Several experimental procedures were developed to enhance the understanding of the aerodynamic properties of tennis balls. Four test methods were tried as quantitative assessments of the aerodynamic forces that act on tennis balls, whilst an additional two methods were introduced for qualitative purposes. A computational trajectory model was developed to predict the effect of any modifications to tennis balls proposed in the study. The test methods adopted utilised two different wind tunnels, projection devices, dropper devices, aerodynamic load cells and motion analysis techniques using high-speed digital cameras. Several different tennis balls were tested: some had the nap modified to investigate changes in aerodynamic forces that may occur during play, others were oversized to investigate the options available for slowing the game down. CD and CL profiles were obtained for a normal sized ball with unmodified nap and then used to develop a set of equations that enable the CD and CL of a tennis ball to be calculated at any speed and spin rate. When used in a trajectory model, a 6.5% larger ball was shown to decelerate 5% faster than a normal sized ball when projected with the same initial elevation angle, speed and spin rate. This results in the larger ball landing 1.5 metres shorter and taking more than 19ms longer to arrive at the receiver. Initial testing showed that the CD of all tennis balls with unmodified naps was similar and remained constant at around 0.53 up to a wind speed of around 63ms'. The nap of the tennis ball was modified to represent early wear characteristics (fluffed) and extensive wear characteristics (shaved). It was found that the CD of a ball with a fluffed nap is higher than that of a ball with an unmodified nap, which in turn is greater than the CD of a ball with a shaved nap. The CD of a ball almost twice the size of a normal tennis ball was found to be independent of Reynolds number up to 5x105, which is clear evidence that the boundary layer around a tennis ball turns turbulent at a low Reynolds number. The ball with the shaved nap was shown to be similar to a classic rough ball however, with boundary layer transition occurring at a low Reynolds number. The flow around a tennis ball was assessedu sing pressurep rofiles and smokep articles, and the separation of flow for all balls was shown to be near the poles. Pressure profile testing provided clear separation details, and showed how the pressure around the ball differs for subcritical and postcritical Reynolds number regimes. Flow through and over the fibres causes the elevated CD over and above that associated with separation at the apex of a sphere.

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The aim of the current work is to investigate the stability of supersonic and subsonic triple deck flows and liquid layer and jet double deck flows. In particular we aim to investigate the stability properties of the separated flows when coming into contact with a concave or convex corner. The work presented uses two distinct approaches to investigate the stability of the flow. First, a global linear stability analysis, taking disturbances proportional to eAt, is performed. Secondly, numerical simulations have been carried out with the linearised unsteady equations, linearised about a steady state, using forced disturbances. The governing equations are solved in primitive variables format using two different discretization methods. The first method uses finite differences in the streamwise direction and Chebyshev collocation in the wall normal direction and the second method uses finite differences in both spatial directions. The numerical results obtained show good agreement for the global stability analysis and the bifurcation results, with the global stability analysis predicting the bifurcations and indicating the loss of stability at such parameters. Though, in reasonably separated flow, the temporal simulation results disagree with the global stability behaviour. The temporal simulations indicate that the separated flows are unstable with respect to some non-modal instability and that round-off error alone is sufficient enough to trigger this instability.

