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 D13: Biofluids: Native and Prosthetic Cardiac Valves |
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Chair: Morteza Gharib, California Institute of Technology Room: 316 |
Sunday, November 20, 2011 2:10PM - 2:23PM |
D13.00001: The Decay of Vortex Ring Circulation in Left Ventricular Filling Kelley Stewart, Sunghwan Jung, William Little, Pavlos Vlachos Radially confined vortex rings have been previously investigated and shown to exhibit a decay in circulation after vortex ring pinch-off. A semi-empirical model for the evolution of the vortex ring circulation subject to the effect of confinement was previously developed and displayed strong agreement with experimental observations. In this work the model was applied to clinical phase contrast Magnetic Resonance Imaging data to track and potentially predict the rate of vortex ring circulation decay within the filling left ventricle (LV) subject to changing physiological characteristics in normal and diseased conditions. From our previous work and clinical observations, we hypothesize that variations in vortex ring dynamics within the LV are caused by changes in ventricular geometry. Therefore impaired ventricular relaxation causes a decreased LV volume during filling, results in a more rapid decay of early diastolic hydrodynamic circulation. This model may be used to more completely understand the filling dynamics and potentially lead to improved diagnostic techniques. [Preview Abstract] |
Sunday, November 20, 2011 2:23PM - 2:36PM |
D13.00002: Measuring Heart Filling Propagation Velocity using the Cross Wavelet Transform Casandra Niebel, Takahiro Ohara, Pavlos Vlachos, William Little During early diastole, a pressure gradient is formed across the mitral valve as the left ventricle (LV) relaxes, forcing blood from the left atrium into the LV. This process generates a rapid filling wave and creates an unsteady flow environment within the LV. A continuous wavelet transform is capable of dealing with non-stationary and noisy signals and is therefore ideal for measuring the wave speed of the early diastole rapid filling wave. This wave speed, or propagation velocity (Vp), is used clinically to evaluate diastolic function and is conventionally measured from a Color M-Mode (CMM) echocardiogram. A CMM scan gives a spatiotemporal map of the blood velocity in the left ventricle and is used to visualize flow patterns and manually measure the Vp. In this work, a moving cross wavelet transform is used to measure the phase shift between consecutive time steps in a CMM echocardiogram, providing a more robust and repeatable measurement of Vp, less sensitive to noise, aliasing boundaries, and user inputs. [Preview Abstract] |
Sunday, November 20, 2011 2:36PM - 2:49PM |
D13.00003: Experimental investigation of the flow field past a bileaflet mechanical heart valve in pulsatile flow within an anatomical aorta model Laura Brown, Stavros Tavoularis A bileaflet mechanical heart valve (BMHV) has been mounted at the inlet of an anatomical model of the human aorta, and placed within a mock circulation loop that simulates physiological flow conditions. The working fluid matches the refractive index of silicone, from which the aorta model and other parts of the test section are made, and the viscosity of blood. Flow characteristics past the BMHV are measured using stereoscopic and planar particle image velocimetry and laser Doppler velocimetry. In contrast to previous experiments, in which heart valves have been tested in simplified aortic geometries, this arrangement permits the study of the dependence of flow past the valve upon recirculation in the sinuses of Valsalva, the flow rate through the coronary arteries, and the aorta curvature. The effect of valve orientation will also be investigated with the objective to determine a hemodynamically optimal configuration with potential benefits to implantation procedures. The measured viscous shear stress distribution will be analyzed towards predicting the initiation of thrombosis in patients and identifying regions of stagnation, which could facilitate thrombus attachment. [Preview Abstract] |
Sunday, November 20, 2011 2:49PM - 3:02PM |
D13.00004: Deformation Differences between Tricuspid and Bicuspid Aortic Valves in Vitro Kai Szeto, Javier Rodriguez-Rodriguez, Peter Pastuszko, Vishal Nigam, Juan C. Lasheras It has been shown in clinical studies that patients with congenital bicuspid aortic valves (CBAVs) develop degenerative calcification of the leaflets at young ages compared to patients with the normal tricuspid aortic valves (TAVs). It has been hypothesized that the asymmetrical geometry of the leaflets in CBAVs, flow shear stresses (SS), disturbed flow, and excessive strain rate levels are possible causes for the early calcification and stenosis. Central to the validation of this hypothesis is the need to quantify the differences in strain rate levels between the BAVs and TAVs. We simulate the CBAVs by surgically stitching two of the leaflets of a porcine aortic valve together. To quantify strain differences, we performed in-vitro experiments in both trileaflet and bileaflet valves by tracking the motion of small ink dots marked on each leaflet surface. We then used phase-locked stereo photogrammetry to reconstruct at each instant of time the 3D surface of the leaflets and measure the strain rates in both radial and circumferential directions during the whole cardiac cycle. Our results indicate that the total strain rate of the simulated BAVs is about 15 to 20\% higher than the normal leaflets of TAVs at systole. In the BAVs' case, the fused leaflet stretches radially up to 25\% higher than the reference length. The excessive stretching in both directions in the fused leaflet results in large changes in the flow patterns and associated wall SS. [Preview Abstract] |
Sunday, November 20, 2011 3:02PM - 3:15PM |
