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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Ultrasonic inspection of highly scattering materials

Van Pamel, Anton January 2015 (has links)
Ultrasonic Non-Destructive Evaluation (NDE) relies on the scattering of waves from discontinuities, such as fractures or voids, to probe media otherwise invisible to the naked eye. Whilst this has been industrially exploited for several decades within acoustically transparent materials, many materials maintain a microstructure that causes scattering of the propagating waves. This undermines the aforementioned premise as it becomes exceedingly difficult to discern the features of interest from the scattering inherent to microstructural features, thereby limiting the range of materials which can be reliably inspected, non-destructively. Experimental investigations confirm the challenges and significant shortcomings for the inspection of future industrial components where such microstructures are desirable for their mechanical properties. It is demonstrated that the rapid increases in scattering with the insonifying frequency severely limit the achievable sensitivity of conventional ultrasound techniques. A review of the latest advances in ultrasound technology, including signal processing and imaging algorithms, explore the opportunities to exceed current limitations and advance the capability of ultrasonic NDE. Establishing these advances, and those of future approaches, requires a rigorous definition of performance. In contrast to commonly adopted strategies, a novel strategy which considers the probabilities of detection and false alarms is proposed as a valuable benchmark that can be used to make objective comparisons in terms of performance between competing algorithms. Future progress will also rely on a better scientific understanding of scattering, which can be provided by powerful modelling tools. Here, Finite Element modelling is established to be very useful; it captures the complex scattering physics and allows an investigative flexibility which can provide extremely useful insights. Whereas previous studies have often been restricted to weak scattering assumptions, the present FE modelling capability now enables the study of more complex, highly scattering environments. This is demonstrated by investigating ultrasonic arrays, where through optimising their engineering, especially in terms of their configuration, significant performance enhancements are shown to be possible. These important scientific tools have enabled the assessment of the latest imaging algorithms, the optimisation of inspection configurations, and increased our understanding of scattering phenomena. Their use in the future enables wide possibilities towards further pursuing the ultrasonic inspection of highly scattering materials.
12

Improved reliability of automated non-destructive evaluation

Tippetts, Trevor January 2014 (has links)
In recent years, Non-Destructive Evaluation (NDE) has trended toward increased automation in data acquisition. Automated scanning has the potential to greatly increase reliability of the NDE results, but it also tends to increase the volume of data that must be inspected manually by a skilled technician. This is a time-consuming task made tedious by the fact that most of the data contains no indication of a defect. There is a great need for software that can partially automate the data analysis by prioritizing regions of interest to the inspector. This thesis describes an approach to that end, laying out a framework that is general enough to fit a wide array of NDE applications. It also describes practical considerations for the specific application of ultrasound inspection of titanium turbine discs.
13

Local rheology of lubricants in the elastohydrodynamic regime

Ponjavic, Aleks January 2014 (has links)
Numerous models have been developed to describe the viscosity and rheology of lubricants in elastohydrodynamic (EHD) lubrication, but little experimental and theoretical work has been done on the flow of lubricants. Due to the high pressures in a tribological contact it is likely that lubricants may undergo structural changes, which would significantly affect their flow. Photobleached-fluorescence imaging velocimetry was applied to a glass-glass EHD contact, lubricated with the oligomer polybutene, which was doped with fluorescent dye. The technique involved tagging a volume inside the contact, by making it dark compared to its surroundings. A model was developed to solve for the through-thickness velocity profile using the experimental data and the technique was validated experimentally using a parallel plate Couette setup. Velocity profiles of polybutene in an EHD contact were measured under various conditions. Three distinct rheological responses could be observed. At low pressures, the velocity profile was mostly linear. At a critical pressure, a low shear rate plug formed in the centre of the film, possibly due to pressure-induced glass transition of the lubricant. The application of a low surface energy coating caused the lubricant to slip at the interface, depending on the applied pressure. The velocimetry studies were supported by film thickness and friction measurements. Laser-induced fluorescence was used to measure the film thickness, showing that the plug flow of polybutene coincided with an anomalous increase in film thickness, while the occurrence of boundary slip resulted in reduced film thickness. Friction measurements showed that plug flow had negligible effects on friction. Boundary slip however caused a decrease in friction (up to 70 %). Results suggest that lubricant flow in an elastohydrodynamic contact is non-trivial and deserves more consideration than is typically given. Direct flow measurements could be useful to elucidate the complex relationship between film thickness, friction and flow.
14

