Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G30: Biofluids: Arterial Flows: Curving, Branching, and Stents |
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Chair: Michael Plesniak, The George Washington University Room: Ballroom IV |
Monday, November 21, 2011 8:00AM - 8:13AM |
G30.00001: Structures in secondary flow under simple harmonic inflow in a 180 degree curved pipe model of an artery Autumn L. Glenn, Sarah L. Seagrave, Kartik V. Bulusu, Michael W. Plesniak Inward centrifuging of fluid in a 180 degree curved pipe leads to development of secondary flow vortical structures. These Dean's vortices have been widely studied in steady flows. Complex secondary flow structures were observed under (unsteady) physiological flow forcing associated with the cardiac cycle, as well as simple harmonic forcing. These structures were investigated under several simple harmonic inflow conditions with phase-locked 2-D PIV measurements to examine the formation of coherent structures in the secondary flow. Experimental velocity field data were acquired at various cross-sectional planes along the bend. Multiple vortex pairs were observed at 90 degrees into the bend for all waveforms investigated. The overarching goal of this study is to understand the effect of driving waveform characteristics, i.e. period, flow acceleration, etc. on secondary flow morphologies and to characterize these morphologies in terms of dimensionless parameters describing the flow. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G30.00002: A numerical study of steady flow through a curved tube with wavy walls Chekema Prince, Mingyao Gu, Sean Peterson Flow through curved tubes has been studied for nearly a century owing to the practical industrial applications and general academic interest. More recently, interest in curved tubes has resurfaced due to the ubiquity of curvature in the vasculature and the resulting need to accurately model arterial vessels. Previous studies have focused primarily on circular cross sections and the roles of the Dean number and curvature ratio on the flow physics. In this study we examine the effect of wavy walls, that is, axially aligned ribs extending the length of the tube, on steady flow through mildly and finite curved tubes using computational fluid dynamics. Analytical work on the subject has been limited to low Dean numbers and small bump heights, thus we primarily focus on the impact of higher Dean number with large protrusions on the flow physics. The results are compared with those in circular cross section tubes at the same Dean number. Particular attention is paid to flow characteristics of interest in the vasculature, such as wall shear stress, that have been shown to stimulate biochemical pathways that trigger cell growth. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G30.00003: Secondary flow structure detection using boundary tracing under physiological inflow through a bent pipe model for curved arteries Shadman Hussain, Kartik V. Bulusu, Autumn L. Glenn, Fangjun Shu, Michael W. Plesniak Experimental data from an investigation of secondary flows in a bent artery model under physiological inflow conditions were obtained using the PIV technique. Continuous wavelet transforms were used to resolve coherent vortical structures in cases with and without an idealized stent model. Boundaries of these structures were traced using MATLAB functions to estimate their circulation and scale. Vortical structures of different scales, possessing approximately equal strength were observed in histograms of phase-locked measurements with the stent model. A circulation threshold was established to facilitate identification of smaller-scale, high-circulation secondary flow vortical structures. A parametric study of circulation thresholding was performed to gain insight into the scales associated with secondary flow structures in curved artery models. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G30.00004: Continuous wavelet analysis of stent-induced perturbations in a bent pipe model for curved arteries Kartik V. Bulusu, Autumn L. Glenn, Fangjun Shu, Shadman Hussain, Michael W. Plesniak Secondary flow vortical structures were observed in a 180 degree circular bend under physiological flow conditions with a stent model installed upstream of the bend. Phase-locked 2-D PIV measurements were made at various cross-sectional planes along the bend. Stent-induced perturbations led to a transient flow regime with a multiplicity of vortical patterns initiated during the deceleration phase of the systolic peak (starting at t/T=0.21). An exploratory investigation of vortical scale-count metrics from continuous wavelet transforms, was performed using a Ricker wavelet. The metrics highlight the evolution of a pair of ordered, coherent, high-circulation, counter-rotating vortical structures (at t/T=0.21) into multiple, disordered, low-circulation, coherent vortical structures (by t/T=0.30). The overarching goal of this study is to create a regime map of secondary flow morphologies based on the driving physiological waveform. An approach to develop a regime map using vortical scale-count metrics is outlined. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G30.00005: The effect of waveform shape on the flow in endovascular stents Amirreza Rouhi, Matthew Ford, Ugo Piomelli, Pavlos Vlachos We studied the effects of the imposed waveform on the flow in an idealized stented artery. Two stents were used in this study, both based on commercially available geometries. The Navier-Stokes equations were solved using a cartesian staggered code, and the stents were modelled via an immersed-boundary method. The Reynolds number and Womersley number are $160$ and $2.09$ respectively. The presence of flow reversal results in the formation of vortices between the stent struts, which detach from the wall and migrate toward the center of the channel. Truncated series representations of the waveform shape were tested to determine the effect of the waveform detail on the flow dynamics. A reduction in the number of Fourier modes did not affect the time-averaged wall shear stress or the distribution of the Oscillatory Shear Index. The formation of vortical structures and their kinematics were insensitive to the geometrical detail. We examined cases with decreased percentages of flow reversal, achieved by increasing the mean flow rate while maintaing the waveform shape. We initially observed two vortex formations and liftoffs per cycle. As the duration of flow reversal was decreased, the vortex creation and migration occurred once with the vortex remaining suspended above the stent wires before disappearing. When the flow was unidirectional, no vortex creation was observed. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G30.00006: Hemodynamics and transient flow reversal in real deployed stents Ralph Metcalfe, Mircea Ionescu Restenosis rates caused by neointimal hyperplasia are relatively high ($\sim30\%$) after stent implantation in stenosed arteries. The flow around stent struts under steady and unsteady conditions using computational hemodynamics (CHD) was studied to identify contributing factors to the formation of low and oscillating wall shear stress regions that have been shown to promote endothelial dysfunction and atherosclerotic plaque formation in arteries. Datasets of the Neuroform, BxVelocity, and Taxus stents deployed in straight polymer tubes were obtained from high resolution micro computed tomography. Finite volume CHD simulations of steady and unsteady flow with and without flow reversal were performed. Stagnation zones were noticed adjacent to the strut junctions as the flow enters and exits the stent cells. The stagnation zones were larger in the case of the stents with larger strut diameter (BxVelocity, Taxus), wider strut junctions and larger angles between the struts. Unsteady flow simulations showed enhanced flow reversal with thicker struts and large regions of recirculation flow developing inside the stent at Reynolds numbers higher than 200. It was shown that alterations in blood flow due to real stent deployment (strut prolapse, junction misalignment) cannot be captured with computer generated stent models, that stent specific geometry, and time dependent flow effects can locally alter the wall shear stress and stagnation zones. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G30.00007: ABSTRACT WITHDRAWN |
Monday, November 21, 2011 9:31AM - 9:44AM |
G30.00008: Effect of rheological property on blood flow in vertebral artery branch Taegee Min, Myungjoon Kim, Taesung Kim, O-Ki Kwon Blocking of an artery is one of mechanisms for cerebral stroke development. If an important cerebral artery is occluded by any reason and if there is no sufficient collaterals, tissue ischemia occurs at brain tissues distal to the occluded artery, which is a well known clinical situation. However, in practice, ischemia or hypoperfusion has also been observed through the branches proximal to the occluded artery. The unexpected ``proximal ischemia'' is not yet known, from which patients could suffer serious complications. In the present study, two patient cases are presented to elucidate this phenomenon from the view point of fluid dynamics, especially with emphasis on the role of rheology in hemodynamics. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G30.00009: ABSTRACT WITHDRAWN |
Monday, November 21, 2011 9:57AM - 10:10AM |
G30.00010: Is the human left ventricle partially a fractal pump? Brandon Moore, Lakshmi Dasi Ventricular systolic and diastolic dysfunctions represent a large portion of healthcare problems in the United States. Many of these problems are caused and/or characterized by their altered fluid-structure mechanics. The structure of the left ventricle in particular is complex with time dependent multi-scale geometric complexity. At relatively small scales, one facet that is still not well understood is the role of trabeculae in the pumping function of the left ventricle. We utilize fractal geometry tools to help characterize the complexity of the inner surface of the left ventricle at different times during the cardiac cycle. A high-resolution three dimensional model of the time dependent ventricular geometry was constructed from computed tomography (CT) images in a human. The scale dependent fractal dimension of the ventricle was determined using the box-counting algorithm over the cardiac cycle. It is shown that the trabeculae may indeed play an integral role in the biomechanics of pumping by regulating the mechanical leverage available to the cardiac muscle fibers. [Preview Abstract] |
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