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 LA: Bio-Fluid Dynamics: Aneurysms |
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Chair: Juan Lasheras, University of California, San Diego Room: Hilton Chicago International Ballroom South |
Tuesday, November 22, 2005 8:00AM - 8:13AM |
LA.00001: Vascular Dynamics of a SMP Foam Aneurysm Treatment Technique Jason Ortega, Duncan Maitland, William Tsai, \"{O}mer Sava\c{s}, David Saloner An aneurysm treatment technique currently under development at LLNL is modeled with CFD simulations. This treatment technique involves the catheter delivery of a piece of shape memory polymer (SMP) foam to an aneurysm. When the foam is heated by laser radiation from a fiber-optic cable embedded within the catheter, the foam expands, filling the aneurysm volume. To determine the effects of the foam upon the arterial walls, the coupled Navier-Stokes and thermal energy equations are solved for steady flow through a generic basilar aneurysm for pre-, during-, and post-treatment stages. Two steady flowrates are applied to the basilar artery corresponding to the mean and peak flowrates over a cardiac cycle. For the mean flowrate boundary condition, the flow through the aneurysm and bifurcation is steady for all treatment stages. However, at the peak systole, the shear layers from the confined jet entering the untreated aneurysm become unstable and roll up into pairs of counter- rotating vortex hoops. The pressure within the aneurysm becomes unsteady as series of these hoop pairs advect into the aneurysm dome. For the post-treatment stage in which the aneurysm is completely filled, the foam eliminates all flow unsteadiness. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. UCRL-ABS-213657. [Preview Abstract] |
Tuesday, November 22, 2005 8:13AM - 8:26AM |
LA.00002: Aortic aneurysms : a mechanical instability? Virginie Duclaux, Francois Gallaire, Christophe Clanet We study experimentally a system composed of an elastic membrane in which water is periodically flushed from a heart-like pump. Our first interest is the occurrence of waves as opposed to an homogeneous mode of deformation of our membrane. These modes may explain differences in the localization of aneurysms in the body. Then we show the case of an inhomogeneous membrane (in terms of Young's modulus) submitted to our flow. In a certain range of parameters, our fake artery is able to gradually develop a swollen region that we call aneurysm, and whose cause is mechanical. [Preview Abstract] |
Tuesday, November 22, 2005 8:26AM - 8:39AM |
LA.00003: Distribution of Wall Stress in Abdominal Aortic Aneurysm (AAA) Juan Lasheras, Rubing Tang, Pierre Badel, Javier Rodriguez, Christian Geindreau Abdominal aortic aneurysm (AAA) rupture is believed to occur when the mechanical stress acting on the wall exceeds the strength of the wall tissue. Therefore, knowledge of the AAA wall stress distribution could be useful in assessing its risk of rupture. In our research, a finite element analysis was used to determine the wall stresses both in idealized models and in a real clinical model in which the aorta was considered isotropic with nonlinear material properties and was loaded with a given pressure. In the idealized models, both maximum diameter and asymmetry were found to have substantial influence on the distribution of the wall stress. The thrombus inside the AAA was also found to help protecting the walls from high stresses. Using CT scans of the AAA, the actual geometry of the aneurysm was reconstructed and we found that wall tension increases on the flatter surface (typically corresponds to the posterior surface) and at the inflection points of the bulge. In addition to the static analysis, we also performed simulations of the effect of unsteady pressure wave propagation inside the aneurysm. [Preview Abstract] |
Tuesday, November 22, 2005 8:39AM - 8:52AM |
LA.00004: In Vivo Quantification of Flow Dynamics in Intracranial Aneurysms Using Intra-Operative Contrast-Specific Ultrasound and PIV Techniques G\'{a}dor Cant\'{o}n, Javier Rodriguez-Rodriguez, Thilo H\"{o}elscher, Juan C. Lasheras The goal of this study is to assess in vivo the hemodynamics of intracranial aneurysms using ultrasound and Digital Particle Imaging Velocimetry (DPIV) techniques. An ultrasound machine, equipped with an intra-operative transducer, was used to visualize the flow features inside an aneurysm with the aid of microbubbles as ultrasound contrast agent. The ultrasound studies were done using a Phase Inversion technique. Operating in the Doppler mode, the flow velocities in the afferent and the downstream vascular segments as well as inside the aneurysm were recorded and assessed. We analyzed the ultrasound data sets with a DPIV technique using backscattered signals from the microbubbles. The spatial and temporal distribution of velocity, vorticity, and stress fields was measured. These quantities were also measured in an in vitro aneurysmal model using the DPIV system. Our study shows that an advanced contrast-specific ultrasound technique in combination with a DPIV technique can be used to quantify in real time the flow-mechanical patterns inside the aneurysm. [Preview Abstract] |
