http://www.dspace.cam.ac.uk:80/handle/1810/213729 Sat, 18 May 2013 20:22:04 GMT 2013-05-18T20:22:04Z http://www.dspace.cam.ac.uk:80/handle/1810/244532 Title: Analysis of amorphous indium-gallium-zinc-oxide thin-film transistor contact metal Authors: Kiani, Ahmed; Hasko, David G.; Milne, William I.; Flewitt, Andrew J. Abstract: It is widely reported that threshold voltage and on-state current of amorphous indiumgallium- zinc-oxide bottom-gate thin-film transistors is strongly influenced by the choice of source/drain contact metal. Electrical characterisation of thin-film transistors indicates that the electrical properties depend on the type and thickness of the metal(s) used. Electron transport mechanisms and possibilities for control of the defect state density are discussed. Pilling-Bedworth theory for metal oxidation explains the interaction between contact metal and amorphous indium-gallium-zinc-oxide, which leads to significant trap formation. Charge trapping within these states leads to variable capacitance diode-like behavior and is shown to explain the thin-film transistor operation. Description: This is the author's accepted manuscript of the publication: http://dx.doi.org/10.1063/1.4801991 Sun, 14 Apr 2013 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244532 2013-04-14T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244504 Title: Turbulent partially premixed combustion: DNS analysis and RANS simulation Authors: Ruan, S. Abstract: Increasingly stringent regulation of pollutant emission has motivated the search for cleaner and more efficient combustion devices, which remain the primary means of power generation and propulsion for all kinds of transport. Fuel-lean premixed combustion technology has been identified to be a promising approach, despite many difficulties involve, notably issues concerning flame stability and ignitability. A partially premixed system has been introduced to remedy these problems, however, our understanding on this combustion mode needs to be greatly improved to realise its full potential. This thesis aims to further the understanding of various fundamental physical processes in turbulent partially premixed flames. DNS data of a laboratory-scale hydrogen turbulent jet lifted flame is analysed in this study. The partially premixed nature of this flame is established by examining the instantaneous and averaged reaction rates and the "Flame Index", which indicate premixed and diffusion burning modes coexisting. The behaviour of turbulent flame stretch and its relation to other physical processes, in particular the scalar-turbulence interaction, the effects of partial premixing on the displacement speed of iso-scalar surface and its correlation with the surface curvature are explored using DNS data. The scalar gradient alignment characteristics change from aligning with the most compressive strain to aligning with the most extensive one in regions of intensive heat release. This alignment change creates negative normal strain rate which can result in negative surface averaged tangential strain rate. The partial premixing affects the flame surface displacement speed through the mixture fraction dissipation rate and a second derivative in the mixture fraction space. The correlation of curvature and displacement speed is found to be negative in general and the effects of partial premixing act to reduce this negative correlation. The combined effects of the normal strain rate and the displacement speed/curvature correlation contribute to the negative mean flame stretch observed in the flame brush. Scalar dissipation rates (SDR) of the mixture fraction ẼZZ, progress variable Ẽcc and their cross dissipation rates (CDR) ẼcZ are identified as important quantities in the modelling of partially premixed flames. Their behaviours in the lifted flame stabilisation region are examined in a unified framework. It is found that SDR of mixture fraction is well below the quenching value in this region while SDR of progress variable is smaller than that in laminar flames. The CDR changes from weakly positive to negative at the flame leading edge due to the change in scalar gradient alignment characteristics. Axial and radial variation of these quantities are analysed and it is found that Ẽcc is an order of magnitude bigger than ẼZZ. ẼcZ is two orders of magnitude smaller than Ẽcc and it can be either positive or negative depending on local flow and flame conditions. Simple algebraic models show reasonable agreement compared to DNS when a suitable definition of c is used. Further statistics of the scalar gradients are presented and a presumed lognormal distribution is found to give reasonable results for their marginal PDFs and a bivariate lognormal distribution is a good approximation for their joint PDF. Four mean reaction rate closures based on presumed PDF and flamelets are assessed a priori using DNS data. The turbulent flame front structure is first compared with unstrained and strained laminar premixed and dif fusion flamelets. It is found that unstrained premixed flamelets give overall reasonable approximation in most parts of this flame. A joint PDF model which includes the correlation between mixture fraction and progress variable using a "copula" method shows excellent agreement with DNS results while their statistical independence does not hold in the burning regions of this partially premixed flame. The unstrained premixed flamelet with the correlated joint PDF method is identified to be the most appropriate model for the lifted jet flame calculation. This model is then used in the RANS simulation of turbulent jet lifted flames. A new model to include the contribution from diffusion burning and the effects of partial premixing due to SDR of mixture fraction is also identified and included in the calculation. These models are implemented in a commercial CFD code "Fluent" with user defined scalars and functions. It is found that both the correlated joint PDF model and the model accounting for the diffusive burning in partial premixing