Predicting the positions of satellites in Low Earth Orbit (LEO) requires a comprehensive understanding of the dynamic nature of the atmosphere. For objects in LEO the most significant orbit perturbation is atmospheric drag, which is a function of the local atmospheric density from a layer in the atmosphere called the thermosphere. For long-term predictions of satellite orbits and ephemerides, any density trend in the thermosphere is a necessary consideration, not only for satellite operators, but also for studies of the future LEO environment in terms of space debris. Numerous studies of long-term thermospheric density change have been performed. Predictions by Roble & Ramesh (2002), along with evidence by Keating (2000), Emmert et al.(2004), Marcos et al. (2005), Qian et al. (2006) and Emmert et al. (2008), strongly suggest the existence of such a phenomenon. Therefore, the objective of the research presented in this thesis is to provide a novel method to evaluate quantitatively thermospheric density change. Satellite drag data is an effective medium through which one can investigate local thermospheric density and changes thereof. There are many ways of determining atmospheric density, but inferring thermospheric density from satellite drag data is a relatively cost-effective way of gathering in-situ measurements. To do this, knowledge about a satellite’s physical properties that are intrinsic to atmospheric drag is required. A study by Saunders et al. (2009) highlighted problems with estimating a satellite’s physical properties directly from data given explicitly by Two-Line Element (TLE) sets. This prompted an investigation into ways to estimate ballistic coefficients: a required satellite parameter associated with drag coefficient and area-to-mass ratio. A novel way of estimating satellite ballistic coefficients was derived and is presented in this thesis. Additionally, novel consideration of atmospheric chemical composition was applied on long-term drag coefficient variability. Using a quantitative estimate of a ballistic coefficient one can propagate numerically a satellite’s orbit and predict the effects of atmospheric drag. Given an initial satellite orbit from TLE data, one approach is to use an orbital propagator to predict the satellite’s state at some time ahead and then to compare that state with TLE data at the same epoch. The difference between the semi-major axes of the initial orbit and that after the orbit propagation is then integrated and can be used to estimate the global average density. The method employed in this study utilises this process. To achieve this, a specially developed, computer-based, numerical orbital propagator was written in the programming language C/C++. The underlying theories and implementation tests for this propagator are presented in this thesis.

A set of plasma based jet actuators were designed for flow control applications. The characteristics of these actuators and their flow control applications were studied experimentally in a low speed wind tunnel. A dielectric barrier discharge (DBD) based jet actuator is designed, which is made of a covered cavity with two spanwise aligned parallel slots. Two-component particle image velocimetry (PIV) measurements were conducted to determine the effect of actuator in quiescent air and on a canonical zero pressure gradient turbulent boundary layer. It was found that the designed plasma jet actuator produced a transverse jet similar to a continuously blowing jet but with no mass addition into the flow field. The device is different from a traditional alternative blowing-and-suction synthetic jet as the current jet is continuously blown. As such, the DBD based jet actuator is different from either a mass injection blowing jet actuator or a traditional diaphragm based synthetic jet actuator. The impact of the actuation with the designed actuator on the boundary layer characteristics was investigated in detail at different Reynolds numbers. Circular cylinder wake flow control using a newly designed five-electrode plasma jet actuator is also presented in this thesis. This plasma actuator configuration mounted on the cylinder model can easily produce either a downward or upward jet into the flow around the circular cylinder by simply adjusting the same five electrodes’ electrical circuits. The experiments were performed at Reynolds numbers from 7,000 to 24,000. Wake profile measurements were made to evaluate the modification to the mean and fluctuation velocities in the cylinder wake. The results shown that the cylinder wake flow and the turbulence levels in the wake were modified under the actuations, sectional drag reduction and drag increment were obtained by different actuator actuation directions. The study suggested that this new designed five-electrode actuator can be applied to practical separation suppression or enhancement control by adjusting the plasma actuator electric circuits conveniently.

In the case of civil transport aircraft, engines were the dominant noise source until the advent of the high-bypass ratio engines in the early 1970s. Since then, airframe noise has become more important, particularly during the approach-to-landing stage of aircraft operations. The main components of airframe noise are the flap side edge, leading edge slat, and the landing gear. Experiments in both the wind tunnel and via fly-over measurements have shown that the slat noise is a major contributor to the overall airframe noise during the landing approach for a commercial aircraft. To achieve the goal of reducing slat noise significantly without adversely affecting the aerodynamic performance of the wing, it is obligatory to improve the understanding of the mechanism of slat noise generation. Experiments and numerical simulations were performed to investigate the phenomena of slat noise. It was found that the slat broadband noise generation is governed by two kinds of mechanism. At a low angle of attack of the wing, the typical circulation region is not formed in the slat cove and the slat noise level is low. As the angle of attack increases to a certain value, vortical structures are intermittently generated due to flow interaction occuring between the shear layer originating from the slat cusp and the flow convected from the stagnation line on the main element. Intense slat noise is produced as the vortical structures approach the slat cove surface. With the angle of attack increasing further, the slat noise becomes weak again. The interaction effect tends to become weaker as the shear layer deviates away from the surface of the main element. Two approaches with the aim of attenuating the slat noise were experimentally and numerically studied. The first approach was to reduce the slat noise using air blown on the suction surface of the slat near its trailing edge. A numerical simulation showed that the slat noise levels over most of the frequencies, especially above a St number of 7, were obviously attenuated. In the second approach, a strip mounted on the pressure surface of the main element model was experimentally proven to be an effective method for reducing the broadband slat noise at an angle of attack of 8 degrees and a freestream velocity of 25 m/s. The position and height of the strip also influenced the level of the reduction. Several tonal noise components appear in the slat noise spectrum at an angle of attack of 4 degrees and a freestream velocity of 25 m/s. The dominant tone is associated with the vortex shedding off the slat cusp through the Kelvin-Helmholtz instability. This tone was successfully suppressed using a plasma actuator employing an open-loop control. A maximum reduction of 11 dB was achieved at a St number of approximately 19.7. A quasi-static feedback control system was also developed, wherein a controller is responsible for calculating the control inputs in terms of ii feedback signals. The experimental results show that the controller can work effectively to suppress the slat noise.

