Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session FB: Hemodynamics I |
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Chair: Joseph Bull, University of Michigan Room: Hilton Chicago International Ballroom South |
Monday, November 21, 2005 8:00AM - 8:13AM |
FB.00001: Normal force exerted on vascular endothelial cells Panagiotis Dimitrakopoulos, Yechun Wang Hemodynamic forces play a pivotal role in the normal and pathological behavior of vascular endothelial cells. A plethora of studies, mainly in the last two decades, has attributed the behavior, or changes in the behavior, of the endothelium as a result of one of the two components of the hemodynamic force, i.e. as effects of the shear stress. For example, shear stress has found to increase the endothelial hydraulic conductivity, to regulate occludin content and phosphorylation, and to affect the ability of cells to induce adhesion of flowing neutrophils. Based on computational investigation and scaling analysis, our study shows that the normal force contributes significantly to the total force on the endothelium cells even in large vessels. Since both normal and shear forces can affect the stretching and bending of the cell membrane, our study suggests that both the normal and the pathological behavior of endothelial cells is affected by the normal force exerted on them. The effects of the normal force are more pronounced for smaller vessels and/or less spread cells. These conclusions are in direct contrast to the common practice of previous studies which attribute the operation of endothelial cells to the shear stress exerted on them, and thus neglect the normal force contribution. [Preview Abstract] |
Monday, November 21, 2005 8:13AM - 8:26AM |
FB.00002: Blood Flow in the Stenotic Carotid Bifurcation Vitaliy Rayz, Stanley Berger The carotid artery is prone to atherosclerotic disease and the growth of plaque in the vessel, leading often to severe occlusion or plaque rupture, resulting in emboli and thrombus, and, possibly, stroke. Modeling the flow in stenotic blood vessels can elucidate the influence of the flow on plaque growth and stability. Numerical simulations are carried out to model the complex flows in anatomically realistic, patient-specific geometries constructed from magnetic resonance images. The 3-D unsteady Navier-Stokes equations are solved in a finite-volume formulation, using an iterative pressure-correction algorithm. The flow field computed is highly three-dimensional, with high-speed jets and strong recirculating secondary flows. Sharp spatial and temporal variations of the velocities and shear stresses are observed. The results are in a good agreement with the available experimental and clinical data. The influence of non-Newtonian blood behavior and arterial wall compliance are considered. Transitional and turbulent regimes have been looked at using LES. This work supports the conjecture that numerical simulations can provide a diagnostic tool for assessing plaque stability. [Preview Abstract] |
Monday, November 21, 2005 8:26AM - 8:39AM |
FB.00003: DNS of transitional pulsatile flow through a constriction: coherent structures and Reynolds stress budgets Nikolaos Beratlis, Elias Balaras Direct Numerical Simulations of pulsatile flow through channels and pipes with a constriction will be presented. Overall our simulations verify some of flow features reported in earlier experiments in the literature. In particular, during the acceleration phase a shear layer that forms at the stenosis becomes unstable and sheds an array of vortices. The vortices break down as they interact with the wall leading to the formation of boundary layers with turbulent-like characteristics shortly after the mean reattachment point. The simulations provide a detailed description of the instantaneous flow and the spatial-temporal evolution of the structures responsible for the generation of turbulence. The relation of theses structures to the phase averaged statistics and especially the budgets of turbulent kinetic energy and the Reynolds stresses will be discussed in detail. Finally the effect of Reynolds number and the frequency and amplitude of the oscillating flow rate on the flow patterns will be given. [Preview Abstract] |
Monday, November 21, 2005 8:39AM - 8:52AM |
