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Structural health monitoring of onshore wind turbine foundations
No full textOngoing concern regarding humanity's impact on the environment and declining fossil fuel reserves has inspired a global adoption and continual support of renewable power generation. Onshore wind is a major contributor to the world's renewable capacity. Ensuring or possibly extending the lifetime of current assets is key in gaining the most efficient power generation. Structural health monitoring (SHM) systems can be employed to identify the health state of a structure and provide information regarding lifetime. Many such systems are already incorporated into various infrastructure.;Regarding onshore wind turbines, the supporting concrete foundations represent an integral structural feature, where failure could cause collapse of the entire turbine. Severe cracks on such foundations are, therefore, of great concern to turbine operators. SHM of such foundation cracks may provide a more detailed insight into the lifetime of the turbine as a whole. Sub-surface cracks are conventionally monitored using intermittent excavation and visual inspections that are carried out during turbine downtime.;Other methods, such as fracture mechanics, consider the critical failure point of a cracked specimen and fail to provide an indication of increasing severity. Research presented in this thesis demonstrates the application of optical fibre Bragg grating (FBG) strain sensors for crack displacement monitoring on an operational wind turbine foundation. The primary original contribution to knowledge of this work is the development of a novel methodology in order to categorize and quantify crack deterioration measured by subsurface crack displacement sensors deployed on an operational asset with visually severe cracks. Results from this methodology should help decision making procedures in regards to acceptable crack displacements, repairs and the overall lifetime of the turbine.;Accompanying investigation into the effectiveness of epoxy and metallic bonding for FBG attachment demonstrates that purely metallic bonding may provide an improved sensor design. Particularly, a humidity dependence was observed in industry standard epoxies in the form of swelling, with the metallic bonding technique immune to such effects but producing similar performance for strain measurement during direct static and fatigue experimentation. To validate industrial results and further test metallic bonding of FBGs, a unique low-cost small-scale fatigue testing machine is designed and demonstrated for cracked concrete beam fatigue tests.;Transitioning from reactive to preventative methods for initial or additional damage is an important topic and could potentially constitute most future SHM work, including for wind turbine foundations. Prediction methodologies are explored using lab test-benches at this early stage, presenting promise for future application in SHM for preventing critical events. Further development of the sensors, deterioration methodology and prevention techniques as part of a limit alert system for cracks in an onshore wind turbine foundations is suggested.Ongoing concern regarding humanity's impact on the environment and declining fossil fuel reserves has inspired a global adoption and continual support of renewable power generation. Onshore wind is a major contributor to the world's renewable capacity. Ensuring or possibly extending the lifetime of current assets is key in gaining the most efficient power generation. Structural health monitoring (SHM) systems can be employed to identify the health state of a structure and provide information regarding lifetime. Many such systems are already incorporated into various infrastructure.;Regarding onshore wind turbines, the supporting concrete foundations represent an integral structural feature, where failure could cause collapse of the entire turbine. Severe cracks on such foundations are, therefore, of great concern to turbine operators. SHM of such foundation cracks may provide a more detailed insight into the lifetime of the turbine as a whole. Sub-surface cracks are conventionally monitored using intermittent excavation and visual inspections that are carried out during turbine downtime.;Other methods, such as fracture mechanics, consider the critical failure point of a cracked specimen and fail to provide an indication of increasing severity. Research presented in this thesis demonstrates the application of optical fibre Bragg grating (FBG) strain sensors for crack displacement monitoring on an operational wind turbine foundation. The primary original contribution to knowledge of this work is the development of a novel methodology in order to categorize and quantify crack deterioration measured by subsurface crack displacement sensors deployed on an operational asset with visually severe cracks. Results from this methodology should help decision making procedures in regards to acceptable crack displacements, repairs and the overall lifetime of the turbine.;Accompanying investigation into the effectiveness of epoxy and metallic bonding for FBG attachment demonstrates that purely metallic bonding may provide an improved sensor design. Particularly, a humidity dependence was observed in industry standard epoxies in the form of swelling, with the metallic bonding technique immune to such effects but producing similar performance for strain measurement during direct static and fatigue experimentation. To validate industrial results and further test metallic bonding of FBGs, a unique low-cost small-scale fatigue testing machine is designed and demonstrated for cracked concrete beam fatigue tests.;Transitioning from reactive to preventative methods for initial or additional damage is an important topic and could potentially constitute most future SHM work, including for wind turbine foundations. Prediction methodologies are explored using lab test-benches at this early stage, presenting promise for future application in SHM for preventing critical events. Further development of the sensors, deterioration methodology and prevention techniques as part of a limit alert system for cracks in an onshore wind turbine foundations is suggested
