Mechanical Engineering - Theses
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ItemNumerical investigation of hemodynamics in stenotic vessels( 2023-08)Stenosis refers to any condition in which a vessel becomes abnormally narrow, so the flow through it is restricted. It is associated with an obstruction of a blood vessel lumen resulting in physiologically abnormal hemodynamics in the vessel and further progression of the obstruction. The presence of stenosis results in an acceleration of the flow past it which is accompanied by a noticeable drop in pressure, while the flow upstream and downstream of the stenosis is commonly disturbed due to rapid change in the geometry of the stenosed vessel. Stenotic flows can occur in various physiological contexts, including atherosclerosis, vascular diseases, inflammation, congenital heart defects and etc. Understanding the hemodynamics of stenotic flows is important for diagnosing and monitoring various cardiovascular diseases, as well as for designing medical devices and improving treatment strategies, encompassing pharmaceutical interventions, stent procedures, and surgical interventions. The objective of this thesis is to conduct a series of numerical experiments on stenotic flows under different geometrical and flow conditions using the direct numerical simulation method. In the thesis, the hemodynamics are numerically investigated in surrogate models that mimic vessels and their connections in the presence of stenosis. The thesis explores different origins of stenosis formation, including atherosclerosis, inflammation, subsequent scar tissue formation, and thrombosis. As an example of stenosis arising from the buildup of atherosclerotic deposits on the inner walls of arteries, the thesis considers a surrogate model that mimics the left anterior descending artery (LAD) affected by stenosis. To investigate hemodynamics in the presence of stenosis caused by scar tissue formation, stenoses at anastomoses between the artificial conduit and vessels in the surrogate model of the total cavopulmonary connection (TCPC) are examined. Furthermore, the thesis focuses on platelet activation triggered by the extensional flow in a channel with a stenosis of a hyperbolic shape. The surrogate LAD model yielded results that included various hemodynamic indices. These indices have the potential to predict the advancement of coronary artery disease. The results showed that alterations in the axisymmetric configuration of the stenosis significantly affect both the flow within the stenosis and downstream, emphasizing the importance of considering the asymmetry of the stenosis in diagnostics to enhance accuracy. The findings of the surrogate TCPC model revealed that the hemodynamics within the model are highly responsive to alterations in conduit stenosis geometry and relative lung perfusion. The most physiologically detrimental scenario, characterized by reduced energy efficiency and heightened wall shear stress, is observed in a TCPC with diffuse conduit stenosis and highly uneven lung perfusion. Finally, a noticeable response of platelet activation to the increased extensional and compressional forces in the flow of a channel with a stenosis of a hyperbolic shape was observed at the entrance and exit of the stenosis. In summary, the thesis delves into the hemodynamic environment of the stenotic flows and gains insights into the flow physics associated with physiologically abnormal hemodynamic metrics. Such insights are crucial for assessing the clinical risks and implications faced by cardiovascular disease patients.
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ItemResolvent analysis pressure modelling for airfoil trailing-edge noise predictions( 2021)The design of quieter airfoils has been stymied by the lack of fast-turnaround, accurate noise prediction tools applicable to new designs. Herein, the suitability of incompressible resolvent analysis for predicting the acoustic source field is investigated. This is done by comparing resolvent predictions to high-fidelity counterparts. Combining resolvent method surface pressure predictions with Amiet's acoustic analogy is shown to be a very promising trailing-edge noise prediction framework. The accuracy of this noise prediction hinges on an accurate prediction of the surface pressure difference, in the region close to the airfoil trailing edge. In the first part of this thesis it shown, for a flat plate and a NACA0012 airfoil, that incompressible resolvent analysis accurately captures the surface pressure footprint of hydrodynamic instabilities that give rise to noise. Resolvent analysis can be relied upon to identify the frequencies at which dominant linear instability mechanisms occur that lead to tonal contributions to TE noise. At these frequencies, resolvent-based Amiet noise predictions replicate DNS-based Amiet noise predictions and are two orders of magnitude faster. Incompressible resolvent analysis is also applied to a controlled diffusion (CD) airfoil, at a high Reynolds number and large incidence. The physics-driven nature of resolvent analysis is demonstrated, as it captures subtle differences in the acoustic source field for changes in the flow conditions. However, the method struggles to capture structures that arise from nonlinear interaction and the breakdown to turbulence. The second part of this thesis consists of an investigation into which components of pressure are accounted for by resolvent analysis of the incompressible Navier-Stokes equations. Firstly, we analyse whether the resolvent method predicts solely one of the incident or scattered pressure fields or whether it accounts for both and their interaction, i.e. to total pressure field. The resolvent method is shown to capture the total pressure field, motivating its use for generating input to Amiet's acoustic analogy. Secondly, we analyse how accurately the incompressible resolvent formulation predicts the fast component of pressure and whether it can be used to retrieve the slow component of pressure. At frequencies where the dominant flow features arise from linear instability mechanisms, the fast pressure far exceeds the slow pressure in magnitude. Resolvent analysis provides accurate approximations of this fast pressure, and by extension of the total pressure. At frequencies where the dominant flow features arise due to the nonlinear interaction of modes from different frequencies, the slow component of pressure is non-negligible. To model the latter, the resolvent forcing needs to contain a non-solenoidal component. This is achieved by approximating the forcing using the triadic interaction of resolvent modes from frequencies where linear mechanisms dominate. Using the true forcing, evaluated from DNS data, we are able to capture the airfoil surface pressure with great accuracy, even at frequencies where the flow physics are dominated by nonlinear interaction, the resolvent operator is not low-rank, and the slow pressure constitutes a large contribution to the total pressure. This proves that even when it is not low-rank, the resolvent operator remains a transfer function capable of predicting the flow-physics in an accurate and low-cost manner.
