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Corrosion of the oil and gas pipeline infrastructure has the potential to lead to catastrophic consequences, including loss of life. By performing continual diagnostic non-destructive evaluation (NDE) assessments of the pipeline infrastructure, corrosion can be tracked, mitigated and controlled. However, the sheer scale of the modern pipeline networks around the world requires new innovative techniques to efficiently and cost-effectively monitor and screen for corrosion. Currently there are two well-established ultrasonic techniques for monitoring and screening corrosion. Spot measurements extract the pipe wall thickness at a single point location. This method can extract an accurate value of wall thickness, but the measurement is highly localised. In contrast, long range guided waves (LRGWs) screen for damage by exciting a pulse which travels axially down a pipe and will reflect off any large areas of corrosion. This method has a large coverage area, but is much less sensitive to corrosion than the spot measurement techniques. The first part of this thesis explores the sensitivity of a circumferential guided wave technique, which is suggested to mitigate against some of the disadvantages of existing methodologies. This method propagates a guided wave around the circumference of a pipe, and uses either the reflected pulse from the defect or the through transmitted pulse, to detect the presence of a defect. The viability of this method was assessed by modelling the ultrasonic response to a range of different defects and using probabilistic techniques. The S0, SH1 and SH0 modes were studied and it was revealed that in reflection the technique is able to detect narrow and deep defects, whereas in transmission wide defects are detectable. Wavelength normalised results are also presented. The second part of this thesis investigates the interaction between spot measurement (shear bulk waves) and hemispherical pits. This is with a view to further understanding the mechanics behind the interaction between shear waves and pitting corrosion, and to assessing current transducer types to the tracking and monitoring of individual pits. It was found that an anisotropic relationship exists between the pit’s location relative to the shearing direction of the bulk wave. This is due to either the inherent anisotropy of the transducer or the mode conversion of the shear wave.

This project consists of a two-pronged computational and experimental approach to the study of flow in closed, thin rectangular ducts with a partial cubic blockage. Results are presented at three different bulk Reynolds numbers, ReD = 5600, 10400 and 15600, based on the channel height, which is also the blockage dimension. The new experimental data produced consists of fluctuating pressure measurements at the cube surface, with 2D-2C PIV snapshots captured simultaneously in the wake region. In addition to this, DNS data is produced at the lowest Reynolds number of ReD = 5600, allowing more detailed comparisons where PIV laser access was not possible. Comparisons are drawn between the data and URANS CFD simulations. A literature review and preliminary testing process narrowed down the considered URANS models to the two-layer k−ε model and the Elliptic Blending Reynolds Stress Model, or EBRSM. In the light of the new data, these two URANS models are compared in order to better understand their strengths and weaknesses. Particular regard is given to the prediction of large-scale unsteady behaviour, with a focus on vortex shedding. This unsteady phenomenon was found to be present and to have a significant effect on the flow in the near-cube and wake regions. Results show that certain aspects of this behaviour are captured with only limited accuracy by the URANS models tested. As a result, inaccuracies are also found in the mean simulated velocity fields. The shortcomings appear more pronounced at higher flow rates. At a given flow rate, they are more severe in regions of the flow where organised unsteadiness is large relative to the mean values. It is suggested that inaccuracies in mean URANS predictions are a result of limitations in model capability for unsteady flows, and that validation cases may be pertinent to address this.

The aim of the project was to improve the ultrasonic array inspection of high-density polyethylene (HDPE) heat-fused pipe joints of cooling water pipework that is installed in EDF Energy's nuclear power stations. Whereas ultrasound array inspection is now established for safety-critical metal components, HDPE poses a hugely challenging problem, that the ultrasound waves are heavily attenuated by the material. This impacts multiple aspects of the inspection and of the modelling that is needed to design and qualify inspection. The thesis reports a range of research that was needed to overcome this challenge. The work of the thesis has: - obtained accurate acoustic properties of HDPE that are necessary for improved simulated or real ultrasonic array imaging of HDPE pipe joints - developed a simulation technique for representing ultrasound in such inspections that has both high accuracy and efficiency - extended analytical analysis of ultrasound scattering from cylindrical voids from elastic media to general, attenuative media - used the Huygens-Fresnel principle to represent ultrasound scattering from volumetric and planar voids, to image sub-wavelength features of these defects in an ideal circular array setup, and to image angled ultrasonic array nondestructive evaluation (NDE) of potential defects occurring in HDPE pipe joints - devised an automated and antidispersive system for reducing coherent and incoherent noise in waveforms with an isolated wave reflection signal - produced an imaging and analysis method for ultrasonic array NDE that can represent defects in a refractive, reflective, and scattering environment in attenuating media, which is applied to data from the above developed simulation technique - applied much of the above imaging and analysis method to defects machined into HDPE pipe material, with an experimental ultrasonic array controller, yet with an array of limited suitability - specified parameters for ultrasonic arrays and a water-filled wedge, which are optimum for HDPE pipe joint inspection, and have been designed and built by Imasonic SAS, France, for research use at Imperial College London.

Three problems are considered, namely (1) the steady flow produced between two infinite torsionally oscillating discs (2) the flow induced by a disc oscillating about a state of steady rotation (3) net transport in water waves. They are all oscillatory fluid motions with the common feature that when one attempts to obtain a solution by small amplitude approximation methods, they prove to belong to the class of singular perturbation problems. Furthermore, the singularities are all of the double boundary layer form, i.e. in certain regions of the flow, the viscous and non-linear terms are comparable. In the first problem, particular interest centres on how the double boundary layer matches on to an inviscid outer region where only the non-linear terms are important. In the second problem, the important singularity is a resonance point which occurs when the frequency of the imposed oscillation is twice the basic rotational frequency. Analysis shows that the form of the solution depends critically on the boundary conditions which are assumed in the far field. The third problem is only analysed to obtain the regular perturbation solution to second order. The singularity that this contains is noted but not analysed further.

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