Tesis sobre el tema "Blood flow - Mathematical models"
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1
Pincombe, Brandon. "A study of non-Newtonian behaviour of blood flow through stenosed arteries /". Title page, contents and summary only, 1999. http://web4.library.adelaide.edu.au/theses/09PH/09php6469.pdf.
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Healy, Timothy M. "Multi-block and overset-block domain decomposition techniques for cardiovascular flow simulation". Diss., Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/15622.
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Carrig, Pauline Elize. "The effect of blood chemistry on the rheological properties of the fluid". Thesis, Virginia Polytechnic Institute and State University, 1986. http://hdl.handle.net/10919/94451.
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A four variable constitutive equation was developed utilizing the method first presented by Schneck and Walburn. Spearman rank correlation coefficients were calculated on whole blood samples within a narrow range of hematocrit to investigate further the effect of the various plasma constituents on whole blood viscosity.
Viscosity measurements were made on one hundred anticoagulated blood samples of known hematocrit and chemical composition. The constitutive equation was developed using a power law functional form similar to that employed by Schneck and Walburn. This equation contains two parameters, the consistency index and the non-Newtonian index. A computerized multiple regression technique with apparent viscosity as the dependent variable was used to determine the particular form of these parameters.
The one, two and three variable models developed confirmed the results of the previous work of Schneck and Walburn. The four variable model included the total lipids in combination with the concentration of total protein minus albumin and hematocrit. Spearman rank correlation coefficients showed the highest correlations between whole blood viscosity and the plasma constituents to be those of the globulins, total protein and fibrinogen.
The constitutive equation developed did not show as high a correlation between experimental data and theory as did the Schneck-Walburn three variable model. The addition of a fourth variable did produce a statistically significant increase over the best three variable model of the present study.M.S.
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Hong, Say Yenh. "Fluid structure interaction modeling of pulsatile blood flow in serial pulmonary artery stenoses". Thesis, McGill University, 2007. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=112571.
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Motivated by the physiological phenomena of collapse and flow limitation for a serial pulmonary artery stenosis, we investigated the three-dimensional influence of spatial configuration on the wall motion and hemodynamic. Our numerical study focused on the effect of two geometrical parameters: the relative distance and the angular orientation between the two stenoses. The collapse of a compliant arterial stenosis may cause flow choking, which would limit the flow reserve to major vital vascular beds such as the lungs, potentially leading to a lethal ventilation-perfusion mismatch. Flow through a stenotic vessel is known to produce flow separation downstream of the throat. The eccentricity of a stenosis leads to asymmetric flow where the high velocity jets impinge on the sidewall, thereby inducing significant dissipation. The additional viscous dissipation causes a higher pressure drop for a flow through a stenotic vessel, than in a straight compliant vessel. It is likely that some particular morphology would have a higher vulnerability to the fluid induced instability of buckling (divergence), under physiological pulsatile flow. It was found that fluid pressure distribution have substantial implication for the downstream wall motion, under conditions of strong coupling between nonlinear vessel geometries, and their corresponding asymmetric flow. The three-dimensional fluid structure interaction problem is solved numerically by a finite element method based on the Arbitrary Lagrangian Eulerian formulation, a natural approach to deal with the moving interface between the flow and vessel. The findings of this investigation reveal that the closeness between stenoses is a substantial indication of wall collapse at the downstream end. Moreover, the results suggest a close link between the initial angular orientation of the distal stenosis (i.e. the constriction direction) and the subsequent wall motion at the downstream end. For cases showing evidence of preferential direction of wall motion, it was found that the constricted side underwent greater cumulative displacement than the straight side, suggestive of significant wall collapse.
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Ang, Keng Cheng. "A computational fluid dynamic study of blood flow through stenosed arteries /". Title page, table of contents and summary only, 1996. http://web4.library.adelaide.edu.au/theses/09PH/09pha5808.pdf.
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Fry, Brendan. "Theoretical Models for Blood Flow Regulation in Heterogeneous Microvascular Networks". Diss., The University of Arizona, 2013. http://hdl.handle.net/10150/293413.
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Proper distribution of blood flow in the microcirculation is necessary to match changing oxygen demands in various tissues. How this coordination of perfusion and consumption occurs in heterogeneous microvascular networks remains incompletely understood. Theoretical models are powerful tools that can help bridge this knowledge gap by simulating a range of conditions difficult to obtain experimentally. Here, an algorithm is first developed to estimate blood flow rates in large microvascular networks. Then, a theoretical model is presented for metabolic blood flow regulation in a realistic heterogeneous network structure, derived from experimental results from hamster cremaster muscle in control and dilated states. The model is based on modulation of arteriolar diameters according to the length-tension characteristics of vascular smooth muscle. Responses of smooth muscle cell tone to myogenic, shear-dependent, and metabolic stimuli are included. Blood flow is simulated including unequal hematocrit partition at diverging vessel bifurcations. Convective and diffusive oxygen transport in the network is simulated, and oxygen-dependent metabolic signals are assumed to be conducted upstream from distal vessels to arterioles. Simulations are carried out over a range of tissue oxygen demand. With increasing demand, arterioles dilate, blood flow increases, and the numbers of flowing arterioles and capillaries, as defined by red-blood-cell flux above a small threshold value, increase. Unequal hematocrit partition at diverging bifurcations contributes to capillary recruitment and enhances tissue oxygenation. The results imply that microvessel recruitment can occur as a consequence of local control of arteriolar tone. The effectiveness of red-blood-cell-dependent and independent mechanisms for the metabolic response of local blood flow regulation is examined over a range of tissue oxygen demands. Model results suggest that although a red-blood-cell-independent mechanism is most effective in increasing flow and preventing hypoxia, the addition of a red-blood-cell-dependent mechanism leads to a higher median tissue oxygen level, indicating distinct roles for the two mechanisms. In summary, flow rates in large microvessel networks can be estimated with the proposed algorithm, and the theoretical model for flow regulation predicts a mechanism for capillary recruitment, as well as roles for red-blood-cell-dependent and independent mechanisms in the metabolic regulation of blood flow in heterogeneous microvascular networks.
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Alirezaye-Davatgar, Mohammad Taghi Graduate School of Biomedical Engineering Faculty of Engineering UNSW. "Numerical simulation of blood flow in the systemic vasculature incorporating gravitational force with application to the cerebral circulation". Awarded by:University of New South Wales. Graduate School of Biomedical Engineering, 2006. http://handle.unsw.edu.au/1959.4/26177.
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Background. Extensive studies have been conducted to simulate blood flow in the human vasculature using nonlinear equations of pulsatile flow in collapsible tube plus a network of vessels to represent the whole vasculature and the cerebral circulation. For non-linear models numerical solutions are obtained for the fluid flow equations. Methods. Equations of fluid motion in collapsible tubes were developed in the presence of gravitational force (Gforce). The Lax-Wendroff and MacCormack methods were used to solve the governing equations and compared both in terms of accuracy, convergence, and computer processing (CPU) time. A modified vasculature of the whole body and the cerebral circulation was developed to obtain a realistic simulation of blood flow under different conditions. The whole body vasculature was used to validate the simulation in terms of input impedance and wave transmission. The cerebral vasculature was used to simulate conditions such as presence of G-force, blockage of Internal Carotid Artery (ICA), and the effects on cerebral blood flow of changes in mean and pulse pressure. Results. The simulation results for zero G-force were in very good agreement with published experimental data as was the simulation of cerebral blood flow. Both numerical methods for solutions of governing equations gave similar results for blood flow simulations but differed in calculation performance and stability depending on levels of G-force. Simulation results for uniform and sinusoidal G-force are also in good agreement with published experimental results, Blood flow was simulated in the presence of a single (left) carotid artery obstruction with varying morphological structures of the Circle of Willis (CoW). This simulation showed significant differences in contralateral blood flow in the presence or absence of communicating arteries in the CoW. It also was able to simulate the decreases in blood flow in the cerebral circulation compartment corresponding to the visual cortex in the presence of G-force. This is consistent with the known loss of vision under increased acceleration. Conclusions. This study has shown that under conditions of gravitational forces physiological changes in blood flow in the systemic and cerebral vasculature can be simulated realistically by solving the one-dimentional fluid flow equations and non-linear vascular properties numerically. The simulation was able to predict changes in blood flow with different configurations and properties of the vascular network.
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Bouchnita, Anass. "Mathematical modelling of blood coagulation and thrombus formation under flow in normal and pathological conditions". Thesis, Lyon, 2017. http://www.theses.fr/2017LYSE1300/document.
