The main aim of this study was to investigate noise reduction techniques for bluff body noise. Three methods were investigated, using a splitter plate on a fairing- strut configuration, applying flow control to the surface of a fairing and fitting a splitter plate behind a isolated bluff body. Aerodynamic tests were performed in wind tunnel facilities using particle image velocimetry (PIV), hotwire anemometry, pressure sensors and a force balance. Acoustic tests using a microphone array, on-surface microphones and freefield mi- crophones were performed to investigate the noise generated by the models. The splitter plate fitted to the fairing-strut configuration was found to be dominated by large scale vortex shedding. The addition of the splitter plate blocked the interaction between the two opposing shear layers aft of the shell's trailing edge thereby reducing their interaction with the downstream strut. Broadband noise reductions were observed as well as reduction in the noise levels of the peaks asso- ciated with the shedding. Applying flow control showed noise reductions for both cases when suction and blowing were applied. These reductions were observed at the lower tested Reynolds numbers (ReDshell = 1.75 x 105), at higher Reynolds numbers (ReDshell = 3.5 x 105) the noise reductions decreased when compared to the baseline case. The splitter plate fitted behind an isolated bluff body modified the wake, decreasing shedding frequency and drag with an increase in the splitter plate length. Broadband noise reductions were observed with all three splitter plate lengths and the tonal peak of the vortex shedding noise was suppressed. The study shed light on the possibility of achieving noise reductions using the three methods. However more research is required to apply these findings on a landing gear.

This study investigates a novel approach to the problem of shock isolation. The questions considered are whether friction produces a better performance in terms of reduced response during a shock compared to viscous damping and a lower residual response after the shock. To gain physical insight, a single degree of freedom model with friction applied to the isolated mass is analysed. It serves as a benchmark to the performance of a two degree of freedom model where friction is applied to a secondary mass. The isolation system performance is then quantified. For the two degree of freedom system with an intermediate secondary spring which connects the primary and secondary mass, it is possible to obtain the reduction in the displacement response as good as the single degree of freedom system and at the same time smoother acceleration response compared to the single degree of freedom system. For the purpose of further improvement, a control strategy is introduced to switch on and off friction in both models depending on some response parameters and this is compared to the passive systems. This is the semi active control strategy where friction is changed within a cycle of vibration (discontinuous). The control strategy provides more displacement reduction to ensure the maximum displacement response is much smaller than the base input which cannot be obtained with the passive systems. The practical implementation and experimental validation is presented only for the first stage of the response during the shock. For the practical implementation of the switchable friction, an electromagnet is applied to separate the friction surfaces. Good agreement with the simple theoretical models for both passive and switchable systems is obtained. The reduced displacement and smooth acceleration response were obtained from the experiments with the system used to represent the two degree of freedom model. The issues and limitations in the practical implementation are identified and discussed.

This thesis addresses the problem of cubesat limitations on transmission power and onboard memory storage. The number of small satellites is continuing to increase. The reduced amount of time and budget required for the development of these satellites has considerable advantages. The short time leads to data becoming available faster than from a larger satellite. Consequently, the communication system is very important to ensure that all data from the cubesat can be transmitted to the receiving station. In this thesis the link budget of a cubesat has been studied to identify the constraints on power and data transmission. As cubesat satellites become more complex, additional constraints and requirements are placed on system components. For more complex missions, greater flexibly of the onboard computer architecture is required to support the mission adaptation or changing specifications of onboard devices. Alternative onboard computer architecture for the next generation of cubesats is presented in this thesis and hybrid onboard computer architecture is proposed. There are many cubesats which have provided remote sensing imagery. An issue is how to store the data onboard and how to transmit these data with limited power. A solution is to reduce the size of the original image by pre-image processing. The potential for using image compression and defining the region of interest to decrease the original satellite image size has been examined in this research. Three approaches are studied and described in the context of the region of interest technique. There is image segmentation based on edge, histogram and texture detection. The presented evaluation is focused on the detection of the land part of the image that contains dynamic information and rejecting the ocean where there is less interest. The technique, however, is equally applicable for any region of interest that can be characterised and this is illustrated by considering some examples. The proposed adaptive image compression system is made up of two parts. The first part consists of the identification of the region of interest and the second part the image compression of this region of interest. The accuracy of the proposed system has been examined by comparing the number of different pixels between the proposed automatic region of interest system and the manual detection of the region of interest. Morphological methods are the main technique that has been used in the system. The morphology structure element has different shapes and size and it is necessary to understand how the shape and size of the structure elements affects the proposed system. A study of structure element has been conducted. In the real implementation of the proposed system on a cubesat, additional power would be required. To quantify this increase, a particular proposed system based on edge segmentation for region of interest automatic detection has been studied. The potential for using the proposed image compression to detect the region of interest and image compression was examined using a standard microcontroller. The result shows that the proposed system could be used on a cubesat satellite with reasonable additional power and mass.

