Mechanical Engineering - Theses
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ItemNo Preview AvailableStudy of the H2/Air/NOx Kinetics Using a Turbulent Flow Reactor( 2023-12)Hydrogen is a promising alternative fuel since it only produces water as a complete combustion product. However, hydrogen oxidation has a complex dependence on the operating pressure. Abundant works have been reported on the well-known reverse S-shaped explosion limit of hydrogen but variation of the oxidation reactivity as a function of pressure has been rarely quantified. Furthermore, the high flame temperature of hydrogen can also lead to higher NOx emission which could significantly alter its auto-ignition behaviour. Neat hydrogen oxidation is investigated firstly in a turbulent flow reactor at high pressures and intermediate temperatures. Pressure shows a weak promoting effect at these conditions, in contrast to the strong inhibiting effect at lower pressures when crossing the second ignition branch. Analysing the hydrogen consumption rate revealed that the oxidation reactivity remains minimal and is barely affected by pressure between 2 and 20 bar, suggesting the existence of a low-reactivity band on the temperature-pressure plane which could be potentially utilised for practical combustion of hydrogen fuels. 50-1000 ppm of NO is then added to the reacting hydrogen/air mixture to study the impact of NO at an extended pressure and temperature range. The presence of NO is found to promote hydrogen oxidation, only to a different extent, under all conditions studied. The low reactivity band appears to be widened due to the addition of NO. In other words, more NO is needed at high pressures to promote hydrogen oxidation to the same degree as it would be at lower pressures. The measured species concentration-time profiles, combined with ignition delay times selected from the literature, are used to develop a new H2/NOx model focusing on hydrogen autoignition behaviour. The model is developed in a hierarchical order which means the neat hydrogen model is determined first and the NOx sub-model is added and optimised afterwards. A global optimisation method incorporating random sampling and high-dimensional model representation is used to identify the most sensitive reactions and to constrain their uncertainty with the selected experimental targets. An optimised model is then obtained which shows excellent agreement with not only the obtained turbulent flow reactor experiments but also a wide range of data reported in the literature.
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ItemInvestigating the Characteristics of Surface Pressure Spectra of CD airfoil through Large Eddy Simulation and Semi-Empirical Modelling( 2023-03)This study focuses on the surface pressure spectrum characteristics of Controlled Diffusion (CD) airfoils under varying Mach numbers and crossflow angles. CD refers to a class of cambered airfoils with characteristics to carefully control the flow and minimize losses around the airfoil surface. High-resolution compressible large eddy simulation datasets are used to analyze the noise generation mechanism and source locations responsible for the observed mid-high frequency hump in the pressure spectrum. The hydrodynamic and acoustic pressure modes travelling at specific speeds are obtained by a wavenumber-frequency decomposition, and the acoustic component originating from the separation bubble region near the leading edge is found to be responsible for the observed hump in the pressure spectrum. The interaction of hydrodynamic and acoustic waves also plays a significant role in the appearance of the hump in the spectrum. The effect of spanwise crossflow on the surface pressure spectrum of the CD airfoil is also studied. The chordwise and streamwise statistics are analyzed to characterize the effect of crossflow on the mean flow parameters, and linear stability is conducted on local profiles to analyze how the crossflow affects the hydrodynamically most unstable frequencies in the leading edge separation bubble. The results show that the hydrodynamic contribution to the surface pressure spectrum with crossflow plays a prominent role. Additionally, the suppression of the hump in the spectrum is due to crossflow effects instead of the curvature-induced pressure gradient. Further, the thesis focuses on the prediction of trailing edge noise in turbulent boundary layers with high Reynolds number flow under incompressible conditions using Gene Expression Programming (GEP). To accomplish this, boundary layer parameters from both experimental and Reynolds Averaged Navier Stokes (RANS) calculations are utilized as inputs. The GEP algorithm is also enhanced with new features that allow for pre-defined mathematical expression structures. The study also develops new semi-empirical models to assess their performance in predicting the surface pressure spectrum on RANS boundary layer inputs. Additional flat plate datasets and airfoil cases with high adverse pressure gradient conditions are included in the database, and a thorough analysis of this diverse dataset is performed. The analysis reveals significant changes in the overall data characteristics, which can aid in achieving better modelling results. A new semi-empirical model is developed using a novel approach to multi-scaling for frequency terms, demonstrating improved performance compared to the single-scale framework. A brief comparison is also conducted with a Neural Network model for surface pressure spectrum prediction to assess performance when test data falls outside the training range, providing insight into the prediction power of the newly developed models. In conclusion, this study provides a comprehensive investigation of the surface pressure spectrum characteristics of a CD airfoil, the effect of Mach number and crossflow on the pressure spectrum, and an improved mathematical expression for semi-empirical wall pressure spectra modelling. Accurate and efficient prediction of trailing edge noise is crucial to improving the performance and reducing noise levels of aircraft, fans and wind turbines.