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Cette thèse est consacrée à la modélisation mathématique de la coagulation sanguine et de la formation de thrombus dans des conditions normales et pathologiques. La coagulation sanguine est un mécanisme défensif qui empêche la perte de sang suite à la rupture des tissus endothéliaux. C'est un processus complexe qui est règlementé par différents mécanismes mécaniques et biochimiques. La formation du caillot sanguin a lieu dans l'écoulement sanguin. Dans ce contexte, l'écoulement à faible taux de cisaillement stimule la croissance du caillot tandis que la circulation sanguine à fort taux de cisaillement la limite. Les désordres qui affectent le système de coagulation du sang peuvent provoquer différentes anomalies telles que la thrombose (coagulation exagérée) ou les saignements (insuffisance de coagulation). Dans la première partie de la thèse, nous présentons un modèle mathématique de coagulation sanguine. Le modèle capture la dynamique essentielle de la croissance du caillot dans le plasma et le flux sanguin quiescent. Ce modèle peut être réduit à un modèle qui consiste en une équation de génération de thrombine et qui donne approximativement les mêmes résultats. Nous avons utilisé des simulations numériques en plus de l'analyse mathématique pour montrer l'existence de différents régimes de coagulation sanguine. Nous spécifions les conditions pour ces régimes sur différents paramètres pathophysiologiques du modèle. Ensuite, nous quantifions les effets de divers mécanismes sur la croissance du caillot comme le flux sanguin et l'agrégation plaquettaire. La partie suivante de la thèse étudie certaines des anomalies du système de coagulation sanguine. Nous commençons par étudier le développement de la thrombose chez les patients présentant une carence en antihrombine ou l'une des maladies inflammatoires. Nous déterminons le seuil de l'antithrombine qui provoque la thrombose et nous quantifions l'effet des cytokines inflammatoires sur le processus de coagulation. Puis, nous étudions la compensation de la perte du sang après un saignement en utilisant un modèle multi-échelles qui décrit en particulier l'érythropoïèse et la production de l'hémoglobine. Ensuite, nous évaluons le risque de thrombose chez les patients atteints de cancer (le myélome multiple en particulier) et le VIH en combinant les résultats du modèle de coagulation sanguine avec les produits des modèles hybrides (discret-continues) multi-échelles des systèmes physiologiques correspondants. Finalement, quelques applications cliniques possibles de la modélisation de la coagulation sanguine sont présentées. En combinant le modèle de formation du caillot avec les modèles pharmacocinétiques pharmacodynamiques (PK-PD) des médicaments anticoagulants, nous quantifions l'action de ces traitements et nous prédisons leur effet sur des patients individuelsThis thesis is devoted to the mathematical modelling of blood coagulation and clot formation under flow in normal and pathological conditions. Blood coagulation is a defensive mechanism that prevents the loss of blood upon the rupture of endothelial tissues. It is a complex process that is regulated by different mechanical and biochemical mechanisms. The formation of the blood clot takes place in blood flow. In this context, low-shear flow stimulates clot growth while high-shear blood circulation limits it. The disorders that affect the blood clotting system can provoke different abnormalities such thrombosis (exaggerated clotting) or bleeding (insufficient clotting). In the first part of the thesis, we introduce a mathematical model of blood coagulation. The model captures the essential dynamics of clot growth in quiescent plasma and blood flow. The model can be reduced to a one equation model of thrombin generation that gives approximately the same results. We used both numerical simulations and mathematical investigation to show the existence of different regimes of blood coagulation. We specify the conditions of these regimes on various pathophysiological parameters of the model. Then, we quantify the effects of various mechanisms on clot growth such as blood flow and platelet aggregation. The next part of the thesis studies some of the abnormalities of the blood clotting system. We begin by investigating the development of thrombosis in patients with antihrombin deficiency and inflammatory diseases. We determine the thrombosis threshold on antithrombin and quantify the effect of inflammatory cytokines on the coagulation process. Next, we study the recovery from blood loss following bleeding using a multiscale model which focuses on erythropoiesis and hemoglobin production. Then, we evaluate the risk of thrombosis in patients with cancer (multiple myeloma in particular) and HIV by combining the blood coagulation model results with the output of hybrid multiscale models of the corresponding physiological system. Finally, possible clinical applications of the blood coagulation modelling are provided. By combining clot formation model with pharmacokinetics-pharmacodynamics (PK-PD) models of anticoagulant drugs, we quantify the action of these treatments and predict their effect on individual patients
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Lucas, Claire. "An anatomical model of the cerebral vasculature and blood flow". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:37d408b6-b804-4085-b420-a9704aeb97eb.
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The brain accounts for around 2 % of human adult bodyweight but consumes 20 % of the resting oxygen available to the whole body. The brain is dependent on a constant supply of oxygen to tissue, transported from the heart via the vasculature and carried in blood. An interruption to flow can lead to ischaemia (a reduced oxygen supply) and prolonged interruption may result in tissue death, and permanent brain damage. The cerebral vasculature consists of many, densely packed, micro-vessels with a very large total surface area. Oxygen dissolved in blood enters tissue by passive diffusion through the micro-vessel walls. Imaging shows bursts of metabolic activity and flow in localised brain areas coordinated with brain activity (such as raising a hand). An appropriate level of oxygenation, according to physiological demand, is maintained via autoregulation; a set of response pathways in the brain which cause upstream or downstream vessels to expand or contract in diameter as necessary to provide sufficient oxygen to every region of the brain. Further, autoregulation is also evident in the response to pressure changes in the vasculature: the perfusing pressure can vary over a wide range from the basal-state with only a small effect on flow due to the constriction or dilation of vessels. Presented here is a new vasculature model where diameter and length are calculated in order to match the data available for flow velocity and blood pressure in different sized vessels. These vessels are arranged in a network of 6 generations each of bifurcating arterioles and venules, and a set of capillary beds. The input pressure and number of generations are the only specifications required to describe the network. The number of vessels, and therefore vessel geometry, is governed by how many generations are chosen and this can be altered in order to create more simple or complex networks. The flow, geometry and oxygen concentrations are calculated based on the vessel resistance due to flow from geometry based on Kirchoff circuit laws. The passive and active length-tension characteristics of the vasculature are established using an approximation of the network at upper and lower autoregulation limits. An activation model is described with an activation factor which governs the contributions of elastic andmuscle tension to the total vessel tension. This tension balances with the circumferential tension due to pressure and diameter and the change in activation sets the vessel diameter. The mass transport equation for oxygen is used to calculate the concentration of oxygen at every point in the network using data for oxygen saturation to establish a relationship between the permeability of the vessel wall to oxygen and the geometry and flow in individual vessels. A tissue compartment is introduced which enables the modelling of metabolic control. There is evidence for a coordinated response by surrounding vessels to local changes. A signal is proposed based on oxygen demand which can be conducted upstream. This signal decays exponentially with vessel length but also accumulates with the signal added from other vessels. The activation factor is therefore set by weighted signals proportional to changes in tissue concentration, circumferential tension, shear stress and conducted oxygen demand. The model is able to reproduce the autoregulation curve whereby a change in pressure has only a small effect on flow. The model is also able to replicate experimental results of diameter and tissue concentration following an increase in oxygen demand.
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Bevan, Rhodri L. T. "A locally conservative Galerkin approach for subject-specific biofluid dynamics". Thesis, Swansea University, 2010. https://cronfa.swan.ac.uk/Record/cronfa42314.
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In this thesis, a parallel solver was developed for the modelling of blood flow through a number of patient-specific geometries. A locally conservative Galerkin (LCG) spatial discretisation was applied along with an artificial compressibility and characteristic based split (CBS) scheme to solve the 3D incompressible Navier-Stokes equations. The Spalart-Allmaras one equation turbulence model was also optionally employed. The solver was constructed using FORTRAN and the Message Passing Interface (MPI). Parallel testing demonstrated linear or better than linear speedup on hybrid patient-specific meshes. These meshes were unstructured with structured boundary layers. From the parallel testing it is clear that the significance of inter-processor communication is negligible in a three dimensional case. Preliminary tests on a short patient-specific carotid geometry demonstrated the need for ten or more boundary layer meshes in order to sufficiently resolve the peak wall shear stress (WSS) along with the peak time-averaged WSS. A time sensitivity study was also undertaken along with the assessment of the order of the real time step term. Three backward difference formulae (BDF) were tested and no significant difference between them was detected. Significant speedup was possible as the order of time discretisation increased however, making the choice of BDF important in producing a timely solution. Followed by the preliminary investigation, four more carotid geometries were investigated in detail. A total of six haemodynamic wall parameters have been brought together to analyse the regions of possible atherogenesis within each carotid. The investigations revealed that geometry plays an overriding influence on the wall parameter distribution. Each carotid artery displayed high time-averaged WSS at the apex, although the value increased significantly with a proximal stenosis. Two out of four meshes contained a region of low time-averaged WSS distal to the flow divider and within the largest connecting artery (internal or external carotid artery), indicating a potential region of atherosclerosis plaque formation. The remaining two meshes already had a stenosis in the corresponding region. This is in excellent agreement with other established works. From the investigations, it is apparent that a classification system of stenosis severity may be possible with potential application as a clinical diagnosis aid. Finally, the flow within a thoracic aortic aneurysm was investigated in order to assess the influence of a proximal folded neck. The folded neck had a significant effect on the wall shear stress, increasing by up to 250% over an artificially smoothed neck. High wall shear stresses may be linked to aneurysm rupture. Being proximal to the aneurysm, this indicated that local geometry should be taken into account when assessing the rupture potential of an aneurysm.
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Hye, Md Abdul. "Simulation of transient blood flow in models of arterial stenosis and aneurysm". Thesis, University of Glasgow, 2012. http://theses.gla.ac.uk/3836/.