No description available.

The hostile conditions encountered in flames makes it difficult to obtain measurements by conventional techniques. This work describes attempts to obtain time-resolved fluid mechanical information in jet diffusion flames using laser-optical techniques. Several different instruments were used in this study. Laser Doppler anemometry, using a tracking filter processor, was developed for measurement of the time varying velocity structure of these flames. The limitations and applicability of such a technique are investigated. Measurements of various statistical quantities, such as rms intensities and power spectra, are presented. A quantitative Schlieren technique was used to obtain information of flow structure over a wider range of experimental conditions than was possible using the LDA. This provided an insight into the larger scale processes within the flame. A hybrid correlation technique using both Schlieren and LDA was developed. This was used to investigate the near field of both jets and flames. The experiments show that significant differences in structure exist between isothermal and combusting flows. This manifested itself in several ways, including changes in vortex structure and a well defined low frequency instability of the flame.

The main objective of this thesis is to understand and predict jet noise installation effects for engines mounted below aircraft wings. This is done through a variety of empirical, analytical and computational methods. Aspects of the jet source are examined and a jet source model, suitable for determining installation effects is derived. As part of this research programme a novel and extensive set of model scale jet noise installation effects experiments were undertaken. These results are presented and analyzed in this thesis. A new semi-empirical method, which can readily predict installation effects for heated coaxial jets is presented and validated using experimental data. A new 3D ray theory jet propagation method for sources in a steady inhomogeneous moving medium is presented. This method is benched marked using an analytical solution of the Lilley equation. The 3-D method is further enhanced by combing it with realistic CFD jet velocity profiles, and bench marked using the data from the experimental programme

The general aim of this thesis is to investigate experimentally airfoil trailing edge noise reduction using various trailing edge geometries. The work presented in this thesis is part of the FP7 European Project FLOCON. This thesis focuses on sawtooth serrations and a detailed study is conducted in which thirty seven sawtooth trailing edges are tested for reducing the noise at various flow velocities and angles of attack. Broadband noise reductions of up to 5 dB are obtained below some critical frequency above which the noise is increased. The mechanisms by which the noise is changed in the presence of sawtooth serrations are also investigated experimentally by measuring the changes introduced in the unsteady surface pressure near the edge, the turbulence in the boundary layer and in the near wake, and also using Howe's model [66] as a reference for comparisons. Generally, it is shown that noise reductions occur due to an attenuation of the interaction between incident and scattered pressures, which results in a decrease of up to a half of the phase speed along the edges compared with the corresponding straight edge. The noise increase is shown to be caused by a cross-flow being forced through the valleys of the serrations by the pressure difference between the two sides of the airfoil near the trailing edge. Four novel trailing edge geometries are also tested to address the high frequency noise increase observed with sawtooth serrations. These are the slits, the sawtooth with holes, the slitted sawtooth and the random trailing edges. The slitted sawtooth are shown to provide a good alternative to sawtooth serrations, and afford similar levels of noise reductions while limiting the high frequency noise increase to no more than 1 dB. Random trailing edges also show reasonable levels of broadband noise reductions of up to 3 dB and no increase at high frequencies. Finally, serrations are used simultaneously at the trailing edge of an upstream airfoil and at the leading edge of a downstream airfoil to reduce trailing edge noise and interaction noise of the airfoils in a tandem configuration. Broadband reductions of up to 8.5 dB are obtained using the slitted sawtooth trailing edge and the leading edge serrations designed by ONERA. It is shown that most of the noise reduction is provided by a reduction of the airfoil leading edge response due to the leading edge serrations, but that sawtooth slitted serrations provide up to about 3.5 dB additional broadband noise reductions due to a reduction in its wake turbulence.