The current work details an experimental study that attempts to introduce significant alterations to the level of unsteady loading experienced by the lifting surface of a delta wing when vortex breakdown (also known as vortex burst) is present. Specifically, the study looks to make use of an array of synthetic jet actuators along the length of the wing leading edge, to alter the characteristic spectral energy signature of the spiral vortex filament known to be present in many burst flows. The experimental model is a delta wing with a 60° angle of sweep with a leading edge profile of 20mm diameter. 137 pressure tappings are distributed over the lifting surface for the measurement of steady pressure coefficients, unsteady pressures and power spectral density distributions. The leading edge has 18 discrete synthetic jet actuators distributed along its length, each with a 1.2mm diameter orifice and a piezoceramic diaphragm. Three different waveforms are used to drive the actuator array: a single sine wave, two summed sine waves and a pulsed wave, each with frequencies based on measurements of the characteristic spectra of the burst flow. Sine wave actuation with a frequency an order of magnitude greater than the characteristic burst frequencies is found to be particularly effective at altering the burst spectra and is seen to be able to produce changes in the unsteady pressure levels of the order of 30% to 40%. The general effect of this actuation is seen to be an increase in unsteady loading in the immediate post burst region but a significant decrease in loading in the far-burst region. Surface flow visualisation results show that actuation introduces time-averaged delays in the leading edge separation line, local to each orifice, resulting in localised variations in vortex diameter. Also, PlV data shows that in the initial burst region, actuation moves the vortex core toward the surface. Based on these results, an interpretive hypothesis is formulated that offers an explanation for the changes seen. Firstly, it is suggested that movement of the core toward the surface is responsible for the loading increases seen in the initial burst region. Secondly, it is postulated that the local changes in vortex diameter, seen in the flow visualisation results, introduce kinks in the spiralling vortex filament of the burst flow. These kinks will augment the rate of dissipation of the energy of the spiral due to increased levels of Biot-Savart self-induction with the result being lower levels of unsteady loading on the wing surface. Finally, based on the large amount of potential actuation parameters and the interpretive nature of the hypothesis suggested, a series of suitable avenues for future work are suggested.

With rising global fuel prices impacting on airline operating costs and increasingly stringent environmental targets being enforced on the aviation industry as a whole, interest in the implementation of laminar flow control techniques has been renewed. However, there are many difficulties associated with the design of practical profiles with extended regions of laminar f low, in particular the trade-off between empirical transition prediction methods versus the computational intensiveness of the more accurate physics based methods. The aim of the work documented in this thesis is to demonstrate how improvements in both the robustness and efficiency of the design of aerodynamic components with extended regions of laminar flow can be achieved through the use of a hybrid physics-based laminar flow optimisation approach, implementing the linear form of the Parabolised Stability Equations. Hybridisation of the optimisation algorithm to improve the efficiency of a global search genetic algorithm with an adaptive low fidelity local search is discussed, and benchmarking against the more traditional gradient based and genetic algorithms demonstrates that the bespoke algorithm consistently outperforms the traditional search methods. Initial tests indicated that the success of the algorithm could be enhanced through increased populations, and extension of the algorithm to use trust region methods to exploit the solution speed of the solver is demonstrated. This is achieved while retaining the accuracy of the parabolised stability evaluations to facilitate larger population searches without sacrificing solution robustness. The viability of using an adaptive low fidelity local search is ensured through an updatable N factor scheme, the accuracy of the lower fidelity solver is improved through iterating the N factor to give a solution closer to that had the high fidelity solver been implemented. In all instances tested, the newly developed algorithm resulted in increased quality of the profiles generated in relation to the defined objective function

This research addresses the difficulties in determining the manoeuvring characteristics of a slender body through a fluid. This work has direct applicability in many areas of hydrodynamics and aerodynamics. For example the manoeuvring of ships and submarines through water or the manoeuvring of missiles or aircraft through the atmosphere for cases where the compressibility of the air is not a significant factor. The research is approached by looking at both a slender body theory in potential flow (inviscid) and a slender body theory in Oseen flow (viscous). The differences between these two approaches are considered and the force expressions are found for both cases. This work gives rise to a discrepancy between the lift force expressions (sometimes referred to as the normal or side force) for the two approaches which leads to some interesting comparisons and conclusions. This discrepancy in the lift force calculation has in fact been noticed separately by both aerodynamicists, Alien [3], and hydrodynamicists, Clarke [12] and Newman [27]. The work presented here looks at the reasons for this force discrepancy and links slender wing theory and slender body theory together.

