$294 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)

7371 We, 08:45-09:00 (P28) Flow behavior and blockage ef fects in stenosed arteries M. Griffith 1,3, T. Leweke 3, K. Hourigan 1 , M. Thompson 1 , W. Anderson 2. 1Fluids Laboratory for Aeronautical and Industrial Research (FLAIR), Dept. of Mechanical Engineering, and 2School of Biomedical Sciences, Monash University, Melbourne, Australia, 31RPHE, CNRS/Universit6s Aix-Marseille, France

Flows, both steady and pulsatile, through a circular tube with an axisymmetric blockage of varying size are studied experimentally and numerically. Selected as an idealized model of a stenosed artery at various points in its development, the geometry consists of a long straight tube and a blockage, semi-circular in cross-section. The stenosis has been simplified to a single-parameter blockage in order to highlight fundamental behaviors of constricted flows. Experimentally, a water flow is considered inside a tube of 19mm diameter, which has an unblocked length of 2 m both upstream and downstream of the blockage. The flow is characterized using dye visualisations and Particle Image Velocimetry. These results are complemented by spectral-element numerical simulations. The study initially looks at steady inlet flows, using them as a limiting case for pulsatile flow. At low Reynolds numbers, the flow is characterized by a jet ema- nating from the constriction, surrounded by an axisymmetric recirculation zone, the length of which increases linearly with Reynolds number. Our numerical results indicate a critical Reynolds number threshold for absolute instability, while our experiments point to the existence of convective instabilities at lower Reynolds numbers. Flows subject to a pulsatile inlet condition, which more closely describe the type of flow found in the cardio-vascular system are also investigated. With a particular focus on the effects of the blockage size, we investigate the stability of such flows, transition to turbulent flow and also other physical properties pertinent to cardiovascular fluid mechanics, such as wall shear stress.

4640 We, 09:00-09:15 (P28) Patient-specif ic MR image-based studies of stenosed carotid b i furcat ions N.B. Wood 1 , G. Soloperto 1 , S.E. Bashford 2, X.~ Xu 1 , A.D. Hughes 2, S.A. Thorn 2. 1Chemical Engineering, South Kensington Campus, Imperial College London, UK, 2 NHLI, International Centre for Circulatory Health, Imperial College London, UK

The carotid bifurcation is a site of particular interest for studies of atheroscle- rosis, since it is a site where focal atherosclerotic plaques are common. As the atherosclerotic plaque forms, it may become vulnerable to rupture, releasing emboli and promoting distal thrombosis leading to stroke. Because of its importance in arterial disease, and its easy accessibility for imaging, the biomechanics of the carotid bifurcation have been widely studied [1-2]. Moreover, because the anatomy of the region varies so much between subjects [3], patient-specific modelling is particularly important. Thirteen patients with stenoses of the left internal carotid artery (ICA) ranging from 40-50% to 70-80% (European Carotid Surgery Trial criteria) are scanned using a 2D contrast-enhanced time-of-flight (TOF) MR sequence to provide geometry data. A gated 2D cine phase contrast sequence is used to acquire velocity inlet boundary data from the common carotid artery (CCA) and velocity outlet boundary data superior to the bifurcation apex. Geometry is reconstructed using in-house software and CFD modelling performed with CFX-10 with a rigid wall assumption.. The flow simulations are carried out under patient-specific pulsatile flow conditions. The effects of turbulence on flow patterns are also investigated for some cases. Stenoses characteristically alter the flow and stress patterns in the carotid arteries, and local arterial geometry exerts important influences on shear stress patterns in individual patients.

References [1] Ku, DN., Giddens, DP, Zarins, CZ, Glagov S. Arteriosclerosis 1985; 5: 293-302. [2] Ariff B, Stanton A, Barratt DC, Augst A, Glor FP, Poulter N, Sever P, Xu XY,

Hughes AD, & Thorn SA. J. Renin Angiotensin Aldosterone Syst. 2002; 3: 116-22.

[3] Goubergrits L, Affeld K, Fernandez-Britto J, Falcon L. Biorheology 2002; 39: 519-524.

7307 We, 09:15-09:30 (P28) Unsteady and three-dimensional s imulat ion o f b lood f low in aor t ic d issect ion reconstructed from CT images M. Watanabe, T. Matsuzawa. Center for Information Science, Japan Advanced Institute of Science and Technology, Ishikawa, Japan

The aortic dissection is a kind of an arteriosclerosis disease. We have studied blood flow through the shape of an aortic dissection reconstructed from CT images [1]. An aortic dissection is a tear in the inner lining of the aorta, creating a space between the inner and outer layers called a false lumen. The pipe where the usual bloodstream flows is called a true lumen. The entrance of the dissection is called Entry and the exit of dissection is called Re-entry.

