Filter Results
32 results
  • Estimates of wave energy fluxes in the solar photosphere and chromosphere suggest that gravity waves carry more energy than co-spatial acoustic waves. This work presents an exploration of the upward propagation of gravity waves from the photosphere to the chromosphere, in a realistic model of the solar atmosphere. The purpose is to determine what happens to upward propagating gravity waves when they reach regions where the magnetic forces dominate. Wave motions are assumed linear and the atmosphere is static and horizontally invariant. Dispersion and ray diagrams provide insight into the mode con¬version mechanism. Numerical solution of the linear adiabatic wave equations quantify wave energy fluxes. The analysis shows that typically even weak magnetic fields cause the gravity wave to reflect back downwards as a slow magneto-acoustic wave well before the Alfven-acoustic equipartition level, and it fails to reach the chromosphere. However, highly inclined magnetic fields allow gravity waves to penetrate the equipartition level and experience substantial mode conversion to up-going Alfven waves, or field-guided acoustic waves. Wave energy fluxes are sensitive to the magnetic field orientation and short radiative damping times, but are insensitive to the magnetic field strengths over the range considered (lOG -100 G). The mode conversion pathways found in adiabatic analyses are preserved in the presence of damping. Mode conversion of gravity waves to Alfven waves is a possible pathway to the upper atmosphere for wave energy. The cause of the high frequency enhancement (the acoustic glory) in seismic power surrounding large active regions with complex field structures is not yet understood. This work presents an investigation of the properties of the seismic emission power and acoustic power of large active regions that have acoustic glories. Helioseismic holography is used to create seismic emission power maps for six large active regions with complex magnetic field structures. Acoustic glories are visible in high-frequency seismic emission power maps (5.0 mHz -7.0 mHz) for all active regions studied. The analysis of acoustic power about large active regions, in high and low frequency regimes, confirms behaviours that are consistent with the findings of previous analyses of smaller active regions. Several new properties of high frequency seismic emission power associated with large active regions are identified: The maximum enhancements in seismic emission and acoustic power occur at different spatial locations; acoustic glories are most prominent in seismic emission power maps in the 5.0 mHz -6.0 mHz bandwidth; the radius of the acoustic glory reduces as frequency increases; and the mean value of high frequency (above 5.0 mHz) seismic emission power for the active regions studied was enhanced at intermediate line-of-sight magnetic flux densities (50 G -300 G).
    Data Types:
    • Document
  • The current state of the art lung imaging tools are incapable of assessing lung function and respiratory airflows during highly complex and dynamic biological events. To better understand the mechanics of lung function, we must first possess the appropriate tools for measuring lung function. The literature review conducted within this thesis highlighted a lack of imaging tools that can simultaneously provide excellent temporal and spatial resolution in relation to measurements of lung function. This lack of combined temporal and spatial resolutions limits the use of imaging for assessments of complex biological events. The aim of this research was to develop imaging tools that are able to assess lung tissue motion and respiratory mechanics during respiratory events that involve high frequencies or complex out of phase motions. Of particular interest were respiratory events that involved internal airflow oscillations. Traditionally these respiratory events have been notoriously difficult to obtain regional data at appropriate sampling rates. To achieve simultaneous high temporal and high spatial resolutions of measurements, a synchrotron based imaging set-up was developed that was optimism for respiratory imaging. The set-up involved the use of high-sensitivity detectors and propagation based phase contrast imaging. To extract appropriate and valuable data from the images a modified particle image velocimetry analysis method was developed. As the lung is a dynamic organ that functions through motion, imaging of the lung is difficult especially when small amplitude and high frequency oscillations are occurring, such as during high frequency ventilation. The first application for this novel imaging and analysis method was to assess lung volume distributions in rabbit pups during high-frequency ventilation. It was found that decreasing tidal volume and increasing frequency of ventilation could maintain minute volume whilst simultaneously minimising lung tissue excursion. This shows that HFV is able to provide sufficient gas flow throughout the lungs without causing overdistention to the lung tissue. Pressure oscillations imparted at the airway opening can be used for assessing lung mechanics as well as for ventilation, yet the penetrance of these input signals is not well understood. Using a modified version of the synchrotron-based small animal imaging and analysis method, regional measurements of lung tissue oscillations were captured whilst murine subjects underwent forced oscillation tests. The spatial distribution of lung tissue oscillations identified that there is an uneven signal penetration into the lungs and as a result the measurements may be biased towards particular regions of the lung. In humans and mice, the heart is almost entirely encased by the lungs. As the heartbeats it imparts oscillations onto the surrounding lung tissue. By using a modified imaging and analysis method reconstructions of the airflow generated within the lungs due to the beating heart were generated. This was the first time cardiogenic oscillations have been mapped throughout the airway tree. In mice, the majority of gas mixing that occurs over a single breath is as a result of the physical action of the heart rather than from ventilation. This research into cardiogenic gas mixing has built knowledge in what was previously a poorly understood phenomenon. The techniques developed throughout this thesis represent critical advances in the field of lung function imaging. The techniques describe in this thesis were able to quantify the distribution of ventilation during high-frequency ventilation strategies, measure the airflow that results from the action of the heart compressing the neighbouring lung tissue, and assess the effectiveness of current forced oscillation testing techniques. This thesis comprises three peer-review publications in combination with a significant literature review. The research conducted throughout this thesis not only resulted in peer-reviewed publications, but also resulted in the invention of two new lung function imaging techniques, numerous patent applications and the formation of a startup company for the commercialisation of the imaging inventions.