are important in order to accurately predict flame lift-off height compared to the experiments. Tue, 12 Mar 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244504 2013-03-12T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244408 Title: Fast, low-artifact speech synthesis considering global variance Authors: Shannon, Matt; Byrne, William Abstract: Speech parameter generation considering global variance (GV generation) is widely acknowledged to dramatically improve the quality of synthetic speech generated by HMM-based systems. However it is slower and has higher latency than the standard speech parameter generation algorithm. In addition it is known to produce artifacts, though existing approaches to prevent artifacts are effective. We present a simple new theoretical analysis of speech parameter generation considering global variance based on Lagrange multipliers. This analysis sheds light on one source of artifacts and suggests a way to reduce their occurrence. It also suggests an approximation to exact GV generation that allows fast, low latency synthesis. In a subjective evaluation our fast approximation shows no degradation in naturalness compared to conventional GV generation. Description: Copyright 2013 IEEE. Sun, 26 May 2013 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244408 2013-05-26T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244407 Title: Autoregressive models for statistical parametric speech synthesis Authors: Shannon, Matt; Zen, Heiga; Byrne, William Abstract: We propose using the autoregressive hidden Markov model (HMM) for speech synthesis. The autoregressive HMM uses the same model for parameter estimation and synthesis in a consistent way, in contrast to the standard approach to statistical parametric speech synthesis. It supports easy and efficient parameter estimation using expectation maximization, in contrast to the trajectory HMM. At the same time its similarities to the standard approach allow use of established high quality synthesis algorithms such as speech parameter generation considering global variance. The autoregressive HMM also supports a speech parameter generation algorithm not available for the standard approach or the trajectory HMM and which has particular advantages in the domain of real-time, low latency synthesis. We show how to do efficient parameter estimation and synthesis with the autoregressive HMM and look at some of the similarities and differences between the standard approach, the trajectory HMM and the autoregressive HMM. We compare the three approaches in subjective and objective evaluations. We also systematically investigate which choices of parameters such as autoregressive order and number of states are optimal for the autoregressive HMM. Description: Copyright 2013 IEEE. Fri, 01 Mar 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244407 2013-03-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244406 Title: The effect of using normalized models in statistical speech synthesis Authors: Shannon, Matt; Zen, Heiga; Byrne, William Abstract: The standard approach to HMM-based speech synthesis is inconsistent in the enforcement of the deterministic constraints between static and dynamic features. The trajectory HMM and autoregressive HMM have been proposed as normalized models which rectify this inconsistency. This paper investigates the practical effects of using these normalized models, and examines the strengths and weaknesses of the different models as probabilistic models of speech. The most striking difference observed is that the standard approach greatly underestimates predictive variance. We argue that the normalized models have better predictive distributions than the standard approach, but that all the models we consider are still far from satisfactory probabilistic models of speech. We also present evidence that better intra-frame correlation modelling goes some way towards improving existing normalized models. Fri, 26 Aug 2011 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244406 2011-08-26T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244379 Title: Combustion characteristics of alternative liquid fuels Authors: Chong, Cheng Tung Abstract: Envisaged application of biodiesel in gas turbine engines or furnaces requires extensive tests on the deflagration properties of biodiesel. The laminar flame speeds of Palm Methyl Esters (PME) and blends of PME with conventional fuels are determined using the jet-wall stagnation flame configuration. The same technique is also used to measure the laminar flame speed of diesel, Jet-A1, n-heptane, acetone, methane and methane/acetone. The spray atomization characteristics of a plain-jet airblast atomizer are investigated using a phase Doppler anemometry (PDA) under non-reacting conditions. The droplet size and velocity distribution of biodiesels are compared to conventional fuels. For spray combustion investigations, a generic gas turbine-type combustor is developed to compare the spray flame established from PME, rapeseed methyl esters (RME), diesel, Jet-A1 and biodiesel blends. The spray droplet characteristics in the flame and the flow field in the combustor are investigated. Chemiluminescence imaging of OH* and CH* are applied to capture the global flame structure and heat release region. Flame spectroscopy and long bandpass filtered imaging at > 550 nm are performed to evaluate the tendency of soot formation. In general, biodiesels exhibit flame shapes and spray droplet characteristics that are comparable to conventional fuels. In spite of the higher fuel specific consumption, the emission of NOx is found to be lower for biodiesels compared to conventional fuels. The results show that biodiesels can potentially be used as alternative fuels for gas turbine operation. Tue, 08 Nov 2011 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244379 2011-11-08T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244367 Title: Deformation mechanisms beneath shallow foundations Authors: McMahon, Brendan Abstract: Shallow