In recent years, there has been growing interest in contra-rotating open rotors (CRORs) for use as the power plants on aircraft, since they are estimated to burn 20% to 30% less fuel than equivalent turbofan engines for short-haul flights. However, one of the main challenges for their introduction is their very high levels of noise emissions. There is therefore a need for schemes by which the noise from CRORs can be predicted and hence reduced. Nearly all of the research effort on the prediction and reduction of CROR noise has been so far focused on the tonal component, whereas broadband noise emissions remain poorly understood. Nevertheless, it is shown that broadband noise emissions from CRORs can be signifcant, hence the necessity of the present research. In this thesis, broadband noise emissions from uninstalled CRORs are investigated for the frst time. It is assumed that the two most signifcant sources of broadband noise in uninstalled CRORs are the broadband rotor-wake/rotor interaction noise (BRWI) and the broadband rotor trailing edge noise (BRTE). Fast semi-analytical prediction schemes are developed for these sources of broadband noise, which exhibit good agreement with noise measurements from a scaled model CROR. The relative importance of the BRWI and BRTE noise sources is investigated for a realistic CROR confguration at assumed take-off, cruise and approach-type conditions. It is predicted that both broadband noise sources are signifcant at assumed take-off, whereas only the BRTE noise contributes to the total broadband noise emissions at assumed cruise and approach. A parameter study is conducted to investigate the effects on CROR broadband noise emissions of variations in rotor-rotor separation distance, rotor speed and blade number at constant engine power, torque split and solidity. The validity of two widely used approximations for fan broadband noise predictions is also studied: Amiet's approximate BRTE model and the use of isolated airfoil theory for turbulence-cascade interaction noise. Criteria for the validity of Amiet's approximate model are established by comparing it to the general BRTE noise model developed in this work. The two models are shown to differ in the low and high frequency limits, but excellent agreement is observed for realistic rotor configurations over most of the audible frequency range. In addition, sound power predictions from isolated airfoils and blade cascades in a turbulent flow are compared. Excellent agreement is observed between an isolated airfoil model and a cascade model for frequencies higher than a critical frequency. Below this critical frequency, agreement is poor for high solidity cascades but is reasonable for low solidity cascades, typical of CRORs, thus validating the use of isolated airfoil theory in the BRWI model.

Honeycomb sandwich structures (commonly referred to as honeycomb sandwich panels) have found wide spread application in the aerospace industry thanks to their excellent properties, in particular their high strength-to-weight and high stiffness-to-weight ratios. Surrey Satellite Technology Ltd. (SSTL), like many other space companies, often use honeycomb sandwich panels as part of the primary and secondary structures of the small satellites they develop. Although honeycomb panels have been used for the past 50 years gaining a better understanding of these sandwich structures, and the methods and solutions used to produce structural assemblies from them is still a major concern in the aerospace industry. Whether directly or indirectly, there are still significant research efforts ongoing that affect these areas. This work focuses on some of these issues and covers several research fields including material science, tribology and adhesive bonding technology. The first area of focus of this work deals with the structural performance of honeycomb panels alone and mainly concentrates on hexagonal honeycomb cores. An experimental investigation using the rail shear test was conducted to study the shear behaviour of hexagonal honeycomb cores. This involved both static and fatigue tests using numerous honeycomb panel test samples with the loading direction at various angles to the core ribbon. From these tests it was found that core shear strength did not have a linear relationship with loading orientation and that contrary to what is commonly assumed the transverse direction (to the ribbon) is not always necessarily the weakest orientation. The optimal design and performance of the load introduction points was the second area of focus for this work which covers equipment inserts and bolted joints. Two types of inserts where investigated in this work: hot bonded inserts and cold bonded inserts. A study on hot bonded and cold bonded inserts was conducted to assess their performance and effectively compare the two insert systems. A large portion of the study was experimental and involved carrying out numerous insert pull-out tests to measure static pull strength capability. From the study it was found that contrary to what was expected cold bonded potted inserts outperformed the hot bonded inserts in terms of static strength capability. Using finite element it was found that this was due to the different filler materials used for the two insert systems. The last area covered in this work concerns friction grip bolted joint between honeycomb panels. Here a simple method to analyze the efficiency of shear joint units is proposed. An extensive test campaign was also carried out to determine the influence of various parameters on the friction coefficient. Surface abrasion was found to be a reliable way of achieving high values of friction coefficient.