    Data Types:
    • Document
  • Lab-on-a-chip and micro-total-analysis systems are vital analytical tools used in both the biotechnology and nanotechnology industries. One of the most important aspects of this type of system is the ability to reliably control and manipulate the system itself or the sub-components located within the system such as cells. Audible frequency acoustofluidic actuation can provide a number of potential benefits for microfluidic procedures and is thusly investigated within this thesis. This thesis concentrates on two fundamental facets of manipulation of a microscale droplet system. The first aspect involves a previously undiscovered mechanism allowing manipulation of particles sized down to the nanoscale. The oscillatory motion of the fluid causes a time averaged linear relationship between particle and fluid flow. The intricate interplay between the hydrodynamic focussing and steady streaming effects must be controlled to optimise particle handling. The reduction in particle size has been achieved by minimising the strength of the acoustic streaming function and optimising the multiple-pass hydrodynamic focusing that acts on the particles. The magnitude of the scale reduction presented is quite significant, as particles as small as 190nm in diameter have been manipulated. The other aspect elucidates the harmonic fluidic motion’s ability to modify the wettability of a microscale droplet. The research conducted for this thesis has found that the forced spreading mechanism can be linked to the change in contact angle over an oscillation cycle. Over an oscillation period the temporal contact angle will spend more time in the advancing or receding segment depending on the direction of spreading. Consequently, this single oscillation period effect can result in droplet spreading over hundreds or thousands of oscillation cycles. The connection between the amplitude and the degree of spreading is via the oscillation mode. The behaviour of spreading changes when the droplet reaches larger accelerations. Distinct regions are defined each having different degrees of spreading. Moreover, the work conducted for this thesis has found that a droplet experiencing extremely large oscillations can spread so much that the drop can exceed its hysteretic limits. This mechanism can compel multiple droplets to evenly spread over a desired area and, subsequently, aid imaging of those drops. This thesis aims to highlight the prospective advantages that audible frequency actuation has over ultrasonic methods. These benefits include simpler instrumentation, higher transmission of particles, negligible temperature variation and synchronised manipulation of multiple samples.
    Data Types:
    • Document
  • In the 50 years of helioseismology, we have gained an extensive understanding into the physical processes present within our sun. With the aid of high resolution observations and increased computational power, the current body of understanding is rapidly growing. However, there are still many questions left answered today. In this thesis, we will address two phenomena in order to shed light on their related open questions. In the first part, we will examine the scattering regimes that exist within bundles of thin magnetic flux tubes. In particular, we will address the question of how magnetic plage can absorb large amounts of wave energy and whether the resultant scattered wave field can be used to infer the magnetic field structure. The second phenomenon concerns the seismic sources that are situated within acoustic power halos and what role of the magnetic field has in enhancing these sources. In addressing the multiple scattering regime, a semi-analytical model was developed in order to model the scattering between numerous thin flux tubes situated within a stratified atmosphere. We have used a method originally designed for oceanic wave scattering, and applied it to a solar context. While to some extent this has been done in the past, we further develop the model to include the scattering of all possible modes between thin magnetic flux tubes. In doing so, we outline and address a mathematical error in the original formalism, that was not apparent until previous results were compared to recent numerical studies. Numerous case studies are then examined, ranging from the simple case of two interacting tubes, to large numbers of closely packed tubes. Various parameters are explored and the effect these have on the scattering regime is reported. Our results compare quite well with numerical and observational studies, and this model presents a significant step forward in understanding how the scattered wave field can be used to infer the internal constitution of a slender magnetic field structure. On addressing the second phenomenon, we employ the helioseismic holography technique to examine the enhanced seismic sources that are situated within the acoustic power halo that surrounds complex active regions. We examine three active regions using SDO data and apply strict statistical precautions in our analysis. The relationship between the seismic sources and the magnetic field is explored, with a strong correlation found between seismic enhancement and quasi-horizontal fields of intermediate strength. Additionally, the most intense seismic emitters (acoustic glories) were found to be located within fields very close to horizontal. The relationship between the seismic source halos and the commonly used local acoustic power maps is also explored, with large similarities reported. We found that the greatest difference between the two types of halos occurs within the high frequency (9 mHz) regime. The results of this observational study agree with other recent studies, however this study presents a significant advancement on previous holography studies.