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ItemNumerical simulation of coronary blood flow: from idealised to patient-specific studies( 2021)Haemodynamic indices such as endothelial shear stress (ESS) resulting from disturbances in arterial blood flow are associated with pathological processes underlying atherosclerosis and other complications after stent implantation. Computational fluid dynamics (CFD) allows in vivo estimation of ESS and other flow indices, offering insights into the progression of coronary artery diseases and optimising treatment. Stenting is an effective treatment method to unblock the artery. Unfortunately, stents are not always placed perfectly. Incomplete stent apposition (ISA), defined as a lack of contact between the stent and arterial wall, can lead to complications after stenting, such as late stent thrombosis and restenosis. Simplified 2-dimensional simulations show that ISA leads to recirculation bubbles behind the stent struts, which is one of the leading causes of abnormal ESS. The faster the heart pumps, the quicker blood flow accelerates and decelerates and hence larger flow recirculation bubbles. This undesirable effect, however, can be improved by changing the strut profiles (e.g. from square to circular). As atherosclerotic plaque accumulates, artery cross-sections may change from circular to elliptical. The simulations of the full-scale stent model in elliptical cross-sectional arteries demonstrate high shear rates at ISA regions, which increases the risk of platelet activation. When activated platelets accumulate at low ESS regions, it may indicate a site for thrombosis. The knowledge gained from the analysis in idealised models can be applied to study the haemodynamics inside a patient’s artery. Using high-resolution intravascular imaging such as Optical Coherence Tomography (OCT), a total of 59 cases of coronary arteries with active lesions including plaque rupture and erosion has been analysed. The CFD results showed active lesions with higher ESS compared to those of non-lesions and ESS change between baseline and follow-up cases. Temporal ESS change (baseline vs. follow-ups) showed correlations to plaque erosion, but other factors, such as medications etc., were effective during the time and could affect the lesion condition and should also be taken into consideration.
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ItemHigh-fidelity numerical investigation of different mechanisms of aerofoil self-noise( 2020)The self-noise of an isolated controlled-diffusion aerofoil is investigated using direct noise computations. The motivation is to investigate the multiple sources of aerofoil self-noise on a realistic compressor blade geometry, the physical mechanisms leading to the generation of sound, and the influence of compressibility effects. The use of direct noise computation allows to obtain a detailed and accurate picture of both the hydrodynamic and acoustic fields. The high-order finite difference solver HiPSTAR is used to conduct the numerical simulations. In the context of the present study, the capabilities of HiPSTAR have been extended with an overset grid framework. The framework employs a novel algorithm for the generation of the composite grid, i.e. to identify the discretisation, interpolation and non-physical points. This algorithm was designed to minimise and simplify the user input, while maintaining the flexibility to handle complex setups. An explicit fourth-order Lagrange interpolation scheme is used, so that the formal order of accuracy of the finite difference scheme used in the flow solver is matched. An instability linked to the possible presence of uncoupled numerical solutions on separate grids sharing the same physical location is discussed, and a modification of the composite grid generation algorithm that prevents this instability is introduced. The final overset grid method is validated with two test cases: the convection of an isentropic vortex and the Taylor-Green vortex. The solver is used to conduct four large eddy simulations and one direct numerical simulation of the flow around the controlled-diffusion aerofoil. The angle of attack is 8 degrees, the chord-based Reynolds number is 10000, and results obtained for four values of the free-stream Mach number [0.2, 0.3, 0.4, 0.5] are compared. For those flow parameters, the pressure side is fully laminar, whereas a separation bubble is present on the suction side close to the leading edge that promotes transition to turbulence. The size of the separation bubble is found to increase with the Mach number. Two noise sources are observed, one at the trailing edge and one in the leading edge transition/reattachment region. The first has a broadband, low frequency spectrum, while the second displays a tone whose frequency depends on the local Mach number. Because the leading edge separation bubble is very small, the associated tone frequency is high and requires a significantly finer grid to faithfully resolve the acoustic propagation than what is typically deemed sufficient. Finally, cross correlations between the surface pressure and the far-field pressure reveal that the pressure fluctuations reaching the trailing edge are initially generated in the transition/reattachment region, which indicates that the trailing edge noise is a consequence of the pressure fluctuations generated by the separation bubble.