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The noise produced by low Mach number (M ≤ 0.4) laminar and turbulent flows is studied using computational and experimental techniques. The emphasis is on the development and application of numerical methods to further the understanding of noise generation and far field radiation. Numerical simulations are performed to investigate the tonal noise radiated by two- and three-dimensional cavities submerged in low-speed turbulent and laminar flows. A numerical approach is developed that combines near field flow computations with an integral radiation model to enable the far field signal to be evaluated without the need to directly resolve the propagation of the acoustic waves to the far field. Two basic geometries are employed in these investigations: a plane rectangular cavity and a rectangular cavity with a lip. Results for the two geometries show good agreement with available experimental data, and highlight the sensitivity of the amplitude and directivity of the radiated sound to geometry, flow speed and the properties of the incoming boundary layer. The cavity with a lip is shown to behave as a Helmholtz resonator. The plane cavities are characterized by the more familiar Rossiter modes. Both geometries are characterized by intense near field oscillations and strong noise radiation. To quantify the effects of three-dimensional phenomena on the generation and radiation of sound, a fully three-dimensional simulation is performed. The Navier-Stokes equations are solved directly using an optimized prefactored compact scheme for spatial discretization. Results are compared with those from a two-dimensional simulation and the effects of the three-dimensional phenomena are discussed. Finally, wind tunnel tests are performed to quantify the effects of geometry and flow speed on the velocity and pressure fields within a plane rectangular cavity. Velocity measurements are made using the Laser Doppler Anemometry and Particle Image Velocimetry techniques. Instantaneous and statistical data are employed to probe the flows. Although coherent vortical structures are found to characterize the shear layer, their intermittent nature prevents self-sustaining oscillations developing and consequently the pressure field is broadband in nature.

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Optical measurements of small-scale auroral structures are here combined with spectrographic data in order to study the relationship between auroral morphology and the energy characteristics of the precipitating population. It is shown that rayed auroral structures are associated with precipitating electrons with a broad range in energy, including a significant population at energies of around 100 eV. In comparison, observations of fast-moving auroral arc elements are shown to result from precipitation energy distributions peaking at several keV with a very small low-energy component. This spectrographic information feeds directly into the study of naturally enhanced ion-acoustic lines, or NEIALs, which have been observed by incoherent-scatter radars at high-latitudes. It has been proposed that these radar enhancements result from natural plasma instability, causing the generation of ion-acoustic waves through the decay of unstable Langmuir waves, themselves driven by low-energy electron streams. Using multi-spectral imaging in combination with radar observations, a direct link is shown between ion-acoustic wave enhancements and precipitating electrons at 100 eV energies. Wave enhancements at the radar wavevector which are three orders of magnitude above the thermal level, are successfully modelled using the Langmuir decay interpretation for the time of observation. Electron populations with a broad energy range are thought to result from Alfv´enic acceleration mechanisms, which play an important role in the generation of smallscale auroral structure. With the recent advancements in multi-spectral imaging, it is now possible to resolve auroral filaments of a few hundred meters width. An interferometric imaging capability is under development for the EISCAT Svalbard Radar system, in order to resolve scattering wave structures on similar spatial scales within the radar beam. A technique is demonstrated by which to calibrate the position of coherent echoes detected by the interferometer. This will be of great use in clarifying the role of precipitating electron beams in turbulent plasma processes on small scales.

Compressor blades are subtle aerodynamic shapes designed after years of research and insight. They inevitably show deviations from their desired shapes due to manufacturing errors, erosion or foreign object damage. In the present study we focus on seeking compressor blade geometries, that are robust in performance in the presence of geometric uncertainty. Sophisticated tools for representing and propagating uncertainty are employed. Novel method for modeling eroded blade geometry and simulating manufacturing variations with process capability data are presented. These are combined with an automatic meshing routine and a high fidelity viscous flow solver for performance analysis. A combination of Design of Experiment techniques and Gaussian Process emulators are employed to develop efficient surrogate models for uncertainty analysis and exploring the design space. Efficient multiobjective optimization based robust design methodologies are presented. The robust design methods in conjunction with the surrogate model are used to seek blades that have less variation in performance in the presence of erosion and manufacturing variations. Main effects and sensitivity analysis are also performed to understand the effect of each noise variable on the performance. The performance of the robust blades obtained are compared to that of deterministic optimal blades in the presence of the uncertainties. The robust optimal blades exhibit considerably less variability and mean shift in performance as compared to the optimal blades. Finally, a probabilistic framework is developed to deal with randomness in objectives during multiobjective optimization and is applied in conjunction with Gaussian Process emulators for robust design.