    Data Types:
    • Document
  • Radio Frequency Identification (RFID) systems are currently a major research area globally. Most of the RFID tags available in the market use application-specific integrated circuits (ASICs) which are expensive compared to other tagging techniques. RFID can only compete with, even and replace barcodes if they are made chipless and printed like the barcodes. Chipless RFID tags reduce the manufacturing costs and enable the use of the technology in high volume applications. Much research has been carried out on the development of chipless RFID tags. However, only a limited amount of work has been carried out on the development of chipless RFID readers. This thesis presents the design of three novel, very low-cost chipless RFID readers for reading spectral signature-based chipless RFID tags. Two of the readers use frequency domain-based techniques to decode data from the chipless tags. The Gen-1 reader is capable of detecting the features of amplitude and phase signature of a chipless RFID tag. The reader requires a calibration measurement. The detection process is more hardware-based and fewer signal processing techniques are used. The Gen-2 reader reconstructs the amplitude and phase responses using the signals received from the chipless RFID tags. The reader does not need a calibration measurement, which offers a major improvement over the predecessors. The voltage controlled oscillator (VCO) of the reader generates a linear chirp (swept) frequency interrogation signal. The Gen-2 reader is even lower-cost compared to the Gen-1 and has a simpler RF section. The detection process uses a Hilbert transform-based signal processing technique to re-construct the amplitude and phase responses of the chipless tag. The operation of both Gen-1 and Gen-2 readers are validated experimentally. The tag reading speed is hindered by the performance of the VCO and the number of data points required in frequency domain-based readers. A novel high-speed tag reading technique based on ultra-wideband RF pulses is proposed in this research. The proposed method is validated with simulations. The integrated reader is a complete system with an RF section, a digital section and a graphical user interface (GUI) and software interface. Most of the existing UWB antenna designs are not suitable for chipless RFID applications due to their low gain or physical size. Hence, in addition to the readers, a design of novel UWB antenna is also proposed in this research work to use with the readers. The antenna is compact and high gain and provides UWB operation with over 9 dB gain and 3.9-10 GHz operating frequency band. The unique features of the developed chipless RFID reader systems are (i) low cost, (ii) secure and (iii) remote and non-line of sight operability. The importance of these developments lies in the fact that they enable the development of low-cost chipless RFID systems comparable to other cheap tagging systems such as optical barcodes.
    Data Types:
    • Document
  • This thesis investigates cross-flow flow-induced vibration of elastically mounted rigid circular and square cylinders with low mass and damping ratio over a range of low and vibration parameters. In particular, two typical body oscillator phenomena of flow-induced vibration, vortex-induced vibration and galloping, are studied experimentally. Thus, the study consists of two main parts. The first part, using simultaneous displacement, force and vorticity measurements, investigates the dynamic response of a circular cylinder undergoing transverse free vortex-induced vibrations and controlled trajectory-following vibrations. The second part characterises the amplitude and frequency responses of a square cylinder with angle of attack variation. Despite extensive work has been undertaken to understand the fundamental characteristics of free and forced vibrations of a circular cylinder, there are still a number of unresolved questions in the literature concerning the similarities and differences between the free and forced vibration cases. In order to directly compare these two vibration cases, a low-friction air-bearing rig and a real-time feedback control system were therefore designed and implemented in experiments. The major results obtained show that the transverse lift force and the decomposed vortex force experienced by a cylinder forced to follow the trajectory of free VIV are identical to the free vibration case, when both the total phase and the vortex phase relative to the body motion are constantly stable. However, significant differences between the two vibration cases are found at a reduced velocity located in the middle upper branch of VIV where largest-scale body vibration occur with complex switching phenomena in the total phase and the vortex phase. The results from the second part, flow-induced vibration of a square cylinder with angle of attack variation, show that the cylinder, as expected, experiences galloping at the zero angle of attack orientation (0 degree) and vortex-induced vibration at the diamond orientation (45 degree). As the angle of attack is varied from 45 degree to 0 degree, the body can undergo combinations of both vortex-induced vibration and galloping in a mixed response region. In this region, the oscillation amplitude response exhibits a new branch that exceeds the responses resulting from the two body-oscillator phenomena independently. For velocities above this resonant region, the body oscillation frequency splits into two diverging branches. Further, analysis of the amplitude reveals that the transition between galloping and vortex-induced vibrations occurs over a narrow range of angle of incidence. Despite the rich set of states found in the parameter space the vortex shedding modes remain very similar to those found previously in vortex-induced vibration.