Synthetic jet actuators have previously been defined as having potential use in both internal and external aerodynamic applications. The formation of a jet flow perpendicular to the surface of an aerofoil or in a duct of diffuser has a range of potential flow control benefits. These benefits can include both laminar to turbulent transition control, which is associated with a drag reduction in aerodynamic applications. The formation and development of zero-net-mass-flux synthetic jets are investigated using highly accurate numerical methods associated with the methodology of Direct Numerical Simulation (DNS). Jet formation is characterised by an oscillating streamwise jet centreline velocity, showing net momentum flux away from the jet orifice. This momentum flux away from the orifice takes the form of a series of vortex structures, often referred to as a vortex train. Numerical simulations of the synthetic jet actuator consist of a modified oscillating velocity profile applied to a wall boundary. The Reynolds numbers used vary from 85 ≤ Re ≤ 300. A complete numerical study of both axisymmetric and fully three-dimensional jet flow is performed. A parametric axisymmetric simulation is carried out in order to study the formation criterion and evolution of zero-net-mass-flux synthetic jets under variations in actuator input parameters. From the results of these simulations the conditions necessary for the formation of the synthetic jet along with the input parameters that provide an optimal jet output are deduced. Jet optimisation is defined by the mass flow, vortex strength and longevity of the vortex train as it travels downstream. Further investigations are carried out on a fully three-dimensional DNS version of the optimised axisymmetric case. Comparisons between the jet evolution and flow-field structures present in both the axisymmetric and three-dimensional configurations are made. This thesis examines the vortex structures, the jet centreline velocities along with time dependent and time averaged results in order to deduce and visualise the effects of the input parameters on the jet formation and performance. The results attained on altering the oscillation frequency of the jet actuator indicated that synthetic jets with zero mean velocity at the inflow behave significantly differently from jets with non-zero mean velocity at the inflow. A study into the evolution and formation of the train of vortex structures associated with the formation of a synthetic jet is performed. This study is accompanied with a series of time averaged results showing time dependent flow-field trends. The time history of the jet centreline velocity, showing the net momentum flux of the fluid away from the orifice of a fully developed synthetic jet, is analysed for both axisymmetric and three-dimensional cases. Differences in the fluid dynamics between the idealised axisymmetric configuration and the three-dimensional case have been identified, where three-dimensional effects are found to be important in the region near the jet nozzle exit. The effect of a disturbance introduced into the three-dimensional simulation in order to break its inherent symmetr around the jet centreline is examined by altering the input frequency of the disturbance. It was found that the effect of this relatively minor disturbance had a major effect on the jet flow field in the region adjacent to the orifice. The effect of which was deemed to be caused by discontinuities in the surface of the jet orifice due to manufacturing tolerances. Although the effects of these disturbances on the jet flow-field are large, they seem to have been neglected from numerical simulations to date. The effect of a synthetic jet on an imposed cross-streamwise velocity profile was examined. It was found that the synthetic jet flow-field resulted in a deformation of the velocity profile in the region downstream of the synthetic jet. It is suggested that this region of deformed flow could interact with coherent structures in a transitional boundary layer in order to delay flow transition to turbulence. The effect of varying the Strouhal number of a synthetic jet in a cross-flow is also analysed. It is clear from the results presented that, in the presence of a cross-flow velocity the Strouhal number effect on the synthetic jet flow field evolution, while dominant in a quiescent fluid is surpassed by the effect of the cross-flow.

An experimental investigation has been undertaken in a wind tunnel to study the induced airflow and drag reduction capability of AC glow discharge plasma actuators. Plasma is the fourth state of matter whereby a medium, such as air, is ionized creating a system of electrons, ions and neutral particles. Surface glow discharge plasma actuators have recently become a topic for flow control due to their ability to exert a body force near the wall of an aerodynamic object which can create or alter a flow. The exact nature of this force is not well understood, although the current state of knowledge is that the phenomenon results from the presence of charged plasma particles in a highly non-uniform electric field. Such actuators are lightweight, fully electronic (needing no moving parts or complicated ducting), have high bandwidth and high energy density. The manufacture of plasma actuators is relatively cheap and they can be easily retrofitted to existing surfaces. The first part of this study aims at characterising the airflow induced by surface plasma actuators in initially static air. Ambient air temperature and velocity profiles are presented around a variety of actuators in order to understand the nature of the induced flow for various parameters such as applied voltage, frequency, actuator geometry and material. It is found that the plasma actuator creates a laminar wall jet along the surface of the material on which it is placed. The second part of the study aims at using plasma actuators to reduce skin-friction drag in a fully developed turbulent boundary layer. Actuators are designed to induce spanwise forcing near the wall, oscillating in time. Thermal anemometry measurements within the boundary layer are presented. These show that the surface plasma can cause a skin-friction drag reduction of up to 45% due to the creation of streamwise vortices which interact with, and disrupt the near-wall turbulence production cycle.