This thesis reports on investigations undertaken into the reliability, availability and maintenance of offshore wind farms when considering different maintenance strategies, an understanding of which is fundamental when considering the technical and economical viability of existing and future offshore wind farms. A comprehensive literature review has been undertaken in the areas of offshore wind farm maintenance strategies, offshore wind turbine reliability and accessibility issues, and CO2 emissions associated with maintenance expeditions for offshore wind farms. The limitations and disadvantages of current maintenance practices are identified and a planned intervention maintenance policy is proposed and examined in detail. To help design a planned intervention maintenance policy, the offshore wind farm parameters that affect its technical and economical viability have been identified, which become the foundation for developing computer based models using Monte Carlo simulations to quantify the maintenance practices of the planned intervention maintenance policy. Different scenarios of the proposed solution are investigated to help quantify the technical and economical benefits over the current maintenance practises, in terms of wind farm availability, cumulative energy output, production cost of energy and CO2 emissions. The research on the reliability of offshore wind turbines has shown that the power converter system is a critical item that suffers from high failure rates. This thesis reports upon the investigation of a hot standby redundancy on the wind turbine power converter system. A redundancy model is deployed to simulate the planned intervention maintenance policy for different offshore wind farm case studies in order to establish the effects of the hot standby redundancy on the offshore wind turbine operational performance. The novel contribution of this work is claimed to be in the development of dedicated models for the reliability, availability and maintenance of offshore wind farms, which should lead in establishing a technical and economic benchmark for the parameters affecting offshore wind farms.

Tumour blood flow plays a critical role in tumour growth and cancer therapies. Computational fluid dynamics is an efficient method to study blood behaviour by modelling fluid flow through numerical simulations. A mathematical model is developed to study the blood flow through a three-dimensional permeable vascular network embedded in a solid tumour, and its transvascular movement and spread within tumour interior in context with cancer therapies. The vasculature is described by the parametric equations in terms of vessel centre lines. The flow through each tumour vessel is approximated with the leading component in the longitudinal direction of the vessel, and its governing equation becomes an ordinary differential equation based on the parameter of the parametric equation for the vessel centre line. The pressure continuity and mass conservation conditions are imposed at every junction within tumour vascular network. The interstitial flow is described by the Darcy’s law which is converted into the Laplace equation. The coupling effect between the flows through tumour vasculature and within tumour interstitial due to the vascular permeability is described by the Starling’s law. A coupling mathematical model is then developed. Based on mass conservation, a differential equation for pressures on both sides of vascular surface is obtained. Transforming the Laplace equation into the boundary-integral form by using the Green’s function offers another equation linking the pressures inside and outside vessels. The numerical procedure is developed, and the discretised differential and integral equations are solved by finite difference method and boundary element method respectively. The model is applied to investigate how different types of physical parameters and special characters of tumour vasculature affect tumour blood flow. Finally, an approximation model by ignoring the term with small value of the fully coupling model is developed, and its validity and simulation efficiency are examined.

Electromechanical devices are used in a wide range of dynamic applications, from hard disk drives to levitating trains; therefore any improvements in the dynamic performance of such devices would be of significant academic and commercial benefit. This thesis presents theoretical and experimental analyses of shorted turns (conductive loops inductively coupled to a coil). The effect of shorted turn design parameters was investigated using electromagnetic actuator models, measuring force and electrical impedance, with the aims of increasing dynamic range and stability. It was found that a shorted turn reduces the motor constant at high frequencies; it does however significantly improve the transient response of the total coil current. This would suggest that although a shorted turn is likely to reduce the peak force, with the correct shorted turn configuration an increase in acceleration may be apparent; this would be particularly beneficial for positioning actuators with fast response time, such as those found within hard disk drives. A shorted turn was also found to provide damping, attributed to eddy-currents, which reduced resonance in tests. Utilising an inductively-coupled secondary coil it was possible to control the shorted turn effect, switching it on and off as desired. This could be applied in practice: the shorted turn could be passive in the acceleration phase and active in the deceleration phase, potentially leading to improvements in the dynamic performance.

No description available.