Tuesday, November 22, 2005 8:52AM - 9:05AM |
LA.00005: Experimental study of a flow in a saccular aneurysm William Tsai, Omer Savas, Duncan Maitland, Jason Ortega, David Saloner Better understanding of cerebral aneurysms will lead to better techniques for their detection and treatment. Presently, a majority of patients who suffer hemorrhaging as a result of a ruptured cerebral aneurysm experience reduced quality of life, long term brain damage, or death. We will present research examining the underlying fluid dynamics of cerebral aneurysms. Experiments are conducted on a simplified model of a saccular cerebral tip aneurysm at a bifurcation. Flow visualization and PIV are used at steady physiological input flow conditions. The results show the formation of confined jets and vortical structures are observed within the aneurysm dome. The final goal of this research is to characterize the effects of laser-activated shape memory polymer devices on blood flow and the aneurysm. [Preview Abstract] |
Tuesday, November 22, 2005 9:05AM - 9:18AM |
LA.00006: Effect of Turbulence on the Temporal Variation of Hemodynamic Stresses in Aneurysm Model under Resting and Exercise Conditions Khalil Khanafer, Ramon Berguer, Joseph Bull A numerical model is developed to analyze pulsatile turbulent flow in axisymmetric abdominal aortic aneurysm models (AAM) using realistic physiological resting and exercise waveforms. The transport equations are solved using the finite element formulation based on the Galerkin method of weighted residuals. The $\kappa -\varepsilon $ model is used in this work to simulate turbulence characteristics of the convective flow by incorporating Boussinesq eddy-viscosity model. A number of interesting features of the flow field resulting from using realistic physiological waveforms are obtained for various pertinent parameters. Such parameters include Reynolds number, size of aneurysm, flexibility of aneurysm's wall, and the propagation of pressure and flow waves through AAM. The effect of non-Newtonian behavior of blood on hemodynamic stresses and compared with Newtonian behavior through AAM is investigated in the present study. The results of the present work illustrate that maximum turbulent shear stress occurs at the distal end of the AAA model. Furthermore, turbulence is found to have a significant effect on the pressure distribution along AAA wall for both physiological waveforms. This work paves the road for researchers in the area of AAA risk rupture to improve their understanding on the mechanics of aneurysm rupture enhanced by increased flow turbulence. [Preview Abstract] |
Tuesday, November 22, 2005 9:18AM - 9:31AM |
LA.00007: The effect of hemodynamics on the failure of endovascular coiling in cerebral aneurysms Kyung Se Cha, Elias Balaras, Baruch B. Lieber Today the treatment of intracranial aneurysms with endovascular coils is an established procedure which has several advantages compared to surgical clipping. However, coil compaction with recanalization remains a long term problem and is observed in approximately $50\%$ of large and giant aneurysm cases over a 5-6 year follow-up period. Clinical data suggest that the coil packing density and the location and size of the aneurysm are important parameters in the long term outcome, suggesting that the repeated impulses exerted by the impingement of the pulsatile blood flow on the coil are mainly responsible for coil compaction. To test this hypothesis we will present: 1. patient specific simulations of two different clinical cases having high and low coil compaction risk respectively; 2. a systematic study on the effects of various geometrical parameters (bifurcation angle, ratio of aneurysm neck size to parent vessel diameter) on the magnitude of the total force on the coil, using idealized configurations. In all cases the three-dimensional laminar flow computations have been carried out using an unstructured, finite-element, Navier-Stokes solver. It will be shown that the ratio of aneurysm neck size to parent vessel diameter has the largest influence on the maximum force on the coil, which is less sensitive to the bifurcation angle. [Preview Abstract] |
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