Development of an energy filtering direct electron detector for diffraction studies in the SEM
Get PDFThis thesis was previously held under moratorium from 22nd June 2018 until 25th March 2020.This thesis describes the application of an energy filtering digital direct electron detector for diffraction studies of materials in the field emission scanning electron microscope (SEM). The main aim was the development of the digital complementary metal-oxide-semiconductor hybrid pixel detector, \Timepix" for electron backscatter diffraction (EBSD), a technique which allows the acquisition of precise crystallographic information from the surface of a sample, such as crystallographic orientation, phase and strain. EBSD results from nitride semiconductor, silicon and diamond thin films and tungsten-carbide cobalt samples are presented and used to illustrate the advantages of acquiring EBSD patterns with the Timepix detector, in particular to demonstrate the improvement in the contrast and increase in the detail contained in the EBSD patterns as consequence of the energy filtering. Alongside EBSD, new applications were developed such as re ection high energy electron diffraction (RHEED) in the SEM. RHEED is a very surface sensitive technique which in principle could allow the study of ultrathin samples where conventional SEM based methods are limited. The combination of RHEED and Kikuchi diffraction, led furthermore to the development of surface wave resonance electron channelling contrast imaging (SWRECCI), which allows crystalline defects such as surface steps, grain boundaries, dislocations and stacking faults to be imaged with a high level of surface sensitivity, iii extending furthermore the application of ECCI to non-continuous surfaces. This is obtained by selecting experimental geometries which stimulate the surface wave resonance at the specimen surface. Transmission diffraction in the SEM was also explored, resulting in the acquisition of transmission diffraction patterns and in the generation of images of the sample obtained under experimental conditions analogous to scanning transmission electron microscopy. This allowed for example, bright and dark field images of the specimen to be obtained. The resulting images exhibited crystalline contrast not often observed in the SEM. The Timepix sensor is constructed from a piece of single crystal silicon. Diffraction effects within this single crystal were found to result in the Timepix detector response exhibiting an underlying diffraction pattern; that is a detector diffraction pattern (DDP). The DDP provides a watermark from which the location of the camera relative to the position of the electron beam on the sample may be precisely and accurately determined. This opens up new opportunities for improved mapping of the strain distribution in materials for example. The development of all the novel techniques summarized above opens up new horizons which need to be explored.This thesis describes the application of an energy filtering digital direct electron detector for diffraction studies of materials in the field emission scanning electron microscope (SEM). The main aim was the development of the digital complementary metal-oxide-semiconductor hybrid pixel detector, \Timepix" for electron backscatter diffraction (EBSD), a technique which allows the acquisition of precise crystallographic information from the surface of a sample, such as crystallographic orientation, phase and strain. EBSD results from nitride semiconductor, silicon and diamond thin films and tungsten-carbide cobalt samples are presented and used to illustrate the advantages of acquiring EBSD patterns with the Timepix detector, in particular to demonstrate the improvement in the contrast and increase in the detail contained in the EBSD patterns as consequence of the energy filtering. Alongside EBSD, new applications were developed such as re ection high energy electron diffraction (RHEED) in the SEM. RHEED is a very surface sensitive technique which in principle could allow the study of ultrathin samples where conventional SEM based methods are limited. The combination of RHEED and Kikuchi diffraction, led furthermore to the development of surface wave resonance electron channelling contrast imaging (SWRECCI), which allows crystalline defects such as surface steps, grain boundaries, dislocations and stacking faults to be imaged with a high level of surface sensitivity, iii extending furthermore the application of ECCI to non-continuous surfaces. This is obtained by selecting experimental geometries which stimulate the surface wave resonance at the specimen surface. Transmission diffraction in the SEM was also explored, resulting in the acquisition of transmission diffraction patterns and in the generation of images of the sample obtained under experimental conditions analogous to scanning transmission electron microscopy. This allowed for example, bright and dark field images of the specimen to be obtained. The resulting images exhibited crystalline contrast not often observed in the SEM. The Timepix sensor is constructed from a piece of single crystal silicon. Diffraction effects within this single crystal were found to result in the Timepix detector response exhibiting an underlying diffraction pattern; that is a detector diffraction pattern (DDP). The DDP provides a watermark from which the location of the camera relative to the position of the electron beam on the sample may be precisely and accurately determined. This opens up new opportunities for improved mapping of the strain distribution in materials for example. The development of all the novel techniques summarized above opens up new horizons which need to be explored