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The Large Eddy Simulation (LES) technique with the Smagorinsky-Lilly dynamic subgrid model and two-equation Standard k-ω Transitional turbulence model are applied to investigate non-spiral and spiral blood flow through three dimensional models of arterial stenosis and aneurysm. A spiral pattern of blood flow is thought to have many beneficial effects on hemodynamics. Previous computational studies on spiral blood flow involve only steady spiral flow in a straight stenosed pipe without considering an upstream curved section of the artery. But a spiral pattern in the blood flow may exist due to the presence of an upstream curved section in the artery. On the other hand, pressure is generally considered a constant quantity in studies on pulsatile flow through either arterial stenosis or aneurysm; however, blood pressure is a waveform in a physiological flow. Although cosine-type or smooth regular stenoses are generally taken in investigations of blood flow in a three-dimensional model of arterial stenosis, in reality, stenoses are of irregular shape. Besides stenosis and aneurysm, another abnormal condition of the artery is the presence of stenosis with an adjacent aneurysm in the same arterial segment, especially in the posterior circulation. A study on (steady or pulsatile) flow through such arterial stenosis with an adjacent aneurysm in the same arterial segment is not available so far. Therefore, taking above things into consideration, thorough investigations of steady and unsteady pulsatile non-spiral and spiral blood flow in three-dimensional models of stenosis and aneurysm are needed to give a sound understanding of the transition-to-turbulence of blood flow due to stenosis and aneurysm and to study the the effects of spiral velocity on the transition-to-turbulence. The LES technique has mostly been used to investigate turbulent flow in engineering fields other than bio-fluid mechanics. In the last decade, LES has seen its excellent potential for studying the transition-to-turbulence of physiological flow in bio-fluid mechanics. Though the k-ω Transitional model is used in few instances, mainly LES is applied in this study. Firstly, investigations of steady non-spiral and spiral blood flow through threedimensionalmodels of cosine-type regular stenosed tube without and with upstream curved segment of varying angles of curvature are performed by using the k-ω Transitional model and LES. A fully developed Poiseuille velocity profile for blood is introduced at the inlets of the models. To introduce a spiral effect at the inlet, onesixth of the bulk velocity is taken as the tangential velocity at the inlet along with the axial velocity profile there. Secondly, physiological pulsatile non-spiral and spiral blood flow through a three-dimensional model of a straight tube having cosine-type regular stenosis are investigated by using mainly LES. A two-equation k-ω Transitional model is also used in one non-spiral flow case. The first four harmonics of the Fourier series of pressure pulse are used to generate physiological velocity profiles at the inlet. At the outlet, a pressure waveform is introduced. The effects of percentage of area reduction in the stenosis, length of the stenosis, amplitude of pulsation and Womersley number are also examined. Thirdly, transient pulsatile non-spiral and spiral blood flow through a threedimensional model of irregular stenosis are investigated by applying LES and comparison is drawn between non-spiral flow through a regular stenosis and that through an irregular stenosis. Lastly, pulsatile non-spiral and spiral blood flow through a three-dimensional model of irregular stenosis with an adjacent post-stenotic irregular aneurysm in the same arterial segment are studied by applying LES and the k-ω Transitional model. The effects of variation in spiral velocity are also examined. The results presented in this thesis are analysed with relevant pathophysioloical consequences. In steady flow through the straight stenosed tube, excellent agreement between LES results for Re = 1000 and 2000 and the corresponding experimental results are found when the appropriate inlet perturbations are introduced. In the models with an upstream curved segment, no significant effect of spiral flow on any flow property is found for the investigated Reynolds numbers; spiral pattern disappears before the stenosis – which may be due the rigid wall used in the models and/or a steady flow at the inlet. The effects of the curved upstream model can be seen mainly in the maximum turbulent kinetic energy (TKE), the maximum pressure drop and the maximum wall shear stress (WSS), which in the curved upstream models generally increase significantly compared with the corresponding results in the straight stenosed tube. The maximumcontributions of the SGS motion to the large-scale motion in both non-spiral and spiral flow through a regular stenosis, an irregular stenosis and an irregular stenosis with an adjacent post-stenotic irregular aneurysm are 50%, 55%and 25%, respectively, for the highest Reynolds number investigated in each model. Although the wall pressure and shear stress obtained from the k-ω Transitional model agree quite well with the corresponding LES results, the turbulent results obtained from the k-ω Transitional model differ significantly from the corresponding LES results – this shows unsuitability of the k-ω model for pulsatile flow simulation. Large permanent recirculation regions are observed right after the stenosis throat in both non-spiral and spiral flow, which in the model of a stenosis with an adjacent post-stenotic aneurysm are stretched beyond the aneurysm and the length of the recirculation regions increases with spiral velocity. This study shows that, in both steady and unsteady pulsatile flow through the straight tube model having either a stenosis (regular or irregular) or an irregular stenosis with an adjacent post-stenotic irregular aneurysm, the TKE rises significantly at some locations and phases if a spiral effect is introduced at the inlet of the model. However, the maximum value of the TKE in a high spiral flow drops considerably compared with that in a low spiral flow. The maximum wall pressure drop and shear stress occur around the stenosis throat during all the phases of the pulsatile cycle. In the model of a stenosis only, the wall pressure rises in the immediate post-stenotic region after its drop at the stenosis throat. However, in the model of a stenosis with an adjacent aneurysm, the wall pressure does not rise to regain its undisturbed value before the start of the last quarter of the aneurysm. The effects of the spiral flow on the wall pressure and WSS are visible only in the downstream region where they take oscillatory pattern. The break frequencies of energy spectra for velocity and pressure fluctuations from −5/3 power slope to −10/3 power slope and −7/3 power slope, respectively, are observed in the downstream transition-to-turbulence region in both the non-spiral and spiral flow. At some locations in the transition region, the velocity spectra in the spiral flow has larger inertial subrange region than that in non-spiral flow. The effects of the spiral flow on the pressure spectra is insignificant. Also, the maximum wall pressure drop, the maximum WSS and the maximum TKE in the non-spiral flow through the irregular stenosis rise significantly compared with the corresponding results in the non-spiral flow through the regular stenosis. When the area reduction in the stenosis is increased, the maximum pressure drop, the maximumWSS and the TKE rise sharply. As for the effects of the length of the stenosis, the maximum WSS falls significantly and the maximum TKE rises sharply due to the increase in the length of the stenosis; but the maximum pressure drop is almost unaffected by the increase in the stenosis length.
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Yu, Xiaohong y 于曉紅. "Hemodynamic analysis of blood flows in carotid bifurcations". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2007. http://hub.hku.hk/bib/B3864700X.
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Jordan, Sumanas W. "A mathematical model of tissue factor-induced blood coagulation: discrete sites of initiation and regulation under conditions of flow". Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/33907.
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A mathematical model of blood coagulation under defined flow conditions, initiated and modulated by spatially discrete regions of surface bound tissue factor (TF) and thrombomodulin (TM), respectively, is presented. The model incorporates fluid phase and surface-associated reactions of the extrinsic, intrinsic, and common pathways, as well as three inhibitory pathways. The spatially heterogeneous model is formulated by finite element method, and an effective prothrombotic zone, which quantifies the spatial propagation of thrombin generation is defined. Characteristic features of coagulation are simulated under physiologic conditions, and the behavior of the system in response to perturbations in TF and TM surface densities, TF site dimensions, and wall shear rate is explored. The major findings of these studies include: (i) The model system responds in an 'all-or-none', threshold-like manner to changes in model parameters. (ii) It was found that prothrombotic effects may extend significantly beyond the dimensions of the spatially discrete site of TF expression in both axial and radial directions. (iii) The relationship between the length of the effective prothrombotic zone and the interval distance between tandem sites of TF expression dictate the net response of the system. Additive prothrombotic effects of sub-clinical lesions as well as suppressive antithrombotic effects of intervening TM-containing regions were observed. Secondly, the computational model is applied to calculate an individualized, systems-based metric of clotting potential for 210 pre-menopausal women in the Leiden Thrombophilia Study (LETS). The simulated variable was found to be a highly predictive parameter for deep venous thrombosis risk.
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Khalil, Adil. "Processing of laser speckle contrast images : study of mathematical models and use of nonlinear analyses to investigate the impact of aging on microvascular blood flow". Thesis, Angers, 2017. http://www.theses.fr/2017ANGE0006/document.