foundations can provide the most economical solution for supporting small-scale structures. The design approach is quite simple considering the ultimate bearing capacity and working-load settlement. Research has shown that settlement calculations, determined using a linear-elastic approach, usually govern the design but this approach is inappropriate because soil is highly non-linear, even at small strains. The result is that signifi cant discrepancies are observed between predicted and actual settlements. This uncertainty has seen the development of settlement-based approaches such as Mobilisable Strength Design (MSD). MSD uses an assumed undrained mechanism and accounts for soil non-linearity by scaling a triaxial stress-strain curve to make direct predictions of footing load-settlement behaviour. Centrifuge experiments were conducted to investigate the mechanisms governing the settlement of shallow circular foundations on clay and saturated sand models. Clay model tests were performed on soft or rm kaolin beds, depending on its pre-consolidation. Sand model tests were performed on relatively loose Hostun sand saturated with methyl-cellulose to slow consolidation. One-dimensional actuators were developed to apply footing loads through dead-weight or pneumatic loading. A Perspex window in the centrifuge package allowed digital images to be captured of a central cross-section, during and after footing loading. These were used to deduce soil displacements by Particle Image Velocimetry which were consistent with footing settlements measured directly. Deformation mechanisms are presented for undrained penetration, consolidation due to transient flow, as measured by pore pressure transducers, and creep. A technique was developed for discriminating consolidation settlements from the varying rates of short and long-term creep of clay models. Using MSD, a method for predicting the undrained penetration of a spread foundation on clay was proposed, using database results alone, which then provided estimates of creep and consolidation settlements that follow. The importance of the undrained penetration necessitated further investigation by using the observed undrained mechanism as the basis of an ellipsoidal cavity expansion model. An upper-bound energy approach was used to determine the load-settlement behaviour of circular shallow foundations on linear-elastic and non-linear clays, with yield defined using the von Mises' yield criterion. Linear-elastic soil results were consistent with those obtained from nite element analyses. The non-linear model, as described by a power-law, showed good agreement with both centrifuge experiment results and some real case histories. The single design curve developed through this model for normalised footing pressure and settlement could be used by practising engineers based on existing soil correlations or site investigations. Tue, 05 Feb 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244367 2013-02-05T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244274 Title: On the performance of base-isolated buildings: a generic model Authors: Talbot, James P. Abstract: Ground-borne vibration has existed ever since the development of urban road and rail networks. Vibration generated by the moving traffic propagates through the ground and into buildings, resulting in unacceptable levels of internal noise and vibration. A common solution to this increasingly significant problem is the base-isolation of buildings by incorporating vibration isolation bearings between the buildings and their foundations. This technique has been employed for over forty years but the exact performance of base isolation remains uncertain. This dissertation is concerned with the development of a generic computational model; generic in that it accounts for the essential dynamic behaviour of a typical base-isolated building in order to make predictions of isolation performance. The model is a linear one, formulated in the frequency domain, and consists of a two-dimensional portal-frame model of a building coupled to a three-dimensional boundary-element model of a piled-foundation. Both components of the model achieve computational efficiency by assuming they are infinitely long and using periodic structure theory. The development of the model is described systematically, from the modelling of a building and its isolation bearings to that of its foundation. The majority of the work is concerned with the piled-foundation model, which is comprehensive in that it accounts for the vertical, horizontal and rotational motion of the pile heads due to both direct pile-head loading and interaction through wave propagation in the surrounding soil. It is shown that this level of detail is important in the prediction of base isolation efficiency. A key question facing designers is not only how but on what basis base isolation should be assessed, since fundamental problems exist with the existing measures of isolation performance. Power flow analysis is explored and the concept of power flow insertion gain, based on the total mean vibrational power flow entering a building, is introduced as a useful measure of isolation performance. This is shown to offer clear benefits by providing a single measure of performance that is suitable for design purposes. Finally, the development of a prototype force-sensitive vibration isolation bearing is described as a contribution to verifying base-isolation theory with experiments. Tue, 12 Feb 2002 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244274 2002-02-12T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244245 Title: GREEN IPTV: a resource and energy efficient network for IPTV Authors: Ramos, Fernando M. V. Abstract: The distribution of television is currently dominated by three technologies: over-the-air broadcast, cable, and satellite. The advent of IP networks and the increased availability of broadband access created a new vehicle for the distribution of TV services. The distribution of digital TV services over IP networks, or