorithm. Measurements of the bubble interface velocity, void fraction, bubble frequency, time of flight, and Sauter mean diameter were obtained by using the optical probe. The duration of the probe measurements for all three flow rates and both regions lasted approximately 33 hours. The tracking algorithm was used to analyze the experimental data for two visual methods: shadowgraphy and Particle Tracking Velocimetry (PTV). Shadowgraphy provided gas-phase measurements of the bubble centroid velocity and its fluctuations, void fraction, bubble size, and Reynolds stresses. Five data sets were acquired for each flow rate, resulting in a total of 327,540 shadowgraphy images. Liquid parameters such as the velocity, fluctuations in the velocity, and the Reynolds stresses were provided by PTV. Only one data set containing 10,918 images was obtained from liquid measurements for each flow rate. One data set was sufficient to provide reliable statistics since tracking two consecutive images lead to approximately 15,000 velocity vectors. The data obtained from this study was an effort to assist in the verification, validation, and improvement of two-phase flow simulations.

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The author developed a computational framework for the study of the correlation between airway morphology and aerosol deposition based on a population of human subjects. The major improvement on the previous framework, which consists of a geometric airway model, a computational fluid dynamics (CFD) model, and a particle tracking algorithm, lies in automatic geometry construction and mesh generation of airways, which is essential for a population-based study. The new geometric model overcomes the shortcomings of both centerline (CL)-based cylindrical models, which are based on the skeleton and average branch diameters of airways called one-dimensional (1-D) trees, and computed tomography (CT)-based models. CL-based models are efficient in terms of pre- and post-processing, but fail to represent trifurcations and local morphology. In contrast, in spite of the accuracy of CT-based models, it is time-consuming to build these models manually, and non-trivial to match 1-D trees and three-dimensional (3-D) geometry. The new model, also known as a hybrid CL-CT-based model, is able to construct a physiologically-consistent laryngeal geometry, represent trifurcations, fit cylindrical branches to CT data, and create the optimal CFD mesh in an automatic fashion. The hybrid airway geometries constructed for 8 healthy and 16 severe asthmatic (SA) subjects agreed well with their CT-based counterparts. Furthermore, the prediction of aerosol deposition in a healthy subject by the hybrid model agreed well with that by the CT-based model. To demonstrate the potential application of the hybrid model to investigating the correlation between skeleton structure and aerosol deposition, the author applied the large eddy simulation (LES)-based CFD model that accounts for the turbulent laryngeal jet to three hybrid models of SA subjects. The correlation between diseased branch and aerosol deposition was significant in one of the three SA subjects. However, whether skeleton structure contributes to airway abnormality requires further investigation.

La thèse se place dans le contexte du programme de physique prévu auprès du collisionneur Beijing Electron Positron Collider II, pour lequel l'expérience BESIII (Beijing Electron Spectrometer III) doit subir une jouvence. Mon travail concerne l'amélioration des performances de la trajectométrie centrale de l'expérience, en explorant une nouvelle approche utilisant les capteurs à pixels CMOS (CPS) en remplacement partiel de la chambre à dérive actuelle. Il est validé, qu'un pixel de 39.0X50.8 µm2 permet d'atteindre une résolution spatiale ~10 µm. Cette confirmation ouvre la voie pour le développement d'échelle de grande longueur. Un logiciel générique proposant une stratégie d'optimisation pour un trajectomètre pixellisé a été mis au point et appliqué au cas particulier du BESIII. Il a permis de montrer qu'une configuration à trois couches optimise la résolution sur les trajectoire alors qu'une option incluant une couche double-face atteint une efficacité de reconstruction supérieure. / The thesis addresses the upcoming particle physics program foreseen at the Beijing Electron Positron Collider II, for which the Beijing Electron Spectrometer III (BESIII) is being upgraded. The work presented focused on the upgrade of the central tracker of the experiment by exploring the possibilities offered by a new approach using CMOS Pixel Sensors (CPS) instead of the present gas drift chamber. The performance of CPS with a larger pixel (39.0X50.8 µm2) and a lower power consumption has been validated, which will help to develop long ladders. A general strategy and software to optimise the design of a barrel pixelated tracker has also been developed, and its application on the BESIII silicon tracker indicates that the layout of three evenly placed single-sided layers is favoured by BESIII physics for the best momentum resolution, with an option using one double-sided ladder to achieve a higher tracking efficiency.