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Le vieillissement est un facteur de risque des maladies cardiovasculaires. Il est associé à des altérations fonctionnelles et structurelles du système vasculaire.Une étude approfondie du processus de vieillissement et le développement de systèmes d’imagerie et des traitements de données associés deviennent donc une priorité. Par l’analyse d’images de contraste par speckle laser (LSCI), l’objectif de cette thèse est d’étudier l’influence de l’âge sur la micro circulation.Pour ce faire, des données de LSCI ont été acquises sur l’avant-bras de sujets sains jeunes et âgés. A partir de modèles mathématiques, nous avons déterminé la vitesse des érythrocytes de la micro circulation chez les deux groupes de sujets. Par ailleurs, nous avons également mené une étude de la complexité de séries temporelles d’ LSCI s’appuyant sur des mesures d’entropie multi échelle. Nos résultats montrent que : 1) le groupe de sujets plus âgés présente des valeurs de vitesse des globules rouges significativement plus élevées que celles des sujets jeunes à l’hyperémie réactive post-occlusive; 2) les fluctuations des séries temporelles de LSCI dans le groupe des sujets jeunes ont une complexité supérieure à celles du groupe de sujets âgés. Ces modifications observées sur la micro circulation pourraient être attribuées à des modifications du système vasculaire dans son ensemble. La compréhension de ces altérations pourrait conduire à de nouvelles perspectives en matière de prévention et de traitement des pathologies liées à l’âgeAging is a primary risk factor for cardiovascular diseases. It is associated with functional and structural alterations in the vascular system. Therefore, a deep study of the aging process and the development of imaging systems and associated processing become of the utmost importance. By processing laser speckle contrast images (LSCI), this PhD work aims at studying the influence of age on microcirculation. In our work, LSCI data were acquired from the skin forearm of healthy subjects, subdivided into two age groups (younger and older). From mathematical models, we determined red blood cells velocity in microcirculation in the two groups of subjects. Moreover, we applied multiscale entropy-based algorithms to LSCI time series in order to study the complexity of microvascular signals. Our main findings are: 1) the older group has significantly higher velocity values than the younger group at post-occlusive reactive hyperaemia; 2) LSCI fluctuations in the younger group have significantly higher complexity than those of the older group. Age-related changes in skin microcirculation can be attributed to alterations in the vascular system as a whole. Understanding these changes in the microcirculatory system may give new insights for prevention and treatment of age-related diseases
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El-Bouri, Wahbi K. "Multi-scale modelling of the microvasculature in the human cerebral cortex". Thesis, University of Oxford, 2017. http://ora.ox.ac.uk/objects/uuid:8a9409a6-6279-4f7b-a975-b70149732378.
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Cerebrovascular diseases are by far the largest causes of death in the UK, as well as one of the leading causes of adult disability. The brain's healthy function depends on a steady supply of oxygen, delivered through the microvasculature. Cerebrovascular diseases, such as stroke and dementia, can interrupt the transport of blood (and hence oxygen) rapidly, or over a prolonged period of time. An interruption in flow can lead to ischaemia, with prolonged interruptions leading to tissue death and eventual brain damage. The microvasculature plays a key role in the transport of oxygen and nutrients to brain tissue; however, its role in diseases such as dementia is poorly understood, primarily due to the inability of current clinical imaging techniques to resolve microvessels, and due to the complexity of the underlying microvasculature. Therefore, in order to understand cerebrovascular diseases, it is necessary to be able to resolve and understand the microvasculature. In particular, generating large-scale models of the human microvasculature that can be linked back to contemporary clinical imaging is important in helping plug the current imaging gap that exists. A novel statistical model is proposed here that generates such large-scale models efficiently. Homogenization theory is used to generate a porous continuum capillary bed (characterised by its permeability) that allows for the efficient scaling up of the microvasculature. A novel order-based density-filling algorithm is then developed which generates morphologically accurate penetrating arterioles and venules, also demonstrating that the topology of the vessels only has a minor influence on CBF compared to diameter. Finally, the capillary bed and penetrating vessels are coupled into a large voxel-sized model of the microvasculature from which pressure and flux variations through the voxel can be analysed. A decoupling of the pressure and flux, as well as a layering of flow, was observed within the voxel, driven by the topology of the penetrating vessels. Micro-infarctions were also simulated, demonstrating the large local effects they have on the pressure and flux, whilst only causing a minor drop in CBF within the voxel.
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Wayman, Brian H. "Arterial Response to Local Mechanical Variables: The Effects of Circumferential and Shear Stress". Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/22611.
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Arteries respond to changes in global mechanical parameters (pressure, flow rate, and longitudinal stretching) by remodeling to restore local parameters (circumferential stress, shear stress, and axial strain) to baseline levels. Because a change in a single global parameter results in changes of multiple local parameters, the effects of individual local parameters on remodeling remain unknown. This study uses a novel approach to study remodeling in organ culture based on independent control of local mechanical parameters. The approach is illustrated by studying the effects of circumferential and shear stress on remodeling-related biological markers. Porcine carotid arteries were cultured for three days at a circumferential stress of 50 kPa or 150 kPa or, in separate experiments, a shear stress of 0.75 Pa or 2.25 Pa. At high circumferential stress, matrix synthesis, smooth muscle cell proliferation, and cell death are significantly greater, but matrix metalloproteinase-2 (MMP-2) and pro-MMP-2 activity are significantly less. In contrast, biological markers measured were unaffected by shear stress. Applications of the proposed approach for improved understanding of remodeling, optimizing mechanical conditioning of tissue engineered arteries, and selection of experimentally motivated growth laws are discussed.
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Myers, Lance Jonathan. "Mathematical modeling of foetal arterial blood flow". Doctoral thesis, University of Cape Town, 2001. http://hdl.handle.net/11427/5143.
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Bibliography: leaves 187-211.The aim of this thesis was to develop an accurate and comprehensive computer model of the foetal circulatory system and to use this model to investigate influences of various haemodynamic viriables on common Doppler blood flow velocity waveform indices. The foetal model consists of an number of vascular compartments, cascaded together using electrical transmission line analogies.
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Arciero, Julia. "Theoretical Models of Blood Flow Regulation". Diss., The University of Arizona, 2008. http://hdl.handle.net/10150/195903.
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In normal tissues, blood supply is closely matched to tissue demand for wide ranges of oxygen demand and arterial pressure. This suggests that multiple mechanisms regulate blood flow. Theoretical models can be used to analyze these interacting mechanisms. One proposed mechanism for metabolic flow regulation involves the saturation-dependent release of ATP by red blood cells, which triggers an upstream conducted response signal and arteriolar vasodilation. To analyze this mechanism, oxygen and ATP levels are calculated along a flow pathway of seven representative segments, including two vasoactive arteriolar segments. The conducted response signal is dependent on ATP concentration. Arteriolar tone depends on the conducted response signal, local wall shear stress and wall tension. Arteriolar diameters are calculated based on vascular smooth muscle mechanics. The model can account for increases in perfusion consistent with experimental findings at low and moderate oxygen consumption rates despite the opposing effects of the myogenic and shear-dependent responses. Autoregulation, the maintenance of nearly constant blood flow as arterial pressure varies, is assessed in the presence or absence of the myogenic, shear-dependent and/or metabolic responses. The model results indicate that the combined effects of myogenic and metabolic regulation overcome the vasodilatory effect of the shear-dependent response to generate autoregulatory behavior. Capillary recruitment has been shown to increase the capacity for oxygen delivery during exercise. In the model, capillary density is assumed to depend on small arteriole diameter. The model predicts a significant increase in the range over which perfusion can be regulated when recruitment is included. Oscillations in diameter and tone are predicted under certain conditions, suggesting a novel mechanism for vasomotion. The conditions that give rise to oscillations are analyzed. It is shown that the appearance of oscillations depends in a complex way on a number of system parameters. In summary, the theoretical model provides a quantitative assessment of the myogenic, shear-dependent and metabolic responses that affect blood flow regulation and identifies a role for capillary recruitment and vasomotion in the control of blood flow.
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Johnson, Kevin Robert. "In Vivo Coronary Wall Shear Stress Determination Using CT, MRI, and Computational Fluid Dynamics". Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/14482.
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Wall shear stress (WSS) has long been identified as a factor in the development of atherosclerotic lesions. Autopsy studies have revealed a strong tendency for lesion development at arterial branch sites and along the inner walls of curvature areas that, in theory, should experience low WSS. Calculations of coronary artery WSS have typically been based upon average models of coronary artery geometry with average flow conditions and then compared to average lesion distributions. With all the averaging involved, a more detailed knowledge of the correlation between WSS and atherosclerotic lesion development might be obscured. Recent advancements in hemodynamic modeling now enable the calculation of WSS in individual subjects. An image-based approach for patient-specific calculation of in vivo WSS using computational fluid dynamics (CFD) would allow a more direct study of this correlation. New state-of-the-art technologies in multi-detector computed tomography (CT) and 3.0 Tesla magnetic resonance imaging (MRI) offer potential improvements for the measurement of coronary artery geometry and blood flow.
The overall objective of this research was to evaluate the quantitative accuracy of multi-detector CT and 3.0 Tesla MRI and incorporate those imaging modalities into a patient-specific CFD model of coronary artery WSS. Using a series of vessel motion phantoms, it has been shown that 64-detector CT can provide accurate measurements of coronary artery geometry for heart rates below 70 beats per minute. A flow phantom was used to validate the use of navigator-echo gated, phase contrast MRI at 3.0 Tesla to measure velocity of coronary blood flow. Patient-specific, time-resolved CFD models of coronary WSS were created for two subjects. Furthermore, it was determined that population-average velocity curves or steady state velocities can predict locations of high or low WSS with high degrees of accuracy compared to the use of patient-specific blood flow velocity measurements as CFD boundary conditions. This work is significant because it constitutes the first technique to non-invasively calculate in vivo coronary artery WSS using image-based, patient-specific modeling.
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Park, Chang Sub. "Mathematical techniques for the analysis of unsteady blood flow". Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.532000.
