Fluid mechanics of pulsatile blood flow in abdominal aortic aneurysms

The cardiovascular system is an internal flow loop with multiple ramifications in which a fluid of complex properties circulates. Additionally, blood flow in human arteries is dominated by time-dependent transport phenomena. In particular, blood in the abdominal segment of the aorta under a patient's average resting conditions exhibits laminar flow patterns that are influenced by secondary flows in adjacent branches and in irregular vessel geometries. The flow dynamics becomes more complex when there is a pathological condition that causes changes in the normal structural composition of the vessel wall, for example, in the presence of an aneurysm. An aneurysm is an irreversible dilation of a blood vessel accompanied by weakening of the vessel wall. Abdominal Aortic Aneurysms (AAAs) are usually located in the infrarenal segment of the abdominal aorta, between the renal arteries and the iliac bifurcation. AAAs are a health risk of significant importance since they are largely asymptomatic until the onset of rupture, an event that carries a mortality rate of 90% and which is ranked as the 13th most common cause of death in the United States. Aneurysm rupture is a biomechanical phenomenon that occurs when the mechanical stress acting on the inner wall exceeds the failure strength of the diseased aortic tissue. Since the internal mechanical forces are initiated and maintained by the dynamic action of blood flow within the aneurysm, the hemodynamics of AAAs becomes an important element of study for the description of the biomechanical environment of aneurysms.; The principal objective of this investigation is to characterize the pulsatile flow dynamics in AAAs for average resting conditions by means of identifying regions of disturbed flow and quantifying the disturbance through the evaluation of flow-induced stresses at the aneurysm wall. To achieve this goal, computational models of AAAs ranging from two-dimensional axisymmetric to patient-specific are developed for the Direct Numerical Simulation (DNS) of the momentum transport equations. The Spectral Element Method (SEM) and the Finite Element Method (FEM) are utilized for the simulation of blood flow and the evaluation of wall pressure, wall shear stress and wall shear stress gradients. A novel technique has been developed for the numerical evaluation of wall shear stresses in two-dimensional and three-dimensional models of deformed arteries. For average resting conditions, pulsatile blood flow in individual AAAs is laminar and exhibits a highly complex vortex dynamics characterized by four distinct flow phases: (i) downstream ejection of residual vortices and attached flow during systolic acceleration, (ii) flow separation at the proximal neck, single-vortex growth and its translation downstream during systolic deceleration, (iii) partial vortex shedding and vortex weakening during early diastole, (iv) significant disturbance caused by a jet of principal flow along the posterior wall and a low-velocity vortex inside the aneurysm during late diastole. Wall shear stress is complexly distributed throughout the cardiac cycle, reaching a maximum at peak systolic flow. Wall pressure is increased by an order of magnitude in the vicinity of the distal neck when the flow reattaches to the wall. High wall shear stress gradients are obtained at the distal end when low negative and high positive wall shear stresses coexist. An initial prediction of mechanical wall stresses is provided by performing fluid-structure interaction simulations of pulsatile blood in two-dimensional AAA models.