Session HE: Biofluids VI: Cardiovascular

Flow Instability and Wall Shear Stress Ocillation in Intracranial Aneurysms

We investigate the flow dynamics and oscillatory behavior of wall shear stress (WSS) vectors in intracranial aneurysms using high-order spectral/hp simulations. We analyze four patient- specific internal carotid arteries laden with aneurysms of different characteristics : a wide-necked saccular aneurysm, a hemisphere-shaped aneurysm, a narrower-necked saccular aneurysm, and a case with two adjacent saccular aneurysms. Simulations show that the pulsatile flow in aneurysms may be subject to a hydrodynamic instability during the decelerating systolic phase resulting in a high-frequency oscillation in the range of 30-50 Hz. When the aneurysmal flow becomes unstable, both the magnitude and the directions of WSS vectors fluctuate. In particular, the WSS vectors around the flow impingement region exhibit significant spatial and temporal changes in direction as well as in magnitude.   

Flow patterns and shear stress waveforms in intracranial aneurysms: The effect of pulsatility

The wall shear stress on the dome of intracranial aneurysms has been hypothesized to be an important factor in aneurysm pathology and depends strongly on the hemodynamics inside the dome. The importance of patient-specific geometry on the hemodynamics of aneurysms has long been established but the significance of patient-specific inflow waveform is largely unexplored. In this work we seek to systematically investigate and quantify the effects of inflow waveform on aneurysm hemodynamics. We carry out high resolution numerical simulations for an anatomic intracranial aneurysm obtained from 3D rotational angiography (3DRA) data for various inflow waveforms. We show that both the vortex formation process and wall-shear stress dynamics on the aneurysm dome depend strongly on the characteristics of the inflow waveform. We also present preliminary evidence suggesting that a simple non-dimensional number (named the Aneurysm number), incorporating both geometry and inflow waveform effects, could be a good qualitative predictor of the general hemodynamic patterns that will arise in a given aneurysm geometry for a particular waveform.   

Hemodynamic simulations in coronary aneurysms of children with Kawasaki disease

Kawasaki disease (KD) is a serious pediatric illness affecting the cardiovascular system. One of the most serious complications of KD, occurring in about 25\% of untreated cases, is the formation of large aneurysms in the coronary arteries, which put patients at risk for myocardial infarction. In this project we performed patient specific computational simulations of blood flow in aneurysmal left and right coronary arteries of a KD patient to gain an understanding about their hemodynamics. Models were constructed from CT data using custom software. Typical pulsatile flow waveforms were applied at the model inlets, while resistance and RCR lumped models were applied and compared at the outlets. Simulated pressure waveforms compared well with typical physiologic data. High wall shear stress values are found in the narrow region at the base of the aneurysm and low shear values occur in regions of recirculation. A Lagrangian approach has been adopted to perform particle tracking and compute particle residence time in the recirculation. Our long-term goal will be to develop links between hemodynamics and the risk for thrombus formation in order to assist in clinical decision-making.   

Analyzing Transient Turbuelnce in a Stenosed Carotid Artery by Proper Orthogonal Decomposition

High resolution 3D simulation (involving 100M degrees of freedom) were employed to study transient turbulent flow in a carotid arterial bifurcation with a stenosed internal carotid artery (ICA). In the performed simulation an intermittent (in space and time) laminar-turbulent-laminar regime was observed. The simulation reveals the mechanism of the onset of turbulent flow in the stenosed ICA where the narrowing in the artery generates a strong jet flow. Time- and space-window Proper Orthogonal Decomposition (POD) was applied to quantify the different flow regimes in the occluded artery. A simplified version of the POD analysis that utilizes 2D slices only - more appropriate in the clinical setting - was also investigated.   

Numerical Simulation of the Flow in Vascular Grafts for Surgical Applications

Numerical simulation of the human blood vessels, is becoming an important tool in surgical planning and research. Accurate vascular simulations might grant physicians the predictive capability to perform pre-surgical planning. We focus our attention on the implantation of vascular grafts. The high rate of failure of this common vascular interaction is intimately related to the fluid mechanics in the affected region and the subsequent wall tissue remodeling. Here, we will present our current work in developing a methodology for the numerical simulation of vascular grafts which incorporates physiologically realistic geometries and flow boundary conditions. In particular, we seek to correlate the wall shear stress and its spatial (WSSG) and temporal (OSI) variability to wall remodeling as observed in patient specific longitudinal studies. The pulsatility ($Re_{mean} = 800$ , $Re_{peak} = 2000$, $Wo = 2$) of the flow gives rise to additional fluid dynamics phenomena such as instability, flow separation, transition, and unsteadiness. Our goal is to describe and evaluate their effect on the wall physiology.   