This thesis shows how utilising dynamic simulation to estimate unit costs and manufacturing resources, can aid design decisions. A framework specification is introduced that integrates Computer Aided Design (CAD), Discrete Event Simulation (DES) and Activity Based Cost (ABC) methodology. The framework aids a design team in understanding the consequences of design decisions in terms of unit cost and manufacturing resources, by returning aggregated unit cost and manufacturing based data, directly to the design team, within the design environment. Dynamic Resource Estimation System (DRES) has been developed to implement the framework and conduct two case studies based on representative aerospace components. The purpose of the first case study is to determine the benefits and applications of integrating a dynamic supply chain simulation and unit cost estimation. The second case study is used to show that the framework is capable of handling significantly different components and to highlight the effort required to implement a new component within the framework. This thesis concludes that there are three primary benefits provided by the framework, which are: firstly, the framework can accurately predict required resources to fulfil a supply chain for a specific production rate, which can be utilised by manufacturing engineers to aid production planning; secondly, the framework increases refinement of a component unit cost estimate, by including manufacturing time and dynamically determined resource requirements into an ABC cost model; and thirdly, the framework has the ability to compare multiple supply chain options and different supply chain types at the same time from component geometry.

The acoustic environment in an automobile cabin has a significant effect on the perceived quality of the vehicle. There are two components of the acoustic environment in the automobile cabin: the noise due to automotive processes and the sound produced by the car audio system. In both cases active methods can be employed to improve the acoustic environment and this thesis presents an investigation of both active control of automotive noise and active sound reproduction systems in cars. In the context of active noise control, cost-effective systems are investigated for the control of both engine noise and road noise. A model of structural-acoustic coupling is first derived and the effects of coupling on the performance of feedforward and feedback controllers is investigated. Feedforward control has been highlighted as a cost-effective method of controlling engine noise. In order to achieve low-cost active control of road noise, however, a number of multi-source, multi-sensor feedback active noise control systems are investigated. A modal feedback control system is investigated and shown to offer significant levels of control of a single dominant resonance. In general, however, a fully multi-input multi-output (MIMO) feedback controller is necessary to achieve control of the multiple resonances present in a road noise disturbance. Measurements have been conducted in a small city car to validate the proposed MIMO feedback controller and a real-time feedforward engine order control system has been implemented. Active control of sound reproduction has also been investigated with the aim of producing independent listening zones in the front and rear seats of the automobile cabin. Simulations have first been used to investigate the effect of implementing a personal audio system in a car cabin and to compare the performance of acoustic contrast and least squares optimisation strategies. In order to achieve control of the sound field over the full audio bandwidth two arrays are implemented, one employing the four standard car audio loudspeakers and one employing phase-shift loudspeakers mounted to the headrests. The performance of the system is validated using a real-time implementation in a people carrier.