The PANDA experiment at FAIR is an antiproton experiment and it will offer unique possibilities for the study of hadron physics including hyperon physics. With the PANDA detector, it will be possible to exclusively reconstruct hyperon-antihyperon events. Hyperons are relatively long-lived particles (10−10s), which can travel for a distance of up to a few meters before decaying into their reconstructible decay products. In this project, I focus on the reaction p̅ p → Ξ̄+ Ξ− → Λ̅ π+ Λ π− → p̅ π+π+ p π− π−. The Ξ− (cascade) hyperons take a special role as they are the lightest hyperons to introduce a sequential decay topology, i.e., Ξ− → Λ π− → p π− π− (and c.c.). This constitutes a substantial challenge for the reconstruction algorithms. In this thesis, a Monte-Carlo simulation study of realistic track and event reconstruction of this Ξ̄+ Ξ− reaction in the region near the production threshold is done, and the event generation of the Ξ̄+ Ξ− reaction channel was performed by the EvtGen event generator in PandaRoot. For various realistic tracking algorithms, track and event reconstruction performance levels were investigated in PandaRoot.

M. Tech. (Department of Electronic Engineering, Faculty of Engineering and Technology) --Vaal University of Technology| / Global warming is of increasing concern due to several greenhouse gases. The combustion of fossil fuels is the major contributor to the greenhouse effect. To minimalise this effect, alternative energy sources have to be considered. Alternative energy sources should not only be environmentally friendly, but also renewable and/or sustainable. Two such alternative energy sources are hydrogen and solar energy. The regenerative fuel cell, commonly known as a hydrogen generator, is used to produce hydrogen. The current solar/hydrogen system at the Vaal University of Technology’s Telkom Centre of Excellence makes use of PV array to supply power to an inverter and the inverter is connected to the hydrogen generator. The inverter provides the hydrogen generator with 220VAC. The hydrogen generator has its own power supply unit to convert the AC power back to DC power. This reduces the efficiency of the system because there will be power loss when converting DC power to AC power and back to DC power. The hydrogen generator, however, could be powered directly from a PV array. However, the hydrogen generator needs specific input parameters in order to operate. Three different input voltages with their own current rating are required by the hydrogen generator to operate properly. Thus, a DC-DC power supply unit needs to be designed to be able to output these parameters to the hydrogen generator. It is also important to note that current PV panel efficiency is very low; therefore, the DC-DC power supply unit also needs to extract the maximum available power from the PV array. In order for the DC-DC power supply unit to be able to extract this maximum power, a maximum power point tracking algorithm needs to be implemented into the design. The DC-DC power supply is designed as a switch mode power supply unit. The reason for this is that the efficiency of a switch mode power supply is higher than that of a linear power supply. To reach the objective the following methodology was followed. The first part of the research provided an introduction to PV energy, charge controllers and hydrogen generators. The problem statement is included as well as the purpose of this research and how this research was to be carried out. The second part is the literature review. This includes the background study of algorithms implemented in MPPT’s; it also explains in detail how to design the MPPT DC-DC SMPS. The third part was divided into two sections. The first section is the design, programming and manufacturing of the MPPT DC-DC SMPS. The second section is the simulation of the system as a whole which is the simulation of the PV array connected to the MPPT DC-DC SMPS and the hydrogen generator. The fourth part in the research compared the results obtained in the simulation and practical setup. The last part of the research provided a conclusion along with recommendation made for further research. The simulation results showed that the system works with an efficiency of 40,84%. This is lower than expected but the design can be optimised to increase efficiency. The practical results showed the efficiency to be 38%. The reason for the lower efficiency is the simulation used ideal components and parameters, whereas the practical design has power losses due to the components not being ideal. The design of the DC-DC switch mode power supply, however, indicated that the hydrogen generator could be powered from a PV array without using an inverter, with great success.