Oral Presentations

Currently, the examination of the flow in the aorta dissociation by patient's geometry just started. Under these circumstances, we thought that it would be useful to examine the stress that joins the blood vessel wall by numerical simulation, in order to changes after the onset of the disease. The inlet flow from the heart was set to the entrance of the aortic arch, and the flow was set to approximate the real profile at the entrance of heart. The maximum Reynolds number was about 6000, 3000. In results, a common flow in each case was a fast flow by the influence that the blood vessel in the true lumen narrows by the false lumen. J. Chang measured the flow in the aorta dissociation by velocity encoded cine MRI. The flow velocity-time curve by each case (with Re-Entry, without Re-Entry) was approximated well with the results of J. Chang. A high wall shearing stress was distributed in the area where the diameter of the blood vessel in a true lumen was narrow. And the wall shear stress values were high in the re-entry portion and entry portion in systole. The pressure value became high regardless of the position of entry and the dissociation of Re-entry at the systole.

References [1] M. Watanabe, T. Matsuzawa, Computational Simulation of Flow in A Dissecting

Aortic Aneurysm Reconstructed from CT Images. Proc. of 5th International Sym- posium on Computational Technologies for Fluid/Thermal/Chemical Systems with Industrial Applications, ASME 2004; pp.83-89.

[2] J. Chang, K. Friese, G.R. Caputo, C. Kondo, C.B. Higgins, MR Measurement of Blood Flow in the True and False Channel in Chronic Aortic Dissection. J. Computer Assisted Tomography, 1991.

6452 We, 09:30-09:45 (P28) Numerical simulat ion o f b lood f low in a side-to-end f istula for hemodia lys is

Z. Kharboutly 1 , J.M. Treutenaere 2, M. Fenech 1 , T. Chambon 2, I. Claude 1 , C. Legallais 1 . 1Universit6 de Technologie de Compiegne, UMR CNRS 6600, Biom6canique et G~nie Biom6dical, Compiegne, France, 2Service de Radiologie, Polyclinique Saint-C6me, Compiegne, France

Patients with end stage renal disease (ESRD) are treated by hemodialysis which requires a vascular access with high blood flow. This can be achieved by the creation of an arterio-venous fistula (AVF) through a surgical oper- ation connecting an artery to a vein. Imposing the vein into its new non physiological (high pressure) and hemodynamical environment subjects it to potential vascular remodeling and modifies its mechanical characteristics. We propose a computational fluid dynamics (CFD) investigation protocol to understand the correlation between pathology and hemodynamical changes in this patient-specific AVE Realistic 3D reconstructed AVF geometry is the result of segmenting CT angiographic images. These images are rich in information, precisely locate the calcification zones and identify the stenosis degree. As AVF is superficial, it can be easily accessed by clinicians to measure the flow and qualitatively evaluate the vascular structure. In a previous study we presented a simplified realistic end-to-end AVF at the upper arm, meshed with hexahedral elements. In this study we investigate another type of AVE side artery-to-end vein located at the forearm. The reconstructed geometry is much more realistic and preserves its complex morphology. The tortuosity of the AVF makes it unique in form and not comparable to carotid or aorta, and requires the evaluation of different meshing strategies (Hexahedral, HexCore or Tetrahedral). Simulations permit the choice of the best meshing approach after the comparison of numerical results with the patient's echo Doppler exam at predefined locations. When appropriately conducted, CFD corroborates echo Doppler data and offer additional information to this exam in the anastomosis region. Our present investigation protocol is intended to give a deeper look on the AVF hemodynamics, like recirculation zones and oscillatory shear area and help for diagnostic.

7465 We, 11:00-11:30 (P31) Patient-specif ic pu lmonary hemodynamic s imulat ions l inking lumped-parameter boundary condi t ions to morphometr ic data

R.L. Spilker 1, I.E. Vignon-Clementel 1, H.J. Kim 1, J.A. Feinstein 2, C.A. Taylor 1,3. 1Department of Mechanical Engineering, Stanford University, Stanford, CA, USA, 2Department of Pediatrics, Stanford University, Stanford, CA, USA, 3Departments of Bioengineering and Surgery, Stanford University, Stanford, CA, USA

We have previously described the solution of one-dimensional equations governing blood flow in image-based pulmonary arteries using tuned, morphometry-based impedance boundary conditions at each outlet to rep- resent the downstream arterial trees. To account for nonperiodic pulmonary hemodynamic phenomena associated with physiological changes and con- genital abnormalities, we have developed a method to replace the previous impedance boundary condition, which was tied to the periodicity of flow and pressure at each outlet, with a lumped-parameter model having an impedance spectrum that approximates that of a morphometry-based arterial

