Numerical Simulation of Blood Flow Past Irregular Solid Obstacles (blood clotting) Through Veins / Arteries

Abstract

The research work presented in this thesis focuses upon mild and severe stenosis in human common carotid artery and femoral artery bifurcation. The intensity of the effects of these stenoses is investigated on different aspects. In this thesis, the main attention is focused upon intensity of shear stress, flow separation region and length of reattachment. Results of numerical simulation are compared with the experimental results for a slightly higher than a critical Reynolds number. For results regarding reattachment length, it has been observed that the results obtained using numerical technique considering laminar flow simulations are consistent. The development of atherosclerotic disease has been investigated by taking various constriction ratios of stenosis. The Common Carotid artery has also been taken into account for the purpose of numerical simulations. The numerical calculations are performed on the stenosed carotid artery bifurcation under laminar flow conditions. The results are obtained at mean Reynolds number (=200) with flow division ratio of about 70:30, which depicts the entire systolic and diastolic pulse wave. Here, we have considered two mesh geometries having various ratios of stensosis ranging from 20% to 80%. At stenosis 20% or above, we found that two flow zones emerged in the internal carotid artery i.e the area where wall shear stresses are very high that may cause the damage to the endothelial cells. The other zone is relevant with low shear stresses, known as elongated flow recirculation area, where the duration of the reversal of flow is amplified during pulse cycle. The recirculation zone may cause the retardation of mass transport through the arterial wall and may consequently speed up evolution of atherosclerosis near the stenosis at downstream. Furthermore, from the results of streamlines, velocity vectors, velocity contours, reverse flow duration and line plots, it has been observed that the atherosclerotic lesions may evolve rapidly between 30% and 50% stenosis. However, the trend of development of these atherosclerotic lesions may continue beyond 50% stenosis, but at below the above reported rate. This suggests that the rate of development of atherosclerotic lesions decreases with increasing percentage of stenosis beyond 50% and upto 70%. In a very interesting result at 75% stenosis, we observed a significant change in the flow behavior that causes the development of atherosclerotic lesions at a very fast rate. On the basis of these findings, we focused our studies upon stenosis above 75%. The narrower the flow regime may cause the laminar flow into turbulent flow. This required a stable and consistent numerical scheme that may not diverge in the turbulent region. In this regard, a robust Semi-Implicit Pressure-Correction Taylor-Galarkin Finite Element scheme is applied to capture accuracy, and to maintain stability and consistency of the numerical algorithm at severe stenosis which may cause the laminar flow region into turbulent flow region. In the present study, it is observed that the presence of 80% plaque altered the flow regime from laminar to turbulent. However, the presence of 45% to 55% constriction in both test models has not changed the flow properties notably from laminar to turbulence flow behavior. Using more practical settings in the calculations and applying proper numerical technique as used in this research work; one may understand the trend of development of the plaque and its measurement in Carotid Artery Bifurcation.