Microvibrations are a critical concern on satellites equipped with instruments with high stability requirements. Amongst many sources of microvibration onboard, reaction wheel and momentum wheel assemblies are often considered the most significant. This thesis presents the development and validation of microvibration models for a cantilever configured wheel assembly designed with a soft-suspension system. Wheel assembly induced microvibrations under hard-mounted and coupled boundary conditions are studied. In particular, the wheel assembly semi-analytical microvibration model in a hard-mounted boundary condition is developed with harmonic excitations and the traditionally ignored broadband noise excitations are included. Some peculiar dynamics such as nonlinearity in the motor and high damping of the soft-suspension system are observed from the hard-mounted measurements conducted on a bespoke dynamometer. Modeling strategies for these peculiar dynamics are developed and implemented in the wheel assembly microvibration modeling. This includes a systematic approach to extract stiffness and damping values of the suspension system, considering nonlinearity and high damping from measurements. The microvibrations produced by the wheel assembly in a coupled boundary condition are studied using a seismic mass to support the wheel assembly. A coupled microvibration measurement system, which allows the wheel assembly interface loads to be reconstructed by measuring the response accelerations on the seismic mass, is designed, built and validated. In addition, the wheel assembly driving point static and dynamic accelerance are measured and analytical expressions of the driving point dynamic accelerance are derived. The coupled microvibrations are predicted with wheel assembly static accelerance, dynamic accelerance and the standard method (i.e. no wheel accelerance). The predicted results have shown that the method developed in this thesis which uses the wheel assembly dynamic accelerance accurately simulates the microvibrations observed in practice.

The subject of acoustic fatigue first came to prominence with the advent of using jet engines on commercial aircraft in the 1950’s. Despite the period of time which has elapsed since then and the work carried out to help develop our understanding of the response of structures subject to high intensity noise, acoustic fatigue problems still occur. The novel contributions which this thesis makes to knowledge in the area have been in the application of the dynamic stiffness method which has been used to predict stress and strain response due to acoustic loading. The dynamic stiffness method can under certain circumstances provide very accurate results for natural frequency and displacement. Indeed for certain configurations it can provide exact solutions. It is largely independent of the number of degrees of freedom necessary to give an accurate result unlike the finite element method. The thesis documents how the dynamic stiffness method can offer a very favourable alternative to available analysis techniques. An alternative method of formulating the dynamic stiffness method is presented and is further extended to the analysis of orthotropic plates. The response of actual structures is discussed and previously unpublished data is also presented.

The work contained within this thesis concerns the behaviour of poly-dispersed and electrically charged particles in turbulent flows. Fundamental investigation has considered effects of momentum two-way coupling between particles and turbulence within such scenarios, and practical investigation has examined the potential utilisation of charge on fuel droplets to improve pre-combustion spray dynamics internal to marine Diesel combustion engines. A spectral formulation is derived for the mesoscopic Eulerian transport equation describing momentum and kinetic energy transport for mono-dispersed electrically charged particles suspended at isotropic or near-isotropic conditions. This spectral transport equation specifically takes into account momentum contributions from the random uncorrelated particle velocity field. An in-house pseudo-spectral direct numerical simulation (DNS) code has been extended and used to numerically investigate high order spectral statistics associated with momentum and kinetic energy transport equations for homogeneous and isotropic turbulence with particulate suspensions. Key results show that poly-dispersity can reduce the attenuation level of turbulent kinetic energy relative to mono-dispersed suspensions at similar mass loading ratios. In addition, two-way coupled charged particle suspensions were found to exhibit in their mesoscopic spectra, large scale augmentation of kinetic energy and small scale attenuation of kinetic energy, analogous to the behaviour of turbulence spectra in the presence of polymer chain additives. Furthermore, in the presence of gravity, two-way coupled charged particle suspensions were found under certain conditions, to fall with a velocity slower than their own Stokes settling velocity. A droplet charge-diameter distribution model suitable for attributing electrical charge to poly-dispersed droplets in electrostatically atomized dielectric liquid sprays, has successfully been validated against experimental data. A methodology has been developed that uses the charge-diameter distribution model to simulate and successfully predict characteristics of electrostatically atomized dielectric sprays for low pressure spray systems. A computational methodology has also been developed, suitable for predicting the characteristics of high pressure electrostatically charged sprays. This methodology has also been validated against existing experimental data, and is found to be reliable at predicting secondary atomization processes for both uncharged and charged spray plumes at elevated injection conditions.