Mechanistic models to simulate slug flow in horizontal and vertical pipes

Di Salvo, Fabio Robert January 2014 (has links)
While numerous studies have been conducted on using the one-dimensional, two-fluid model to simulate a range of flow regimes in horizontal and nearly horizontal pipes, no work has been conducted thus far on using the model to simulate intermittent flow in vertical pipes, specifically in the slug flow regime where large gas bubbles are separated by rising liquid slugs. This thesis presents the development of the model to accurately simulate this flow regime. For the first time, it has been shown that the model can capture the underlying physics behind slug generation in vertical flow: that of a falling liquid film leading to a bridging of the pipe, thereby resulting in the formation of slugs. Closure relations for the interfacial shear force are proposed, tested and developed, where it was found that the choice of model used in the flow development region has a significant effect on the flow downstream. A new correlation has been developed that is able to accurately reproduce results and trends seen experimentally. The effects of the viscous diffusion term, a pressure loss model at the slug front and the surface tension term, all previously introduced into the model, were tested for the vertical flow cases. The effects of mesh size and the influence of the inlet boundary conditions on the characteristics of the generated slugs were also investigated. As well as the vertical slug flow work, the thesis also presents results obtained in testing the models capabilities to simulate two other effects found in two-phase flows in pipes. The first is the hysteresis phenomena found in horizontal pipes, where the point of transition from stratified flow to slug flow and vice versa is found to shift depending on the starting flow regime. The second is terrain-induced slugging, where bends in the pipe can cause a localised build-up of liquid, causing undesired fluctuations in flow rates and pressures at the pipe outlet.
15

Simultaneous temperature and velocity imaging in turbulent flows using thermographic phosphor tracer particles

Fond, Benoit January 2014 (has links)
Combined measurements of velocity and temperature are essential to improve our understanding of turbulent flows involving heat transfer or chemical reactions. However, performing such measurements is a very difficult task. The presence of particles, which are seeded into the flow as tracers for the flow velocity, strongly interferes with classic optical thermometry techniques such as Rayleigh scattering. A review of the current approaches shows that a technique that can measure both quantities simultaneously, in two dimensions and over a wide range of flow conditions is yet to be found. An alternative approach to this problem, presented in this dissertation, uses tracer particles made of temperature-sensitive luminescent material, which are capable of also indicating the gas temperature. Thermographic phosphors are shown to be clear candidates for this concept. Made of ceramic material, they are chemically inert and survive low and high temperature environments. The temperature has a strong influence on the luminescence process allowing various ways to perform thermometry Currently, phosphors are used for surface temperature measurements, but a phosphor suitable for two-dimensional measurements in turbulent flows must meet stringent requirements in terms of luminescence properties. In this respect, the temperature dependence of the emission spectrum, a high quantum efficiency and a short lifetime are essential. Micrometre-size refractory particles are widely used for PIV and are able to follow the fluid motion without slip for a wide range of fluid velocities and turbulence intensities However, for the concept to be valid, the ability of phosphor particles to follow fluctuations in the gas temperature must be demonstrated. Using theoretical heat transfer models, it is shown that the temperature response of a particle is faster than its velocity response irrespective of the gas temperature. These response times have a quadratic dependence on the particle diameter so only small particles can be used. Various aspects of the practical implementation of the flow measurement concept, such as the excitation, particle seeding, detection, image processing and calibration, are considered, tested and developed, with the objective of providing high signal levels and to permit precise, accurate, and highly resolved measurements. In order to determine whether a sufficient signal level can be obtained for a reasonable particle seeding density, i.e. that does not have any effect on the gas properties, a particle counting tool is implemented. This system is used to characterise the phosphorescence intensity of 2 μm diameter particles made of BAM:Eu2+, a phosphor with very advantageous properties for flow measurements. It is shown that a seeding density comparable to that of conventional PIV and relatively small laser fluence provide sufficient signal levels for precise single shot measurements. The technique is demonstrated in a turbulent heated jet from 300 K to 700 K. Single shot measurements of temperature and velocity are presented with a single-shot, single-pixel temperature precision of 2-5 %, a temperature accuracy of 2%, and a spatial resolution of 400 μm. An additional concept is explored. By seeding two streams with different materials, the phosphorescence signal can be used to visualise the turbulent mixing between the streams. This concept is demonstrated in the same turbulent heated jet. Future developments and applications of the thermographic phosphor tracer particle concept are discussed. Owing to the very wide variety of thermographic phosphors, the results presented in this dissertation constitute a solid foundation for the expansion of this promising technique.
16