FB.00004: Effects of the flexibility of the arterial wall on the wall shear stresses and wall tension in Abdominal Aortic Aneurysms. Anne-Virginie Salsac, Miguel Fernandez, Jean-Marc Chomaz, Patrick Le Tallec As an abdominal aortic aneurysm develops, large changes occur in the composition and structure of the arterial wall, which result in its stiffening. So far, most studies, whether experimental or numerical, have been conducted assuming the walls to be rigid. A numerical simulation of the fluid structure interactions is performed in different models of aneurysms in order to analyze the effects that the wall compliance might have on the flow topology. Both symmetric and non-symmetric models of aneurysms are considered, all idealistic in shape. The wall mechanical properties are varied in order to simulate the progressive stiffening of the walls. The spatial and temporal distributions of wall tension are calculated for the different values of the wall elasticity and compared to the results for the rigid walls. In the case of rigid walls, the calculation of the wall shear stresses and pressure compare very well with experimental results. [Preview Abstract] |
Monday, November 21, 2005 8:52AM - 9:05AM |
FB.00005: Analysis of Turbulent flow in early stages of atherosclerosis of coronary artery Kiran Bhaganagar During the early stages of atherosclerotic heart disease, fatty material accumulates in the coronary artery resulting in development of streaks of plaque and creating high levels of turbulence, and with significantly modified flow parameters. Diagnostic measures performed during this early stage may not show any evidence of coronary artery disease, because the lumen of the coronary artery has not decreased in caliber. These streaks do not obstruct the flow of blood but alter the flow characteristics, even at this preclinical stage. This talk presents the preliminary results for the analysis of turbulent flow characteristics for a range of atherosclerotic plaque configurations in the left main coronary artery. For this purpose a CAD/medical imaging based direct-simulation (DNS) tool has been developed. The Navier-stokes equations are solved in the vertical vorticity-velocity formulation. The plaque is introduced using immersed body technique. The geometric acquisition of the artery geometry and plaque morphology is obtained using CAD based commercial software. [Preview Abstract] |
Monday, November 21, 2005 9:05AM - 9:18AM |
FB.00006: Transition to Turbulence in Stenotic Flows Sonu Varghese, Steven Frankel, Paul Fischer Direct numerical simulations (DNS) of pulsatile flow through $75\%$ (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. DNS predicts a laminar flowfield downstream of an axisymmetric stenosis and comparison to previous experiments showed good agreement in the immediate post-stenotic region. However, the introduction of a geometric perturbation at the stenosis throat, in the form of an eccentricity that was $5\%$ of the main vessel diameter, resulted in jet breakdown and transition to turbulence in the post-stenotic flowfield. Transition to turbulence was achieved as a starting vortex structure that formed during early acceleration broke up into elongated streamwise structures that subsequently underwent turbulent breakdown during peak inlet flow, confirmed by turbulent statistics and broadband velocity spectra. The impact of the flow on a hemodynamically significant parameter like wall shear stress was also studied. [Preview Abstract] |
Monday, November 21, 2005 9:18AM - 9:31AM |
FB.00007: Bubble adhesion in a microfluidic model of cardiovascular microbubble sticking James Stephen, Joseph Bull Motivated by a developmental gas embolotherapy that uses selectively formed gas bubbles for therapy, we investigated bubble sticking in two geometries: planar surfaces and circular cross-section microchannels. It was hypothesized that both surface tension and adhesion forces between the vessel wall and adsorbed molecules on the bubble surface are responsible for bubble sticking in microvessels. The planar geometry experiments demonstrated the effect of serum albumin concentration on surface tension, the tendency for bubbles to resist adhesion to an endothelial cell monolayer, and the apparent role of geometry in bubble adhesion. Microchannels (140 to 800$\mu $m in diameter) were constructed from polydimethylsiloxane (PDMS), and bubbles of various volumes were introduced into the fluid-filled channel to occlude the channel's full diameter. Once the bubble was lodged within the channel, the pressures at the inlet and outlet were slowly adjusted, and bubble motion, including the contact angle, was recorded using a microscope and CCD camera. The effects of bubble volume, microchannel diameter, serum albumin concentration, and endothelialized vs. non-endothelialized channel walls were investigated. Noting that surface tension force scales with contact line length and molecular adhesion force scales with contact area, we examined the relative contributions of the two sticking mechanisms. This work is supported by NIH grant EB003541 and Whitaker Foundation grant RG-03-0017. [Preview Abstract] |
Monday, November 21, 2005 9:31AM - 9:44AM |