    Data Types:
    • Document
  • Radio frequency (RF) technology plays a significant role in the development of portable devices. Well-designed RF circuits may reduce the dimensions of portable medical devices, which will provide a more comfortable monitoring environment and may lead to more accurate diagnosis. This research is focused on the use of RF technology to develop a wearable wireless multi-parameter human monitoring system suitable for sleep apnoea research. In this thesis, an innovative system prototype employing RF and microwave technology is proposed. The thesis focusses on research into specified passive and active microwave circuits, 5.8 GHz signal propagation and concept approval at the system level. Specially-designed microwave bandpass (BPF) and lowpass filters (LPF) are presented in the thesis. These filters are designed based on the required specifications for the system. The designed BPF uses a combination of compact microstrip resonant cell (CMRC) and defected ground structure (DGS) technologies, which is able to satisfy a narrow bandwidth of 60 MHz and a selectivity of 0.22 dB/MHz, as required in the system specification. The LPF designs also use CMRC and DGS technologies. The achieved LPFs have the characteristics of sharp roll-off, low insertion loss, compact size and wide stop-bandwidth. 2.4 GHz and 5.8 GHz circular patch antennas are designed for the proposed wireless on-body transducer system, and the 5.8 GHz signal penetration capability has been tested in a simulated monitoring environment. The results prove that the 5.8 GHz microwave signal can be applied in the wireless monitoring of sleep apnoea patients. In the active design section, the design principles of a Class AB power amplifier and Class E oscillator using GaN HEMT technology are introduced. The designed circuits are able to achieve an appropriate output power level with a high power-added efficiency. These designs can be applied in a wireless power transmission system. Therefore, the battery can be completely removed from the on-body transducer, leading to significant saving of space. The cost of the portable transducer device may also be reduced at the same time. The sleep apnoea wireless monitoring system consists of two building blocks: an RF-based wireless on-body transducer and remote base station. The on-body prototype has a microstrip circular patch antenna, a project-oriented microstrip BPF, a frequency mixing circuit, a modulation circuit and a differential circuit for biomedical signal detection. The development of the on-body prototype was divided into five stages. For each of the developmental stages, the prototype underwent the phases of design, modelling, simulation, fabrication and measurement. The testing results for each phase and stage are very promising. The proposed prototype has been tested from baseband to 2.4 GHz RF band with excellent results. The research will benefit both sleep apnoea patients and hospitals by reducing the cost of the device, and the reduced size of the portable wireless patient monitoring device will enable patients to be more comfortable during diagnosis. The research reported in this thesis has made significant contributions in the following fields: (1) the development of a novel design of microstrip filters using CMRC and DGS, (2) successful 5.8 GHz signal penetration capability in a simulated environment for sleep apnoea monitoring, (3) the development of a high efficiency and high output power amplifier and oscillator for wireless power transmission and (4) concept approval of the proposed novel wireless on-body transducer.
