A deeper understanding of the different factors that influence the laminar-turbulent transition in supersonic boundary layers will help the design of efficient high-speed vehicles. In this work we study the effects of surface roughness on the stability and transition to turbulence of supersonic boundary layers. The investigation is carried out by direct numerical simulations (DNS) of the compressible Navier-Stokes equations and focuses on the modifications introduced in the transition process by localised roughness elements, for Mach numbers M∞ = 6.0 and M∞ = 2.5, and distributed slender pores at M∞ = 6.0. The first part of the investigation into the effects of localised roughness deals with the receptivity and initial exponential amplification of disturbances in boundary layers subjected to small external perturbations. Different transition scenarios are investigated by considering different free-stream disturbances and roughness elements with different heights. The results show that, for roughness heights approaching the local displacement thickness, transition is dominated by the growth of a number of instability modes in the roughness wake. These modes are damped by wall cooling and their receptivity is found to be more efficient in the case of free-stream disturbances dominated by sound. At M∞ = 6 the growth of Mack modes in the boundary layer is found to play a crucial role in the excitation of the most unstable wake modes. An investigation into the nonlinear stages of transition shows that the breakdown to turbulence starts with nonlinear interactions of the wake instability modes. This leads to the formation of a turbulent wedge behind the roughness element, which spreads laterally following mechanisms similar to those observed for the evolution of compressible turbulent spots. An oblique shock impinging on the transitional boundary layer significantly accelerates the breakdown process and leads to a wider turbulent wedge. The study ends with an analysis of porous walls as a passive method for transition control, which is carried out using a temporal DNS approach. The results show damping of both the primary, of second or Mack mode type, and secondary instabilities and indicate that, despite the high Mack number, first mode waves regain importance in this modified transition scenario.

No description available.

Vibratory micro-machined gyroscopes utilize Coriolis force to detect the rotation rate. Recently it has been proved that embedding the gyroscope in an electromechanical ΣΔ modulator (EMSDM), results in increased linearity, rate detection range, and immunity to fabrication variations. In addition, this architecture can be deployed as a ΣΔ modulator (SDM) analogue to digital converter (ADC), providing a digital bit stream that can be used directly by any digital signal processing system (e.g. micro-processors). Furthermore, recent research has proved that higher order EMSDMs proved to deliver better performance in terms of signal to noise ratio while retaining linearity, dynamic range, fabrication tolerance and bandwidth advantages. Furthermore, in the view of ADC performance, higher order SDMs achieve higher resolution performance which is a desired feature for an ADC. Considering all these advantages, there have been attempts to deploy this approach in designing micro-machined gyroscopes interface in form of low-pass and band-pass EMSDM in order to achieve detection of high angular rate motions and angular motions with faster variation (which requires higher band width). However, the fabrication process of vibratory micro machined gyroscopes just like any other micro fabrication process is prone to flaws and inaccuracies. One of these flaws in the case of vibratory gyroscopes is the root cause of a mechanical coupling that occurs between the excitation direction and detection direction. This coupling results in appearance of an error signal in the detection direction which is known as quadrature error. Existence of this mechanical error results in reduction of performance in this type of gyroscopes and most importantly it limits the dynamic range of the sensor. In this work, a novel interface is proposed that eliminates the quadrature error while retaining the advantages of EMSDM for micro-machined gyroscopes system. The approach is a combination of quadrature amplitude modulation technique which is quite mature in communication systems, time division modulation in digital systems and the fundamental theory of EMSDM. A system level model of the novel architecture has been developed by using Matlab and Simulink. The system level simulation of the novel interface indicates that attenuation of -80dB can be achieved for the quadrature error signal. Furthermore, circuit level simulation model has been developed using Orcad/Pspice, in order to verify the consistency of the system level simulation. Finally a prototype PCB has been built characterized to evaluate the practical system performance. The measurement results on the hardware implementation show that the quadrature error power spectral density is attenuated by -70dB. In another words, the quadrature error is attenuated by about 3 orders of magnitude in the hardware implementation prototype.

An experimental and numerical investigation was performed in order to firstly establish the aerodynamics and aeroacoustics of a landing gear torque link configuration, and secondly to assess the effectiveness of both passive and active flow control as noise attenuation methods. Experimental and numerical data showed that afixing a torque link on to a half cylinder strut resulted in the suppression of vortex shedding in between the torque arms. This resulted in a narrower wake with reduced drag. PIV wake profiles showed the suppression of shedding to result in reduced velocity fluctuations, as shown by reductions in shear and normal Reynolds stresses.