Multilayer techniques, in conjunction with thick-film technology have been applied to the design and fabrication of several multilayer microwave structures to achieve the low cost and high performance goals set by modern microwave circuits and systems. To provide accurate material parameters for the design of multilayer thick-film components, a novel slit cavity resonator method has been developed that enables the relative permittivity and loss tangent of dielectric samples to be measured easily, and with high accuracy. A particular feature of this method is that it can be used to measure thick-film samples that are normally only available in relatively thin layers in a two-layer format. Rigorous electromagnetic analysis on a slit cavity has been performed that accounts for the effect of the fringing fields and the radiation from the slits. The method has been verified through measurement on several thick-film materials over X-band. Both the analytical methods and the fabrication techniques for multilayer microwave microstrip structures are presented. Several multilayer thick-film microstrip line test structures have been designed and characterised, and these provide a basic database for the design of multilayer microstrip components. A new design procedure for the multilayer end-coupled filter has been developed that enables the designer to arrive at the physical dimensions of the multilayer structure based on the filter specification. This design technique is effective as it combines the accuracy of electromagnetic (EM) analysis and the efficiency of circuit simulation. The multilayer gap, which is the most critical element of multilayer end-coupled filters, has been characterised using EM analysis and the data is incorporated into a circuit simulator. Measured and simulated results are presented that verify the new design technique. A 40% bandwidth has been achieved experimentally, which shows a very significant improvement over conventional single layer structures, where the bandwidth achievable is normally less than 5%. Novel, octave band DC blocks have been designed, fabricated and tested using a new multilayer format. The tight coupling required between the coupled lines in this component was realized by overlapping these lines in a multilayer structure. Very good agreement was obtained between measured and simulated data. The multilayer approach was also applied to the design of coupled line bandpass filters where a measured 80% bandwidth was achieved. For the first time, the properties of multilayer coupled lines using a range of different thick-film dielectrics are examined using their coupled-mode parameters. Design curves for multilayer coupled lines are obtained, that provide important information on the design of multilayer directional couplers. A practical design strategy for multilayer directional couplers is developed, which overcomes the problem of excessive computation that is normally associated with the electromagnetic optimization of multilayer circuit designs. The methodology has been verified through the design and measurement of wide bandwidth 2dB and 3dB directional couplers that were fabricated using multilayer, thick-film technology. New techniques for the design and fabrication of multilayer microwave thick-film components have thus been established, both theoretically and through practical circuit fabrication and measurement.

No description available.

No description available.

Sports played with an ovoid ball may be considered as minority sports in comparison to the numerous games played with spherical balls, however the ovoid ball market is considerable, with $84million spent on the purchasing of American footballs in the US alone (SGMA 2007b). In comparison to spherical balls, it is apparent that little research has been performed on ovoid types, which presented an opportunity for a detailed study into their dynamic properties in game related situations. With the development of this knowledge new ball design concepts have been investigated to improve consistency and performance, allowing manufacturers to create balls with improved physical characteristics. Experimental procedures have been created which allow the dynamic behaviour of an ovoid ball to be characterised. It was found that the measured parameters varied depending upon the position of impact, orientation angle of the ball and the position of the valve at impact. The inclusion of the valve within a rugby ball creates a non uniform mass distribution resulting in unstable rotation about the axis with the intermediate moment of inertia. This unstable rotation results in the ball performing a series of half twists, thus increasing the drag force during the flight and wobble. Prototype rugby balls have been manufactured with various mass distributions, allowing the effect on the unstable rotation to be analysed. Results showed that the inclusion of multiple valves, within the dynamically balanced bladder, resulted in stable rotation about all ptinciple axes. FE models have been created with isotropic and anisotropic material properties, with all models validated using experimental procedures. Results suggest that the anisotropic FE simulation accurately predicts the coefficient of restitution, contact time and deformation during experimental testing, and as a result it can be used to predict the dynamic behaviour of a rugby ball during various impact scenarios. Using a thermo-bonded construction, it was shown that a novel rugby ball could be developed with a unique carcass configuration and outer panels with increased design flexibility. A number of carcass designs, based upon mathematical shapes, have been created which increase the consistency of the dynamic behaviour of the ball when impacted at different locations.

For a narrowband, instantaneous mixing multi-input, multi-output (MIMO) communications system, the channel is represented as a scalar matrix. In this scenario, singular value decomposition (SVD) provides a number of independent spatial subchannels which can be used to enhance data rates or to increase diversity. Alternatively, a QR decomposition can be used to reduce the MIMO channel equalization problem to a set of single channel equalization problems. In the case of a frequency selective MIMO system, the multipath channel is represented as a polynomial matrix. Thus conventional matrix decomposition techniques can no longer be applied. The traditional solution to this broadband problem is to reduce it to narrowband form by using a discrete Fourier transform (DFT) to split the broadband channel into N narrow uniformly spaced frequency bands and applying scalar decomposition techniques within each band. This describes an orthogonal frequency division multiplexing (OFDM) based system. However, a novel algorithm has been developed for calculating the eigenvalue decomposition of a para-Hermitian polynomial matrix, known as the sequential best rotation (SBR2) algorithm. SBR2 and its QR based derivatives allow a true polynomial singular value and QR decomposition to be formulated. The application of these algorithms within frequency selective MIMO systems results in a fundamentally new approach to exploiting spatial diversity. Polynomial matrix decomposition and OFDM based solutions are compared for a wide variety of broadband MIMO communication systems. SVD is used to create a robust, high gain communications channel for ultra low signal-to-noise ratio (SNR) environments. Due to the frequency selective nature of the channels produced by polynomial matrix decomposition, additional processing is required at the receiver resulting in two distinct equalization techniques based around turbo and Viterbi equalization. The proposed approach is found to provide identical performance to that of an existing OFDM scheme while supporting a wider range of access schemes. This work is then extended to QR decomposition based communications systems, where the proposed polynomial approach is found to not only provide superior bit-error-rate (BER) performance but significantly reduce the complexity of transmitter design. Finally both techniques are combined to create a nulti-user MIMO system that provides superior BER performance over an OFDM based scheme. Throughout the work the robustness of the proposed scheme to channel state information (CSI) error is considered, resulting in a rigorous demonstration of the capabilities of the polynomial approach.