Blast resistance of laminated glass facades

Del Linz, Paolo January 2014 (has links)
The aim of this thesis is to improve the understanding of the behaviour of Polyvinyl Butyral (PVB) laminated annealed glass façade panels subjected to blast loading. A full scale blast test was performed. During this, deflection and strain data were collected employing digital image correlation techniques (DIC). Local reaction forces were measured using several pairs of strain gauges on the support. The full field deflection and strain data obtained were in line with those observed in historical tests. The strain gauge data available showed that the reaction forces varied along the edge, with higher values being reached at the quarter length gauge locations. The results from this test and from other historical experiments were used to calculate the reaction forces along the entire perimeter of the glass pane. The results showed that the forces reach an early peak before the glass failure, and then rise gradually approaching a plateau at high central deflections. To explain the specific form of this force time history, the detailed behaviour of the laminated material after the glass skins failed was studied. Existing experimental data was employed to fit a material model to the PVB material. Two Prony series models with different hyperelastic springs and a model employing a full finite deformation viscoelastic law derivation were employed. It was found that the finite deformation viscoelastic model could represent the material's behaviour more accurately and fully include its rate dependency. One of the PVB models was employed to study the delamination between the glass and the membrane. Delamination energies were found for different speeds of deformation, and these parameters were employed to study the delamination of samples presenting different crack arrangements. The results showed that these had only a limited influence on the behaviour of the composite.
17

Thermal management of the permanent magnets in a totally enclosed axial flux permanent magnet synchronous machine

Malloy, Adam January 2014 (has links)
Elevated magnet temperature in Axial Flux Permanent Magnet Synchronous Machines (AF PMSM) adversely affects torque production, material cost, and the risk of demagnetisation. These machines show promise in applications requiring high power density, however the factors which affect magnet temperature have rarely been investigated. This is therefore the focus of the thesis. A multiphysics numerical model was formulated which predicted the loss, flow, and temperature fields within an AF PMSM. A criterion for estimating the relative importance of the fluctuating component of a periodic heat source on the temperature response of a device was proposed and validated. In this work it was used to justify a steady state, rather than transient, thermal analysis. Thermometric and electrical measurements were taken from an instrumented AF PMSM to validate the numerical predictions. A novel magnet loss measurement technique was implemented; losses were determined by measuring the initial temperature rise rate of the magnets. This was achieved via a calibration relating temperature rise to voltage constant. It was found that 99% of the heat generated in the magnets was convected to the inner cavity of the machine, due to the inner cavity's recirculating flow structure this heat was dissipated to the casing and core. As a proportion of all heat entering the inner cavity 56-62% left to the casing while 28-41% left to the core. Magnet hot spots were found to be up to 13% greater than the mean temperature rise. Their location was influenced by the distribution of losses and the direction of shaft rotation. Temperature gradients within the inner cavity caused the magnet's trailing edge to incur a 10% greater temperature rise than the leading edge. As increasing temperature decreases the coercivity of magnet materials these findings are a crucial contribution to the understanding of devices where local demagnetisation is of concern.
18

A method for development and validation of multi-agent systems using accurate communication network modelling

Perkonigg, Fidelis January 2014 (has links)
There has been a considerable amount of research on multi-agent system (MAS) technologies for a wide variety of industrial applications. One application domain is the power industry, for which multi-agent systems are widely suggested as a promising method for the realisation of highly distributed, flexible, fault tolerant management and control applications. Multi-agent systems make extensive use of digital communication, which can significantly influence the overall system performance. However, no general solutions have been proposed for the difficult tasks of multi-agent system development and validation that would fully account for the underlying communication network performance, before it is first deployed on the target system. This work proposes a new method for this purpose and presents a novel platform that consists of a federation of a standardised multi-agent system development framework (JADE) and an industry standard network simulator (OPNET Modeler). It was realized through generic extensions of the JADE framework to provide discrete event scheduling capabilities, while the OPNET Modeler was extended to provide a generic method of relating network nodes with agents running in JADE. The federation adheres to the High Level Architecture standard. The multi-agent systems analysed using this platform may be deployed on the target system without manual modifications. An example of a time-critical, agent-based protection system for the Smart Grid is presented and its performance analysed with respect to candidate agent behaviours and different communication scenarios. The results clearly show that the feasibility of the multi-agent system critically depends on the application design as well as the communication infrastructure. The developed multi-agent system was shown to be directly deployable on target hardware, which proves that the proposed method not only supports analysis through simulation but also subsequent deployment. The new platform can be used to rapidly develop a wide range of agent-based applications and validate them for different communication technologies before deployment.
19

Impact of electrically assisted turbocharging on the transient response of an off-highway diesel engine