No description available.

Instability, flow separation and transition are essential aspects of high speed flows with shock-wave/boundary-layer interaction. Supersonic flows with jet injection, compression ramp and cavity, either individually or in combination, are numerically studied by solving directly the compressible Navier-Stokes equations to better understand the transition of high speed laminar boundary layer to turbulence and the influence of flow separation. A sonic jet injected into a Mach 6.69 crossflow is studied through both two-dimensional(2D) and three-dimensional (3D) simulations. Effects of the momentum flux ratio (Jp) are evaluated. A 3D global instability is observed with a critical value of Jp below but close to 0.1. The most unstable mode is found to have a spanwise wavelength of 8 times the incoming boundary layer displacement thickness. Streamwise vortices are observed at the saturated stage. Ramp flow is studied at Mach number 4.8 and Reynolds numbers of 6,843 and 3,422. The flow is stable in two dimensions. Simulations in 3D show that the flow is globally unstable at higher Reynolds number, while being stable at the lower Reynolds number, suggesting that a critical Reynolds number for instability exists between these two Reynolds numbers. The most unstable mode for the ramp flow has a spanwise wavelength of 12 times the incoming boundary layer displacement thickness. For the ramp flow with higher Reynolds number, breakdown to turbulence is observed, however this occurs downstream of the region where the global mode is active, suggesting that the global mode does not lead to transition directly but provides a disturbance seed which gives rise to transient growth, leading to streaks which subsequently breakdown to turbulence. Effects of an upstream cavity and a downstream sonic jet injection on the Mach 4.8 ramp flow are studied. It is found that the jet could greatly increase flow separation while the cavity has little effect. Similar configuration is studied for a supersonic ramp flow at Mach 5.3 with a Mach 3.6 jet injection for a practical application to rocket stage separation. Effects of cavity and jet on laminar boundary layer separation are firstly evaluated through a 2D parametric study. Ramp flow with turbulent boundary layer is simulated at greater detail in 3D. Then a slot jet is switched on to evaluate the laminar and turbulent inflow effect on the flow separation. A shorter separation bubble is observed, though the rates at which the separation point moves upstream are comparable for laminar and turbulent inflow.

As a result of the last 60 years of satellite launches, a significant amount of space debris has been generated in Earth orbit. Growing consensus amongst experts over the last decade, has suggested that removing existing debris, alongside mitigation efforts, can assist in controlling the size of the low Earth orbit (LEO) population. However, no objective or long-term strategy exist to ensure the most effective use of active debris removal (ADR). The way we utilise near-Earth space, and the way the space environment will behave in the future will directly affect the number of debris objects required to be removed. This then, makes it difficult to identify any potential future ADR strategy that will perform effectively in all possible future cases. This thesis explores a novel adaptive strategy that determines how many debris objects should be removed to control the size of the LEO debris population. The strategy adapts and adjusts the number of removals performed by ADR in response to the evolution of the debris population. The framework for the strategy was inspired by the methods incorporated in adaptive management and control engineering. The University of Southampton’s Debris Analysis and Monitoring Architecture to the Geosynchronous Environment (DAMAGE) model was used to represent the space environment, whilst a new debris model entitled the Computational Adaptive Strategy to Control Accurately the Debris Environment (CASCADE) was used to predict the evolution of DAMAGE, and required removal rate. Predictions using DAMAGE were run under a variety of launch, explosion, mitigation and solar activity for both the ≥10 cm and ≥5 cm LEO populations. Two key parameters of the adaptive strategy were also investigated: modifying the frequency of implementation and exploring different high-level objeives for the strategy. Using the adaptive strategy increased the probability of achieving its objective and required fewer removals, as each prediction had a bespoke number of removals. On average, 3.1 removals (standard deviation: 1.2) were required to provide an 88% probability in preventing the growth of the ≥10 cm LEO population. Whereas, implementing realistic variations in launch, explosion, mitigation and solar activity, on average, 6.3 removals (standard deviation: 6.8) were required to prevent the growth of the ≥5 cm LEO population with 76% confidence. This compared with a “traditional” strategy of removing five objects per year that only provided 49% confidence. This approach then, represents a rational method to calculate the number of removals required to ensure the future sustainability of outer space activities.