With the growing interest in life cycle costs for heavy-duty gas turbines, equipment operators are investigating the trade off between performance improvements and associated maintenance costs. One of the key factors leading to performance degradation during plant operation is compressor fouling especially in harsh environments. These results from the adherence of dust or sand particles mixed with small oil droplets to compressor blade surfaces. The result is a reduction in compressor pressure ratio and an overall loss in mass flow, compressor efficiency and, therefore, overall power output. To restore this power loss, an increased fuel flow rate with the consequent higher firing temperature (TET) is necessary. This will seriously reduce the creep life of the HP turbine blades. The research described here is divided into three sections: The first section focuses on the simulation and diagnostic analyses of a typical clean and fouled industrial gas turbine. The analysis tool is a performance code (Turbomatch) developed at Cranfield University. The performance degradation was based on the following engine which is currently operating in the Libyan Desert, using field data available from the company. A single shaft industrial gas turbine (11 MW) engine The study comprised a simulation of the "clean" engine performance of a typical small gas turbine used for power generation. The study also examined the economic impact of compressor fouling on operating costs due to increased fuel flow rate and the reduced creep life of the HP turbine. Additionally, the study included an economical analysis of various types of compressor online washing techniques. The real engine data of a Sulzer Type 7 gas turbine (11MW) have been collected from a Libyan oil field over a period of four months without compressor washing. Corrected to standard ambient conditions the results are then compared with the clean engine performance. A case study shows that as a result of compressor fouling, the compressor efficiency decreases about 2.5%, and Heat Rate increases by 7% and power output falls by 10%. Consequently, the cost of power losses in a one year period is around $600,000. The second section involves high fidelity CFD (two dimensional and three dimensional) simulations of the fouling mechanism of the ninth stage of an axial compressor. This HP9 stage was tested in No. 3 Compressor Test Facility at Rolls-Royce. The comparison between the experimental data and the ANSYS CFX is shown in chapter 4. This research contains a description of the numerical analyses carried out on the HP9 test case to include the effects of variable amounts of roughness both in terms of spatial distribution and equivalent grain size. The examination of the effects of adding roughness to selected regions of the rotor blade suction side has shown the relative importance of the leading edge end on the blade suction side over other regions of the blade. The relative impact was quantified in terms of the wake velocity defect, magnitude and wake width. However when the radial distribution is considered the differences between clan and roughened blades is less apparent excepting for the case where the entire suction side of the blade is roughened. The pressure distribution around the blades was also examined. This part of the work has shown the benefits of flow analysis using CFD in a very small region of the flow field. In this case close the analysis is confined to the leading edge and trailing edge of a compressor blade. It also demonstrates clearly the disadvantages of fouling in reducing pressure rise through increases of total pressure loss and therefore reduced efficiency. It has been established that the leading edge and the concave surfaces of the rotor blade are the most sensitive regions to the fouling. It is emphasised that experimental verification of this trend would be very difficult to achieve with a cascade rig because both rotor and stator would need static pressure tapings very close to one another at the leading edge of the blade. This would be extremely difficult to achieve in practice. In the third section experimental work has been undertaken on a cascade facility in the Gas Turbine Laboratory. This details measurements of the cascade the pressure losses as a result of increasing levels of fouling on the blades. In addition, further experiments are undertaken to recover performance loss due to fouling through a washing technique. This has facilitated a rigorous estimation of the magnitude of compressor stage inefficiency caused by fouling. By observing the actual distribution of fouling in a typical compressor, it is now possible to estimate more accurately the overall impact of the fouled performance on overall compressor efficiency. Furthermore an experimental study has been carried out based on a need in practice for online compressor washing; an investigation was undertaken by using a different spray nozzle pressure. The detergent used for the washing experiments was a water based cleaner diluted with demineralised water in a ratio of 1: 4. The fouled compressor efficiency after cleaning with the current project washing scheme has been improved by 2.2%.