Hemodynamics in abdominal aorta aneurysms
This thesis presents results from numerical simulations of pulsatile fluid flow in canonical channel geometry and patient-specific abdominal aortic aneurysms (AAA). An AAA is a dilatation of the aortic segment between the renal arteries and the iliac bifurcation. The continuing expansion of an AAA weakens the wall and potentially leads to rupture. Intraluminal thrombus (ILT) deposition is the coagulation of platelets and often accumulates along the wall in AAA. The presence of ILT may decrease the flow of oxygen to the wall and the region of the AAA with ILT could be more susceptible to further expansion or rupture. Knowledge of AAA shapes that cause adverse hemodynamics could be used to establish criteria to help determine when to operate on a particular AAA to prevent rupture. It was determined that some AAA shapes experience non-disturbed hemodynamics that do not significantly deviate from normal-sized aorta. For other AAA shapes a turbulent jet forms distal to the aneurysms neck and impinges against the AAA wall. It was found that at the location the jet impinges the wall was devoid of ILT and away from this location ILT often accumulated. It was suspected that the high wall-tangent shearing force from the impact of flow vortexes prevents attachment of cellular material to the wall. Although impingement prevents ILT, in all cases the AAA was found to be expanding in the direction the neck angled the flow vortexes and it was reasoned that the wall-normal pressure force from vortex impact causes the expansion. The triple decomposition method is used to segment blood flow into its periodic and turbulent components. Another segmentation method, dynamic mode decomposition (DMD) is used to segment blood flow into modes oscillating at specific frequencies. A low-order representation of the velocity field was then reconstructed using only the modes oscillating at the pulse frequency. This procedure removes both the higher-frequency turbulent and periodic oscillations from the velocity field. The resulting reconstruction shows a simplified representation of the blood flow dynamics in the AAA, such that it consists of a single large-scale vortex.
Abdominal aorta aneurysm, Dynamic mode decomposition, Direct numerical simulation, Turbulence