The efficiency of travelling wave thermoacoustic system, for a given operating temperature difference, is determined by the acoustic wave transmission through its feedback resonator loop system. Curved duct bends are one of the most repeated components used in the construct of these feedback resonator systems and thus require acoustic transmission optimisation. This research investigates the transmission of low frequency, high amplitude acoustic waves propagating through duct bends with different radius of curvatures using Particle Image Velocimetry (PIV). The experimental PIV investigation was conducted on the axial plane of the bend. The velocity vector maps obtained from each run was analysed using both the newly developed Velocity based Wave Decomposition (VWD) technique as well as the Proper Orthogonal Decomposition (POD) technique. The POD technique was shown to successfully separate the different flow component of the acoustic wave in the respective Proper Orthogonal Modes (POMs). The acoustic transmission was thus computed based on the strength of these POMs. The POMs also allowed for the flow visualisation of the different loss mechanism that exists within the wave propagating through the bend (most notably, the energy cascade loss mechanism). Based on the quantitative measurement of the acoustic transmission as well as the qualitative flow observation of the different loss mechanism, a non-dimensional parameter was developed in order to characterise the acoustic transmission through curved duct bend systems. This parameter is known as the Strouhal-Dean number. Based on this parameter, the acoustic transmission can be characterised into 3 acoustic flow regimes: Viscous dominated oscillation, Inertia dominated oscillation and the Transition regime between the first two oscillation regimes. The optimum acoustic transmission range corresponded to the transition regime where the inertia generated secondary circulation was balanced by its viscous loss suppression. The optimal Strouhal Dean number for acoustic transmission was found to be approximately 10.8.

This thesis concerns an analysis of an Integral Compressed Air Wind Turbine (ICWT), in which energy is extracted from a slow-moving renewable source through the use of compressed air. This concept is particularly applicable to large offshore wind turbines, and can be readily integrated with compressed air energy storage methods. The ICWT has a very large rotor with free pistons travelling within the rotor blades, inducting and compressing air to high pressures at each end of the stroke. The compressed air can be stored and expanded when the energy is required, solving the intermittency issue of wind energy. By gathering energy along the rotor blades, rather than at the hub, it also avoids the very high torques associated with extremely large turbines. This thesis investigates optimal control strategies for ICWTs. Firstly, an initial model of the system using coupled ordinary differential equations (ODEs) is constructed to simulate a single piston pair of an ICWT system. This framework utilises several `modes' which the system passes through in the course of each stroke, with movement between modes controlled by simple algorithms. Calculations of potential and required energy are developed to allow basic control of the valve timings. The simulation is then extended to include thermal modelling of the walls of the compression tube, using orthonormal polynomials. A long-duration instance of the model is used to identify steady-state values for the orthonormal parameters, which demonstrates that the wall temperatures would remain within 15~K of the ambient temperature. One possible solution to the high temperatures caused by the near-adiabatic conditions of the compression is added to the model; namely, the injection of water droplets to the cylinder at the start of the compression stage. A method to efficiently simulate a phase transition in MATLAB is developed and implemented, allowing an analysis of the optimum mass balance of water to be injected to reduce the exhausted air temperature. An appendix examines several of the assumptions built into the model, in particular the rigidity of the components and variations in the rotational velocity of the rotor due to Coriolis and gravitational forces. Two valve control schemes are developed and implemented into the model; firstly, a simple proportional and derivative controller, which acts according to a rule dictating a gradual reduction in the energy surplus. This option proves to be limited in the degree to which it can avoid wasting compressed air. A second scheme, involving a simple version of sliding-mode control with two controllers operating at different timescales, is instead developed and shown to be significantly more effective at improving the system efficiency. Finally, an optimisation study is carried out on the `kick' stage, in which stored compressed air is used to accelerate the piston before compression. A large dataset of simulations allows for the specification of a set of optimum parameters based on a balance between power extraction from the rotating frame and net power generation.

Turning operation is a very popular process in producing round parts. Vibration and chatter noise are major issues during turning operation and also for other machining processes. Some of the effects of vibration and chatter are short tool life span, tool damage, inaccurate dimension, poor surface finish and unacceptable noise. The basic dynamic model of turning operation should include a rotating work piece excited by a force that moves in the longitudinal direction. Dynamic interaction between a rotating work piece and moving cutting forces can excite vibration and chatter noise under certain conditions. This is a very complicated dynamic problem. Vibration and chatter in machining is one example of moving load problems as the cutter travels along the rotating work-piece. These moving cutting forces depend on a number of factors and regenerative chatter is the widely accepted mechanism and model of cutting forces which then introduce time delays in a dynamic model. In this investigation, the work piece is modelled as a rotating Rayleigh beam and the cutting force as a moving load with time delay based on the regenerative mechanism. The mathematical model developed considers work piece and cutting tools both as a flexible. Without doubt, this dynamic model of vibration of work piece in turning operation is more realistic than previous ones as the dynamic model has multiple-degrees-of-freedom and considers the vibration of the cutter with regenerative chatter. It is found that the cutting force model of regenerative chatter which introduces time delay in a dynamic model leads to interesting dynamic behaviour in the vibration of rotating beams and a sufficient number of modes must be included to sufficiently represent the dynamic behaviour. The effects of depth of cut, cutting speed and rotational speed on the vibration and chatter occurrence are obtained and examined. Simulated numerical examples are presented. These three different parameters are vital and definitely influence the dynamic response of deflection in the y and z directions. The depth of cut is seen to be the most influential on the magnitude of the deflection. In addition, higher cutting speed combined with high depth of cut promotes chatter and produces a beating phenomenon whereas rotational speeds have a moderate influence on the dynamic response. Furthermore, several turning experiments are conducted that demonstrate vibration and chatter in the machining operations. There is fairly good qualitative agreement between the numerical results and the experimental ones.