Computational modelling of the effect of surfaces on polyvinyldenedifluoride
Get PDFThe physical properties of polyvinyldenediluoride (PVDF) polymorphs and the effect of surfaces on PVDF properties have been investigated with computational modelling to address crystallinity issues that such semi-crystalline polymer presents. Indeed, PVDF has the potential to support new technology generation of lexible electronic devices, but to preparere liable devices made with PVDF, such polymer needs to be sampled at high crystalline grade.;As PVDF is a semi-crystalline polymer its intrinsic lexibility represents a major advantage for lexible electronics which also increases manufacturing complexity of such material. To understand the crystalline behaviour of PVDF it is necessary to computationally investigate its fundamental physical properties per each of its crystal phase and the main behaviour of PVDF in conditions of inite temperature.;Density functional theory (DFT) calculations has been used as a quantum mechanical (QM) tool to solve the electronic structures of PVDF polymorphs obtaining structural information such as geometries, energetics, spontaneous polarisation and vibrational frequencies. Furthermore the impact of including van derWaals (vdW) forces in DFT was evaluated showing that the vdW-DF DFT functional had the best physical properties prediction agreement with experimental observations.;The vibrational frequencies of all PVDF polymorphs were computationally determined to verify the metastability of every crystal phase determined in the present study. Furthermore, the vibrational frequencies determination allowed to enrich the knowledge about adsorption peaks that each PVDF structure possesses to ease the computation to experimental IR spectra comparison.;The optimised geometries of PVDF crystals obtained from the DFT investigation have been scaled to molecular dynamics (MD) since it represents a time consistent methodology to follow the evolution of molecular interactions between particles. The interest was to compute the inite temperature dynamics, ensuring the use of the best performing force field and to gather new knowledge about the crystalline phase formation of PVDF liquid melts under different conditions such as bulk and confined between surface layers.;The effect of polymer confinement and surface/polymer electrostatics interaction were evaluated in such study showing that electrostatics played a main role in driving the formation of highly crystalline PVDF systems.The physical properties of polyvinyldenediluoride (PVDF) polymorphs and the effect of surfaces on PVDF properties have been investigated with computational modelling to address crystallinity issues that such semi-crystalline polymer presents. Indeed, PVDF has the potential to support new technology generation of lexible electronic devices, but to preparere liable devices made with PVDF, such polymer needs to be sampled at high crystalline grade.;As PVDF is a semi-crystalline polymer its intrinsic lexibility represents a major advantage for lexible electronics which also increases manufacturing complexity of such material. To understand the crystalline behaviour of PVDF it is necessary to computationally investigate its fundamental physical properties per each of its crystal phase and the main behaviour of PVDF in conditions of inite temperature.;Density functional theory (DFT) calculations has been used as a quantum mechanical (QM) tool to solve the electronic structures of PVDF polymorphs obtaining structural information such as geometries, energetics, spontaneous polarisation and vibrational frequencies. Furthermore the impact of including van derWaals (vdW) forces in DFT was evaluated showing that the vdW-DF DFT functional had the best physical properties prediction agreement with experimental observations.;The vibrational frequencies of all PVDF polymorphs were computationally determined to verify the metastability of every crystal phase determined in the present study. Furthermore, the vibrational frequencies determination allowed to enrich the knowledge about adsorption peaks that each PVDF structure possesses to ease the computation to experimental IR spectra comparison.;The optimised geometries of PVDF crystals obtained from the DFT investigation have been scaled to molecular dynamics (MD) since it represents a time consistent methodology to follow the evolution of molecular interactions between particles. The interest was to compute the inite temperature dynamics, ensuring the use of the best performing force field and to gather new knowledge about the crystalline phase formation of PVDF liquid melts under different conditions such as bulk and confined between surface layers.;The effect of polymer confinement and surface/polymer electrostatics interaction were evaluated in such study showing that electrostatics played a main role in driving the formation of highly crystalline PVDF systems
Fundamental atomic data and prototype techniques for a generalised collisional-radiative model of medium-weight elements in fusion and astrophysical plasma