A set of wind tunnel experiments were performed to study the effect of large surface roughness on circular cylinder noise, with the goal of improving landing gear noise predictions. Roughness increases vortex shedding noise levels, and shifts the peak to a lower Strouhal number. The noise levels in the fall-off range also increase, but no significant change in the fall-off rate is observed. The decrease of the vortex shedding peak frequency has been associated with early detachment caused by the effect of roughness on the TBLs, which is in agreement with previous experimental studies with smaller roughness. The high frequency range of the spectrum revealed a broadband, Strouhal-based peak, which is caused by roughness noise generated on the upstream face of the cylinder. The peak Strouhal number is well predicted by Howe's model using the maximum outer velocity around the cylinder. Cylindrical roughness presents a weaker roughness noise peak, but higher noise levels for higher frequencies, and is thought to be caused by sharp edge separation. A bluff body roughness noise model has been developed based on the model of Howe and a Green's function tailored to the bluff body geometry, calculated using the Boundary Element Method. The application to rough circular cylinders using a at wall (ZPG) TBL model shows good agreement with experiments for downstream observers, but the model overpredicts the levels in over-head observers. The disagreement is thought to be due to inaccuracy of the at wall TBL model. The transition from smooth regime to rough regime was studied experimentally by partially covering the cylinder with distributed roughness in spanwise uniform configurations. Transition regarding vortex shedding happens mainly when roughness is added or removed around the separation region. The results agree with the fact that roughness changes the separation location by perturbing the TBL close to separation. Sparse and dense two-dimensional roughness on a circular cylinder, studied using CFD, have similar effects than distributed roughness regarding the vortex shedding peak level and frequency.

A gas turbine disc has three critical regions for which lifing calculations are essential: the assembly holes or weld areas, the hub region, and the blade-disc attachment area. Typically, a firtree joint is used to attach the blades to the turbine disc instead of a dove-tail joint, which is commonly used for compressor discs. A firtree joint involves contact between two surfaces at more than one location which makes the joint more difficult to design. Large loads generated due to the centrifugal action of the disc and associated blades are distributed over multiple areas of contact within the joint. All of the contacts in a firtree joint are required to be engaged simultaneously when the blades are loaded. However, slight variations in the manufacture of these components can have an impact on this loading. It is observed that small changes in the geometric entities representing contact between the two bodies can result in variations in the stress distribution near contact edges and the notch regions. Even though manufacturing processes have advanced considerably in the last few decades, the variations in geometry due to these processes cannot be completely eliminated. Hence, it is necessary to design such components in the presence of uncertainties in order to minimise the variation observed in their performance. In this work, the variations in geometry due to the manufacturing processes used to produce firtree joints between a gas turbine blade and the disc are evaluated. These variations are represented in two different ways using measurement data of firtree joints obtained from a coordinate measuring machine (CMM): (i) the variation for the pressure angle in the firtree joint is extracted from a simple curve fit and (ii) using the same measurement data, the unevenness of the pressure surfaces is represented using a Fourier series after filtering noise components. A parametric computer aided design (CAD) model which represents the manufacturing variability is implemented using Siemens NX. Non-smooth surfaces are also numerically generated by assuming the surface profile to be a random process. Two- and three-dimensional elastic stress analysis is carried out on the firtree joint using the finite element code, Abaqus and the variations observed in the notch stresses with changing pressure angle are extracted. A surrogate assisted multiobjective optimisation is performed on the firtree joint based on the robustness principles. Kriging based models are used to build a surrogate for notch stresses and the non-dominated sorting genetic algorithm-II (NSGA-II) is implemented to perform a multiobjective optimisation in order to minimise the mean and standard deviation of the notch stresses. An iterative search algorithm that updates the Kriging models with equally spaced infill points from the predicted Pareto front is adopted. Finally, a new design of the firtree joint is obtained which has better performance with respect to the variation in the notch stresses due to manufacturing uncertainties.

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This study investigates the aerodynamics of nil inverted wing in ground effect, a race car wheel and the interaction between the two components, using numerical and experimental methods. The wheels were located behind the wing at flU overlap and gap of 20mm, and the wing ride height. iu the vertical direction was the primary variable. Models of 50% scale were used , giving a Reynolds number of 5.8 x 105 based on the wing chord . The Detached-Eddy Simulation model was validated against wind tunnel measurements including PIV, surface pressures and forces , where it was found to outperform a Reynolds averaged Navier-Stokes approach which used the Spalart-Allmaras turbulence model. It accurately predicted the wing vortex breakdown at low ride heights, which is of the bubble type with a spiralling tail, and the wake of the wheel. A mesh sensitivity study revealed that a finer mesh increased the amount of structures captured with the DES model, improving its accuracy.

Topics in wind turbine aerodynamics are reviewed. These include the effect of freestrearn turbulence all the flows over wind turbine blades; dynamic stall phenomenon; and rotational augmentation. The advantages of numerical studies on these topics are highlighted and large-eddy simulation (LES) is selected to overcome the defects for other numerical approaches, e.g. Reynolds Average Navier-Stokes (RANS) , all such applications