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Abstract

An abdominal aortic aneurysm (AAA) is a vascular disorder that emerges when the aortic wall degenerates and the abdominal aorta undergoes a permanent expansion of at least 1.5 times its normal diameter. If an AAA is left undiagnosed and untreated, it can eventually rupture with an associated mortality risk of 81%. Clinically, a maximum diameter exceeding 5 cm is considered the decisive criterion to identify at-risk AAAs. The present work investigates the flow dynamic variations associated with aneurysm experimentally, using compliant AAA models and time-resolved particle image velocimetry (TR-PIV). For this purpose, an in vitro setup is designed and developed to simulate the flow inside 6 idealized phantoms mimicking in vivo physiological properties. The instantaneous flow fields in two orthogonal planes reveal that the flow behavior in the early stages of an aneurysm does not deviate significantly from that of the healthy aorta. An increase in the size and number of bulges triggered the evolution of features such as flow detachment, swirling motion, wall impingement and the collapse of the vortical structures within the aneurysm sac. All these characteristics are more noticeable in the AAA model with a maximal diameter equal to the clinical threshold. Therefore, in the context of fluid dynamics, this perception can provide physical support to the Dmax=5 cm criterion to nominate an AAA patient for elective surgery. It is assumed that the onset and propagation of large vortical structures in the deceleration phase and the formation of stagnant flow regions near the proximal neck of the largest model can lead to unfavorable hemodynamic changes that escalate the weakening, enlargement and rupture of the aortic wall.
Following recent applications of viscous energy dissipation in the severity stratification of cardiovascular diseases, as part of this thesis, the associated energy loss is quantified in order to examine its correlation with changes in the geometry and flow topology of each model. Although the two largest models were identified with the highest energy losses, which can be attributed to their higher shear stresses, this parameter does not follow a clear increase with the aneurysm expansion. In the other four cases, the kinetic energy was almost equally well preserved. The experimental AAA flow data in this work is also inspected from a modal analysis perspective, in particular proper orthogonal decomposition (POD), to identify and extract the dominant structures, which can provide more insights on the disease progression. The Shannon (global) entropy demonstrates that as the aneurysm expands, the modal energy distribution becomes dispersed, meaning that more modes are required to describe and model the flow. This parameter, in turn, can potentially be applied in clinical assessment of the severity of patient-specific AAAs.

Available Versions of this Item

• Flow Characteristics in Abdominal Aortic Aneurysms: An in vitro Study. (deposited 23 Jun 2021 16:33)
  • Flow Characteristics in Abdominal Aortic Aneurysms: An in vitro Study. (deposited 30 Jun 2021 15:06) [Currently Displayed]