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 NA: Bio-Fluid Dynamics: Heart Valves |
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Chair: H.S. Udaykumar, University of Iowa Room: Hilton Chicago International Ballroom South |
Tuesday, November 22, 2005 11:01AM - 11:14AM |
NA.00001: High-fidelity large eddy simulation of blood flow through a mechanical heart valve Min Zhou, Foluso Ladeinde, Danny Bluestein Bileaflet heart valves are currently the most commonly implanted type of mechanical heart valve (MHV). However, the current designs are far from being optimal and, due to non-physiological flow characteristics, significant complications often arise after implantation. We carry out a high-fidelity large eddy simulation (LES) of blood flow through a bileaflet MHV for a better understanding of the dynamics of flow in both 2-D and 3-D models. For this purpose, we employ a sixth-order Pad\'{e} approximant compact scheme that is coupled with an eleventh-order filtering procedure for removing high wave number noise. To our knowledge, this is one of the most complicated applications of high-order CFD methods, from the standpoint of geometry, and the first attempt at simulating blood flow in 3-D mechanical heart valves using such methods, with and without the overset grid methodology. The results clearly show the need for a full 3-D model for MHV. The formation of vortices and shear layers is discussed, as are their physiological implications. [Preview Abstract] |
Tuesday, November 22, 2005 11:14AM - 11:27AM |
NA.00002: A 3D method for modeling the fluid-structure interaction of heart valves Patrick Anderson, Raoul van Loon, Frans van de Vosse The behaviour of heart valves, which should cause no resistance to the flow during systole (ejection), but need to sustain transvalvular pressure gradients during diastole, is not easily captured. Therefore, a new approach for computing the fluid-structure interaction (FSI) problems associated with flow past heart valves is developed. The primary motivation for the development of this model is to capture the large movements and deformations of the valve leaflet. Using a finite element method, an Eulerian and Lagrangian description were adopted for the constituent blood and valve, respectively, the former modeled by the instationary Navier-Stokes equation, the latter by an incompressible Neo-Hookean solid. Both domains are coupled at the boundary of the solid domain by means of a Lagrange multiplier and the resulting set of equations is fully coupled Since a closed valve separates a fluid domain into two subparts, velocity fields at either side of the valve can differ considerably. The proposed method is therefore able to compute the shear stresses along both sides of the valve. [Preview Abstract] |
Tuesday, November 22, 2005 11:27AM - 11:40AM |
NA.00003: Direct Numerical Simulation of turbulent flow induced by prosthetic heart valves Antonio Cristallo, Elias Balaras, Roberto Verzicco The complex turbulent flow patterns downstream of mechanical bileaflet valves are to a large extend responsible for the thromboembolic complications that remain a major concern after surgery. To illuminate the detailed dynamics of flow in the vicinity of such valves we performed Direct Numerical Simulations in a simplified configuration. The selected shape and size of the leaflets roughly mimics the SJM Standard bi-leaflet. Also, the housing was a straight pipe with rigid walls which expands and then contracts to mimic the geometry of the aortic root. The overall set-up resembles the one commonly used in in-vitro experiments. The computation of the fluid structure interaction problem is performed using a fully coupled, embedded boundary formulation at physiologic flowrates. The valves open at the beginning of the systole and close before the start of the diastole. The interaction of vortices originating from the leaflets and the housing dominate the flow in the downstream proximal area and are responsible for most of the production of turbulent stress. [Preview Abstract] |
Tuesday, November 22, 2005 11:40AM - 11:53AM |