This research project focussed on the low-velocity oblique impact response of glass fibre-reinforced epoxy laminates and sandwich structures with a range of polymeric cores of linear PVC and PET with nominal densities in the range of 90-140 kg/m3, conducted at normal (0°), 10° and 20° inclination angles, at energies up to 40 J. For the laminated composites and the linear PVC sandwich structures, at maximum impact energies, the damage area reduced whilst the energy absorbed increased with increasing inclination angle. Damage took the form of matrix cracking, due to bending and shear, combining with fibre fracture due to tensile loading. In the case of the higher density foam-core sandwich structures (PVC and PET), the maximum damage area occurs at 10° and less severe damage occurs at 20°, suggesting an effect of the combination of tensile, compression and shear occurred at 10°. Interestingly, the absorbed energy reduced with increasing inclination angle for these structures. The threshold energy in which visible damage occurs was observed at 14 J and 10 J for the laminated composites and sandwich structures, respectively. At higher energy levels (40 J), full perforation occurred. Contrary to the observations at relatively low energies, the PET-based sandwich structures showed increased damage with increasing inclination angle. An energy-balance model was established and used to successfully predict the maximum impact force (Pmax) values, showing good agreement with the experimental results up to the threshold energy. In addition, these findings also showed that core density has a great influence on the impact response of the sandwich structures, whereby the contact stiffness, C, and the maximum impact force (Pmax), increased with an increase in core density.

Practically every engineering product with any degree of complexity uses threaded fasteners. Although threaded fasteners are generally considered a mature technology, significant problems exist with their use. This study has investigated a number of issues with the tightening and self-loosening of threaded fasteners. - It was found that upon repeated tightening of electro-zinc plated fastners significant wear of the contact surfaces of the bolt/nut thread and nut face occured. This wear was accompanied by an increase in the friction coefficient causing a reduction in the clamp force provided for an assembly when tightened to a specific torque value. - The self-loosening characteristics of prevailing torque nuts were also investigated. It was found that there was a significant loss of prevailing torque when a fastner self-loosened when compared to the prevailing torque when being deliberately disassembled. The current standard test for prevailing torque nuts on re-use does not reflect this surprising result and leads to a significant over-estimate of the capability of this class of nut to resist self-loosening. This is a contribution to knowledge on this topic. - A further major original finding of this study has been that if an axial load is also acting on a joint which is experiencing transverse slip, prevailing torque nuts can continue to self-loosen leading to their possible detachment from bolts. A number of accidents have been caused by such detachments, but the cause was not understood partly because this detachment could not be reproduced on the standard loosening test. Work reported in this thesis has been found that if an external axial load is acting whilst the joint is experiencing transverse slip, under the approprioate conditions, the loosening process will continue until nut detachment occurs. - A series of tests has been completed in which the forces needed to tighten and loosen threaded fastners were measured whilst the joint was being subjected to transverse slip/vibration. Measurements were made of the frictional resistance forces in the circumfrential direction and the loosening torque acting on a fastner under transverse slip conditions. It was found that the loosening torque range varied between two positive limits rather than between zero and an upper limit as anticipated by the theory. It was also found that the friction coefficient in the circumfrential direction in the threads is greated than that on the nut face bearing surface during conditions of transverse slip.

A new type of cavitation (damage) has been recently discovered from the Three Gorges turbines, that suggests a complicated inception mechanism involving the boundary-layer K-mode instability triggering cavitation nucleation. As one of the elements, the level of free-stream turbulence increased by wrongly designed guide-plate on the Three Gorges turbines could be a primary concern in the sense of receptivity and the transient growth of K-mode instability. The investigation focuses on the flow analysis of the on-coming flow influenced by the guide plate in terms of flow structure(s) and turbulence level variation. Firstly, CFD has been performed to obtain the main flow features in the whole turbine passage with more detailed analysis on the free-stream pressure-fluctuations near the lower surface of the guide vanes. The unsteady flow characteristics, especially the pressure fluctuations of low-frequency spectrum have been studied that significantly alternate the turbulence intensities and spectrum in the free-stream flow. The results verify that the addition of the guide-plate increases the free-stream turbulence, particularly contributing to the occurrence of a strongest component of extremely low-frequency pressure-fluctuation (i.e. the 0.336 Hz for the case 2 with opening of 16°). These low-frequency fluctuations readily transmit throughout the entire flow passage of the turbine. An extremely large-scale and united unsteady vortex structure that occupies the whole flow passage of a Francis turbine, has been identified for the first time. That is, a vortex-ring structure triggered by the guide-plate, through the connection of the vortices in the stay-vane and guide-vane channels plus those in the runner channels, further interacts with the helical vortex-rope in the draft tube at part-load conditions (e.g. 16° and 30°), forming such an united giant vortex structure. This extremely large-scale vortex structure is thus responsible for the components of extremely low-frequencies (i.e., 0.336 Hz for 16°, 0.15 Hz for 30°) which have been identified from two part-load conditions. These convincing results have proved how this wrongly designed guide-plate increases the free-stream turbulence by inducing extra unsteadiness with gust-like low-frequency and explained that the flow upstream the cone of draft-tube could affect the cone flow significantly, especially under part-load operation conditions.

