Evangelos Boutsianis Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland Michele Guala Institute of Environmental Engineering, ETH Zurich, 8092 Zurich, Switzerland Ufuk Olgac Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland Simon Wildermuth Institute of Diagnostic Radiology, University Hospital of Zurich, Raemistrasse 100, 8091 Zurich, Switzerland Klaus Hoyer Institute of Environmental Engineering, ETH Zurich, 8092 Zurich, Switzerland Yiannis Ventikos Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK Dimos Poulikakos 1 Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland e-mail: dimos.poulikakos@ethz.ch CFD and PTV Steady Flow Investigation in an Anatomically Accurate Abdominal Aortic Aneurysm There is considerable interest in computational and experimental flow investigations within abdominal aortic aneurysms (AAAs). This task stipulates advanced grid genera- tion techniques and cross-validation because of the anatomical complexity. The purpose of this study is to examine the feasibility of velocity measurements by particle tracking velocimetry (PTV) in realistic AAA models. Computed tomography and rapid prototyping were combined to digitize and construct a silicone replica of a patient-specific AAA. Three-dimensional velocity measurements were acquired using PTV under steady aver- aged resting boundary conditions. Computational fluid dynamics (CFD) simulations were subsequently carried out with identical boundary conditions. The computational grid was created by splitting the luminal volume into manifold and nonmanifold subsections. They were filled with tetrahedral and hexahedral elements, respectively. Grid independency was tested on three successively refined meshes. Velocity differences of about 1% in all three directions existed mainly within the AAA sack. Pressure revealed similar variations, with the sparser mesh predicting larger values. PTV velocity measurements were taken along the abdominal aorta and showed good agreement with the numerical data. The results within the aneurysm neck and sack showed average velocity variations of about 5% of the mean inlet velocity. The corresponding average differences increased for all velocity components downstream the iliac bifurcation to as much as 15%. The two do- mains differed slightly due to flow-induced forces acting on the silicone model. Velocity quantification through narrow branches was problematic due to decreased signal to noise ratio at the larger local velocities. Computational wall pressure and shear fields are also presented. The agreement between CFD simulations and the PTV experimental data was confirmed by three-dimensional velocity comparisons at several locations within the in- vestigated AAA anatomy indicating the feasibility of this approach. DOI: 10.1115/1.3002886 Keywords: abdominal aortic aneurysm, particle tracking velocimetry, computational fluid dynamics, grid generation, hemodynamics Introduction Abdominal aortic aneurysms AAAs form secondary to athero- sclerosis due to atrophy or necrosis of the arterial media. Several biomechanical parameters can be taken into account to formulate case specific indicators of the risk of rupture 1. Computational fluid dynamics CFD is used to quantify relevant hemodynamics indices based on velocity, wall shear stress WSS, and pressure within the aneurismal sack 2,3. This is a task of increased diffi- culty necessitating advanced grid generation techniques and ex- perimental cross-validation because of the complexity of the un- derlying anatomy and the need for in vivo physiological and/or pathological boundary conditions. The plausible connection of atherosclerosis with regions of low and oscillating WSS 4,5 sus- tains the wide interest in arterial fluid mechanics. Hemodynamics in the Physiologic Abdominal Aorta. Studies in the physiological abdominal aorta preceded those of AAA cases. Recent CFD investigations include Lee and Chen 6 who produced steady three-dimensional simulations in a rigid model of the abdominal aorta that included tapering, the celiac trunk, the renal and iliac bifurcations, and the superior and inferior mesen- teric arteries. Steady flow simulations revealed basic branching flow characteristics, i.e., WSS maxima located around bifurcation apexes and low WSS regions with possible recirculation along the lateral walls depending on the applied mass discharge ratios. Fur- ther regions of low WSS with or without recirculation are pre- dicted along the posterior aortic wall at the level of the diaphragm due to the visceral branches. Taylor et al. 7,8 conducted pulsatile simulations under resting and exercise conditions in a similar rigid 1 Corresponding author. Contributed by the Bioengineering Division of ASME for publication in the JOUR- NAL OF BIOMECHANICAL ENGINEERING. Manuscript received July 18, 2007; final manu- script received June 27, 2008; published online November 21, 2008. Review con- ducted by B. Barry Lieber. Journal of Biomechanical Engineering JANUARY 2009, Vol. 131 / 011008-1 Copyright © 2009 by ASME Downloaded 01 Dec 2008 to 136.142.183.22. 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