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Guvenen, Haldun. "Aerodynamics of bodies in shear flow". Diss., The University of Arizona, 1989. http://hdl.handle.net/10150/184917.
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This dissertation investigates spanwise periodic shear flow past two-dimensional bodies. The flow is assumed to be inviscid and incompressible. Using singular perturbation techniques, the solution is developed for ε = L/ℓ ≪ 1, where L represents body cross-sectional size, and ℓ the period of the oncoming flow U(z). The singular perturbation analysis involves three regions: the inner, wake and outer regions. The leading order solutions are developed in all regions, and in the inner region higher order terms are obtained. In the inner region near the body, the primary flow (U₀, V₀, P₀) corresponds to potential flow past the body with a local free stream value of U(z). The spanwise variation in U(z) produces a weak O(ε) secondary flow W₁ in the spanwise direction. As the vortex lines of the upstream flow are convected downstream, they wrap around the body, producing significant streamwise vorticity in a wake region of thickness O(L) directly behind the body. This streamwise vorticity induces a net volume flux into the wake. In the outer region far from the body, a nonlifting body appears as a distribution of three-dimensional dipoles, and the wake appears as a sheet of mass sinks. Both singularity structures must be included in describing the leading outer flow. For lifting bodies, the body appears as a lifting line, and the wake appears as a sheet of shed vorticity. The trailing vorticity is found to be equal to the spanwise derivative of the product of the circulation and the oncoming flow. For lifting bodies the first higher order correction to the inner flow is the response of the body to the downwash produced by the trailing vorticity. At large distances from the body, the flow takes on remarkably simple form.
22
Fiechter, Jerome. "Numerical study of platelet transport in flowing blood". Thesis, Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/16770.
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Lamponi, Daniele. "One dimensional and multiscale models for blood flow circulation /". [S.l.] : [s.n.], 2004. http://library.epfl.ch/theses/?nr=3006.
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Lowdermilk, Jennifer L. "Mathematical modeling of incompressible flow through a collapsible tube /". Online version of thesis, 1992. http://hdl.handle.net/1850/10859.
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Wong, Ka-chung Colin y 黃家聰. "Hydraulics of bottom rack chamber for supercritical flow diversion". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2009. http://hub.hku.hk/bib/B42664469.
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Henry, Eric James. "Contaminant induced flow effects in variably-saturated porous media". Diss., The University of Arizona, 2001. http://hdl.handle.net/10150/191256.
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Dissolved organic contaminants that decrease the surface tension of water (surfactants) can have an effect on unsaturated flow through porous media due to the dependence of capillary pressure on surface tension. One and two-dimensional (1D, 2D) laboratory experiments and numerical simulations were conducted to study surfactant-induced unsaturated flow. The 1D experiments investigated differences in surfactant-induced flow as a function of contaminant mobility. The flow in a system contaminated with a high solubility, mobile surfactant, butanol, was much different than in a system contaminated with a sparingly soluble, relatively immobile surfactant, myristyl alcohol (MA). Because surface tension depression caused by MA was confined to the original source zone, the MA system was modeled using a standard unsaturated flow model (HYDRUS-1D) by assigning separate sets of hydraulic functions to the initially clean and source zones. To simulate the butanol system, HYDRUS-1D was modified to incorporate surfactant concentration-dependent changes to the moisture content-pressure head and unsaturated hydraulic conductivity functions. Following the 1D study, a two-dimensional flow cell (2.4 x 1.5 x 0.1 m) was used to investigate the infiltration of a surfactant contaminant plume from a point source on the soil surface, through the vadose zone, and toward a shallow aquifer. Above the top of the capillary fringe the advance of the surfactant solution caused a drainage front that radiated from the point source. Upon reaching the capillary fringe, the drainage front caused a localized depression of the capillary fringe and eventually a new capillary fringe height was established. Horizontal transport of surfactant in the depressed capillary fringe caused the propagation of a wedge-shaped drainage front in the downgradient direction. The numerical model HYDRUS-2D was modified to account for surfactant concentration-dependent effects on the unsaturated hydraulic functions and was successfully used to simulate the surfactant infiltration experiment. The extensive propagation of the drying front and the effect of vadose zone drainage on contaminant breakthrough time demonstrate the potential importance of considering surface tension effects on unsaturated flow and transport in systems containing surface-active organic contaminants or in systems where surfactants are used for remediation of the vadose zone or unconfined aquifers.
27
Zeidan, Dia Hussein Abdulhameed. "Mathematical and numerical study of two-phase flow models". Thesis, Manchester Metropolitan University, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.396533.
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Wang, Ying y 王瑩. "A study of mutual fund flow and market return volatility". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2003. http://hub.hku.hk/bib/B26843572.
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Pathmanathan, Saidharshini. "Development of in vitro models of cerebral ischaemia". Thesis, University of Oxford, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249162.
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Sempere, Alfonso FerraÌndez. "Computational models of blood flow in the circle of willis". Thesis, University of Leeds, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.427838.
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BASEGHI, BEHDAD. "THREE-DIMENSIONAL SEEPAGE THROUGH POROUS MEDIA WITH THE RESIDUAL FLOW PROCEDURE". Diss., The University of Arizona, 1987. http://hdl.handle.net/10150/184107.
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The purpose of this study is to present the development and application of residual flow procedure for analysis of three-dimensional (3-D) steady-state and transient seepage. The finite element equations are derived using a pseudo-variational principle which leads to a transient residual flow (load) vector that, in turn, is used to correct the position of the free surface iteratively. The procedure involves a fixed mesh which requires no mesh regeneration during transient analysis and during iterations. The procedure is also capable of handling material nonhomogeneities and anisotropy with relative ease. Several applications are made including verification with respect to closed-form solutions, and with results from a laboratory glass bead model simulating three-dimensional situations. For these glass beads, the coefficients of permeability and specific storage are also evaluated experimentally.
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Chan, Johnson Lap-Kay. "Numerical procedure for potential flow problems with a free surface". Thesis, University of British Columbia, 1987. http://hdl.handle.net/2429/28637.
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A numerical procedure based upon a boundary integral method for gravity wave making problems is studied in the time domain. The free-surface boundary conditions are combined and expressed in a Lagrangian notation to follow the free-surface particle's motion in time. The corresponding material derivative term is approximated by a finite difference expression, and the velocity terms are extrapolated in time for the completion of the formulations. The fluid-body intersection position at the free surface is predicted by an interpolation function that requires information from both the free surface and the submerged surface conditions. Solutions corresponding to a linear free-surface condition and to a non-linear free-surface condition are obtained at small time increment values. Numerical modelling of surface wave problems is studied in two dimensions and in three dimensions. Comparisons are made to linear analytical solutions as well as to published experimental results. Good agreement between the numerical solutions and measured values is found. For the modelling of a three dimensional wave diffraction problem, results at high wave amplitude are restricted because of the use of quadrilateral elements. The near cylinder region of the free surface is not considered to be well represented because of the coarse element size. Wave forces calculated on the vertical cylinder are found to be affected by the modelled tank length. When the simulated wave length is comparable to the wave tank's dimension, numerical results are found to be less than the experimental measurements. However, when the wave length is shorter than the tank's length, solutions are obtained with very good precision.Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate
33
Liu, Peng y 刘鹏. "Flow mechanisms in horizontal sediment-laden jets". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2012. http://hub.hku.hk/bib/B49799496.
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Particle-laden jets are an important type of multiphase flow which can be found in various natural and technical processes. This study focuses on the flow mechanisms in a horizontally discharging sediment-laden jet that is of particular interest in environmental science and engineering.
Experimental techniques and mathematical models are developed to investigate horizontal sediment-laden jets, both for the buoyant and non-buoyant jet discharge cases. In the laboratory, the separation of images of the fluid and the particulate phases is achieved by harnessing light signals of visualization at different wavelengths. Whole field measurements of velocities of the two phases are made by the adoption of particle image velocimetry (PIV) algorithms. Numerical models are developed in two approaches with regard to the treatment of the particulate phase. In the Lagrangian approach, individual sediment particles are tracked while the flow field of the fluid phase is computed with large-eddy simulation (LES). This simulation successfully captures the transient nature of the particle-laden flow. In the Eulerian approach, a two-phase model is used to obtain steady flow simulations in a much shorter computation time.
The experimental and numerical results for the horizontal momentum jets show that, at low initial particle concentrations, the sediment particles generally follow the jet flow but with some levels of deficit velocities. In the upper layer of the jet the particles do not follow the fluid flow as well as in its lower layer. More particles are observed in the lower layer than in the upper one. For the momentum-dominated zone of a horizontal buoyant jet, the flow exhibits similar behaviors as the horizontal particle-laden momentum jet, except that there are some slight modifications from the effects of buoyancy. In the bending zone of the buoyant jet, the effects of buoyancy become significant. Notably, the locations of maximum velocity magnitude and those of maximum turbulence intensity are well separated in this zone. A strong correlation of particle abundance and high turbulence intensity is observed in the lower outer jet layer in this bending zone.
Significant modifications to the global behaviors of horizontal sediment jets are observed as the particle concentration increases to relatively high levels. The jet trajectories are brought downwards by the particle loads and the jet widths are also increased. For the flow regime being investigated, turbulence intensity in the fluid flow is found to be increased by the presence of sediment particles.