NA.00004: A Fluid-Structure Interaction Model for Artificial Tissue Heart Valves Using a Sharp Interface Fixed Grid Method Sarah Vigmostad, Brian Jeffrey, Saikrishna Marella, Jia Lu, H.S. Udaykumar, Krishnan B. Chandran A tissue heart valve can be described as a deformable hyperelastic structure surrounded by a viscous fluid. The modeling of this complex system requires a fluid-structure interaction approach. A sharp interface, fixed Cartesian grid method is presented which accurately computes the motion and stresses of both the valve and the surrounding fluid. The fluid-structure interaction model couples the fluid and leaflet motions and stresses. This model has been developed to incorporate the normal and shear stresses developed in the leaflet as jumps in the pressure and shear stresses of the surrounding fluid. Stresses in the leaflet result from deformation, where the motion of the leaflet takes into account its experimentally derived material properties. A finite element solver calculates the leaflet deformation and stresses based on the conditions of surrounding fluid. Validations of the fluid-structure interaction model have been performed, and this method is currently being extended to three dimensions. [Preview Abstract] |
Tuesday, November 22, 2005 11:53AM - 12:06PM |
NA.00005: Investigation of Flow Structures Downstream of a Bileaflet Mechanical Heart Valve using Particle Image Velocimetry Juan Mejia, Peter Oshkai Turbulent flow downstream of a bileaflet mechanical heart valve mounted in a non-compliant cylindrical duct is investigated using digital particle image velocimetry. The study focuses on the forward and back flow phases of a cardiac cycle, represented by a unidirectional inflow of constant flow rate. Global quantitative images corresponding to multiple planes of data acquisition provide insight into the three-dimensional nature of the flow. Turbulent flow structures including jet-like regions, shed vortices, and recirculation regions are characterized in terms of patterns of instantaneous and time-averaged velocity, vorticity, and streamline topology. The flow downstream of the valve, during the forward flow phase, features four separated shear layers that form at the leading and trailing edges of the valve leaflets. It is shown that the large-scale transverse oscillations of these shear layers dominate the near-wake of the valve. During the back flow phase, the flow field is dominated by the jets that form at the hinges and between the closed leaflets. [Preview Abstract] |
Tuesday, November 22, 2005 12:06PM - 12:19PM |
NA.00006: Manipulation of the closing transients of bileaflet mechanical heart valves using passive, surface-mounted elements Helene Simon, Lakshmi Dasi, Ajit Yoganathan, Ari Glezer The time-periodic closing of bileaflet mechanical heart valves is accompanied by a strong flow transient that is associated with the formation of a counter-rotating vortex pair near the b-datum line of leaflet edges. The strong transitory shear that is generated by these vortices may be damaging to blood elements and may result in platelet activation. In the present work, these flow transients are mitigated using miniature vortex generator arrays that are embedded on the surface of the leaflets. The closing transients in the absence and presence of the passive vortex generators are characterized using PIV measurements that are phase locked to the leaflet motion. The study utilizes a 25 mm St. Jude Medical valve placed in the aortic position of the Georgia Tech left heart simulator. The valve is subjected to physiological flow conditions: a heart rate of 70 bpm; a cardiac output of 5 l/min; and a mean aortic pressure of 90 mmHg. Measurements of the velocity field in the center plane of the leaflets demonstrate that the dynamics of the transient vortices that precede the formation of the leakage jets can be significantly altered and controlled by relatively simple passive modifications of existing valve designs. [Preview Abstract] |
Tuesday, November 22, 2005 12:19PM - 12:32PM |
NA.00007: A novel high temporal resolution phase contrast MRI technique for measuring mitral valve flows Abram Voorhees, Katja Bohmann, Kelly Anne McGorty, Timothy Wei, Qun Chen, Vinay Pai Mitral valve flow imaging is inherently difficult due to valve plane motion and high blood flow velocities, which can range from 200 cm/s to 700 cm/s under regurgitant conditions. As such, insufficient temporal resolution has hampered imaging of mitral valve flows using magnetic resonance imaging (MRI). A novel phase contrast MRI technique, phase contrast using phase train imaging (PCPTI), has been developed to address the high temporal resolution needs for imaging mitral valve flows. The PCPTI sequence provides the highest temporal resolution to-date (6 ms) for measuring in-plane and through-plane flow patterns, with each velocity component acquired in a separate breathhold. Tested on healthy human volunteers, comparison to a conventional retrogated PC-FLASH cine sequence showed reasonable agreement. Results from a more rigorous validation using digital particle image velocimetry technique will be presented. The technique will be demonstrated \textit{in vitro }using a physiological flow phantom and a St. Jude Medical Masters Series prosthetic valve. [Preview Abstract] |