Track 14. Cardiovascular Mechanics 14.6. Computational Modelling $295

tree. A four-element lumped-parameter model was selected to approximate the morphometry-based impedance boundary condition (Stergiopulos et al. Am. J. Physiol. 276: H81-88, 1999). We applied this approach to study pulmonary arterial hemodynamics at rest and with exercise-associated vasodilation of the distal vasculature. The lumped-parameter model was coupled to each outlet of an image-based model of the proximal pulmonary arteries. The parameters of each lumped model were obtained using the Levenberg-Marquardt algorithm. This approach provides a link between the values of the parameters and changes in the morphometry of the pulmonary vasculature, such as the change in impedance due to vasodilation. A lumped-parameter model representing the entire pulmonary arterial tree was employed to determine the initial state of each outlet's downstream lumped-parameter model. Acknowledgment: This work was supported by NSF ACI-0205741.

ventricular fluid flow. In both ventricles an asymmetric growth of the symmetric initial ring-vortex can be observed and appears to be a typical characteristic of ventricular flow.

References [1] H. Oertel, Biofluid Mechanics of blood circulation, Prandtl's Essentials of Fluid

Mechanics, Ed. H. Oertel, Springer 2004. [2] H. Oertel, Modelling the Human Cardiac Fluid Mechanics. University press

Karlsruhe, 2005. [3] T. Schenkel, M. Reik, M. Malve, M. Markl, B. Jung, H. Oertel, MRI based CFD

analysis of flow in a human left ventricle. Annals of Biomedical Engineering, submitted 2006.

[4] M. Reik, G. Meyrowitz, M. Schwarz, S. Donisi, T. Schenkel, U. Kiencke, A 1D circulation model as boundary condition for a 3D simulation of pumping human ventricle. IFMBE Proceedings EMBEC'05 2005; Volume 11.

4794 We, 11:30-11:45 (P31 ) Coupling a bond graph model of the left ventricle with a CFX model of the aorta: a first approach V. Diaz-Zuccarini, D.R. Hose, P.V. Lawford. Medical Physics Department, University of Sheffield, Royal Hallamshire Hospital, Sheffield, United Kingdom

Despite significant advances in numerical techniques, the coupling of 3D physiological models of extreme complexity remains at the frontier of what can be done today with specialized software. For this reason other approaches are needed. This paper describes the first attempt to couple a simple ventricular Bond Graph model with a 3D model of the aorta. Work on the coupling of linear lumped parameter models with detailed 3D models [1], [2] has been carried out in the past with the arterial network commonly represented by Windkessel-type models. In this work, the ventricular model employed [3], [4] is non-linear and very different to the DC models widely used today. The model of the ventricle is coupled to a CFX ® model of the aorta (an elastic tube) using the CFX ® User Fortran facility. This model allows the characteristics of the pressure wave to be explored by modifying; biochemical aspects at cellular level, input conditions or time-based mechanisms. This work is part of the Marie Curie project C-CAReS for which the ultimate goal is to determine the closure force of an aortic valve as a function of patient physiology.

References [1] K. Lagan&, R. Balossino, F. Migliavacca, G. Pennati, E.L. Bove, M.R. de Leval,

G. Dubini. Multiscale modeling of the cardiovascular system: application to the study of pulmonary and coronary perfusions in the univentricular circulation. Journal of Biomechanics 2005; 38:1129-1141.

[2] D.M. Jones, D.R. Hose, EV. Lawford, D.L. Hill, R.S. Razavi, D.C. Barber. Creation of patient-specific CFD models by morphing a previously-meshed reference geometry using image registration. MIUA Proceedings, 2004 September; 173- 176.

[3] J. Lefevre, L. Lefevre, B. Couteiro. A bond graph model of the chemo-mechanical transduction in the mammalian left ventricle. SIMPRA 1999; 7: 531-552.

[4] V. Diaz-Zuccarini. Etude des Conditions d'Efficacite du Ventricule Gauche par Optimisation Teleonomique d'un Modele de son Fonctionnement. Ph.D Thesis. EC Lille, 2003.