IPTV, offers carriers flexibility and added value in the form of additional services. It causes therefore no surprise the rapid roll-out of IPTV services by operators worldwide in the past few years. IPTV distribution imposes stringent requirements on both performance and reliability. It is therefore challenging for an IPTV operator to guarantee the quality of experience expected by its users, and doing so in an efficient manner. In this dissertation I investigate some of the challenges faced by IPTV distribution network operators, and I propose novel techniques to address these challenges. First, I address one of the major concerns of IPTV network deployment: channel change delay. This is the latency experienced by users when switching between TV channels. Synchronisation and buffering of video streams can cause channel change delays of several seconds. I perform an empirical analysis of a particular solution to the channel change delay problem, namely, predictive pre-joining of TV channels. In this scheme each Set Top Box simultaneously joins additional multicast groups (TV channels) along with the one requested by the user. If the user switches to any of these channels next, switching latency is virtually eliminated, and user experience is improved. The results show that it is possible to eliminate zapping delay for a significant percentage of channel switching requests with little impact in access network bandwidth cost. Second, I propose a technique to increase the resource and energy efficiency of IPTV networks. This technique is based on a simple paradigm: avoiding waste. To reduce the inefficiencies of current static multicast distribution schemes, I propose a semi-dynamic scheme where only a selection of TV multicast groups is distributed in the network, instead of all. I perform an empirical evaluation of this method and conclude that its use results in significant bandwidth reductions without compromising service performance. I also demonstrate that these reductions may translate into significant energy savings in the future. Third, to increase energy efficiency further I propose a novel energy and resource friendly protocol for core optical IPTV networks. The idea is for popular IPTV traffic to optically bypass the network nodes, avoiding electronic processing. I evaluate this proposal empirically and conclude that the introduction of optical switching techniques results in a significant increase in the energy efficiency of IPTV networks. All the schemes I present in this dissertation are evaluated by means of trace-driven analyses using a dataset from an operational IPTV service provider. Such thorough and realistic evaluation enables the assessment of the proposed techniques with an increased level of confidence, and is therefore a strength of this dissertation. Tue, 08 Jan 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244245 2013-01-08T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244235 Title: Autoignition in turbulent two-phase flows Authors: Borghesi, Giulio Abstract: This dissertation deals with the numerical investigation of the physics of sprays autoigniting at diesel engine conditions using Direct Numerical Simulations (DNS), and with the modelling of droplet related effects within the Conditional Moment Closure (CMC) method for turbulent non-premixed combustion. The dissertation can be split in four different sections, with the content of each being summarized below. The first part of the dissertation introduces the equations that govern the temporal and spatial evolution of a turbulent reacting flow, and provides an extensive review of the CMC method for both single and two-phase flows. The problem of modelling droplet related effects in the CMC transport equations is discussed in detail, and physically-sound models for the unclosed terms that appear in these equations and that are affected by the droplet presence are derived. The second part of the dissertation deals with the application of the CMC method to the numerical simulation of several n-heptane sprays igniting at conditions relevant to diesel engine combustion. Droplet-related terms in the CMC equations were closed with the models developed in the first part of the dissertation. For all conditions investigated, CMC could correctly capture the ignition, propagation and anchoring phases of the spray flame. Inclusion of droplet terms in the CMC equations had little influence on the numerical predictions, in line with the findings of other authors. The third part of the dissertation presents a DNS study on the autoignition of n-heptane sprays at high pressure / low temperature conditions. The analysis revealed that spray ignition occurs first in well-mixed locations with a specific value of the mixture fraction. Changes in the operating conditions (initial turbulence intensity of the background gas, global equivalence ratio in the spray region, initial droplet size distribution) affected spray ignition through changes in the mixture formation process. For each spray, a characteristic ignition delay time and a characteristic droplet evaporation time could be defined. The ratio between these time scales was suggested as a key parameter for controlling the ignition delay of the spray. The last part of the dissertation exploits the DNS simulations to perform an a priori analysis of the applicability of the CMC method to autoigniting sprays. The study revealed that standard models for the mixing quantities used in CMC provide poor approximations in two-phase flows, and are partially responsible for the poor prediction of the ignition delay time. It was also observed that first-order closure of the chemical source terms performs poorly during the onset of ignition, suggesting that second-order closures may be more appropriate for studying spray autoignition problems. The contribution of the work presented in this dissertation is to provides a detailed insight into the physics of spray autoignition at diesel engine conditions, to propose and derive original methods for