The three-dimensional numerical simulation of a Mach 6.7 perfect gas, with a unit Reynolds number of 7.6 x 106m-1, over several configurations of a blunt-fin attached to a flat plate are carried out. The resulting interference flowfield is reported in this thesis. The laminar Navier-Stokes code developed by Narvarro-Martinez [47] has been modified to solve any general three-dimensional problem, and the complete Navier-Stokes equations. The numerical scheme is operator split, allowing independent numerical schemes to be used on each of the individual contributions to the Navier-Stokes, which can be combined later to advance the entire solution in time. The inviscid part uses a first order Godunov method with a HLLC approximate Riemann solver; second order accuracy is achieved through the MUSCL approach. The viscous contribution is modeled by a centered difference scheme. An iterative matrix solver is used to advance the implicit solution in time. To handle large three-dimensional grids, the code is implicit and run on a parallel computer cluster. The three-dimensional results from the various blunt-fins simulated show a complex rich three-dimensional structure, with several horseshoe vortices formed within the separated flow. Extremely large heat transfer rates have been measured along the path of these vortices on the plate surface, and on the leading edge of the unswept blunt-fin. In particular cases heat transfer rates as high as (h/hu)60 were measured for the 5mm diameter fin. The 5mm fin results show remarkable similarity to the experimental results obtained by Schuricht [53]. The results obtained using a swept fin, and a fin of doubled fin diameter also show good agreement with the trends observed by Schuricht and others for a laminar interaction.

This work focuses on the development of a hyperthermal, neutral atomic oxygen (AO) sensor that can be used on a wide variety of spacecraft platforms and in ground-based atomic oxygen environment simulators. Carbon has been identified as the sensitive medium for sensing the AO and one of the most important aspects of this work was selecting the most appropriate type of carbon for a particular AO dose. This work fabricates carbon films by physical vapour deposition (PVD) and screen-printing techniques to provide different thicknesses and erosion rates, which affect the sensitivity and life of the sensor. Screen-printed films provided a useful means of detecting large AO doses (fluences), whilst the thinner PVD films provide a more sensitive film for smaller AO fluences. Attempts are also made at interpreting the data to measure the rate of AO (flux). A combination of characterisation techniques confirm that the carbon films react by chemical removal of the carbon, which is also detected by measuring changes in electrical resistance. This work also postulates that the disorder of the carbon films (measured by Raman spectroscopy) can have an effect on the erosion rate of the material. Results from this work will eventually be compared with two low Earth orbiting spacecraft experiments: STORM on the International Space Station and CANX-2. These experiments are described and engineering details relevant to the sensors are also included.

This thesis contains original research into the propagation of sound in acoustically lined ducts with flow. The motivation for this work is the requirement to predict the sound attenuation of acoustic liners in the bypass duct of modern turbofan aeroengines. The liners provide the most effective means with which to suppress the rear fan noise. It is therefore important to make the best possible use of the available lined area by optimising the liner configuration. A set of analytic and numerical methods for predicting the liner attenuation performance have been developed, which are suitable for use in intensive liner optimisation studies, or as preliminary design tools. Eigenvalue solvers have been developed to find modal solutions in rectangular ducts with uniform flow and annular ducts with sheared flow. The solvers are validated by replicating results from the scientific literature and the Finite Element method. The effect of mean core flow radial profile and boundary layers on the mode eigenfunctions and axial decay rates are considered. It is shown that solutions for thin boundary layer flows converge to those based on the commonly used slip flow boundary condition. It is demonstrated that realistic flow profiles should be used to assess acoustic mode propagation in bypass ducts. The flow profile can have strong effects upon low order modes and surface waves, and in fact at high frequencies, the profile can affect all the modes. Mode-matching schemes are developed to assess the power attenuation performance and modal scattering of finite length liners. The results of the schemes are used to show that refraction of sound by boundary layers increases attenuation at high frequency. Power attenuation is higher where the mean core flow gradient refracts sound towards the liner. It is found that asymmetric liners can provide improved attenuation, depending on the direction of the mean flow shear gradient. The optimisation of axially-segmented liners for single and multi-mode sources is demonstrated. It is found that potentially large improvements in the attenuation of tonal noise is possible, whilst benefits for broadband noise are more difficult to achieve.