6230 We, 11:45-12:00 (P31) Numerical flow simulation in the human heart M. Reik, M. Malv~, H. Oertel. Institute for Fluid Mechanics, University Karlsruhe, Germany

Since no in-vivo structural myocardium data of human ventricles are available, the KAHMO (KArlsruhe Heart MOdel), a patient-specific prescribed geometry model of the human heart for flow simulations, has been developed. Numerical simulations of the pulsatile flow in the pumping ventricles were per- formed. To determine the losses, a correct pressure standard is a precondition. With a simplified circulation model, which was developed using the MATLAB toolbox Simulink and coupled with the three dimensional flow simulation, the heart, consisting of left and right ventricle, atria, aorta, venae cavae and arteria pulmonalis, can be simulated with physiologically modelled boundary conditions. Flow simulations were performed using a commercial Navier-Stokes finite- volume (FVM) code (STAR-CD®). The movement of the ventricular geometry is prescribed by segmentation from MRI recording sets, generation of topo- logically identical grids for each MRI trigger step and approximation between these grids. Blood was regarded incompressible and Non-Newtonian. Since the position and time dependent shape of the valve leaflets were not extractable from MRI scans, the valves were modelled as a simplified two- dimensional planar model with temporarily and spatially variable resistance, realised by boundary conditions. The opening area is derived from echocar- diograph Doppler scans and anatomical literature. With the unsteady boundary conditions provided by the circulation model, the simulations have revealed the three dimensional fluid structures in both ventricles. For the first time, interest was focused on the bifurcation of the right

5878 We, 12:00-12:15 (P31) Computational and experimental analysis of flow in lymphatic vessels

A. Macdonald 1 , G. Tabor 1 , P. Winlove 2, K. Arkill 2, N. McHale 3. 1Schools of Engineering and 2Schools of Physics, University of Exeter, Exeter, UK, 3 Dundalk Institute of Technology, Exeter, UK

The fluid mechanics of the lymphatic system have been less extensively investigated than other components of the circulatory system, although it constitutes a complicated, efficient, active drainage network with feedback capabilities (Schmid-SchSnbein 1990, McHale 1998) and its dysfunction in conditions such as lymphoedema has serious consequences. The conducting elements comprise sequences of pumps like 'mini-hearts' separated by one- way valves. The interplay between the contractility of the segments and the function of the valves is critical in pumping against a pressure gradient and is the subject of a computational and experimental study. A ld numerical model of a section of lymphatic vessel consisting of a series of compliant chambers separated by valves has been developed from the work of Reddy et al (1975) and implemented in MATLAB Software (Version 6.5.1, release 13). The pumping behaviour has been found to be critically dependent on the properties assigned to the valves. A numerical model of valve and fluid motion has therefore been developed using FLUENT Software (Graphics Version 9.20-1, Release 6.2.16) and a realistic geometry taken from the literature. A programme of experimental work to determine the mechanical and fluid mechanical parameters required for the models and to validate their predictions has been initiated using vessel segments excised from the horse mesentery and mounted in a perfusion bath.

References Reddy NP, Krouskop TA, Newell PH (1975). The biomechanics of a lymphatic

vessel. Blood Vessels 12(5): 261-278. Schmid-Sch6nbein GW (1990). Microlymphatics and lymph flow. Physiological

Review 70(4): 987-1028. McHale NG (1998). Update on interstitial drainage and lymphatic contractility.

J. Vasc. Res. 35: 14.

6687 We, 12:15-12:30 (P31) Capillary filtration may explain the delayed onset of presyncope in astronauts during postfl ight stand tests J. Broskey, M.K. Sharp. Biofluid Mechanics Laboratory, Department of Mechanical Engineering, University of Louisville, KY, USA

Postflight orthostatic intolerance (POI), which describes the dizziness that astronauts experience when they return from spaceflight, often afflicts over half the space shuttle crew. POI delays resumption of normal activities and could have serious consequences in an emergency during landing. POI was not reported after landing in the 1/6 G (earth gravity) environment on the moon, but may occur in 3/8 G on Mars. A perhaps revealing characteristic of POI is that during 10-min stand tests conducted on landing day, the average time that nonfinishers can stand is about 7 min. Therefore, initial cardiovascular responses to upright posture are adequate, but a mechanism of accumulated deficiency eventually causes presyncope. This phenomenon was studied with a five-compartment model of the cardiovascular system. The model included 28 independent parameters, including factors characterizing cardiac performance, vascular resistance, intrathoracic pressure, nonlinear venous compliance, intravascular hydrostatic pressure and circulating blood volume, and 12 dependent parameters, including cardiac output and cardiac and vascular compartment pressures and volumes. First, parameter sensitivity of hemodynamic indicators of presyncope was studied. Results demonstrated that both cardiac output and arterial pressure were most sensitive to volume- related parameters, particularly total blood volume, and less sensitive to periph- eral resistance. Next, a simulated postflight stand test showed that plasma loss due to capillary filtration, particularly from the caudal region where transmural pressure is high during standing, is a plausible mechanism of POI that also explains the delayed onset of symptoms in nonfinishers. An accumulated drop