    Data Types:
    • Document
  • This work investigates the slipstream, the airflow induced by a vehicles movement, of high-speed trains (HSTs). Such flows can be hazardous to waiting commuters at platforms, track-side workers and infrastructure. Scaled wind-tunnel and moving-model experimental methodologies were developed to assess the slipstream risk of HSTs in their prototype design phase. The slipstream of an Inter-City-Express 3 (ICE3) HST was measured and compared to full-scale experimental data. The scaled methodologies replicated the full-scale slipstream profiles and gust results when measured at low positions. However, they appeared less accurate at high measurement positions. This difficulty is proposed to be caused by the presence of ambient wind in the full-scale results and the higher Reynolds numbers, which reduces the overall coherence of the wake topology responsible for the peak slipstream velocities. Analysis of the scaled moving-model and full-scale results provided insight into the causal flow physics of the peak slipstream velocities. These results indicated that the fluctuation and resulting peak instantaneous velocity are a result of alternating `types' of individual slipstream runs that could be explained by capturing a periodic wake at different phases. A model of the salient time-averaged and unsteady features of the wake has been developed from numerous experimental wind-tunnel techniques and analysis. The wake features a pair of streamwise counter-rotating vortices that move downwards and outwards causing the largest slipstream velocities. The unsteady wake was found to contain vortex shedding from the train's sides and weaker vortex shedding from the roof that interact and merge with the streamwise vortices. This results in an anti-symmetric spanwise oscillation of the streamwise vortices with additional vertical translation at a frequency of St= 0.2. These dynamics cause the slipstream velocity to oscillate and results in the maximum instantaneous slipstream velocities measured, as the near-wake oscillated towards a stationary observer. The effect of modelling a reduced length train on the slipstream and wake was also investigated. Increasing L/H resulted in a thicker surface boundary layer that increased slipstream velocity above the streamwise vortices. The slipstream at low measurement positions and the time-average and unsteady wake appeared to be insensitive to the L/H modelled.
    Data Types:
    • Document
  • Future indoor wireless networks will need alternative RF spectral resources to support the multi-gigabit per second (Gb/s) data speeds demanded by next generation multimedia applications. Although RF spectra in millimeter-wave, terahertz and optical bands are relatively uncongested, the communication systems operating in these frequency bands are difficult to implement, since they often suffer from stability problems due to the very high carrier frequencies. Hence, new physical layer technologies that can offer stable and low-complexity transceivers need to be developed. In this research work, non-coherent orthogonal frequency division multiplexing (OFDM) is studied to meet this demand. Non-coherent OFDMs can mitigate inter-symbol interference caused by channel frequency selectivity and achieve high spectral efficiency. Moreover, compared to conventional OFDMs, non-coherent OFDM uses simple passive direct detection without the need of complex RF frontend components such as mixers and oscillators. In this study, various non-coherent OFDM schemes with new detection enhancements are proposed to improve the performance of millimeter-wave, terahertz, optical, and optical wireless communication systems. It is shown analytically and by simulation that the proposed non-coherent OFDMs offer better bit error rate performance with much lower complexity, when compared to conventional OFDMs. In addition, a simple non-linear pre-distortion technique is explored to further improve the spectral efficiency of non-coherent OFDMs. Finally, space-time block coded (STBC) multiple-input multiple-output (MIMO) transmission schemes are incorporated with the proposed non-coherent OFDMs to offer improved system performance.
    Data Types:
    • Document
  • Advanced particle manipulation techniques with synergistic effects of low cost, high degrees of controllability, precision, and delicateness have been developed. In particular, the one-dimensional pressure fields in a microfluidic channel device driven by a piezoelectric plate have been investigated. Particles lines were observed along the channel when corresponding resonant frequencies applied. The more complex two-dimensional pressure fields excited by one electrode in the microfluidic channel were also investigated. Single array of particles clumps have been achieved by switching between two frequencies. Additionally, two arrays of particles clumps were observed by sweeping frequency rapidly. Furthermore, particles levitation was observed under certain excitation frequency. Based on the knowledge gained from the microfluidic channel, a microfluidic chamber was conducted in the development of ultrasonic technique. Cavitation bubbles driven by the standing wave generated in the chamber have been studied. Various oscillation modes of the bubbles were also studied. Additionally, the vibrating bubbles as size-based particle selective mechanism were examined. Size varied particles either been attracted (larger particles dominated by Bjerknes force) or repelled (smaller particles dominated by drag force) by the bubble were achieved. As an alternative to the ultrasonic particle manipulation methods, the development of particles forming in lines by capillary flow due to water evaporation has also been demonstrated in this thesis. Particles behaviour has been investigated in a capillary cell formed by a parallel glass slide and a glass cover slip. Particles remaining in hydrated while assembled and harvested in batches were shown. Finally, the establishment of advanced strategies for using the float-sink scheme to selecting single fragile particles has been conducted. A droplet dispensed directly above the selected particle floating on the liquid surface was demonstrated to cause the particle to sink even when the particle was within a floating cluster.
    Data Types:
    • Document