The results suggest that turbulence helps suspend sediment particles in horizontally discharging jets. A Stokes number is proposed to represent the ability of particles to follow the fluid flow. It is defined as St=W_s/U_j , where ws is the particle settling velocity in still fluid and Uj is the jet exit velocity, which indirectly governs the turbulence characteristics of the jet flow.
The advecting large eddies in a turbulent jet are found to play the role of organizing particles in patches. Interaction and coalescence between particle-concentrated eddies may result in the sudden drop of a group of particles, which contributes to sediments falling from a horizontal jet in the form of particle-rich “fingers”.published_or_final_version
Civil Engineering
Doctoral
Doctor of Philosophy
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Zoebelein, Till. "Development of an LU-scheme for the solution of hypersonic non-equilibrium flow". Thesis, Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/12509.
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Wong, Ching-chi y 黃精治. "Flow and pollutant dispersion over idealized urban street canyons using large-eddy simulation". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hdl.handle.net/10722/206698.
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Flows and pollutant dispersion over flat rural terrain have been investigated for decades. However, our understanding of their behaviours over urban areas is rather limited. Most cases have either focused on street level or in the roughness sub-layer (RSL) of urban boundary layer (UBL). Whereas, only a handful of studies have looked into the coupling between street-level and UBL-core dynamics, and their effects on pollutant dispersion.
In this thesis, computational fluid dynamics (CFD) is employed to examine the flows and pollutant transport in and over urban roughness. Idealised two-dimensional (2D) street canyons are used as the basic units fabricating hypothetical urban surfaces. A ground-level passive and chemically inert pollutant source is applied to simulate the flows and pollutant dispersion over rough surfaces in isothermal condition. Large-eddy simulation (LES) with the one-equation subgrid-scale model is used to solve explicitly the broad range of scales in turbulent flows. Arrays of idealized street canyons of both uniform and non-uniform building height are used to formulate a unified theory for the flows and pollutant dispersion over urban areas of different morphology. The geometry of roughness elements is controlled by the building-height-to-street-width (aspect) ratio (0.083 ≤ AR ≤ 2) and/or the building height variability (BHV = 0.2, 0.4 and 0.6), in which the characteristic regimes of skimming flow, wake-interference and isolated roughness are covered.
A detailed analysis on the roof-level turbulence structure reveals parcels of low-speed air masses in the streamwise flows and narrow high-speed down-drafts in the urban canopy layer, signifying the momentum entrainment into the street canyons. The decelerating streamwise flows in turn initiate up-drafts carrying pollutants away from the street canyons, illustrating the basic pollutant removal mechanism in 2D street canyons. Turbulent transport processes, in the form of ejection and sweep, are the key events governing the exchanges of air and pollutant of street canyon. Air exchange rate (ACH) along the roof level is dominated by turbulent transport, in particular over narrow street canyons.
The LES results show that both the turbulence level and ACH increase with increasing aerodynamic resistance defined in term of the Fanning friction factor. At the same AR, BHV greatly increases the friction factor and the ACH in dense built areas (AR ≤ 0.25). The turbulence intensity is peaked on the windward side of street canyons that does not overlap with the maximum velocity gradient near the leeward building corners, suggesting the importance of background turbulence in street-level ventilation. Over the building roughness, pollutant plume dispersion after the ground-level area source in cross flows resumes the self-similar Gaussian shape in the vertical direction in which the vertical plume coverage is proportional to the square root of downwind distance in the streamwise direction. Moreover, the vertical dispersion coefficient is proportional to the one-fourth power of friction factor over idealised street canyons. Conclusively, friction factor can be used to parametrise ventilation and pollutant dispersion over urban areas.published_or_final_version
Mechanical Engineering
Doctoral
Doctor of Philosophy
36
Grm, Aleksander. "Mathematical analysis of macroscopic models for slow dense granular flow". [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=98408214X.
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Peng, Jixian y 彭继娴. "Macroscopic characteristics of dense road networks". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hdl.handle.net/10722/195994.
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In the continuum modeling of traffic networks, a macroscopic cost-flow function (MCF) and macroscopic fundamental diagram (MFD) can be used to represent the fundamental relationships between traffic quantities such as speed, flow, and density. The MCF governs the steady-state cost-flow relationship, whereas the MFD represents the instantaneous inter-relationship between speed, flow, and density of traffic streams. This thesis explores the influence of network topologies on the MCF and MFD.
The Hong Kong road system is divided into unit-sized road networks with various physical characteristics for which the network structure and signal timings are reserved. By universally scaling the origin-destination (OD) matrices of the morning peak, traffic conditions ranging from free-flow to congestion are created for microscopic simulation. From the simulation results, an MCF that relates the average journey time and the number of vehicles traveling through the network in one hour and an MFD that relates space to the mean speed and average density aggregated across 300s intervals are derived. The MCF and MFD are calibrated with mathematical models for each network.
The density of roads, junctions, and signal junctions all influence the value of the macroscopic parameters in the MCF and MFD, and predictive equations are constructed that relate the macroscopic parameters to the network topological characteristics. Based on the fitting performance of the mathematical models, recommendations are made for selecting MCF and MFD models for continuum modeling.published_or_final_version
Civil Engineering
Master
Master of Philosophy
38
Yan, Li y 顏理. "On the traffic flow control system". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2007. http://hub.hku.hk/bib/B39431174.
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Marth, Wieland. "Hydrodynamic Diffuse Interface Models for Cell Morphology and Motility". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-204651.
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In this thesis, we study mathematical models that describe the morphology of a generalized biological cell in equilibrium or under the influence of external forces. Within these models, the cell is considered as a thermodynamic system, where streaming effects in the cell bulk and the surrounding are coupled with a Helfrich-type model for the cell membrane. The governing evolution equations for the cell given in a continuum formulation are derived using an energy variation approach. Such two-phase flow problems that combine streaming effects with a free boundary problem that accounts for bending and surface tension can be described effectively by a diffuse interface approach. An advantage of the diffuse interface approach is that models for e.g. different biophysical processes can easily be combined. That makes this method suitable to describe complex phenomena such as cell motility and multi-cell dynamics. Within the first model for cell motility, we combine a biological network for GTPases with the hydrodynamic Helfrich-type model. This model allows to account for cell motility driven by membrane protrusion as a result of actin polymerization. Within the second model, we moreover extend the Helfrich-type model by an active gel theory to account for the actin filaments in the cell bulk. Caused by contractile stress within the actin-myosin solution, a spontaneous symmetry breaking event occurs that lead to cell motility. In this thesis, we further study the dynamics of multiple cells which is of wide interest since it reveals rich non-linear behavior. To apply the diffuse interface framework, we introduce several phase field variables to account for several cells that are coupled by a local interaction potential. In a first application, we study white blood cell margination, a biological phenomenon that results from the complex relation between collisions, different mechanical properties and lift forces of red blood cells and white blood cells within the vascular system. Here, it is shown that inertial effects, which can become of relevance in various parts of the cardiovascular system, lead to a decreasing tendency for margination with increasing Reynolds number. Finally, we combine the active polar gel theory and the multi-cell approach that is capable of studying collective migration of cells. This hydrodynamic approach predicts that collective migration emerges spontaneously forming coherently-moving clusters as a result of the mutual alignment of the velocity vectors during inelastic collisions. We further observe that hydrodynamics heavily influence those systems. However, a complete suppression of the onset of collective migration cannot be confirmed. Moreover, we give a brief insight how such highly coupled systems can be treated numerically using finite elements and how the numerical costs can be limited using operator splitting approaches and problem parallelization with OPENMPDiese Dissertation beschäftigt sich mit mathematischen Modellen zur Beschreibung von Gleichgewichts- und dynamischen Zuständen von verallgemeinerten biologischen Zellen. Die Zellen werden dabei als thermodynamisches System aufgefasst, bei dem Strömungseffekte innerhalb und außerhalb der Zelle zusammen mit einem Helfrich-Modell für Zellmembranen kombiniert werden. Schließlich werden durch einen Energie-Variations-Ansatz die Evolutionsgleichungen für die Zelle hergeleitet. Es ergeben sie dabei Mehrphasen-Systeme, die Strömungseffekte mit einem freien Randwertproblem, das zusätzlich physikalischen Einflüssen wie Biegung und Oberflächenspannung unterliegt, vereinen. Um solche Probleme effizient zu lösen, wird in dieser Arbeit die Diffuse-Interface-Methode verwendet. Ein Vorteil dieser Methode ist, dass es sehr einfach möglich ist, Modelle, die verschiedenste Prozesse beschreiben, miteinander zu vereinen. Dies erlaubt es, komplexe biologische Phänomene, wie zum Beispiel Zellmotilität oder auch die kollektive Bewegung von Zellen, zu beschreiben. In den Modellen für Zellmotilität wird ein biologisches Netzwerk-Modell für GTPasen oder auch ein Active-Polar-Gel-Modell, das die Aktinfilamente im Inneren der Zellen als Flüssigkristall auffasst, mit dem Multi-Phasen-Modell kombiniert. Beide Modelle erlauben es, komplexe Vorgänge bei der selbst hervorgerufenen Bewegung von Zellen, wie das Vorantreiben der Zellmembran durch Aktinpolymerisierung oder auch die Kontraktionsbewegung des Zellkörpers durch kontraktile Spannungen innerhalb des Zytoskelets der Zelle, zu verstehen. Weiterhin ist die kollektive Bewegung von vielen Zellen von großem Interesse, da sich hier viele nichtlineare Phänomene zeigen. Um das Diffuse-Interface-Modell für eine Zelle auf die Beschreibung mehrerer Zellen zu übertragen, werden mehrere Phasenfelder eingeführt, die die Zellen jeweils kennzeichnen. Schließlich werden die Zellen durch ein lokales Abstoßungspotential gekoppelt. Das Modell wird angewendet, um White blood cell margination, das die Annäherung von Leukozyten an die Blutgefäßwand bezeichnet, zu verstehen. Dieser Prozess wird dabei bestimmt durch den komplexen Zusammenhang zwischen Kollisionen, den jeweiligen mechanischen Eigenschaften der Zellen, sowie deren Auftriebskraft innerhalb der Adern. Die Simulationen zeigen, dass diese Annäherung sich in bestimmten Gebieten des kardiovaskulären Systems stark vermindert, in denen die Blutströmung das Stokes-Regime verlässt. Schließlich wird das Active-Polar-Gel-Modell mit dem Modell für die kollektive Bewegung vom Zellen kombiniert. Dies macht es möglich, die kollektive Bewegung der Zellen und den Einfluss von Hydrodynamik auf diese Bewegung zu untersuchen. Es zeigt sich dabei, dass der Zustand der kollektiven gerichteten Bewegung sich spontan aus der Neuausrichtung der jeweiligen Zellen durch inelastische Kollisionen ergibt. Obwohl die Hydrodynamik einen großen Einfluss auf solche Systeme hat, deuten die Simulationen nicht daraufhin, dass Hydrodynamik die kollektive Bewegung vollständig unterdrückt. Weiterhin wird in dieser Arbeit gezeigt, wie die stark gekoppelten Systeme numerisch gelöst werden können mit Hilfe der Finiten-Elemente-Methode und wie die Effizienz der Methode gesteigert werden kann durch die Anwendung von Operator-Splitting-Techniken und Problemparallelisierung mittels OPENMP
40
Poon, Hao-chi Cynthia y 潘顥之. "Numerical simulation of turbulent flow and microclimate within and above vegetation canopy". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2010. http://hub.hku.hk/bib/B45589677.