D13.00005: Bicuspid aortic valve hemodynamics: a fluid-structure interaction study Santanu Chandra, Clara Seaman, Philippe Sucosky The bicuspid aortic valve (BAV) is a congenital defect in which the aortic valve forms with two leaflets instead of three. While calcific aortic valve disease (CAVD) also develops in the normal tricuspid aortic valve (TAV), its progression in the BAV is more rapid. Although studies have suggested a mechano-potential root for the disease, the native BAV hemodynamics remains largely unknown. This study aimed at characterizing BAV hemodynamics and quantifying the degree of wall-shear stress (WSS) abnormality on BAV leaflets. Fluid-structure interaction models validated with particle-image velocimetry were designed to predict the flow and leaflet dynamics in idealized TAV and BAV anatomies. Valvular function was quantified in terms of the effective orifice area. The regional leaflet WSS was characterized in terms of oscillatory shear index, temporal shear magnitude and temporal shear gradient. The predictions indicate the intrinsic degree of stenosis of the BAV anatomy, reveal drastic differences in shear stress magnitude and pulsatility on BAV and TAV leaflets and confirm the side- and site-specificity of the leaflet WSS. Given the ability of abnormal fluid shear stress to trigger valvular inflammation, these results support the existence of a mechano-etiology of CAVD in the BAV. [Preview Abstract] |
Sunday, November 20, 2011 3:15PM - 3:28PM |
D13.00006: The effect of mitral orifice eccentricity on the left ventricular hemodynamics Trung Le, Fotis Sotiropoulos We investigate the left ventricular hemodynamics using high resolution Direct Numerical Simulation. The LV geometry is reconstructed from Magnetic Resonance Imaging (MRI) data of a healthy volunteer. The diastolic kinematics of the LV wall is modelled using a cell-based electrical activation methodology, which yields global left-heart motion parameters well within the physiologic range of an adult. By prescribing the kinematics and the physiologic mitral valve waveform, numerical simulations are carried out to investigate the intraventricular flow patterns during the diastolic filling. The results show that the intraventricular flow is dominated by the formation and breakdown of a vortex ring originating from the mitral orifice. The eccentricity of the mitral orifice is found to be the determining factor controlling the dynamics of vortex formation and rotational flow patterns at the end of diastole. [Preview Abstract] |
Sunday, November 20, 2011 3:28PM - 3:41PM |
D13.00007: Biofluid Dynamics in Cardiovascular System Hansol Chung, Su Jung Yoo, Richard Kyung Biofluid dynamics is characterized by the study of fluids in biological systems. Common biofluid systems include blood flow in the cardiovascular system and airflow in the lungs. The mathematical modeling of blood flow through the complex geometry of a prosthetic heart valve is a difficult task. In such a problem the complex geometries of the valve must be modeled properly so that they can be studied numerically. The present analysis is performed on a disk-type prosthetic heart valve. The valve is assumed to be in the aortic position and observed the structure of the valve cage influence the flow field near an aortic valve. For the purpose of mathematical modeling, the laminar incompressible two-dimensional steady flow of a homogeneous Newtonian fluid with constant viscosity is assumed. The flow is considered during the greater part of systole when the valve is fully open. Convergent numerical solutions are obtained for Reynolds numbers of 30, 180, 900 and 4500. Stream function, horizontal velocity, vertical velocity and shear stress solutions are computed at every grid point. [Preview Abstract] |
Sunday, November 20, 2011 3:41PM - 3:54PM |
D13.00008: Comparison of tricuspid and bicuspid aortic valve hemodynamics under steady flow conditions Clara Seaman, James Ward, Philippe Sucosky The bicuspid aortic valve (BAV), a congenital valvular defect consisting of two leaflets instead of three, is associated with a high prevalence of calcific aortic valve disease (CAVD). CAVD also develops in the normal tricuspid aortic valve (TAV) but its progression in the BAV is more severe and rapid. Although hemodynamic abnormalities are increasingly considered potential pathogenic contributor, the native BAV hemodynamics remain largely unknown. Therefore, this study aims at comparing experimentally the hemodynamic environments in TAV and BAV anatomies. Particle-image velocimetry was used to characterize the flow downstream of a native TAV and a model BAV mounted in a left-heart simulator and subjected to three steady flow rates characterizing different phases of the cardiac cycle. While the TAV developed a jet aligned along the valve axis, the BAV was shown to develop a skewed systolic jet with skewness decreasing with increasing flow rate. Measurement of the transvalvular pressure revealed a valvular resistance up to 50{\%} larger in the BAV than in the TAV. The increase in velocity between the TAV and BAV leads to an increase in shear stress downstream of the valve. This study reveals strong hemodynamic abnormalities in the BAV, which may contribute to CAVD pathogenesis. [Preview Abstract] |
Sunday, November 20, 2011 3:54PM - 4:07PM |
D13.00009: Deformation of a membrane in a pulsatile flow: implications in heart valve design C. Hernandez, J.E.V. Guzman, R. Zenit Current designs of heart valves prosthetics have serious disadvantages and health issues for patients who use them. For this reason, a new design that combines durability (mechanical valves) and biocompatibility (biological valves) has to be conceived. Natural valves have very complex geometry because their leaflets have two principal curvatures, one imposed by the holding ring and a second one imposed by the bending of the closing arrangement. The objective of this research is to study the effects of both curvatures on the performance of a leaflet. It is well known that the increase of the curvature results in a larger stiffness, which, in turn, reduces the deflection of a leaflet. We conducted a study to determine the effect of changing the curvature (in two directions) of a flexible membrane when exposed to a steady and pulsatile flows. A study of the flow field that results from this interaction is also conducted by PIV measurements. Preliminary results of the leaflet deflection for many stiffnesses, curvatures and flow conditions will be presented and discussed. [Preview Abstract] |
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