Get PDFQuantifying the influence of heavy impurities upon plasma power balance, while seizing the opportunities they offer for passive spectroscopy, requires generalised collisional-radiative (GCR) population modelling to produce high-quality ionisation balances and cooling curves. ADAS provides a well-established framework of codes and data for the GCR modelling of light species and has been applied extensively to these scenarios. The extension of GCR modelling to medium and heavyweight elements within the ADAS framework imposes a number of updates and modifications. First, a lift of the ADAS baseline atomic structure and collision data is proposed using autostructure with the distorted-wave approximation, configuration sets selected by optimising on radiated power, and a novel, algorithmic strategy for optimising the radial scaling parameters. The truncation error of the configuration sets is bounded between an order of magnitude and 10%, while three figures of merit prove that the scaling parameter optimisation has eliminated the 20-30% structure error relative to the Cowan code. Second, fully relativistic, partially radiation-damped, Dirac R-matrix calculations of the W44+ ion are performed to showcase the challenges of generating fundamental data for heavy species. The calculations use a configuration interaction and close-coupling expansion that opens up the 3d-subshell, yielding previously unexplored transition arrays, [3d104s2-3d94s24f] and [3d104s2-3d94s4p4d], which contribute 50% of the total radiated line power coefficient (PLT ) near the temperature of peak abundance. Third, collisional excitation by ion projectiles, not just electrons, must now be considered. A broad baseline of ion-impact excitation data is fulfilled by the restoration of a code, a2iratbt, that uses semi-classical, first-order perturbative equations with a limiting function, to prevent transition probability overestimates at intermediate energies, and a radial cutoff, which ensures the xiv infinite-energy Born limit is approached at high energies. The majority of the error in this baseline comes from the neglect of close coupling, accounting for ~ 20% in triplets and < 5% in doublets. Fourth, and most importantly, the resolution of GCR modelling must be moved to intermediate coupling. A prototype is built upon the LS-resolved analogue, predominantly by statistically splitting relevant quantities onto the intermediate-coupling manifold. Comparison to the unresolved fractional abundances in the literature reveal density effects of over an order of magnitude for the near neutrals, decreasing gradually towards complete agreement for the highly ionised stages. The total radiated power function and PLT s showed better agreement, generally within 50% for the higher quality sources. Investigations into the effects of ion-impact excitation and resolution upon the GCR results are performed, showing that neither can be ignored. Also, a true set of metastable terms and levels is established. In the final analysis, the real impact of this new model can only be completely assessed by applying its results in subsequent plasma transport modelling.Quantifying the influence of heavy impurities upon plasma power balance, while seizing the opportunities they offer for passive spectroscopy, requires generalised collisional-radiative (GCR) population modelling to produce high-quality ionisation balances and cooling curves. ADAS provides a well-established framework of codes and data for the GCR modelling of light species and has been applied extensively to these scenarios. The extension of GCR modelling to medium and heavyweight elements within the ADAS framework imposes a number of updates and modifications. First, a lift of the ADAS baseline atomic structure and collision data is proposed using autostructure with the distorted-wave approximation, configuration sets selected by optimising on radiated power, and a novel, algorithmic strategy for optimising the radial scaling parameters. The truncation error of the configuration sets is bounded between an order of magnitude and 10%, while three figures of merit prove that the scaling parameter optimisation has eliminated the 20-30% structure error relative to the Cowan code. Second, fully relativistic, partially radiation-damped, Dirac R-matrix calculations of the W44+ ion are performed to showcase the challenges of generating fundamental data for heavy species. The calculations use a configuration interaction and close-coupling expansion that opens up the 3d-subshell, yielding previously unexplored transition arrays, [3d104s2-3d94s24f] and [3d104s2-3d94s4p4d], which contribute 50% of the total radiated line power coefficient (PLT ) near the temperature of peak abundance. Third, collisional excitation by ion projectiles, not just electrons, must now be considered. A broad baseline of ion-impact excitation data is fulfilled by the restoration of a code, a2iratbt, that uses semi-classical, first-order perturbative equations with a limiting function, to prevent transition probability overestimates at intermediate energies, and a radial cutoff, which ensures the xiv infinite-energy Born limit is approached at high energies. The majority of the error in this baseline comes from the neglect of close coupling, accounting for ~ 20% in triplets and < 5% in doublets. Fourth, and most importantly, the resolution of GCR modelling must be moved to intermediate coupling. A prototype is built upon the LS-resolved analogue, predominantly by statistically splitting relevant quantities onto the intermediate-coupling manifold. Comparison to the unresolved fractional abundances in the literature reveal density effects of over an order of magnitude for the near neutrals, decreasing gradually towards complete agreement for the highly ionised stages. The total radiated power function and PLT s showed better agreement, generally within 50% for the higher quality sources. Investigations into the effects of ion-impact excitation and resolution upon the GCR results are performed, showing that neither can be ignored. Also, a true set of metastable terms and levels is established. In the final analysis, the real impact of this new model can only be completely assessed by applying its results in subsequent plasma transport modelling
A digital aid to support adults with mild learning disabilities during clinical consultations