Globular clusters (GCs) are among the densest and oldest stellar aggregates in the Galaxy, and are thought to date from around the time that the Galaxy first formed. The high central densities that characterise GCs lead to frequent stellar interactions and the formation of exotic stellar populations, making GCs excellent laboratories for studying the stellar dynamics of dense environments. The ability to observe many stars which are equidistant and (approximately) the same age makes GCs invaluable in understanding stellar structure and evolution. This thesis describes surveys of two Galactic GCs: far- and near-ultraviolet data from the Advanced Camera for Surveys (ACS) on-board the Hubble Space Telescope (HST) were used to study the core region of M80, and far-ultraviolet to Iband data from the Space Telescope Imaging Spectrograph (STIS), ACS and the Wide Field Camera 3 (WFC3) on-board HST were used to carry out an in-depth, multi-wavelength survey of NGC6752. In both studies, the properties of key stellar populations resident in globular clusters are investigated. In M80, it was discovered that the fainter (redder) blue straggler stars are more centrally concentrated than the brighter (bluer) ones. This is contrary to expectations, and suggests that blue stragglers might get a ‘kick’ at formation, before settling back towards the core. In a search for counterparts to known X-ray sources in M80, one X-ray source was shown to be the remnant of the classical nova T Scorpii. This source was undergoing a dwarf nova outburst during the observations. A variability study of the GC also revealed three variable sources, including an RR Lyrae that was observed around maximum brightness, an SXPhoenicis star with a ≈ 55minute period, and a longer period variable which might be another RR Lyrae or a Cepheid variable. In NGC6752, two known cataclysmic variables were revealed to be dwarf novae (DNe), which underwent outbursts during the observations. This takes the number of known DNe in NGC6752 to three, more than any other cluster. Some of the global parameters describing NGC6752 were also investigated. A new estimate of the cluster’s centre position was determined and used to show that the stellar density profile cannot be fit using a single ‘King’ profile, indicating that the cluster is in a core-collapsed state. Finally, a search for broadening in the main-sequence of the colour-magnitude diagram found evidence of small-scale broadening, suggesting the presence of multiple populations. A radial trend in the level of observed broadening was also suggested, with more broadening found in the core than the outer parts of the cluster.

Direct numerical simulation (DNS) is used to study some aspects of the dynamics of vortex rings in viscous, incompressible ow at Reynolds numbers (defined as the ratio of the initial circulation to the kinematic viscosity) in the range of 103 to 104. Firstly, the effect of the particular initial core azimuthal vorticity profile of a vortex ring on its subsequent evolution in unbounded ow is studied. Vortex rings with a wide range of initial core vorticity profiles are shown to relax to a common equilibrium state. Additionally the behaviour of the equilibrium vortex ring at large times is studied. When the slenderness ratio of the vortex rings increases beyond a particular limit, the vortex rings diverge from the common equilibrium state and follow paths determined by the viscosity of the uid. Secondly, the interaction of a laminar vortex ring with a non-deformable, free-slip surface at an oblique angle of incidence leading to the phenomenon of vortex reconnection is investigated. Specifically the effect of Reynolds number on the dynamics of the reconnection process is studied. The scaling of the reconnection timescale with the Reynolds number is obtained. At high Reynolds numbers the reconnection process leads to a breakdown of the entire vortex ring structure to a turbulent-like ow. This phenomenon is shown to be related to the mechanics of the reconnection process. Finally, the dynamics of vortex rings with swirl in unbounded ow is studied. Two different types of vortex rings with swirl were considered: i) Vortex rings with Gaussian distributions of core azimuthal vorticity and core azimuthal velocity and ii) Steady state solutions of the Euler equations for vortex rings with swirl. Both types of vortex rings develop an elongated axial vortex after initialisation. The existence of a maximum limit for the swirl on a vortex ring is shown above which the vortex rings undergo a rapid de-swirling readjustment. A helical instability occurring in vortex rings due to swirl at high Reynolds numbers is presented. A relation is shown to exist between one of the modes of the helical instability and the geometric parameters of the vortex ring.

An experimental and computational investigation was carried out to determine the aerodynamics and aeroacoustics of a flap side-edge. A porous side-edge treatment was applied to the flap side-edge in an attempt to reduce airframe noise. Measurements taken as part of the experimental study were forces, on-surface pressures, particle image velocimetry, hotwire anemometry and on-surface microphones. Oil flow was performed to visualise the on-surface flow. A detached eddy simulation was performed on a geometry that consisted of a main element and a half span flap to understand the flowfield. From the experimental and computational investigation four sources of vorticity in the flowfield were identified, i.e. the main element cove, the main element trailing-edge, separation on the flap suction surface, and the flap side-edge vortical system. These sources of vorticity interacted to produce a significantly unsteady flowfield above the solid flap surface. Three potential acoustic sources on the flap were identified. The first two sources were the turbulent shear layers that rolled up to form the flap side-edge vortex, reattaching firstly on the side-edge and secondly on the suction surface of the flap. A mid-frequency broadband hump was measured by an on-surface microphone at the point of reattachment of the turbulent shear layer on the flap side-edge. The third source was a low frequency instability in the off-surface vortex due to non-linear vortical interactions upstream of the flap. This instability was measured by a hotwire in the downstream vortex and by an on-surface microphone in the main element flap cove. The application of a porous flap side-edge had two favourable effects. Firstly, it reduced the magnitude of vorticity in the turbulent shear layer and the vortex. This reduced the magnitude of the hydrodynamic instabilities induced by the flap side-edge vortex. Secondly, it displaced the vortex further away from the flap surface due to the finite mass flux allowed through the porous material. This reduced the magnitude of the disturbances that interacted with the solid flap surface by moving them further away. The effect of applying a porous flap side-edge was most noticeable in reducing the mid frequency broadband hump in the on-surface microphone measurements.

