Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session HL: Biofluids: Physiological Circulatory II |
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Chair: Petia Vlahovska, Brown University Room: Long Beach Convention Center 202A |
Monday, November 22, 2010 10:30AM - 10:43AM |
HL.00001: Subharmonic response from ultrasound contrast microbubbles for noninvasive blood pressure estimation Amit Katiyar, Kausik Sarkar, Flemming Forsberg Estimation of local organ-level blood pressure can help in diagnosing and monitoring heart and vascular diseases. Subharmonic signals from ultrasound contrast microbubbles have been proposed as a noninvasive alternative to the current practice of using manometer-tipped catheter. Approximately 10dB linear decrease in subharmonic component with 25 kPa pressure increase (typical blood pressure variation) has been reported for several contrast microbubbles. Here we report a theoretical investigation of the underlying phenomenon. We first study the well established model of a free microbubble to show that reduction of subharmonic with ambient pressure increase occurs only below a certain excitation frequency. Above this critical frequency, subharmonic signal increases with ambient pressure. Furthermore, where it decreases with ambient pressure, the relationship is linear only above certain excitation pressure. The dependence of the critical frequency on bubble radius and possibly bubble size distribution is discussed. We also report similar behavior for several models for encapsulated contrast microbubbles. [Preview Abstract] |
Monday, November 22, 2010 10:43AM - 10:56AM |
HL.00002: Secondary flow structure from stent-induced perturbations in a bent pipe model for curved arteries Fangjun Shu, Autumn Glenn, Kartik Bulusu, Michael W. Plesniak Secondary flow structures were investigated in a 180-degree circular bend under physiological (pulsatile) flow conditions with a stent model installed upstream of the bend. Upstream Reynolds number ranged from 200 to 1400 and the cardiac cycle period was scaled to match the physiological Womersley number, Wo=4.2. Experimental data were acquired using 2-D PIV at various cross-sectional planes along the bend. Similar to the results in absence of the stent model, symmetric counter-rotating vortex pairs were observed to develop during the cardiac cycle. In addition, transient unstable flow was initiated at the deceleration phase of the systolic peak (t/T=0.21). This complex flow is mainly attributable to perturbations induced by the stent model. It is characterized by breakdown of Dean- and Lyne-type vortices into various multiple-scale vortices. The phase-averaged flow fields were analyzed using the proper orthogonal decomposition (POD) method to gain further insight regarding the structural features of the flow. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HL.00003: Wall shear stress as a stimulus for carotid atherosclerotic plaque progression: An MRI-based CFD pilot study Gador Canton, Bernard Chiu, Tom Hatsukami, William Kerwin, Chun Yuan The aim of this study was to explore the hypothesis that intra-plaque hemorrhage, a feature associated with adverse outcomes and atherosclerotic plaque progression and destabilization, is more likely to occur in plaques with elevated levels of wall shear stress (WSS). We used multi-sequence in-vivo magnetic resonance imaging (MRI) to characterize ten human carotid atherosclerotic plaques and an MRI-based computational fluid dynamics (CFD) model to solve the equations governing the blood flow. Hemorrhage was detected within the necrotic core (intra-plaque hemorrhage) in five of these ten cases. WSS data were extracted from the results of the CFD simulations to compare patterns between the cases with and without hemorrhage. We computed the mean value of the WSS (for each time point of the cardiac cycle) at the region where a necrotic core was detected. The results from this pilot study indicate a possible link between the presence of hemorrhage within a lipid-rich necrotic core in human carotid atherosclerotic plaques and elevated levels of shear stress force acting on the luminal surface. Thus, elevated wall shear stress may be used as a high risk feature in advanced carotid atherosclerotic plaques. [Preview Abstract] |
Monday, November 22, 2010 11:09AM - 11:22AM |
HL.00004: Mass Transport and Shear Stress in the Carotid Artery Bifurcation Riley Gorder, Alberto Aliseda The carotid artery bifurcation (CAB) is one of the leading sites for atherosclerosis, a major cause of death and disability in the developed world. The specific processes by which the complex flow found at the bifurcation and carotid sinus promotes plaque formation and growth are not fully understood. Shear stress, mass transport, and flow residence times are considered key factors. Although the governing equations closely link shear stress and mass transfer, the pulsatile, transitional, and detached flow found at the CAB can lead to differences between regions of WSS and mass transfer statistics. In this study, CAB geometries are reconstructed from patient specific 3D ultrasound medical imaging. Using ANSYS FLUENT, the fluid flow and scalar transport was solved using realistic flow conditions and various mass transfer boundary conditions. The spatial and temporal resolution was validated against the analytical solution of the Graetz-Nusselt problem with constant wall flux to ensure the scalar transport is resolved for a Peclet number up to 100,000. High residence time regions are investigated by determining the number of cardiac cycles required to flush out the carotid sinus. The correlations between regions of low WSS, high OSI, and scalar concentration are computed and interpreted in the context of atherosclerotic plaque origin and progression. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HL.00005: Hemodynamic Simulations in Dialysis Access Fistulae Patrick McGah, Daniel Leotta, Kirk Beach, James Riley, Alberto Aliseda Arteriovenous fistulae are created surgically to provide adequate access for dialysis in patients with End-Stage Renal Disease. It has long been hypothesized that the hemodynamic and mechanical forces (such as wall shear stress, wall stretch, or flow- induced wall vibrations) constitute the primary external influence on the remodeling process. Given that nearly 50\% of fistulae fail after one year, understanding fistulae hemodynamics is an important step toward improving patency in the clinic. We perform numerical simulations of the flow in patient-specific models of AV fistulae reconstructed from 3D ultrasound scans with physiologically-realistic boundary conditions also obtained from Doppler ultrasound. Comparison of the flow features in different geometries and configurations e.g. end-to-side vs. side-to-side, with the \textit{in vivo} longitudinal outcomes will allow us to hypothesize which flow conditions are conducive to fistulae success or failure. The flow inertia and pulsatility in the simulations (mean $Re\approx700$, max $Re\approx2000$, $Wo\approx4$) give rise to complex secondary flows and coherent vortices, further complicating the spatio- temporal variability of the wall pressure and shear stresses. Even in mature fistulae, the anastomotic regions are subjected to non-physiological shear stresses ($>10\hspace{.12pc}Pa$) which may potentially lead to complications. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HL.00006: Hemodynamics in stenotic vessels: synthesis of CFD and PIV results Jenn Rossmann The hemodynamics in atherosclerotic blood vessels have implications for disease progression; fluid mechanical patterns and forces are linked to the risk of plaque rupture. A synthesis of numerical and experimental methods is used to investigate the dynamics in representative stenotic vessels. Detailed understanding of the hemodynamics in these vessels can contribute to prediction of rupture risk for a particular atherosclerotic plaque. Computational Fluid Dynamics (CFD) simulations of blood flow in generic and patient-specific stenotic vessels are performed using commercial software. Results of CFD are compared with those of concurrent PIV experiments to evaluate the significance of arterial wall compliance, flow pulsatility, and turbulence. Aspects of stenosis morphology are identified as useful complements to imaging modalities used in patient diagnosis and treatment. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HL.00007: Three-dimensional Particle Image Velocimetry of Optically Opaque Flows using Ultrasound Contrast Agents Henning Gelshorn, Ana Medina, Daniel Lotz, David J. Fisher, Javier Rodriguez-Rodriguez, Juan C. del Alamo, Thilo Hoelscher Currently, phase contrast magnetic resonance imaging is the only technique that provides time-resolved volumetric velocity maps of optically opaque systems such as blood flow in our vessels. However, this technique is expensive, time consuming and has low resolution. This project constitutes the first step towards the introduction of 3D echo-PIV, a novel ultrasound imaging technique that provides volumetric maps of three-component blood flow velocity almost in real time. 3D echo-PIV is non-invasive, fast, mobile and inexpensive, and therefore has the potential to become a commonly-used modality in the clinical setting. This new modality performs particle image velocimetry on 3D, time-lapse sequences of ultrasound bright-mode frames obtained during contrast agent infusion by tracking the sound backscattered by the contrast agent microbubbles. The present study applies 3D echo-PIV to tubular silicone phantoms in an arterial flow simulator and compares these measurements to the results from computer simulations obtained with commercial codes that have been extensively validated. We vary systematically geometrical parameters and flow rates to model different physiological hemodynamic patterns. [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HL.00008: Secondary flow structures under simple harmonic inflow in a bent pipe model for curved arteries Autumn Glenn, Penelope Seagrave, Fangjun Shu, Kartik Bulusu, Michael W. Plesniak Inward centrifuging of fluid in the inviscid core of a 180 degree curved pipe leads to Lyne-type vortices under zero-mean harmonic oscillations, along with the formation of vortices in the Stokes' layer, that rotate in the same directional sense as their steady flow counterpart (Dean vortices). Under physiological conditions, the development of the Lyne-type vortices is believed to be influenced by the systolic pulse, and its associated rapid acceleration and deceleration. Experimental data acquired using Particle Image Velocimetry (PIV) for three harmonic waveforms of different frequencies clarify the conditions under which Lyne vortices form. Multiple vortex pairs were observed for all waveforms and frequencies investigated, including Dean and Lyne-type vortex structures at a Womersley number of 4.22, much lower than previously reported. Hence, frequency alone is not an adequate governing parameter to characterize secondary flow structures in pulsatile flows. A regime map of the secondary flow was sought by using an acceleration-based parameter and the Dean number. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HL.00009: Relationship between potential platelet activation and LCS Shawn Shadden In the study of blood flow, emphasis is often directed at understanding shear stress at the vessel wall due to its potentially disruptive influence on the endothelium. However, it is also known that shear stress has a potent effect on platelet activation. Platelet activation is a precursor for blood clotting, which in turn is the cause of most forms of death. Since most platelets are contained in the flow domain, it is important to consider stresses acting on the platelet as they are convected. Locations of high stress can correspond to boundaries between different dynamic regions and locations of hyperbolic points in the Eulerian sense. In the computation of LCS, strain in typically considered in the Lagrangian sense. In this talk we discuss the relationship between locations of potential platelet activation due to increased stress and locations of LCS marking increase Lagrangian deformation. [Preview Abstract] |
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