Get PDFSome of the factors that explain poor health outcomes for people with learning disabilities (LDs) include multi-morbidity and underdiagnosed health conditions. Previous studies have suggested that this population face significant communication barriers when interacting with health professionals and this is a major contributing factor to such diagnosis complications. Consequently, such health barriers are often preventable. However, there is a surprising lack of research-based technologies available that intend to promote this communication.;We aim to address this gap by investigating the potential of using mobile technologies to support adults with mild LDs during clinical consultations. To achieve this, we interviewed a number of domain experts including government advisors, academics, support workers and General Practitioners. We then developed a technology probe to inform the information extracted and subsequently created a set of design guidelines for the development of Augmentative and Communicative technologies that target the clinical needs of adults with mild LDs.Some of the factors that explain poor health outcomes for people with learning disabilities (LDs) include multi-morbidity and underdiagnosed health conditions. Previous studies have suggested that this population face significant communication barriers when interacting with health professionals and this is a major contributing factor to such diagnosis complications. Consequently, such health barriers are often preventable. However, there is a surprising lack of research-based technologies available that intend to promote this communication.;We aim to address this gap by investigating the potential of using mobile technologies to support adults with mild LDs during clinical consultations. To achieve this, we interviewed a number of domain experts including government advisors, academics, support workers and General Practitioners. We then developed a technology probe to inform the information extracted and subsequently created a set of design guidelines for the development of Augmentative and Communicative technologies that target the clinical needs of adults with mild LDs
Wind turbine dynamics identification using gaussian process machine learning
Get PDFWind turbine controllers require dynamic information about the turbine for design and operation purposes. These dynamics are currently determined from simulation models during the turbine design stages. Hence, the dynamics for a given operational turbine will not be identical to those assumed by the controller due to manufacturing and construction variations. Furthermore, turbine dynamics are known to change over time due to environmental effects such as blade erosion. There are currently no known methods by which such information can be determined for an operational turbine. This thesis presents such a method.;The determination of sought dynamic information is formulated as a regression problem involving data available to a wind turbine controller. The nature of the dynamics identification problem is shown to necessitate a regression method which is able to process data in batches, updating predictions as new data becomes available.;Gaussian process machine learning is chosen as the regression approach best suited for application in this problem. However, a review of existing batched Gaussian process theory results in the identification of gaps in the current knowledge base which render existing methods unsuitable. A new approach to batched Gaussian process regression is therefore developed, Sufficient-Subset Gaussian process iteration, which addresses the questions for which existing theories come up short. In the process of developing this new method fundamental contributions have been made to the areas of Gaussian process polynomial regression and sparse Gaussian process approximation theory.;Sufficient-Subset Gaussian process iteration is applied to both simulated and real turbine data and shown to be able to identify the sought dynamics to within a 3% error threshold. Additionally, a related regression formulation corresponding to maximum efficiency tracking is shown to present a potential method for turbine monitoring and fault detection.Wind turbine controllers require dynamic information about the turbine for design and operation purposes. These dynamics are currently determined from simulation models during the turbine design stages. Hence, the dynamics for a given operational turbine will not be identical to those assumed by the controller due to manufacturing and construction variations. Furthermore, turbine dynamics are known to change over time due to environmental effects such as blade erosion. There are currently no known methods by which such information can be determined for an operational turbine. This thesis presents such a method.;The determination of sought dynamic information is formulated as a regression problem involving data available to a wind turbine controller. The nature of the dynamics identification problem is shown to necessitate a regression method which is able to process data in batches, updating predictions as new data becomes available.;Gaussian process machine learning is chosen as the regression approach best suited for application in this problem. However, a review of existing batched Gaussian process theory results in the identification of gaps in the current knowledge base which render existing methods unsuitable. A new approach to batched Gaussian process regression is therefore developed, Sufficient-Subset Gaussian process iteration, which addresses the questions for which existing theories come up short. In the process of developing this new method fundamental contributions have been made to the areas of Gaussian process polynomial regression and sparse Gaussian process approximation theory.;Sufficient-Subset Gaussian process iteration is applied to both simulated and real turbine data and shown to be able to identify the sought dynamics to within a 3% error threshold. Additionally, a related regression formulation corresponding to maximum efficiency tracking is shown to present a potential method for turbine monitoring and fault detection