incorporating droplet evaporation effects within CMC in a physically-sound manner, and to assess the applicability and shortcomings of the CMC method to autoigniting sprays. Tue, 08 Jan 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244235 2013-01-08T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244210 Title: Variable horizon model predictive control: robustness and optimality Authors: Shekhar, Rohan Chandra Abstract: Variable Horizon Model Predictive Control (VH-MPC) is a form of predictive control that includes the horizon length as a decision variable in the constrained optimisation problem solved at each iteration. It has been recently applied to completion problems, where the system state is to be steered to a closed set in finite time. The behaviour of the system once completion has occurred is not considered part of the control problem. This thesis is concerned with three aspects of robustness and optimality in VH-MPC completion problems. In particular, the thesis investigates robustness to well defined but unpredictable changes in system and controller parameters, robustness to bounded disturbances in the presence of certain input parameterisations to reduce computational complexity, and optimal robustness to bounded disturbances using tightened constraints. In the context of linear time invariant systems, new theoretical contributions and algorithms are developed. Firstly, changing dynamics, constraints and control objectives are addressed by introducing the notion of feasible contingencies. A novel algorithm is proposed that introduces extra prediction variables to ensure that anticipated new control objectives are always feasible, under changed system parameters. In addition, a modified constraint tightening formulation is introduced to provide robust completion in the presence of bounded disturbances. Different contingency scenarios are presented and numerical simulations demonstrate the formulation’s efficacy. Next, complexity reduction is considered, using a form of input parameterisation known as move blocking. After introducing a new notation for move blocking, algorithms are presented for designing a move-blocked VH-MPC controller. Constraints are tightened in a novel way for robustness, whilst ensuring that guarantees of recursive feasibility and finite-time completion are preserved. Simulations are used to illustrate the effect of an example blocking scheme on computation time, closed-loop cost, control inputs and state trajectories. Attention is now turned towards mitigating the effect of constraint tightening policies on a VH-MPC controller’s region of attraction. An optimisation problem is formulated to maximise the volume of an inner approximation to the region of attraction, parameterised in terms of the tightening policy. Alternative heuristic approaches are also proposed to deal with high state dimensions. Numerical examples show that the new technique produces substantially improved regions of attraction in comparison to other proposed approaches, and greatly reduces the maximum required prediction horizon length for a given application. Finally, a case study is presented to illustrate the application of the new theory developed in this thesis to a non-trivial example system. A simplified nonlinear surface excavation machine and material model is developed for this purpose. The model is stabilised with an inner-loop controller, following which a VH-MPC controller for autonomous trajectory generation is designed using a discretised, linearised model of the stabilised system. Realistic simulated trajectories are obtained from applying the controller to the stabilised system and incorporating the ideas developed in this thesis. These ideas improve the applicability and computational tractability of VH-MPC, for both traditional applications as well as those that go beyond the realm of vehicle manœuvring. Mon, 02 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244210 2012-07-02T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244193 Title: Computations of turbulent premixed flames using conditional moment closure Authors: Amzin, Shokri Abstract: Lean premixed combustion is at present one of the most promising methods to reduce emissions and to maintain high efficiency in combustion systems. As the emission legislation becomes more stringent, modelling of turbulent premixed combustion has become an important tool for designing efficient and environmentally friendlier combustion systems. However, in order to predict these emissions reliable predictive models are required. One of the methods used for predicting pollutants is the conditional moment closure (CMC), which is suitable to predict pollutants with slow time scales. Despite the fact that CMC has been successfully applied to various non-premixed combustion systems, its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modelling of the conditional scalar dissipation rate (CSDR) of the conditioning scalar, the progress variable. In premixed CMC, this term is an important quantity and represents the rate of mixing at small scales of relevance for combustion. The numerical accuracy of the CMC method depends on the accuracy of the CSDR model. In this study, two different models for CSDR, an algebraic model and an inverse problem model, are validated using two different DNS data sets. The algebraic model along with standard k-ε turbulence modelling is used in the computations of stoichiometric and very lean pilot stabilized Bunsen flames using the RANS-CMC method. A first order closure is used for the conditional mean reaction rate. The computed nonreacting and reacting scalars are in reasonable agreement with the experiments and are consistent with earlier computations using flamlets and transported PDF methods for the stoichiometric flames, and transported PDF methods for the very lean flames. Sensitivity to chemical kinetics mechanism is also assessed. Mon, 11 Jun 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244193 2012-06-11T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244077 