The Effect of Aortic Compliance on Left Ventricular Power Requirement

Aortic compliance depends on both geometry and mechanical properties of the aorta. Reduction in arterial compliance has been associated with aging, smoking, and multiple cardiovascular diseases. Increased stiffness of the aorta affects the wave dynamics in the aorta by increasing both pulse pressure amplitude and wave speed. We hypothesized that decreased aortic compliance leads to an increased left ventricular power requirement for a fixed cardiac output due to altered pulse pressure and pulse wave velocity. We used a computational approach using the finite element method for solid and fluid domains coupled to each other by using the direct coupling method. A nonlinear material model was used for the solid wall. The fluid flow model was considered to be Newtonian, incompressible, and laminar. The simulation was performed for a heart rate of 75 beats per minute for six different compliances while keeping the cardiac output and the peripheral resistance constant. The results show a trend towards increased left ventricular energy expenditure per cycle with decreased compliance. The relevance of these findings to clinical observations will be discussed.   

Non-Invasive Measurement of Viscosity in Pulsatile Flow in Elastic Vessels

When laminar pulsatile flows in elastic-walled vessels are fully developed, theoretical solutions can be found relating any two flow variables at a given axial location. When the flow variables are both velocities, such as the time-dependent centerline and area-averaged velocities, their interdependence on the wave speed of pulse propagation cancels. When the wave-speed Reynolds number exceeds 1,000, effects of the vessel wall's compliance on the solution become negligible, leaving the vessel radius and the Newtonian viscosity as the only {\it parameters\/} in the time-dependent problem. A non-invasive method for determining the fluid viscosity based on such solutions is demonstrated in arbitrarily unsteady flow, that is relevant to the critical problem of accurate {\it in vivo\/} measurement of the local viscosity of blood in patients.   

Mass Transport and Shear Stress as Mediators of Flow Effects on Atherosclerotic Plaque Origin and Growth

The carotid artery bifurcation (CAB) is one of the leading site for atherosclerosis, a major cause of mortality and morbidity in the developed world. The specific mechanisms by which perturbed flow at the bifurcation and in the carotid bulge promotes plaque formation and growth are not fully understood. Shear stress, mass transport, and flow residence times are considered dominant factors. Shear stress causes restructuring of endothelial cells at the arterial wall which changes the wall's permeability. Long residence times are associated with enhanced mass transport through increased diffusion of lipids and white blood cells into the arterial wall. Although momentum and mass transfer are traditionally coupled by correlations similar to Reynolds Analogy, the complex flow patterns present in this region due to the pulsatile, transitional, detached flow associated with the complex geometry makes the validity of commonly accepted assumptions uncertain. We create solid models of the CAB from MRI or ultrasound medical images, build flow phantoms on clear polyester resin and use an IOR matching, blood mimicking, working fluid. Using PIV and dye injection techniques the shear stress and scalar transport are experimentally investigated. Our goal is to establish a quantitative relationship between momentum and mass transfer under a wide range of physiologically normal and pathological conditions.   

Evolution of streamwise vortices in a 180$^{\circ}$ circular bend under physiological flow conditions

Due to the complex geometry of the vasculature and the pulsatile nature of blood flow, secondary flows are common in blood vessels. Secondary flow vortices within a 180 $^{\circ}$ circular bend, under physiological flow conditions, with perturbations introduced by model stents were investigated. Reynolds numbers ranged from 200 to 1400 and the cardiac cycle period was scaled to match the physiological Womersley number, Wo=3.6. LDV and PIV were used to measure the flow fields at different locations in the bend. Companion computations were performed using commercial CFD software. The streamwise velocity profile was skewed toward the inner wall at the entrance of the bend and to shift progressively toward the outer wall further downstream. Counter-rotating Dean vortices were observed during the majority of the cardiac cycle. Even though Wo is relatively low, Lyne vortices were observed to develop after 90 $^{\circ}$ of bend, following the systolic peak in the waveform. Because of the strong accelerations, higher frequency harmonic waves are present, which are associated with locally high Wo.   

Flow in an Aortic Coarctation

Coarctation of the aorta is a congenital cardiovascular defect that causes a constriction in the descending thoracic aorta. To gain a better understanding of the cause of post-surgical problems, a rigid glass and a compliant \textit{in vitro }model of the aortic arch and descending aorta with a coarctation were constructed. Near-physiologic compliance was obtained using a silicone elastomer. Stereoscopic PIV was used to obtain 3D velocity maps. Results show a high speed turbulent jet formed at the exit of the coarctation. Flow in the rigid model was significantly different from in the compliant model. In the rigid model, the jet was symmetric, creating a toroidal recirculation area. In the compliant model, the jet was directed towards the medial wall, inducing flow reversal only at the lateral wall. Peak velocities and turbulence intensities were higher in the rigid model, however shear rate values in the compliant model were significantly above both the rigid model and normal \textit{in vivo }values at the medial wall. In both models the reattachment region fluctuated, creating oscillatory shear.