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Pettersson, Ulrika. "Blood Flow Regulation and Inflammatory Response in Experimental Models of Diabetes". Doctoral thesis, Uppsala universitet, Institutionen för medicinsk cellbiologi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-161807.
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Type 2 diabetes is caused by defect pancreatic islet β-cells together with peripheral insulin resistance. The disease is often accompanied by obesity with associated low-grade visceral adipose tissue inflammation, which contributes to insulin resistance. As a consequence of, and a possible compensation for the increased insulin demand, blood flow to the pancreatic islets is increased in animal models of diabetes. This increased blood perfusion might with time affect the vascular network as well as β-cells within the islets. This thesis investigates the role of changes of blood perfusion in pancreatic islets and adipose tissues, as well as the recruitment to and composition of leukocyte subpopulations in insulin-sensitive tissues in experimental models of diabetes. Blood flow measurements in islets and adipose tissues of rats and mice were performed using the microsphere technique, while leukocyte recruitment was studied in the mouse cremaster muscle using intravital microscopy. Increased islet blood flow was observed in the GK rat model of type 2 diabetes, which was decreased by acute as well as continuous 2-week inhibition of β3-adrenoceptors without affecting plasma insulin concentrations. Increased inflammatory leukocyte recruitment was observed in both alloxan-induced and high-fat diet-induced diabetes. However, an impaired bacterial clearance was observed in diabetic mice, which was due to impaired phagocytosis. A gender difference was detected in mice fed a high-fat diet, since obese female mice did not show increased levels of pro-inflammatory circulatory markers or inflammatory leukocytes in the adipose tissue. The main effector cell in the adipose tissue inflammation in high-fat-fed male mice seemed to be the pro-inflammatory macrophage. The Treg population in adipose tissue was increased in female mice, but remained unchanged in male mice on high-fat diet. In conclusion, increased islet blood flow in type 2 diabetes could be reversed by β3-adrenoceptor inhibition, which may maintain islet function. The diabetes-associated hyperglycemia activated leukocytes but impaired their phagocytic ability. High-fat-fed female mice showed less peripheral inflammation due to a smaller number of recruited inflammatory macrophages and a high-fat diet-induced Treg population in intra-abdominal adipose tissues.
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Higa, Mauricio. "Métodos para quantificação da artéria carótida em imagens de ultrassom modo-B e Doppler". Universidade de São Paulo, 2009. http://www.teses.usp.br/teses/disponiveis/3/3142/tde-11082010-163813/.
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No campo da medicina diagnóstica através de imagens, o baixo custo e a característica não-invasiva da ultrassonografia modo-B favoreceram as pesquisas que analisam a relação entre a espessura íntima-média (EIM) da artéria carótida e o desenvolvimento de doenças cardiovasculares. Outra técnica de ultrassonografia, baseada no efeito Doppler, também tem sido extensivamente empregada em ambientes clínicos para a medição da velocidade e do fluxo de sangue das artérias periféricas, oferecendo uma abordagem distinta para estudar as doenças cardiovasculares. Neste trabalho, métodos computacionais foram implementados e analisados para extrair informações quantitativas de artérias a partir de imagens 2D. Os ensaios com imagens em modo-B contemplaram a combinação de técnicas de filtragem e algoritmos de contornos ativos para a detecção das interfaces das paredes arteriais e medições da EIM e do diâmetro da luz (DL). Um modelo matemático e 7 imagens reais da artéria carótida comum foram utilizados neste estudo que, dentre as diversas combinações de filtros e algoritmos, apresentou resultados quantitativos similares no cálculo do DL. Para a EIM, porém, a reduzida dimensão da camada íntima-média proporcionou variações indesejadas dos cálculos. Para medir a velocidade do sangue utilizando o modo Doppler, um método semiautomático foi incorporado a um aplicativo computacional para a detecção da envoltória do espectro do gráfico de velocidades. Uma análise comparativa entre sistemas de ultrassonografia comerciais operados por especialistas e este aplicativo incluiu a velocidade de pico sistólico e a integral de velocidade e tempo (VTI) das artérias carótida comum e braquial e de exames ecocardiográficos. A análise de Bland-Altman e o coeficiente de correlação validaram esta metodologia que, complementada pela interface gráfica amigável do aplicativo, pode auxiliar os clínicos em seus estudos de larga escala, baseados nas imagens Doppler ultrassonográficas, com as seguintes vantagens: redução do tempo operacional e de resultados subjetivos e aumento do grau de reprodutibilidade dos resultados.In the field of image-based diagnostic medicine, low cost and noninvasive B-mode ultrasound technique have supported the researches which investigates the relation between the carotid intima-media thickness (IMT) and the development of cardiovascular diseases. Other ultrasound technique, that offers a different approach to study these diseases, is based on Doppler effect and has also been extensively used in clinical sites to measure blood velocity and flow in peripheral arteries. In this work, computational methods were implemented and analyzed to extract quantitative data from 2D images of arteries. Tests with B-mode images covered the combination of filtering techniques and active contour algorithms, in order to detect the arterial wall interfaces and to measure IMT and lumen diameter (LD). One mathematical model and seven real images of the common carotid artery were used in this study which, among several filters and algorithms combinations, showed similar quantitative results for DL measurements. However, for IMT, the small thickness of intima-media layer led to undesirable variation results. To measure blood velocity by using Doppler mode, a semi-automatic methodology was implemented in a computational tool to detect the spectrum envelope of the velocity graphic. A comparative analysis between commercial ultrasound systems operated by specialists and this tool included systolic peak velocities and velocity-time integral (VTI) of the common carotid and brachial arteries and of echocardiographic exams. Bland-Altmans analysis and the correlation coefficient validated this methodology which, besides the user friendly graphical interface of the tool, may help the clinicians for their large-scale studies based on Doppler ultrasound images, with the following advantages: to save operational time, to lower subjective results, and to support measurement reproducibility.
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Rinaldi, Stephanie. "Experiments on the dynamics of cantilevered pipes subjected to internal andor external axial flow". Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=111615.
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The main objective of this thesis is to study and investigate the dynamics and stability of cantilevered structures subjected to internal, external, or simultaneous internal and external axial flows. This was accomplished, in some cases, by deriving the linear equations of motion using a Newtonian approach and, in other cases, by making the necessary modifications to existing theoretical models. The continuous cantilevered systems were then discretized using the Galerkin method in order to determine their complex eigenfrequencies. Moreover, numerous experiments were performed to compare and validate, or otherwise, the theoretical models proposed. More specifically, the four cantilevered systems studied were the following: (i) a pipe conveying fluid that is fitted with a stabilizing end-piece, which suppresses flutter by blocking the straight-through exit of flow at the downstream end; (ii) a pipe aspirating fluid, which flutters at low flow velocities in its first mode; (iii) a free-clamped cylinder (i.e. with the upstream end free and the downstream end clamped) in confined axial flow, which also flutters at low flow velocities in its first mode and eventually develops a buckling instability; and (iv) a pipe subjected to internal flow, which after exiting the pipe is transformed to a confined counter-current annular flow, that becomes unstable by flutter too.