FB.00008: Bubble Transport in Multiple Arteriole Bifurcations -- Experimental Modeling Brijesh Eshpuniyani, J. Fowlkes, Joseph Bull A bench top vascular bifurcation model is used to investigate the splitting of long bubbles in a series of liquid-filled bifurcations. This study is motivated by a novel gas embolotherapy technique which aims to treat cancer by infarcting tumors with gas emboli that are formed by selective acoustic vaporization of $\sim $6-micrometer, intravascular, perfluorcarbon (PFC) droplets. The resulting gas bubbles are several times larger in volume than the initial PFC droplets, and can extend through several vessel bifurcations. The current bench top experiments examine the effects of gravity and flow on bubble transport through multiple bifurcations. The effect of gravity is varied by changing the roll angle of the bifurcating network about its parent tube. Splitting at each bifurcation is nearly even when the roll angle is zero. At non-zero roll angles, higher flow rates lead to increased homogeneity in splitting. We also find that bubbles can either stick at one of the second bifurcations or in the second generation daughter tubes, even though the flow rate in the parent tube is constant. The findings of this work indicate that both gravity and flow are important in determining the bubble transport, and suggest that a treatment strategy that includes multiple doses may be effective in delivering emboli to vessels not occluded by the initial dose. This work is supported by NSF grant BES-0301278 and NIH grant EB003541. [Preview Abstract] |
Monday, November 21, 2005 9:44AM - 9:57AM |
FB.00009: Bubble sticking and sliding along the wall of a two-dimensional bifurcating channel Joseph Bull, Brijesh Eshpuniyani A two-dimensional bifurcating channel flow in which a bubble initially adheres to one of the walls is studied computationally. This investigation is motivated by a developmental gas embolotherapy technique to occlude blood flow to a tumor and induce necrosis. The bubbles originate as perfluorocarbon droplets that are small enough to pass through capillaries. The droplets are vaporized at the desired location using high intensity ultrasound to produce much larger gas bubbles that can then stick to occlude vessels. One possible scenario is that a bubble contacts the vessel wall and then slides along or sticks to the wall. Since the Reynolds numbers in the microcirculation are small, we use the boundary element method. A Tanner law is used to model the moving contact line dynamics. The evolution of the bubble interface, pressure and velocity fields, and wall normal and shear stresses are examined for a range of inlet to outlet pressure ratios. Preliminary results suggest that the bubble can slide, stick, or detach, depending on the flow parameters. We also observe that the horizontal force acting on the bubble approaches a constant value as the simulation progresses in time. The evolution of bubble interface and it effect on the flow rate through the channel depend significantly on whether the vessel wall is hydrophilic or hydrophobic. This work is supported by NSF grant BES-0301278 and NIH grant EB003541. [Preview Abstract] |
Monday, November 21, 2005 9:57AM - 10:10AM |
FB.00010: A computational model of microbubble transport through a blood-filled vessel bifurcation Andres Calderon, Joseph Bull We are developing a novel gas embolotherapy technique to occlude blood vessels and starve tumors using gas bubbles that are produced by the acoustic vaporization of liquid perfluorocarbon droplets. The droplets are small enough to pass through the microcirculation, but the subsequent bubbles are large enough to lodge in vessels. The uniformity of tumor infarction depends on the transport the blood-borne bubbles before they stick. We examine the transport of a semi-infinite bubble through a single bifurcation in a liquid-filled two-dimensional channel. The flow is governed by the conservation of fluid mass and momentum equations. Reynolds numbers in the microcirculation are small, and we solve the governing equations using the boundary element method. The effect of gravity on bubble splitting is investigated and results are compared with our previous bench top experiments and to a quasi-steady one-dimensional analysis. The effects of daughter tube outlet pressures and bifurcation geometry are also considered. The findings suggest that slow moving bubbles will favor the upper branch of the bifurcation, but that increasing the bubble speed leads to more even splitting. It is also found that some bifurcation geometries and flow conditions result in severe thinning of the liquid film separating the bubble from the wall, suggesting the possibility bubble-wall contact. This work is supported by NSF grant BES-0301278 and NIH grant EB003541. [Preview Abstract] |
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