This work has focused on formulating a experimental/numerical framework for the investigation of spot weld properties and performance. An Inverse temperature measurement approach has been established to predict the thermal history of a spot welded joints using remote thermocouples. This method incorporated the experimental data into an Artificial Neural Network (AAN) to predict cooling curves of the HAZ. Advanced modelling programs have been developed to simulate spot welded joints and thermocouples. Using the programs to investigate the effects of the key dimensional or material parameters on the mechanical or thermal response of spot welded joints of steels and different thermocouple joints relevant to their applications. Graphical User Interface Abaqus plug-ins of spot welded joints have developed using Python scripting and are used to investigate the effect of nugget size and sheet thickness on the stress and deformation of spot welded joints of steel. These works are important to establish an integrated approach to study the electrical, mechanical and thermal process of the spot welding process.

This research compares the dynamic performance of injection moulded and laser sintered poly-ether-ether-ketone (PEEK), outlining the potential of laser sintering as a manufacturing method for high performance polymer gears. It was observed that the mechanical properties of laser sintered PEEK was a significant improvement over previous laser-sintered materials, and comparable with high performance injection-moulded materials. In addition, the coefficient of friction and wear rates were significantly below that of injection moulded PEEK. However, for direct application of the laser sintered material EOS PEEK HP3 to power transmission gears, the predominant form of failure was bending fatigue, limiting the material to low power transmission levels. Observation of the fracture surface highlighted limited amounts of plasticity, although regions of intergranular failure and fast fracture could be identified from the failed surface. In addition, the failure stress path, in the region of progressive intergranuar failure, was governed by the partially sintered particle boundaries in the material; showing a similar failure response to flexure testing. In conclusion, despite the limited amount of plasticity shown in the material failure, the tribological properties of laser sintered PEEK mean that their application to power transmission is still desirable.

Micro and nano filtration is a crucial fundamental process in biological, biomedical and chemical engineering fields. This project aims to establish a multilevel filter for separation and purification complex bio-components mixtures by size in both micro and nano scales; generalize the design methodology of this multi level filter; optimise traditional nanofilter and eventually develop a multi level filter for rapid micro volume sample filtration. During the study and improvement of traditional filter, a new type of multi level filter for sequential separation based on H shaped channels were put forward in this thesis. The methodology of designing this type multi level filter was built and verified with simulations. A general path for multi level filter chip design was established and demonstrates with examples and experiments. A new fabrication process for multi depths channel on the same surface was put forward during the examples design. The fabrication experiments demonstrated the feasibility of this fabrication process and the recipes of these three fabrication processes were given by the experiments. Simulations on both 2D and 3D filters were described and discussed. The angle effect of angular H shaped channels was studied with COMSOL Multiphysics® Modeling Software.

This thesis focuses on the concept, design, fabrication and characterisation of a torsional micro paddle resonator. The ultimate intention is to use the device for rapid detection of anthrax bacteria. A comprehensive research was carried out to review the state of the art in MEMS based mass sensing. Various driving and detection strategies were investigated and discussed. Based on evidence from literature, a novel approach was adopted to realise a device with improved functionality and overcome currently existing drawbacks. The working principle of the proposed device is based on electromagnetic actuation and monitoring of the shift in resonance frequency of a micro paddle. The design of the paddle was optimised using theoretical and finite element methods. Dual beam Focused Ion Beam (FIB) machining techniques were used to fabricate the prototype devices. The chosen substrate is a LPCVD 200 nm thick silicon nitride membrane. Prior to milling the substrate, the sputtering rate of silicon nitride was validated experimentally to ensure machining stability. Different actuating pattern designs were fabricated to generate torque including micro spiral coil, micro dual loop, and single conductive track on the micro paddle. The geometry was finalised for a defined working condition of 1 MHz resonance frequency. Important fabrication parameters were discussed and damage prevention issues were investigated. The sensitivity to the added mass was experimentally characterised and found to be 2.35 fg/Hz. To characterise the asymmetrical paddle resonator, piezoelectric excitation was applied to the device and a laser Doppler vibrometer was used to record the resonant frequency. Resonant frequencies of 0.841 and 0.818 MHz were detected by testing the device in an air medium and a quality factor of about 300 was calculated by applying a Lorentzian curve fit to collected data.