The comfort of a seated person exposed to vibration is known to depend on the magnitude, frequency content, and direction of the excitation. A review of the literature showed that very little is known about the effects of the duration of exposure to vibration on comfort. This thesis investigates the effects of body support, frequency, waveform, and direction of excitation on the Subjective Discomfort Time-Dependency (SDTD) during vibration so as to improve understanding of the mechanisms involved (e.g. the biodynamic responses of the body and muscle activity) and elaborate a model predicting how discomfort evolves with exposure duration. To achieve these objectives, a new method of measuring the discomfort time-dependency was developed and tested. The Subjective Discomfort Time-Dependency has been investigated in 27 experimental sessions, each with twelve subjects seated on a conventional car seat. In each session, subjects were exposed to one stimulus. The new developed method requires the subjects to adjust the magnitude of the vibration in order to keep constant their discomfort. The SDTD was obtained by measuring the platform acceleration over the exposure duration. At specific time-intervals, subjects were also asked to indicate the locations of their discomfort and provide discomfort ratings for these locations. Results showed that the amount of vibration to achieve a constant level of discomfort decreased over time (mainly during the first 15 minutes of exposure). This implies that the sensitivity of vibration increases with duration. Fore-and-aft excitations generated a greater SDTD for most stimuli. For 1-Hz lateral sinusoidal motion, the sensitivity of vibration increased at a greater rate with a harness than without. Stimuli at 1 Hz produced SDTD that were less dependent on the duration of exposure than stimuli at higher frequencies. The waveforms of the vibration had little effect on the SDTD. The discomfort rating showed that prolonged exposure to vibration produced discomfort mainly at the neck. Because discomfort was mainly felt at the neck and that the SDTD depended on the frequency, it was hypothesised that the type of neck muscle activity produced during exposure to vibration depends on the frequency. Neck muscle activity was measured with 12 seated subjects during 10 minutes of fore-and-aft sinusoidal vibration. The r.m.s. magnitudes of the raw EMG and of the phasic and tonic components of the EMG were calculated (it was assumed that phasic muscle activity arose from the periodic vibration whereas the tonic muscle activity was needed to respond to a static load). Results showed that the frequency of vibration had no effect on the EMG r.m.s values but affected the phasic and tonic components of the EMG. Phasic activity was greatest at 1 Hz and decreased as the frequency increased. Tonic activity showed the opposite tendency. As for the SDTD studies, the frequency of excitation seems to have an effect on the phasic and tonic components of the neck muscle activity. Phasic and tonic neck muscle activities represent different types of head motions. Because the content of phasic and tonic activities of the EMG signal seems to be linked with the effects of vibration exposure duration on discomfort, it was hypothesised that predicting the head motions may help estimating the comfort timedependency. A three degree-of-freedom lumped parameter model was developed to predict floor-to-head transmissibility. The model was then calibrated to estimate the head motions using the floor-to-head, seat-tohead, and seat transmissibility measured with 12 subjects, exposed to fore-and-aft sinusoidal, narrow-band random, and broad-band random vibration. Results showed that the model can estimate the head motions around the frequencies of resonances (mode shapes), but requires improvement to be accurate at the other frequencies. The estimated mode shapes showed three types of head motions: at 1.4 Hz the head and neck moved in phase; at 3.5 Hz, there was a resonance of the backrest and the head and neck moved in phase, but with a greater head motion than neck motion; and at 6.9 Hz the head and neck moved out of phase. The subjective, physiological, and biodynamic studies suggest that the SDTD increases when the neck muscles attempt to control head motions by producing greater tonic, and less phasic, activity. The lumped parameter model identified through the mode shapes three types of head motions corresponding to different comfort time-dependencies. It was hypothesized that the phase and modulus of the seat-to-head transmissibility may indicate the amount of phasic and tonic activity produced. Through neck muscle activity, a model predicting seat-to-head transmissibility may also predict the time-dependency of discomfort. This thesis proposes a new method for determining the time-dependency of discomfort caused by whole-body vibration. Discomfort time-dependencies have been shown to depend on the frequency of vibration, direction of excitation, and body support. Mechanisms responsible for the discomfort time-dependency have been proposed.