Investigation of smooth contact angle treatment in porous media
Get PDFSome of the key challenges faced in the oil/gas extraction and carbon dioxide injection/storage processes are the presence of complex geometries and the significant effect of the capillary forces which arise at low capillary numbers. Therefore, the contact angle needs to be carefully treated. Mesoscopic techniques such as lattice Boltzmann methods are capable of dealing with lower capillary numbers as compared to the Navier-Stokes solvers, which can also implicitly capture the interface between two fluids.;To investigate immiscible two-phase ows at low Reynolds and capillary numbers (Re<1 and Ca<1), the colour-fluid model is used i.e. the Rothman-Keller model [1]. This model includes two steps: a perturbation operator from Lishchuk et al [2] (the continuum surface force [3]) or Gunstensen et al [4] approaches and a recolouring operator [5]. However, the lattice Boltzmann implementation employs a Cartesian grid for domain discretisation that is unable to conform with curved surfaces.;It misinterprets those curved surfaces as a series of stair-like patterns. On those surfaces, a non-physical contact angle could be defined which may lead to a numerically flooding of the wetting fluid inside the droplet for a non-spreading drop or outside for a spreading droplet.To remove this unphysical behaviour and take into account the flow field effect on the contact angle, interpolation techniques are employed to estimate the real contact angle on the 'stairs' boundaries. We also employ extrapolations to obtain more accurate density on concave corners, thus the grid resolution can be reduced.;After the code is numerically validated on static droplets, on droplets deformed under a simple shear, and on simple geometries. Finally, we perform simulations on a Berea sandstone sample [6] to understand dynamics behaviour of immiscible fluids in porous media.Some of the key challenges faced in the oil/gas extraction and carbon dioxide injection/storage processes are the presence of complex geometries and the significant effect of the capillary forces which arise at low capillary numbers. Therefore, the contact angle needs to be carefully treated. Mesoscopic techniques such as lattice Boltzmann methods are capable of dealing with lower capillary numbers as compared to the Navier-Stokes solvers, which can also implicitly capture the interface between two fluids.;To investigate immiscible two-phase ows at low Reynolds and capillary numbers (Re<1 and Ca<1), the colour-fluid model is used i.e. the Rothman-Keller model [1]. This model includes two steps: a perturbation operator from Lishchuk et al [2] (the continuum surface force [3]) or Gunstensen et al [4] approaches and a recolouring operator [5]. However, the lattice Boltzmann implementation employs a Cartesian grid for domain discretisation that is unable to conform with curved surfaces.;It misinterprets those curved surfaces as a series of stair-like patterns. On those surfaces, a non-physical contact angle could be defined which may lead to a numerically flooding of the wetting fluid inside the droplet for a non-spreading drop or outside for a spreading droplet.To remove this unphysical behaviour and take into account the flow field effect on the contact angle, interpolation techniques are employed to estimate the real contact angle on the 'stairs' boundaries. We also employ extrapolations to obtain more accurate density on concave corners, thus the grid resolution can be reduced.;After the code is numerically validated on static droplets, on droplets deformed under a simple shear, and on simple geometries. Finally, we perform simulations on a Berea sandstone sample [6] to understand dynamics behaviour of immiscible fluids in porous media
A single-site resolution fermionic quantum-gas microscope
No full textQuantum-gas microscopes have become an important tool in quantum simulation as they enable direct probing of local quantities by single-atom-resolved detection in optical lattices. Recent years have seen many fascinating results from the bosonic quantum-gas microscopes using 87Rb. However, no such device existed for fermionic species.;The goal of this thesis work was to develop and build a quantum-gas microscope setup for fermionic potassium-40. Single-atom-resolved imaging of 40K has proven very challenging due to its smaller mass and smaller excited-state hyperfine splitting compared to 87Rb. In addition, the inverted excited state trapping potential required us to employ electromagnetically induced transparency (EIT) cooling instead of sub-Doppler molasses cooling.;EIT cooling occurs when a coherent driving of a three-level system generates a spectrally narrow Fano-like resonance which can be set to favour red-sideband transitions over blue ones of the quantised vibrational levels in the optical lattice potential. During the cooling process, the fluorescence light is collected by a high-NA objective to image the atomic distribution in a two-dimentional square lattice potential.;Due to the physical constraints of our apparatus, EIT cooling had to be combined with coupling between different motional axes via Raman transitions to achieve cooling of all degrees of freedom. Our imaging method allowed us to collect about 1,000 fluorescence photons per atom within a 1.5 s exposure time. From the analysis of two distinct subsequent fluorescence images, we found that less than 5% of the atoms hopped or were lost during 1s of EIT cooling. Such fidelity will allow fermionic quantum-gas microscopes to investigate fermionic quantum