Tuesday, November 22, 2005 12:32PM - 12:45PM |
NA.00008: Characterization of Fluid Flow through a Simplified Heart Valve Model Kakani Katija, Morteza Gharib, John Dabiri Research has shown that the leading vortex of a starting jet makes a larger contribution to mass transport than a straight jet. Physical processes terminate growth of the leading vortex ring at a stroke ratio (L/D) between 3.5 and 4.5. This has enhanced the idea that biological systems optimize vortex formation for fluid transport. Of present interest is how fluid transport through a heart valve induces flutter of the valve leaflets. An attempt to characterize the fluid flow through a heart valve was made using a simplified cylinder-string system. Experiments were conducted in a water tank where a piston pushed fluid out of a cylinder (of diameter D) into surrounding fluid. A latex string was attached to the end of the cylinder to simulate a heart valve leaflet. The FFT of the string motion was computed to quantify the flutter behavior observed in the cylinder-string system. By increasing the stroke ratio, the amplitude of transverse oscillations for all string lengths increases. For the string length D/2, the occurrence of flutter coincides with the formation of the vortex ring trailing jet. [Preview Abstract] |
Tuesday, November 22, 2005 12:45PM - 12:58PM |
NA.00009: Simulating Prosthetic Heart Valve Hemodynamics: Numerical Model Development Liang Ge, Fotis Sotiropoulos, Ajit Yoganathan Since the first successful implantation of a prosthetic heart valve four decades ago, over 50 different designs have been developed including both mechanical and bio-prosthetic valves. Valve implants, however, are associated with increased risk of blood clotting, a trend which is believed to be linked to the complex hemodynamics induced by the prosthesis. To understand prosthetic valve hemodynamics under physiological conditions, we develop a numerical method capable of simulating flows in realistic prosthetic heart valves in anatomical geometries. The method employs a newly developed hybrid numerical technique that integrates the chimera overset grid approach with a Cartesian, sharp-interface immersed boundary methodology. The capabilities of the method are demonstrated by applying it to simulate pulsatile flow in both bileaflet and tri-leaflet valves moving with prescribed leaflet kinematics. [Preview Abstract] |
Tuesday, November 22, 2005 12:58PM - 1:11PM |
NA.00010: Quantifying the Incoming Jet Past Heart Valve Prostheses Using Vortex Formation Dynamics Olga Pierrakos, Pavlos Vlachos Heart valve (HV) replacement prostheses are associated with hemodynamic compromises compared to their native counterparts. Traditionally, HV performance and hemodynamics have been quantified using effective orifice size and pressure gradients. However, quality and direction of flow are also important aspects of HV function and relate to HV design, implantation technique, and orientation. The flow past any HV is governed by the generation of shear layers followed by the formation and shedding of organized flow structures in the form of vortex rings (VR). For the first time, vortex formation (VF) in the LV is quantified. Vortex energy measurements allow for calculation of the critical formation number (FN), which is the time at which the VR reaches its maximum strength. Inefficiencies in HV function result in critical FN decrease. This study uses the concept of FN to compare mitral HV prostheses in an in-vitro model (a silicone LV model housed in a piston-driven heart simulator) using Time-resolved Digital Particle Image Velocimetry. Two HVs were studied: a porcine HV and bileaflet MHV, which was tested in an anatomic and non-anatomic orientation. The results suggest that HV orientation and design affect the critical FN. We propose that the critical FN, which is contingent on the HV design, orientation, and physical flow characteristics, serve as a parameter to quantify the incoming jet and the efficiency of the HV. [Preview Abstract] |
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