Terdich, Nicola January 2014 (has links)
Engine boosting via turbocharging is a method to increase the engine power output with minimal or no increase in engine parasitic, frictional and pumping losses. Turbocharging in conjunction with engine down-sizing and down-speeding allows a reduction of engine fuel consumption, while maintaining a high engine power output. However, turbocharging introduces a lag in engine transient response, caused by the finite amount of time required by the turbocharger to accelerate, which has to be minimized. Electric turbocharger assistance consists of coupling an electric motor/generator to a standard turbocharger. The scope of the motor/generator is to increase the power available to accelerate the rotor assembly, so that the time to boost is reduced. The motor/generator could also be utilized to brake the turbocharger to control boost and avoid over-speeds, thus replacing the conventional waste-gate. Furthermore, electric assistance allows turbocompounding to be implemented. Turbocompounding improves the engine efficiency by utilizing the turbine and motor/generator to recuperate additional exhaust flow energy. In this thesis, the electric turbocharger assistance impact on the turbocharger and engine performance is studied. An electrically assisted turbocharger prototype has been developed by industrial partners and it has been tested by the author of this thesis. The performance of the turbocharger turbine and motor/generator has been characterized over the full speed range and the impact of the electric assistance on the turbine flow has been investigated experimentally. It has not been possible to characterize the turbine up to choking conditions, so the data has been extrapolated via a mean-line model. The performance data obtained has been utilized to generate a model of the assisted turbocharger, which has been coupled to a one-dimensional model of a non-highway 7-litre diesel engine. This model has been utilized to study the impact of electric turbocharger assistance on the engine transient performance. The electrical machine characterization revealed that the switched reluctance motor/generator operates efficiently up to a speed of 135,000 rev/min, making it one of the fastest running switched reluctance machines of this size. The peak machine efficiency is 93% (excluding the turbocharger bearing losses) and the maximum power output measured is 5.3 kW in generating mode and 4.3 kW in motoring mode. The motor/generator rotor aerodynamic drag loss has been calculated via computational fluid dynamics software and has been found to be 63 W at 140,000 rev/min. Via a novel experimental technique, it has been possible to characterize the turbocharger turbine down to an expansion ratio of 1.00. This experiment revealed that the mass flow rate drops to zero at an expansion ratio higher than unity and that below this critical pressure ratio the turbine flow is reversed. The characterization of the turbine during speed transients showed that the operating point on the performance map deviates from the quasi-steady line. This indicates that minor unsteady effects occur in the turbine and exhaust manifold flow. A further experiment revealed that the motor/generator torque oscillations have a negligible impact on the turbine performance. The engine simulations showed that the ideal electric assistance motoring power for this application is in the 5 to 10 kW range. A 5 kW machine reduces the engine speed drop, which occurs when the engine load is suddenly increased, by up to 83%, depending on the initial load and load step size, and reduces the time to recover the original speed by up to 86%. The simulations also revealed that electric assistance is more effective than the turbine variable geometry system in improving the engine transient response, but the variable geometry system is useful to optimize boost for engine specific fuel consumption over different engine loading conditions.
20

Laser induced incandescence and high speed imaging in hydra optical diesel engine

Hong, Christopher January 2014 (has links)
The aim of this thesis is to investigate ways to reduce soot emission from compression ignition engines by investigating the combustion processes within the diesel engine. The approach is to include computational and experimental work. In order to investigate the trade-off between the amount and stratification of the premixing and the consequent rate of pressure rise, and in particular the effects of changing the autoignition properties of the fuel blend, a multizone code capable of representing the ignition of a premixed charge consequent on multiple injection of fuel is presented. As part of this investigation, this thesis began the process of looking into the potential of gasoline fuel. Experimentally, investigations of soot distribution were carried out in an optically-accessed single cylinder 'Hydra' engine. The soot was visualized using high speed imaging and laser induced incandescence in the optical engine to look at the effect of injection pressure on combustion luminosity and soot distribution. An extensive parametric study was carried out on the Hydra engine to look at the effect of engine load, injection timing and pressure, number of injections, intake temperature and combustion phasing. High speed imaging was carried out to measure injection tip penetration and soot luminosity as a function injection pressure. The combustion luminosity was studied, with proper orthogonal decomposition (POD) analysis, at different injection pressures. It was found that increase of injection pressure decreases combustion luminosity. The technique of laser induced incandescence (LII) was used to visualize the soot. As a preliminary to the use of the LII in the engine, fundamental work on the technique itself was carried out at Heriot-Watt University to isolate the radiative emission component of heat loss from the soot and measure the complex refractive index of soot. Results on soot particle sizes were compared to scanning electron micrograph of the soot collected. The LII measurement was found to be much larger, due to the high signal to noise ratio and the bias towards detection of larger particles. The investigation also showed that the soot agglomerates could undergo micro-explosions on being exposed to laser irradiance. It is speculated that the soot molecules become ionized and mutually repel each other.

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