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ItemComputational Design of Alternate Binders for Tungsten Carbide Hardmetals( 2023-11)Interest in the field of alternate binders for tungsten carbide (WC) hardmetals has increased due to the health implications surrounding the use of cobalt as a binder material. By incorporating the sinterability and mechanical properties of the system simultaneously, alternatives that are manufacturable using existing procedures can be determined. Here, an Integrated Computational Materials Engineering (ICME) approach was used to search for alternate binder compositions using a reduced-order model. The model was derived by combining the densification mechanisms present in cobalt-containing compacts with the rate enhancing factors governing early onset densification. The model incorporates thermodynamic and kinetic components coupled to a multi-objective genetic algorithm. It allows alloys with compositions optimized for sintering to be ranked against those optimized for mechanical properties to form Pareto sets. In this study, I present a computational methodology for the design of alternate binders based on multi-component alloys for the sintering of WC hardmetals. The current state-of-the-art binders used in the production of WC hardmetals have limited resistance to high temperature sintering and can result in unwanted side effects such as grain growth and decreased toughness. To address these limitations, I propose the use of multi-component alloys as binders in the sintering process. This methodology involves the use of first principles calculations to predict the thermodynamic stability and diffusion coefficients of different alloy systems. The results of these calculations are used to identify promising alloy systems for use as binders in the sintering process. Next, reduced-order models coupled with the CalPhaD method are used to speculate on the diffusive transport of the alloy components during sintering to predict the resulting phases of the final product. These simulations allow for the optimisation of the composition and processing conditions of the alternate binders for maximum densification and improved mechanical properties of the WC hardmetals. Finally, the results of the computational methodology are validated against experimental data and are shown to accurately predict the microstructure and mechanical properties of the sintered WC hardmetals. The results demonstrate that multi-component alloys have the potential to serve as effective and efficient binders for the sintering of WC hardmetals and can provide improved performance compared to current state-of-the-art binders. This thesis details the research plan that has been established for the development of a reduced-order design model for the discovery of alternate binders for the tungsten carbide system. The need for an alternate binder material has been highlighted due to health implications regarding the use of cobalt. A review of the literature focussing on the sintering of tungsten carbide was conducted. This led to the formation of three research questions that became the focus of this project. The design model uses an ICME-based approach and was validated using published data of the tungsten carbide cobalt system. The importance of this research in the development and deployment of a reduced-order design model has been highlighted. There have been efforts to find alternate binders for tungsten carbide (WC) hardmetals because of safety and ethical issues related to the continued use of cobalt. However, the process of identifying new binders is often performed using conventional prototype-and-test approaches. Additionally, these approaches are mostly focused on finding binders that result in acceptable mechanical properties, with limited investigation into their processability, particularly their sinterability. Here, I demonstrate a computational method for finding alternate binders for the sintering of WC hardmetals. The methodology involves new design models to describe the sintering performance of WC particles with different binder systems. It is implemented by using a multi-objective optimisation algorithm coupled with the design models. The thermodynamic and kinetic parameters necessary for the models are calculated by using the CalPhaD (Calculation of Phase Diagram) method, which allows the efficient search of the compositional space of alloys. The study presents validation of the developed design models using densification data from literature together with design exercises that resulted in potential alternate binder alloys. The developed method can help in the search for suitable binder metals by identifying alloys, which can facilitate the efficient sintering of WC particles.