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Piechnik, Stefan K. "A mathematical and biophysical modelling of cerebral blood flow and cerebrospinal fluid dynamics". Thesis, University of Cambridge, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269226.
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Yue, Yang y 樂陽. "Spatial-temporal dependency of traffic flow and its implications for short-term traffic forecasting". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2006. http://hub.hku.hk/bib/B35507366.
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Wong, Chung-yin Philip y 黃仲賢. "The development of pedestrian flow model". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2011. http://hub.hku.hk/bib/B48194025.
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This dissertation reported on the findings of the characteristics of the pedestrian
flows in Chinese New Year Eve Fair in Hong Kong and, the recommendations to its
crowd control measures. Since most of the pedestrian flow models were developed
for general purposes under normal condition, special models developed specifically
for major events such as bazaars, fairs and festivals in Hong Kong were required to
understand their pedestrian flow patterns. In this dissertation, pedestrian flows in the
Fair were videotaped and data was extracted for calibrating several pedestrian flow
models. These included the conventional models developed by Greenshields,
Greenberg, Pipes-Munjal, Underwood, Drake, Wong et al. and four modifications of
these models for simulating isotropic and bi-directional pedestrian flow scenarios.
The free flow speed of the pedestrians in the Chinese New Year Eve Fair was found
to be around 0.69-0.84 m/s, slower than those identified in other researches. Besides,
the results of these models showed the relationships between walking speed, density
and flow of the pedestrians in the Fair. Also, the effects of bi-directional flow to
pedestrian flows were assessed and quantified. These findings obtained from models
were then used as a basis for formulating crowd control strategy of major events in
Hong Kong.published_or_final_version
Transport Policy and Planning
Master
Master of Arts in Transport Policy and Planning
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Richard, Paul François. "A computer analysis of the flow of water and nutrients in agricultural soils as affected by subsurface drainage". Thesis, University of British Columbia, 1988. http://hdl.handle.net/2429/29171.
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A computer model was developed in order to determine the effects of drainage practices on nutrient losses from level agricultural soils.
The model performs a daily simulation of the vertical flow of water, nitrogen, phosphorus, and heat, and of the growth of crops. A water flow submodel calculates the depth of the water table based on daily predictions of evaporation, transpiration, flow to drains and ditches, and deep percolation. An original saturated-unsaturated flow algorithm is used to determine moisture infiltration, redistribution, and upward flow in the soil matrix, as well as bypassing flow in the soil macropores and horizontal flux between the soil matrix and the macropores, and surface runoff. Nutrient movement occurs by mass flow. Heat flow, nutrient biochemical transformations, and crop growth are determined by using well established relations.
Field tests were carried out for a period of two years on an experimental site in the Lower Fraser Valley of British Columbia. The water table depth was measured on a continuous basis. Grab samples of drainwater and observation wells were obtained periodically and analyzed for nitrogen (N0₃-N, NH₄-N, and TKN) and phosphorus (P0₄-P and TP). The field results show a decrease in the concentration of all nutrients over the sampling period, and provide evidence that denitrification and bypassing flow are important mechanisms affecting the nutrient balance of this soil.
These results were used to calibrate the model. An excellent fit of the observed water table profile and an adequate fit of the observed drain concentration of nitrate were obtained. The simulation revealed that bypassing flow is a very important transfer mechanism in this soil and must be included in order to obtain a satisfactory fit of the experimental data.
A sensitivity analysis of the model showed that the patterns of moisture flow have a predominant influence on the rate of nutrient leaching. In particular, it was found that the nutrient concentration in drain water is a strong function of the hydraulic conductivity of the soil matrix and of the horizontal distance between the soil macropores, which control the ratio of moisture flow in the soil matrix to the macropore flow and the lateral diffusion of nutrients between the soil matrix and the macropores.
The effects of four different drainage designs on nutrient losses were simulated over a period of two years for three different soils and two different nutrient distributions in the soil. It was found that there is a large difference between the amount of nutrients leached from drainage systems using different drainage coefficients. There was also a large difference in the response of two drainage designs based on the same drainage coefficient but using different depth and spacing of drains. Transient effects, as determined by the initial vertical distribution of the nutrients, were seen to remain dominant over the two year duration of the simulation.
The model was found to be useful in explaining the apparent contradictions found in the literature assessing the effects of subsurface drainage on nutrient losses. The results from the model show these effects to be strongly site and condition specific. Furthermore, the model shows that soils and drainage designs that produce similar volumes of drain flow may exhibit very different leaching responses, and that drainage designs equivalent from a hydraulic standpoint can be very dissimilar in their potential for leaching nutrients. The model provides a tool which can be used to determine the appropriateness of different drainage designs in soils where minimizing nutrient losses is critical.Science, Faculty of
Resources, Environment and Sustainability (IRES), Institute for
Graduate
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Brihi, Sarra. "Mathematical analysis and numerical approximation of flow models in porous media". Thesis, Normandie, 2018. http://www.theses.fr/2018NORMC263/document.
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Cette thèse est consacrée à l'étude des équations du Darcy Brinkman Forchheimer (DBF) avec des conditions aux limites non standards. Nous montrons d'abord l'existence de différents type de solutions (faible, forte) correspondant au problème DBF stationnaire dans un domaine simplement connexe avec des conditions portants sur la composante normale du champ de vitesse et la composante tangentielle du tourbillon. Ensuite, nous considérons le système Brinkman Forchheimer (BF) avec des conditions sur la pression dans un domaine non simplement connexe. Nous prouvons que ce problème est bien posé ainsi que l'existence de la solution forte. Nous établissons la régularité de la solution dans les espaces L^p pour p >= 2.L'étude et l'approximation du problème DBF non stationnaire est basée sur une approche pseudo-compressibilité. Une estimation d'erreur d'ordre deux est établie dans le cas o\`u les conditions aux limites sont de types Dirichlet ou Navier.Enfin, une méthode d'éléments finis Galerkin Discontinue est proposée et la convergence établie concernant le problème DBF linéarisé et le système DBF non linéaire avec des conditions aux limites non standardThis thesis is devoted to Darcy Brinkman Forchheimer (DBF) equations with a non standard boundary conditions. We prove first the existence of different type of solutions (weak, strong) of the stationary DBF problem in a simply connected domain with boundary conditions on the normal component of the velocity field and the tangential component of the vorticity. Next, we consider Brinkman Forchheimer (BF) system with boundary conditions on the pressure in a non simply connected domain. We prove the well-posedness and the existence of a strong solution of this problem. We establish the regularity of the solution in the L^p spaces, for p >= 2.The approximation of the non stationary DBF problem is based on the pseudo-compressibility approach. The second order's error estimate is established in the case where the boundary conditions are of type Dirichlet or Navier. Finally, the finite elements Galerkin Discontinuous method is proposed and the convergence is settled concerning the linearized DBF problem and the non linear DBF system with a non standard boundary conditions
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Khatibi, Rahman Haghi. "Mathematical open channel flow models and identification of their friction parameters". Thesis, University of London, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263145.
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This thesis l concerned with the mathematical modelling of open channel flows governed by the Saint-Venant equations, which are used as a prediction or identification tools. A survey of the literature in these fields identified the problems in need of Immediate research. Numerical test runs were then devised which led to projecting a clear picture as follows. The performance of twn widely used Implicit finite difference schemes, the 4-point box and 6-point staggered schemes were compared In a wide range of circumstances. it is concluded that both schemes produce 'very close results, but the staggered scheme is prone to convergence problems In some extreme cases. It was also noted that a sharp change in geometric configuration of compound channels produced discontinuous features on the aim ulated depth and discharge hydrographs. The inability of the staggered scheme In handling a head-discharge relationship as a downstream boundary condition was tackled by proposing and implementing a scheme of second order accuracy. As model data are generally corrupted withh errors and noise, their effects together with that of other factors on the Identified friction parameters we Investigated. The results demonstte the paramount Importance of the effect of a choice of objective function on the Identified parameters. While the individual values of the identified M2nning n may vary from one flood event to another, their mean is shown both numerically and rigorously to be dependent upon the choice of objective function. It is shown that an objective function formulated by using absolute errors performs ideally and produces reliable results even in the presence of autocorrelated Gaucian noise samples. The mean of the Identified parameters is also found to be adversely affected if the observation station is affected by localized disturbances. Sensitivity of objective functions to the variation In the value of the friction parameter Is also found to be an Important factor, as Insensitivity leads to ill-conditioning.
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Lierkamp, Darren. "Simulating the effects of following distance on a high-flow freeway". Full text available online (restricted access), 2003. http://images.lib.monash.edu.au/ts/theses/Lierkamp.pdf.
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"CP830 Research Project and Thesis 2". Includes bibliographical references (p. 80-93) Electronic reproduction.[S.l. :s.n.],2003.Electronic data.Mode of access: World Wide Web.System requirements: Adobe Acrobat reader software for PDF files.Access restricted to institutions with a subscription.