Title: Transport AC loss in high temperature superconducting coils Authors: Ainslie, Mark Abstract: In this dissertation, the problem of calculating and measuring AC losses in superconducting coils is addressed, with a particular focus on the transport AC loss of coils for electric machines. In order to model the superconducting coil's electromagnetic properties and calculate the AC loss, an existing two dimensional (2D) finite element model that implements a set of equations known as the H formulation, which directly solves the magnetic field components in 2D, is extended to model a superconducting coil, where the cross-section of the coil is modelled as a 2D stack of superconducting coated conductors. The model is also modified to allow the nclusion of a magnetic substrate, which is present in some commercially available HTS wire. The analysis raises a number of interesting points regarding the use of superconductors with magnetic substrates. In particular, the presence of a magnetic substrate affects the penetration of the magnetic flux front within the coil and increases the magnetic flux density within the penetrated region, both of which can increase the AC loss significantly. In order to investigate these findings further, a comprehensive analysis on stacks of tapes with weak and strong magnetic substrates is carried out, using a symmetric model that requires only one quarter of the cross-section to be modelled. In order to validate the modelling results, an extensive experimental setup is designed and built to measure the transport AC loss of a superconducting coil using an electrical method based on inductive compensation by means of a variable mutual inductance. Measurements are carried out on the superconducting racetrack coil and it is found that the experimental results agree with the modelling results for low current, but some phase drift occurs for higher current, which affects the accuracy of the measurement. In order to overcome this problem, a number of improvements are made to the initial setup to improve the lock-in amplifier's phase setting and other aspects of the measurement technique. New measurements are carried out on a single, circular pancake coil and the discrepancies between the experimental and modelling results are described in terms of the assumptions made in the model and aspects of the coil that cannot be modelled. Using the original measured properties of the superconducting tape, there is an order of magnitude difference between the experiment and model. The properties of the superconductor can degrade during the winding and cooling processes, and a critical current measurement of the coil showed that the tape critical current reduced from nearly 300 A, down to around 100 A. Applying this finding to the model, the experimental and modelling results show good agreement, and the difference in the slope of the AC loss curve can be described in terms of the B-dependent critical current dependency Jc(B) used in the model. Finally, methods used to mitigate AC loss in superconducting wires and coils are summarised, and the use of weak and strong magnetic materials as a flux diverter is investigated as a technique to reduce AC loss in superconducting coils. This technique can achieve a significant reduction in AC loss and does not require modification to the conductor itself, which can be detrimental to the superconductor's properties. Mon, 07 May 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244077 2012-05-07T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244052 Title: Design as a strategic resource : design's contributions to competitive advantage aligned with strategy models Authors: Stevens, John Simon Description: There is increasing interest in, and recognition of, the contribution that professional design services can make to a firm's long-term performance. The term strategic design is used ambiguously by design commentators and in empirical literature, with little relation to established theories of business strategy. This thesis documents a study seeking to align the many views of design’s strategic benefits, and to clarify the practical and conceptual relationships of these benefits. The research began with a model development phase, based on literature of corporate strategy and design management, and on exploratory interviews with design practitioners. The second phase sought to test and develop the model derived, through fieldwork interviews and observations. The study identifies nine strategic contributions that can be made by designers or design activity for a firm, and relates them to established models of business strategy. Field interviews and case studies find firms recognising and exploiting some or all of these contributions. However, in some cases – even in highly design-capable firms – these contributions may be recognised as important but are not practiced. Case descriptions serve as examples in context, providing specific, comparable views of design application in two firms, and demonstrate how the design contributions may provide a framework for examining design exploitation in an organisation and identifying opportunities for improvement. Tue, 16 Mar 2010 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244052 2010-03-16T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243981 Title: Supporting material for the paper 'Unified Form Language: A domain-specific language for weak formulations of partial differential equations' Authors: Alnaes, Martin S.; Logg, Anders; Oelgaard, Kristian B.; Rognes, Marie E.; Wells, Garth N. Abstract: Supporting computer code for the paper 'Unified Form Language: A domain-specific language for weak formulations of partial differential equations' Thu, 01 Nov 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243981 2012-11-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243926 Title: Conceptual design for a laminar-flying-wing aircraft Authors: Saeed, Tariq Issam Abstract: The laminar-flying-wing aircraft appears to be an attractive