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ItemCharacterisation of micro-scale flows and devices for high strain rate applications( 2023-10)Since the early 2000s, pursuing single-cell experiments has been crucial for comprehending cell mechanoresponses. Various measurement techniques, such as micropipette aspiration, optical tweezers, and atomic force microscopy, have been extensively employed to assess cell properties. However, these methods risk damaging cell structures due to their intrusive nature. Therefore, microfluidics offers a platform for replicating cell dynamics in vivo using hydrodynamic forces exclusively, employing non-intrusive measurement techniques involving imaging and optics. In this context, two distinct micro-scale devices have been examined for screening blood cells, specifically platelets and red blood cells. In this comprehensive investigation, microfluidic devices were characterised through micro-PIV measurements to understand their performance in high-strain rate applications, particularly in blood handling and hydrodynamic particle trapping. The study employed micropump devices with varying actuation frequencies to mimic pulsatile flow profiles. Interestingly, only one specific channel operating at 3 Hz proved optimal for platelet-based assays, successfully managing small fluid volumes while achieving the critical shear gradient required for platelet activation. Notably, fabrication tolerance variations among devices underscored the importance of device uniformity. Furthermore, the micro-PIV measurements were extended to cross-slot channels to estimate strain rates at different Reynolds numbers. This multifaceted approach enhanced the understanding of microfluidic devices, aiding in the selection of appropriate platforms for applications demanding high strain rates. For the motivation of single-cell experiments, real-time imaging control experiments were conducted for hydrodynamic particle trapping in microfluidic cross-slot channels. Through detailed characterisation in channels of varying widths (0.4 mm and 0.1 mm), the study employed an image-based linear feedback control algorithm implemented in Python-OpenCV to assess the stability of particle confinement. The investigation revealed a distinct interplay between algorithmic delay, particle resolution, and achievable strain rates. Specifically, reductions in algorithmic delay led to notable increases in the maximum attainable strain rates, particularly evident in the 0.1 mm channel, where controlled high strain rates of 250 (1/s) ever achieved in the cross-slot channels. Moreover, the study expanded its scope to investigate the feasibility of trapping individual red blood cells (RBCs) under high strain rates, elucidating the challenges posed by the asymmetrical nature of biological specimens. These investigations underscored the critical need for robust image processing techniques, especially for asymmetric entities like RBCs, to ensure accurate measurements at elevated strain rates. Overall, this research lays a solid foundation for advancing high-strain rate applications in microfluidics, offering insights that extend beyond blood handling to various precision-controlled hydrodynamic scenarios.
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ItemPhysical modelling of riblet performance in yawed and non-yawed flows( 2023-11)We introduce a viscous vortex model to predict the drag and heat transfer of riblets, eliminating the need for expensive direct numerical simulations (DNS) or experiments. The footprint of a typical quasi-streamwise vortex, in terms of the spanwise and wall-normal velocities, is extracted from smooth-wall DNS flow fields in close proximity to the surface. The extracted velocities are then averaged and used as boundary conditions in a Stokes-flow problem, wherein riblets with various cross-sectional shapes are embedded. Here, the same smooth-wall based boundary conditions can be used for riblets, as we observe from the DNSs that the quasi-streamwise vortices remain unmodified apart from an offset. In particular, the position of these vortices remain unpinned above small riblets. This smooth-wall-like behaviour persists up to the optimal size of riblets, and also up to a yaw angle of 15 degrees, enabling us to predict the riblet drag and heat transfer within these size and yaw-angle constraints. The present approach is compared with the protrusion height model of Luchini et al. (J. Fluid Mech., vol. 228, 1991, pp. 87–109), which is also based on a Stokes calculation, but represents the vortex with only a uniform spanwise velocity boundary condition. The key novelty of the viscous vortex model is the introduction of a non-zero wall-normal velocity component into the boundary condition. This approach induces transpiration at the riblet crests, as transpiration becomes relevant at increasing riblet sizes. Consequently, we show that the drag and heat-transfer prediction of the present model agree with our DNS data, as well as published data.
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ItemComputational wake acoustics : effects of freestream disturbances on wake structure and sound emission(University of Melbourne, 2010)
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ItemAnalysis of numerical error on unstructured meshes: with applications to fluid dynamics(University of Melbourne, 2010)
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ItemSensor scheduling for target tracking in sensor networks(University of Melbourne, 2009)
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ItemTrapping and manipulation of small particles using laser lights(University of Melbourne, 2009)
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ItemHybrid methods for the detection of regulatory signals in genomic sequences(University of Melbourne, 2008)