Computational and experimental investigation of steady flow fields, turbulence, and hemodynamic wall stresses in patient-specific abdominal aortic aneurysm models Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6395wb29j

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  • Steady flow fields and flow-induced wall stresses have been evaluated by experimental measurements and computational fluid dynamics (CFD) analysis in a series of patient-specific abdominal aortic aneurysm (AAA) models, over a range of Reynolds numbers (Re) from 125 to 3000 (500 to 3000 for CFD). Experimental methods used particle image velocimetry (PIV) to evaluate velocity flow fields, wall shear stress, flow fields and pressure were predicted in corresponding AAA models. Both laminar and turbulent solutions were obtained at each Re, using k-ω techniques for turbulence simulation. Qualitatively the measurements and predictions were in good agreement, especially with respect to velocity. AAA lumen shape was found to significantly alter flow structure, producing large recirculating vortices in the sacs of bulged lumens, but little to no vortical structure in nearly isodiametric lumens from patients with substantial intraluminal thrombus. Recirculating vortices were associated with adverse wall pressure gradients and retrograde, reduced wall shear stress. For example, CFD predicted at Re = 3000, wall shear stress to be near 8.0 dynes/cm2 in non-dilated aortas but only 2.2 dynes/cm2 within dilations. Quantitative agreement was limited between the measured and predicted wall shear stress and turbulence. Wall shear stress was in reasonable agreement between measurements and predictions at Re = 500, showing global wall shear stress mean values of 0.41 dyne/cm2 for experimental results versus 0.63 dyne/cm2 for computational results. However, at Re = 3000, the difference was much greater as the global wall shear stress mean value was 2.75 dyne/cm2 for experimental results versus 5.78 dyne/cm2 for computational results. Computational results associated separated flows with adverse wall pressure gradients, though experimental measurements did not confirm this. Experimental pressure measurements indicated that under these conditions, wall height was the greatest determinant of wall pressure. The complex flows formed within patient specific lumens seemed to promote turbulence, even in non-dilated sections. Turbulence was measured at all flow rates including Re = 125. At Re = 3000, fluctuating velocities of up to 0.7 times the overall mean velocity were found in the saccular dilation. Computational simulation produced lower turbulence than was found experimentally, with fluctuating velocities generally remaining below 0.1 times the overall mean velocity. This investigation shows the importance of patient-specific analysis, and provides some description of the hemodynamic forces experienced at rest and exercise conditions. The quantitative disagreements between measured and predicted results also suggest that current computational techniques are not yet sufficiently accurate for reliable clinical predictions.
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