The current method of operating space-based assets involves the design and launch of large, monolithic spacecraft. These spacecraft are not responsive to failures or changes in mission requirements, as both require the launch of a completely new spacecraft. The concept of fractionated spacecraft was introduced in 2006 by the US Defence Advanced Research Projects Agency (DARPA) as a way of designing and operating space systems which is more responsive than current methods. The fractionated concept involves the decomposition of the traditional monolithic satellite into a number of free-flying spacecraft connected wirelessly. The free-flying satellites each carry a subset of the required subsystems and share their fractionated resources to achieve the mission objectives. Since 2006, studies of this concept have focused on analyses of the value and utility provided by fractionated spacecraft or on design studies of specific systems. This thesis addresses two key questions: Firstly, is fractionation a mission enabling technology? Secondly, can design rules and guidelines be developed for fractionated satellites to allow continuity of measurements to be maintained and launched mass minimised? The first question is addressed by studying a payload and system for measuring coastal salinity from space. This provides an initial opportunity to assess the fractionated concept from a systems engineering point of view. The second question is addressed by undertaking a more general analysis of fractionated architectures, building on the knowledge gained in development of the fractionated coastal salinity measurement system. A computer model was developed to simulate the lifetime of different fractionated architectures when subjected to subsystem failures. A local search algorithm was used to find fractionated architectures which gave the best compromise between mass launched versus the operational time over a 50 year lifetime. The results from the application of this model showed that architectures that are highly fractionated, containing several homogeneous satellites, best achieved this compromise. These findings provide a contrast to the heterogeneously fractionated architectures proposed by DARPA. When a fractionated architecture is first implemented, the technology required to fractionate all the spacecraft subsystems may not be available. Consequently, the key to the implementation of these first fractionated architectures will be to ensure that there is redundancy in the fractionated subsystems spread across the architecture.

As one of the major contributors to aircraft noise, the noise generated by high-lift devices has been explored for many years. However, the noise related to the slat track system, which includes all the extrusive components connecting the slat and the main element, is still generally studied through experimentation due to the complex geometry. In this project, the aerodynamics and aeroacoustics of the slat track and cut-out, especially the main element cut-out, were investigated through numerical simulations for the first time. Two methods were employed in this work. Noise propagation is first studied via a compact source model to evaluate the contribution of each source individually and to investigate the influence of the slat track system on the noise propagation in the slat region. The APE-IV system was employed but modified by using a more accurate expression of enthalpy perturbation to calculate the acoustic fields. The results show that both the slat track system and the background flow modify the sound propagation path. The energy radiated towards the ground is increased due to the interaction of sound waves with the slat track system and the background flow. Detached eddy simulations were run to investigate the mechanisms of the slat track and cut-out noise generation. Major noise sources in the slat cove region are identified and a noise generation feedback loop is proposed. The results show that the increment of noise levels due to the existence of the slat track system is two-fold. The slat track and the cut-out generate noise individually and they also amplify the noise generated within the slat region when the sound waves propagate though the slat cove area and interact with the slat track and cut-out. The dominant frequencies of the spectrum are seen to shift towards the high frequencies due to these added on components. In this work, two kinds of possible noise attenuation approaches were proposed and studied. Geometries based on replacing the sharp cut-out on the main element leading edge with an edge-rounded or a sealed cut-out have been proved to be able to reduce the cut-out noise significantly. Application of acoustic bulk absorbing material can also attenuate the cut-out noise efficiently for a certain range of frequencies.

No description available.