phases, spin-spin correlations, and out of-equilibrium dynamics of correlated fermionic many-body quantum systems.Quantum-gas microscopes have become an important tool in quantum simulation as they enable direct probing of local quantities by single-atom-resolved detection in optical lattices. Recent years have seen many fascinating results from the bosonic quantum-gas microscopes using 87Rb. However, no such device existed for fermionic species.;The goal of this thesis work was to develop and build a quantum-gas microscope setup for fermionic potassium-40. Single-atom-resolved imaging of 40K has proven very challenging due to its smaller mass and smaller excited-state hyperfine splitting compared to 87Rb. In addition, the inverted excited state trapping potential required us to employ electromagnetically induced transparency (EIT) cooling instead of sub-Doppler molasses cooling.;EIT cooling occurs when a coherent driving of a three-level system generates a spectrally narrow Fano-like resonance which can be set to favour red-sideband transitions over blue ones of the quantised vibrational levels in the optical lattice potential. During the cooling process, the fluorescence light is collected by a high-NA objective to image the atomic distribution in a two-dimentional square lattice potential.;Due to the physical constraints of our apparatus, EIT cooling had to be combined with coupling between different motional axes via Raman transitions to achieve cooling of all degrees of freedom. Our imaging method allowed us to collect about 1,000 fluorescence photons per atom within a 1.5 s exposure time. From the analysis of two distinct subsequent fluorescence images, we found that less than 5% of the atoms hopped or were lost during 1s of EIT cooling. Such fidelity will allow fermionic quantum-gas microscopes to investigate fermionic quantum phases, spin-spin correlations, and out of-equilibrium dynamics of correlated fermionic many-body quantum systems
Particle-scale mechanisms controlling the response of granular and clayey geomaterials at very small strains
Get PDFThis thesis was previously held under moratorium from 18th May 2018 to 18th May 2020.Soil stiffness at very small strains is a fundamental parameter for a wide range of geotechnical applications. The correct evaluation of the soil stiffness parameters at very small strains is essential for the realistic prediction of ground deformations occurring around geotechnical structures under operational conditions (serviceability limit state design) and is used to derive the stiffness degradation curve with increasing strain.The aim of this research is to explore the behaviour of granular and clayey geomaterials at very small strains in cases where the macroscopic response observed experimentally cannot be easily interpreted. In these cases, existing models commonly adopted for the evaluation of the small strain stiffness fail to capture the soil response and may in turn lead to erroneous estimations of ground deformations.Two examples were analysed in this research. Firstly, the stiffness at very small strains of unsaturated granular materials was investigated. Despite a number of recent studies confirming the dependency of the shear modulus at very small strains on suction and degree of saturation, no existing models are able to capture the different trends of variation observed experimentally. Since this dependency has to be accounted for in the design of infrastructure interacting with the atmosphere, there is scope to investigate the effect of these two variables further. Secondly, the macroscopic response at very small strains of saturated clayey geomaterials was explored. Studies on the one-dimensional compression of saturated non-active clays demonstrated how the processes occurring at the particle scale may significantly affect the response observed at the macroscale. Since the evaluation of soil responses in terms of the soil stiffness is generally based on the assumption that soil can be treated as a continuum medium, an attempt was made to take into account the mechanisms occurring at the microscale and their effect on the macroscopic response observed at very small strains.The thesis goals were achieved by carrying out two separate experimental investigations on unsaturated well-graded sand specimens, and on kaolin clay specimens saturated with different pore-fluids. The shear modulus at very small strains, G0, was inferred from the measurement of the velocity of propagation of shear waves through the specimens using the bender element technique.The interpretation of the experimental results was based on the analysis of the micro-mechanisms underlying the macroscopic response. For the case of unsaturated sand, a microscale-based model relating the small strain stiffness with the suction-induced intergranular stress was derived by analysing the stiffness at the contact between sand particles in the presence of water menisci and in the bulk water. The model was successfully validated against the experimental data, and was able to capture the different trends of variation of G0 along a drying or a wetting path observed in the literature. For the case of saturated clay, a DEM model with newly-designed contact laws (accounting for the mechanical and electro-chemical interactions occurring between clay particles) was first introduced. The DEM model was able to reproduce basic aspects of the macroscopic compression behaviour of kaolin clay specimens at a qualitative level. Then, the results of the DEM simulations and the quantitative analysis of the stiffness of the different particle-to-particle interactions were successfully used to elucidate the microscopic mechanisms affecting the velocity of propagation of shear waves, in turn related to the small strain stiffness.Soil stiffness at very small strains is a fundamental parameter for a wide range of