Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session HL: Bio-Fluids: Blood Vessels |
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Chair: Omar Savas, University of California, Berkeley Room: 103A |
Monday, November 24, 2008 10:30AM - 10:43AM |
HL.00001: Vorticity dynamics in an intracranial aneurysm Trung Le, Iman Borazjani, Fotis Sotiropoulos Direct Numerical Simulation is carried out to investigate the vortex dynamics of physiologic pulsatile flow in an intracranial aneurysm. The numerical solver is based on the CURVIB (curvilinear grid/immersed boundary method) approach developed by Ge and Sotiropoulos, J. Comp. Physics, 225 (2007) and is applied to simulate the blood flow in a grid with 8 million grid nodes. The aneurysm geometry is extracted from MRI images from common carotid artery (CCA) of a rabbit (courtesy Dr.Kallmes, Mayo Clinic). The simulation reveals the formation of a strong vortex ring at the proximal end during accelerated flow phase. The vortical structure advances toward the aneurysm dome forming a distinct inclined circular ring that connects with the proximal wall via two long streamwise vortical structures. During the reverse flow phase, the back flow results to the formation of another ring at the distal end that advances in the opposite direction toward the proximal end and interacts with the vortical structures that were created during the accelerated phase. The basic vortex formation mechanism is similar to that observed by Webster and Longmire (1998) for pulsed flow through inclined nozzles. The similarities between the two flows will be discussed and the vorticity dynamics of an aneurysm and inclined nozzle flows will be analyzed.This work was supported in part by the University of Minnesota Supercomputing Institute. [Preview Abstract] |
Monday, November 24, 2008 10:43AM - 10:56AM |
HL.00002: Fluid Characteristics in Abdominal Aortic Aneurysms (AAAs) and Its Correlation to Thrombus Formation Rubing Tang, Pinhas Z. Bar-Yoseph, Juan Lasheras It has been observed that most large Abdominal Aortic Aneurysms (AAAs) develop an intraluminal thrombus as they progressively enlarge. Previous studies have suggested that the build up of the thrombus may be associated with the altered hemodynamic patterns that arise inside the AAA. We have performed a parametrical computational study of the flow patterns inside enlarging AAA to investigate the possible mechanism controlling the thrombus formation. Pulsatile blood flows were simulated in idealized models of fusiform aneurysms with different dilatation ratios and the effects of shear-activated platelet accumulation and platelet/wall interaction were evaluated based on the calculated flow fields. The platelet activation level (PAL) was determined by computing the integral over time of flow shear stresses exerted over the platelets as they are transported throughout the aneurysm. Our results have shown that the values of PAL in AAAs are in fact smaller than the maximum value obtained in a healthy abdominal aorta. However, we show that the transportation of blood cells towards the wall and the formation of stagnation points on the aneurysm's wall play more significant roles in thrombus formation than PAL. [Preview Abstract] |
Monday, November 24, 2008 10:56AM - 11:09AM |
HL.00003: Cardiovascular microbubble transport in vessel bifurcations with pulsatile flow: experimental model and theory Doug Valassis, Robert Dodde, Brijesh Eshpuniyani, J. Brian Fowlkes, Joseph Bull The behavior of long gas bubbles suspended in liquid flowing through successive bifurcations was investigated experimentally and theoretically as a model of cardiovascular bubble transport in gas embolotherapy. In this developmental cancer therapy, perflurocarbon droplets are vaporized in the vasculature and travel through a bifurcating network of vessels before lodging. The homogeneity of tumor necrosis is directly correlated with the transport and lodging of the emboli. An experimental model was used to explore the effects of flow pulsatility, frequency, gravity, and bifurcation roll angle on bubble splitting and lodging. At a bifurcation roll angle of 45-degrees, the most distinct difference in splitting ratios between three physiologic frequencies (1, 1.5, 2 Hz) was observed. As roll angle increased, lodged bubble volume in the first generation channel increased while bubble volume beyond the second bifurcation proportionately decreased. A corresponding time-dependent one-dimensional theoretical model was also developed. The results elucidate the effects of pulsatile flow and suggest the potential of gas embolotherapy to occlude blood flow to tumors. [Preview Abstract] |