long-term prospect for reducing the environmental impact of commercial aviation. In assessing its potential, a relatively straightforward initial step is the conceptual design of a version with restricted sweep angle. Such a design is the topic of this thesis. In addition to boundary layer laminarisation (utilising distributed suction) and limited sweep, a standing-height passenger cabin and subcritical aerofoil flow are imposed as requirements. Subject to these constraints, this research aims to: provide insight into the parameters affecting practical laminar-flow-control suction power requirements; identify a viable basic design specification; and, on the basis of this, an assessment of the fuel efficiency through a detailed conceptual design study. It is shown that there is a minimum power requirement independent of the suction system design, associated with the stagnation pressure loss in the boundary layer. This requirement increases with aerofoil section thickness, but depends only weakly on Mach number and (for a thick, lightly-loaded laminar flying wing) lift coefficient. Deviation from the optimal suction distribution, due to a practical chamber-based architecture, is found to have very little effect on the overall suction coefficient. In the spanwise direction, through suitable choice of chamber depth, the pressure drop due to frictional and inertial effects may be rendered negligible. Finally, it is found that the pressure drop from the aerofoil surface to the pump collector ducts determines the power penalty; suggesting there is little benefit in trying to maintain an optimal suction distribution through increased subsurface-chamber complexity. For representative parameter values, the minimum power associated with boundary-layer losses alone contributes some 80% - 90% of the total power requirement. To identify the viable basic design specification, a high-level exploration of the laminar-flying-wing design space is performed, with an emphasis above all on aerodynamic efficiency. The characteristics of the design are assessed as a function of three parameters: thickness-to-chord ratio, wingspan, and unit Reynolds number. A feasible specification, with 20% thickness-to-chord, 80 m span and a unit Reynolds number of 8 x 10[superscript 6] m[superscript -1], is identified; it corresponds to a 187 tonne aircraft which cruises at Mach 0.67 and altitude 22,500 ft, with lift coefficient 0.14. The benefit of laminarisation is manifested in a high lift-to-drag ratio, but the wing loading is low, and the structural efficiency and gust response are thus likely to be relatively poor. On the basis of this specification, a detailed conceptual design is undertaken. A 220-passenger laminar-flying-wing concept, propelled by three turboprop engines, with a cruise range of 9000 km is developed. The estimated fuel burn is 13.9 g/pax.km. For comparison, a conventional aircraft, propelled by four turboprop engines, with a high-mounted, unswept, wing is designed for the same mission specification and propulsion characteristics, and is shown to have a fuel burn of 15.0 g/pax.km. Despite significant aerodynamic efficiency gains, the fuel burn of the laminar flying wing is only marginally better as it suffers from a poor cruise engine efficiency, due to extreme differences between takeoff and cruising requirements, and is much heavier. The laminar flying wing proposed in this thesis falls short of the performance improvements expected of the concept, and is not worth the development effort. It is therefore proposed that research efforts either be focussed on improving the engine efficiency, or switching to a low aspect ratio, high sweep, design configuration. Mon, 11 Jun 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243926 2012-06-11T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243895 Title: Experiments on flame blow-off Authors: Kariuki, James; Cavaliere, Davide Emiglio; Dawson, James; Mastorakos, Epaminondas Description: The extinction behaviour of flames has been extensively studied from an empirical perspective in the past. Recently, with the advent of fast-response laser diagnostics, the temporal evolution of the blow-off process has been visualised and analyzed. The movies below are taken at 5kHz and monitor the OH* chemiluminescence and OH-PLIF of stable and extinguishing flames of various types (premixed, non-premixed, spray). The work has been published in: (1) Dawson, J. R., Gordon, R. L., Kariuki, J., Mastorakos, E., Masri, A. R. and Juddoo, M. (2011) Visualization of blow-off events in bluff-body stabilized turbulent premixed flames. Proceedings of the Combustion Institute 33, 1559-1566. doi:10.1016/j.proci.2010.05.044. (2) Kariuki, J., Dawson, J. R. & Mastorakos, E. (2012) Measurements in turbulent premixed bluff body flames close to blow-off. Combustion and Flame 159, 2589-2607. doi:10.1016/j.combustflame.2012.01.005. (3) Kariuki, J., Cavaliere, D. E., Letty, C. & Mastorakos, E. (2012) A comparison of the blow-off behaviour of swirl-stabilised premixed and spray flames. Presented at the 50th Aerospace Sciences Meeting, AIAA, Nashville, 4-7 January 2012.. (4) Cavaliere, D.E., Kariuki, J. & Mastorakos, E. (2012) A comparison of the blow-off behaviour of swirl-stablized premixed, non-premixed and spray flames. Submitted to Flow, Turbulence and Combustion. Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243895 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243858 Title: Anharmonic Acoustic Technique for Detection of Surface-bound Particles Authors: Ghosh, Sourav Kumar Abstract: Receptor-based biological detection techniques often suffer from the problem of non-specific interactions. This is largely due to the presence of weak electrostatic and Van der Waals forces between the receptor and the non-target substances in the analyte that are not easily dissociated in practice. Most existing detection techniques are unable to probe the interaction between the bound entity and the surface and differentiate between specific and non-specific interactions in terms of bond strength or activation energy. The resulting false positive responses lead to various issues, such as misdiagnosis and mistreatment in clinical diagnostics and false alarms in biosecurity. The problem is even more significant with direct direction techniques, such as the resonant frequency shift based detection using quartz crystal microbalance (QCM) or micro-cantilevers, which involve minimal sample processing and washing steps. The work presented in this thesis investigates, through modeling and experiments, the mechanical interactions of a resonator with microparticles attached via biomolecular linkers and analyses the resulting nonlinear acoustic modulation of the resonator from the transduced electrical signal. Physisorbed and specific interactions both in air and liquid medium are studied using thickness shear mode quartz crystal resonators and streptavidin-coated polystyrene microbeads (SCPM) of various sizes. It is found that the modification in the transduced electrical signal measured at the third harmonic (3f), or three times the driving frequency f, is significant in presence of the attached particles and approximately proportional to the number of particles. A detection limit of approximately 2 SCPM of 5.6 µm diameter in air and 6700 SCPM of 0.39 µm diameter in liquid is demonstrated, which corresponds to a mass detection limit of ~200 pg. Most interestingly, the deviation in the magnitude of the 3f signal as a function of the resonator oscillation amplitude is found to hold a distinct relationship with the type of particle-surface interaction. This provides a basis for selectivity in detection over and above the efficacy of the receptor. The function is also found to correlate well with the event of SCPM diffusion on the surface. This detection technique, based on the measurement of deviation in magnitude of the transduced electrical signal measured at a higher odd harmonic of the drive frequency due to the presence of surface-bound particles on a resonator, is termed as the anharmonic detection technique (ADT). A feasibility study with Bacillus subtilis spores in phosphate buffer saline (PBS) is carried out successfully where the modeling and experimental results with SCPM are successfully reproduced. A detection limit of 430 spores is demonstrated, which corresponds to a mass detection limit of ~650 pg. Capability for differentiation of the specifically-captured spores from unwashed physisorbed SCPM of similar dimensions is demonstrated using the shape of the ADT signal. These results indicate that the spore immobilization step may be directly followed by the detection step, which are 9 mins and 2 mins respectively in these experiments. ADT thus potentially enables a rapid, sensitive, reliable and direct detection without the need for any sample processing. Moreover, being an entirely electronic technique, ADT suitably lends itself to multiplexing, large scale fabrication and implementation on a miniaturized low-cost point-of-care detection platform that is of immense need in clinical diagnostics, food and environmental monitoring and biosecurity. Furthermore, fitting the experimental results with modeling estimates enables ADT to determine the force-extension characteristics of the binding biomolecular linker. The force-extension characteristics and the estimated unbinding force for a streptavidin-biotin complex estimated using ADT agrees well with those computed using molecular dynamics (MD) simulation at similar loading rates. Thus ADT contributes a unique force-spectroscopic method, which unlike conventional techniques such as the atomic force microscopy (AFM) provides statistically averaged data for multiple biomolecules in a relatively quicker and simpler experimental format. A method for determination of activation energy of the interaction is also proposed using ADT. This potentially enables a method for rapid and large scale biomolecular screening and studying of interaction networks, which have important applications in drug discovery and individualized therapy. Description: This thesis is embargoed pending a patent application. Please contact the author on ghoss3@gmail.com if you have any enquiries Mon, 11 Jul 2011 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243858 2011-07-11T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243845 Title: How PEDOT:PSS solutions produce satellite-free inkjets Authors: Hoath, S.D.; Jung, S.-J.; Hsiao, W.-K.; Hutchings, I.M. Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243845 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243652 Title: Large Eddy Simulations of extinction Authors: Ayache, S; Garmory, A; Tyliszczak, A; Mastorakos, E Description: Large Eddy Simulations, with the Conditional Moment Closure as the combustion sub-model, have been used to capture localised extinction, flame lift-off, and whole flame blow-off for simple geometries like jet flames (Movies 1 and 2 below), swirl-stabilised recirculating flames of natural gas (Movies 3-6), and spray flames (Paper "Spray_LESCMC_PalermoSept2012.pdf". The work is described fully in the following papers: (i) Garmory, A. and Mastorakos, E. (2011) Capturing localised extinction in Sandia Flame F with LES-CMC. Proceedings of the Combustion Institute 33, 1673-1680. doi:10.1016/j.proci.2010.06.065; (ii) Ayache, S. & Mastorakos, E. (2012) Conditional Moment Closure / Large Eddy Simulation of the Delft-III natural gas non-premixed jet flame. Flow, Turbulence and Combustion 88, 207-231. doi: 10.1007/s10494-011-9368-6; (iii) Tyliszczak, A. & Mastorakos, E. (2012) LES/CMC of blow-off in a liquid fuelled swirl burner. To be presented at the THMT7, Palermo, 24-27 September 2012; (iv) Ayache, S. & Mastorakos, E. (2012) Investigation of the “TECFLAM” non-premixed flame using Large Eddy Simulation and Proper Orthogonal Decomposition. Submitted to Flow, Turbulence and Combustion; (iv) S. Ayache, PhD thesis, University of Cambridge, 2012. Tue, 28 Aug 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243652 2012-08-28T23:00:00Z