geotechnical applications. The correct evaluation of the soil stiffness parameters at very small strains is essential for the realistic prediction of ground deformations occurring around geotechnical structures under operational conditions (serviceability limit state design) and is used to derive the stiffness degradation curve with increasing strain.The aim of this research is to explore the behaviour of granular and clayey geomaterials at very small strains in cases where the macroscopic response observed experimentally cannot be easily interpreted. In these cases, existing models commonly adopted for the evaluation of the small strain stiffness fail to capture the soil response and may in turn lead to erroneous estimations of ground deformations.Two examples were analysed in this research. Firstly, the stiffness at very small strains of unsaturated granular materials was investigated. Despite a number of recent studies confirming the dependency of the shear modulus at very small strains on suction and degree of saturation, no existing models are able to capture the different trends of variation observed experimentally. Since this dependency has to be accounted for in the design of infrastructure interacting with the atmosphere, there is scope to investigate the effect of these two variables further. Secondly, the macroscopic response at very small strains of saturated clayey geomaterials was explored. Studies on the one-dimensional compression of saturated non-active clays demonstrated how the processes occurring at the particle scale may significantly affect the response observed at the macroscale. Since the evaluation of soil responses in terms of the soil stiffness is generally based on the assumption that soil can be treated as a continuum medium, an attempt was made to take into account the mechanisms occurring at the microscale and their effect on the macroscopic response observed at very small strains.The thesis goals were achieved by carrying out two separate experimental investigations on unsaturated well-graded sand specimens, and on kaolin clay specimens saturated with different pore-fluids. The shear modulus at very small strains, G0, was inferred from the measurement of the velocity of propagation of shear waves through the specimens using the bender element technique.The interpretation of the experimental results was based on the analysis of the micro-mechanisms underlying the macroscopic response. For the case of unsaturated sand, a microscale-based model relating the small strain stiffness with the suction-induced intergranular stress was derived by analysing the stiffness at the contact between sand particles in the presence of water menisci and in the bulk water. The model was successfully validated against the experimental data, and was able to capture the different trends of variation of G0 along a drying or a wetting path observed in the literature. For the case of saturated clay, a DEM model with newly-designed contact laws (accounting for the mechanical and electro-chemical interactions occurring between clay particles) was first introduced. The DEM model was able to reproduce basic aspects of the macroscopic compression behaviour of kaolin clay specimens at a qualitative level. Then, the results of the DEM simulations and the quantitative analysis of the stiffness of the different particle-to-particle interactions were successfully used to elucidate the microscopic mechanisms affecting the velocity of propagation of shear waves, in turn related to the small strain stiffness
Workers' identities in transition : the impact of deindustrialisation among Scottish steelworkers from the 1990s
No full textWork is intrinsic to the formation of both personal and collective identity. It comprises a major component of an individual's life, with one's occupation often acting as a 'calling card' by which people define themselves and others within society. If we accept this rendition of work as being paramount to identity, then it follows that the removal of work, through redundancy and unemployment, or the transition into new and different employment, will have a consequential impact on an individual's identity. The existing body of literature on the impact of deindustrialisation has primarily outlined the devastation it has wrought upon working-class communities collectively and redundant workers individually.;However, the transitions workers made following their loss of employment, specifically the significance of these transitions in relation to identity, remains largely absent from the literature. This thesis aims to address this absence by analysing the impact that loss of employment and transition into new employment has on workers' identities and perceptions of work.Work is intrinsic to the formation of both personal and collective identity. It comprises a major component of an individual's life, with one's occupation often acting as a 'calling card' by which people define themselves and others within society. If we accept this rendition of work as being paramount to identity, then it follows that the removal of work, through redundancy and unemployment, or the transition into new and different employment, will have a consequential impact on an individual's identity. The existing body of literature on the impact of deindustrialisation has primarily outlined the devastation it has wrought upon working-class communities collectively and redundant workers individually.;However, the transitions workers made following their loss of employment, specifically the significance of these transitions in relation to identity, remains largely absent from the literature. This thesis aims to address this absence by analysing the impact that loss of employment and transition into new employment has on workers' identities and perceptions of work
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