Monday, November 24, 2008 11:09AM - 11:22AM |
HL.00004: Experimental study of physiological flow in a cerebral saccular basilar tip aneurysm William Tsai, Omer Savas, Jason Ortega, Duncan Maitland, David Saloner The subject matter of the research is the flow within cerebral saccular basilar tip aneurysms and exploring correlations with their growth and rupture. The flow phantom consists of an inlet pipe branching out $90^{\circ}$ into two outlets, simulating the basilar artery bifurcation and a nearly spherical dome at the flow divider simulating the aneurysm. Input flow is a physiological waveform for the basilar artery. Flow outlet branching ratios are controlled at will. Experiments are done at Reynolds numbers 221-376 and Sexl-Wormersley number 4.46. Flow visualization and particle image velocimetry are used to study velocity, vorticity, and wall shear stress. All flows can be characterized by an off-center inlet jet and a circulation region, whose transient strength and behavior depends on the outflow ratios. [Preview Abstract] |
Monday, November 24, 2008 11:22AM - 11:35AM |
HL.00005: Non-Newtonian Study of Blood Flow in an Abdominal Aortic Aneurysm with a Stabilized Finite Element Method Victor Marrero, Onkar Sahni, Kenneth Jansen, John Tichy, Charles Taylor In recent years the methods of computational fluid dynamics (CFD) have been applied to the human cardiovascular system to better understand the relationship between arterial blood flow and the disease process, for example in an abdominal aortic aneurysm (AAA). Obviously, the technical challenges associated with such modeling are formidable. Among the many problems to be addressed, in this paper we add yet another complication -- the known non-Newtonian nature of blood. In this preliminary study, we used a patient-based AAA model with rigid walls. The pulsatile nature of the flow and the RCR outflow boundary condition are considered. We use the Carreau-Yasuda model to describe the non-Newtonian viscosity variation. Preliminary results for 200K, 2M, and 8M elements mesh are presented for the Newtonian and non-Newtonian cases. The broad fundamental issue we wish to eventually resolve is whether or not non-Newtonian effects in blood flow are sufficiently strong in unhealthy vessels that they must be addressed in meaningful simulations. Interesting differences during the flow cycle shed light on the problem, but further research is needed. [Preview Abstract] |
Monday, November 24, 2008 11:35AM - 11:48AM |
HL.00006: Pulsatile flow in small arteries G.J. Brereton, A. Majid, M.M. Koochesfahani Womersley's analyses of blood flow in elastic-walled arteries assume solution forms that treat the flow as periodic and continuously unsteady. In smaller arteries such as the popliteal and anterior tibial, measurements of flow variables indicate that they are not continuously unsteady but are characterized more accurately as rapid transient motions followed by recovery towards an almost stationary state, repeated in successive cycles. When the recovery phase exceeds about half a viscous timescale, the flow comes to rest before restarting in the following cycle. As such, its solution is that of an initial value problem and not a continuously unsteady one. In this talk, we discuss the accuracy with which flows measured in these arteries are described by solutions that treat them as initial value problems. [Preview Abstract] |
Monday, November 24, 2008 11:48AM - 12:01PM |
HL.00007: Secondary Flow Structure Induced by a Multiple-Harmonic Pulsatile Waveform at Low Womersley Number in a Curved Tube Chekema Prince, Michael Plesniak, Sean Peterson Under forced harmonic oscillation at sufficiently high Womersley number, the secondary flow pattern in a tube can exhibit Lyne-type vortices, where an inviscid core in the center of the tube experiences inward centrifuging. The present study investigates the evolution of secondary flow development in a curved tube subjected to a physiologically-inspired pulsatile waveform constructed using 10 harmonics. Specifically, the study seeks to address whether a Lyne-type effect is possible at a nominally low Womersley number. Experimental data were acquired using Laser Doppler Velocimetry (LDV) and numerical simulations were conducted using Fluent. Experimental and numerical results show fully developed flow after approximately 150 degrees for all phases of the driving waveform. This is in agreement with previous studies of harmonic forcing. For the lower Reynolds number portions of the waveform, flow development can occur as early as 135 degrees according to experimental data. Lyne-type vortices were observed at portions of the waveform dominated by higher harmonics, such as the peak associated with systole. [Preview Abstract] |