Currently the space community relies on propellants such as hydrazine and its derivatives in propulsion systems aboard satellites and spacecraft. However their highly toxic and carcinogenic nature results in significant costs in handling, storage and transport compared to less toxic propellants. It is due to this benefit that there is a renewed interest in ‘green’ or less toxic propellants. One such green propellant is hydrogen peroxide. This liquid propellant must be catalytically decomposed into steam and oxygen within the thruster. The current PhD is charged with advancing the state of hydrogen peroxide heterogeneous catalysts and associated thruster hardware. To this end a range of assessment techniques have been employed to develop solid catalysts based on metallic gauze, metallic foam and ceramic pellets. In all cases these catalysts must be suitable for the decomposition of up to 99% hydrogen peroxide concentration by mass. This work assesses these catalysts under both laboratory and fully representative conditions using 87.5% hydrogen peroxide concentration by mass. This thesis contains a detailed literature review into heterogeneous catalysts and catalyst supports. This has resulted in the procurement and manufacture of thirty-eight candidate catalysts from all three support types. A selection of these catalysts have undergone physical assessment using a range of surface characterisation techniques in an attempt to understand their performance. The results of a review into experimental hardware have led to the development of two laboratory-based apparatus designed to measure initial performance and catalyst longevity. The results have been described and used to down-select from thirty-eight candidate catalysts to a final selection of four. Additional testing of these four catalysts has been conducted within a highly instrumented catalyst bed to provide performance data under fully representative conditions. With their performance verified an engineering bread-board (EBB) thruster has been developed to investigate the effect of various changes to thruster hardware such as injector and catalyst bed geometry, pre-heating methodology and effect of anti-channelling baffles.

Fan noise is one of the major sources of aircraft noise. This can be modelled by means of frequency and time domain CAA methods. Frequency domain methods based on the convected Helmholtz equation are widely used for noise propagation and radiation from turbofan intakes. However, these methods are unsuited to deal easily with turbofan exhaust noise and presently unable to solve large 3D (three-dimensional) problems at high frequencies. In this thesis the application of time-domain Discontinuous Galerkin Methods (DGM) for solving linearized Euler equations is investigated. The research is focused on large 3D problems with arbitrary mean flows. A commercially available DGM code, Actran DGM, is used. An automatic procedure has been developed to perform the DGM simulations for axisymmetric and 3D intake problems by providing simple control of all the parameters (flow, geometry, liners). Moreover, a new method for integrating source predictions obtained from CFD calculations for the fan stage of a turbofan engine with the DGM code to predict tonal noise radiation in the far field has been proposed, implemented and validated. The DGM is validated and benchmarked for intake and exhaust problems against analytical solutions and other numerical methods. The principal properties of the DGM are assessed, best practice is defined, and important issues which relate to the accuracy and stability of the liner model are identified. The accuracy and efficiency of the CFD/CAA coupling are investigated and results obtained are compared to rig test data. The influence of the 3D intake shapes and the mean flow distortion on the sound field is investigated for static rig and flight conditions by using the DGM approach. Moreover, it is shown that the mean flow distortion can have a significant effect on the sound attenuation by a liner.

The transmission of vibration to people through seats was studied with the final objective of being able to predict the dynamic performance of seats. It was necessary to investigate both the dynamic response and the subjective response of a person sitting on a seat, in addition to the dynamic response of the seat itself. Initial experiments were conducted to investigate the effect of variables, such as the magnitude of the vibration and the posture of the body, on the transmission of vibration through a seat. Seat transmissibilities measured in a laboratory with vertical-only vibration were shown to be similar to those measured in a vehicle with multiple vibration. Frequency response functions were determined for the transmission of multiple axis translational vehicle vibration to the cushion and the backrest of a seat. The apparent mass of the seated body was measured as a pre-requisite for the prediction of seat transmissibilities. It was found that the resonance frequency of the body decreased when the magnitude of the vibration was increased. Increased muscle tension tended to have the opposite effect. The apparent masses of sixty people - men, women and children - were very similar once they were normalised to remove the effect of the different static weights on the seat. Some correlations with the body characteristics were significant - such as that between the normalised apparent mass at resonance and total body mass. A method of measuring the dynamic stiffness of a seat using a rigid indenter instead of a person to load the seat was developed. The results, in conjunction with measurements of the apparent mass of the body, were used to predict seat transmissibilities. Predicted seat transmissibilities were shown to be similar to transmissibilities that were measured with a person sitting on the seat. It was investigated how the weight of a person and the magnitude of the vibration could affect predicted seat transmissibilities. The discomfort caused by vibration transmitted to a seated person was measured using a novel method of adjustment. It was investigated whether the discomfort caused by simultaneous vertical and fore-and-aft vibration could be predicted from the discomfort that would have been caused if the vibration in each axis had occurred separately. A root-sum-square summation procedure was found to be the best.