Monday, November 24, 2008 12:01PM - 12:14PM |
HL.00008: Fluid-structure interaction of oscillating flow in flexible vessels Sebastian Burgmann, Sebastian Gro{\ss}e, Wolfgang Schr{\"o}der The flow field and the wall-shear stress in the respiratory and vascular system are known to be influenced by the flexibility of the walls. Up to now, most experimental biofluidic investigations have been performed in rigid models. In the present work the oscillating flow in a straight vessel with flexible walls is investigated. The model's fluid-mechanical parameters, i.e., the Reynolds and Womersley number, and structure-mechanical characteristics of the flexible wall, i.e., the Young's modulus and the material compliance, represent realistic blood flow in medium blood vessels. High-speed PIV measurements of oscillating water/glycerine flow in elastic vessels at Reynolds numbers based on the non-dilated vessel diameter D and peak velocities Re$_{D}$ range from 1,000 to 1,750. The Womersley number $\alpha $ has been set to 5-15. The measurements are performed using a refractive-index adapted PIV system. The evolution of the unsteady flow field and the dilation of the flexible wall at the different combinations of Reynolds and Womersley numbers will be discussed. Furthermore, findings for the unsteady development of the wall-shear stress will be elucidated. The results are juxtaposed to Womersley's analytic solution and to experimental results of oscillating flow in rigid pipes. [Preview Abstract] |
Monday, November 24, 2008 12:14PM - 12:27PM |
HL.00009: Numerical study of pulsatile flow in s-shaped vessels Kyung Eun Lee, Jung Yul Yoo Blood flow in arteries is dominated by unsteady flow phenomena. The objective of this study is to better understand the effects of vascular geometry and the effect of pulsatility on flow in a double bend geometry which are chosen to loosely model a right coronary artery or femoral artery, neglecting branches. The three-dimensional numerical computations at the Reynolds numbers Re = 125 and 500 and Womersley numbers Wo = 4 and 8, were performed using imcompressible Navier Stokes solver which is based on spectral/hp element method. Pulsatility causes the Lean vortex pattern rather than Dean vortex pattern. Further, pulsatile flow through a double bend can induce more complicated secondary flows. In this numerical study we analyse the haemodynamics in terms of various mechanical factors (i.e., velocity, secondary flows, vorticity, coherent vortical structure and wall shear stress). [Preview Abstract] |
Monday, November 24, 2008 12:27PM - 12:40PM |
HL.00010: Optimal transient disturbances in stenotic flows Hugh Blackburn, Spencer Sherwin, Dwight Barkley Separating flows in stenotic geometries can have extended shear layers that are extremely sensitive to disturbance, leading to very large disturbance energy growth, even if the flow is stable in the large time limit. By adapting numerical methods for analysis of linear transient disturbance growth to cope with arbitrary domain geometries, we have been able for the first time to compute transient growth in comparatively simple (axisymmetric) stenotic geometries, for both steady and pulsatile inflows. At Reynolds numbers well below the stability limit, we find that extremely large (e.g. eight-to-ten magnitude) energy growth is possible for optimal disturbances. For the flows examined, optimal disturbances take the form of sinuous waves that grow on initially axisymmetric shear layers, and exhibit characteristics of local convective instability. The practical implication is that transient response to perturbation, rather than flow instability, is likely to dominate observed features for physiological studies of many separated flows. [Preview Abstract] |
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