Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G16: Biofluids: Aneurysms and Thrombosis |
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Chair: Shawn Shadden, Illinois Institute of Technology Room: 28B |
Monday, November 19, 2012 8:00AM - 8:13AM |
G16.00001: Flow topology in patient-specific abdominal aortic aneurysms during rest and exercise Amirhossein Arzani, Shawn Shadden Abdominal aortic aneurysm (AAA) is a permanent, localized widening of the abdominal aorta. Flow in AAA is dominated by recirculation, transitional turbulence and low wall shear stress. Image-based CFD has recently enabled high resolution flow data in patient-specific AAA. This study aims to characterize transport in different AAAs, and understand flow topology changes from rest to exercise, which has been a hypothesized therapy due to potential acute changes in flow. Velocity data in 6 patients with different AAA morphology were obtained using image-based CFD under rest and exercise conditions. Finite-time Lyapunov exponent (FTLE) fields were computed from integration of the velocity data to identify dominant Lagrangian coherent structures. The flow topology was compared between rest and exercise conditions. For all patients, the systolic inflow jet resulted in coherent vortex formation. The evolution of this vortex varied greatly between patients and was a major determinant of transport inside the AAA during diastole. During exercise, previously observed stagnant regions were either replaced with undisturbed flow, regions of uniform high mixing, or persisted relatively unchanged. A mix norm measure provided a quantitative assessment of mixing. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G16.00002: Virtual Treatment of Basilar Aneurysms Using Shape Memory Polymer Foam J.M. Ortega, J. Hartman, J.N. Rodriguez, D.J. Maitland Numerical simulations are performed on patient-specific basilar aneurysms that are treated with shape memory polymer (SMP) foam. In order to assess the post-treatment hemodynamics, two modeling approaches are employed. In the first, the foam geometry is obtained from a micro-CT scan and the pulsatile blood flow within the foam is simulated for both Newtonian and non-Newtonian viscosity models. In the second, the foam is represented as a porous media continuum, which has permeability properties that are determined by computing the pressure gradient through the foam geometry over a range of flow speeds comparable to those of in vivo conditions. Virtual angiography and additional post-processing demonstrate that the SMP foam significantly reduces the blood flow speed within the treated aneurysms, while eliminating the high-frequency velocity fluctuations that are present prior to treatment. A prediction of the initial locations of thrombus formation throughout the SMP foam is obtained by means of a low fidelity thrombosis model that is based upon the residence time and shear rate of blood. The two modeling approaches capture similar qualitative trends for the initial locations of thrombus within the SMP foam. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G16.00003: PIV Measurement of Wall Shear Stress and Flow Structures within an Intracranial Aneurysm Model Ricky Chow, Eph Sparrow, Gary Campbell, Afshin Divani, Jian Sheng The formation and rupture of an intracranial aneurysm (IA) is a debilitating and often lethal event. Geometric features of the aneurysm bulb and upstream artery, such as bulb size, bulb shape, and curvature of the artery, are two groups of factors that define the flow and stresses within an IA. Abnormal flow stresses are related to rupture. This presentation discusses the development of a quasi-3D PIV technique and its application in various glass models at Re = 275 and 550 to experimentally assess at a preliminary level the impact of geometry and flow rate. Some conclusions are to be drawn linking geometry of the flow domain to rupture risk. The extracted results also serve as the baseline case and as a precursor to a companion presentation by the authors discussing the impact of flow diverters, a new class of medical devices. The PIV experiments were performed in a fully index-matched flow facility, allowing for unobstructed observations over complex geometry. A reconstruction and analysis method was devised to obtain 3D mean wall stress distributions and flow fields. The quasi 3D measurements were reconstructed from orthogonal planes encompassing the entire glass model, spaced 0.4mm apart. Wall shear stresses were evaluated from the near-wall flow viscous stresses. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G16.00004: CFD-based Thrombotic Risk Assessment in Kawasaki Disease Patients with Coronary Artery Aneurysms Dibyendu Sengupta, Ethan Kung, Andrew Kahn, Jane Burns, Alison Marsden Coronary aneurysms occur in 25{\%} of untreated Kawasaki Disease (KD) patients and put patients at increased risk for myocardial infarction and sudden death. Clinical guidelines recommend using aneurysm diameter $>$8mm as the arbitrary criterion for treating with anti-coagulation therapy. This study uses patient-specific modeling to non-invasively determine hemodynamic parameters and quantify thrombotic risk. Anatomic models were constructed from CT angiographic image data from 5 KD aneurysm patients and one normal control. CFD simulations were performed to obtain hemodynamic data including WSS and particle residence times (PRT). Thrombosis was clinically observed in 4/9 aneurysmal coronaries. Thrombosed vessels required twice as many cardiac cycles (mean 8.2 vs. 4.2) for particles to exit, and had lower mean WSS (1.3 compared to 2.8 dynes/cm$^{2})$ compared to vessels with non-thrombosed aneurysms of similar max diameter. 1 KD patient in the cohort with acute thrombosis had diameter $<$ 8mm. Regions of low WSS and high PRT predicted by simulations correlated with regions of subsequent thrombus formation. Thrombotic risk stratification for KD aneurysms may be improved by incorporating both hemodynamic and geometric quantities. Current clinical guidelines to assess patient risk based only on aneurysm diameter may be misleading. Further prospective study is warranted to evaluate the utility of patient-specific modeling in risk stratifying KD patients with coronary aneurysms. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G16.00005: Mechanisms of thrombolysis acceleration by cavitation Hope Weiss, Prashanth Selvaraj, Golnaz Ahadi, Arne Voie, Thilo Hoelscher, Kohei Okita, Yoichiro Matsumoto, Andrew Szeri Recent studies, in vitro and in vivo, have shown that High Intensity Focused Ultrasound (HIFU) accelerates thrombolysis, the dissolution of blood clots, for ischemic stroke. Although the mechanisms are not fully understood, cavitation is thought to play an important role in sonothrombolysis. The damage to a blood clot's fibrin fiber network from cavitation in a HIFU field is studied using two independent approaches for an embedded bubble. One method is extended to the more important scenario of a bubble outside a blood clot that collapses asymmetrically creating a jet towards the clot. There is significantly more damage potential from a bubble undergoing cavitation collapse outside the clot compared to a rapidly expanding bubble embedded within the clot structure. Also, the effects of the physical properties of skull bone when a HIFU wave propagates through it are examined by use of computer simulation. The dynamics of a test bubble placed at the focus is used in understanding of the pressure field. All other things being equal, the analysis suggests that skull thickness can alter the wave at the focus, which in turn can change the nature of cavitation bubble dynamics and the amount of energy available for clot damage. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G16.00006: Evaluation Of Hemolysis Models Using A High Fidelity Blood Model Hussein Ezzeldin, Marco de Tullio, Santiago Solares, Elias Balaras Red blood cell (RBC) hemolysis is a critical concern in the design of heart assisted devices, such as prosthetic heart valves (PHVs). To date a few analytical and numerical models have been proposed to relate either hydrodynamic stresses or RBC strains, resulting from the external hydrodynamic loading, to the expected degree of hemolysis as a function of time. Such models are based on either ``lumped'' descriptions of fluid stresses or an abstract analytical-numerical representation of the RBC relying on simple geometrical assumptions. We introduce two new approaches based on an existing coarse grained (CG) RBC structural model, which is utilized to explore the physics underlying each hemolysis model whereby applying a set of devised computational experiments. Then, all the models are subjected to pathlines calculated for a realistic PHVs to predict the level of RBC trauma. Our results highlight the strengths and weaknesses of each approach and identify the key gaps that should be addressed in the development of new models. Finally, a two-layer CG model, coupling the spectrin network and the lipid bilayer, which provides invaluable information pertaining to RBC local strains and hence hemolysis. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G16.00007: A non-discrete method for residence time calculation as an indicator of thrombus formation in cardiovascular applications Mahdi Esmaily Moghadam, Alison Marsden Cardiovascular simulations provide a promising means to predict risk of thrombosis in grafts, devices, and surgical anatomies in adult and pediatric patients. Although the pathways for platelet activation and clot formation are not fully understood, recent findings suggest that thrombosis risk correlates with the presence of recirculation regions with high residence time (RT). Current approaches for calculating RT are often based on releasing a finite number of Lagrangian particles in the flow and calculating RT by tracking their pathways. However, this method requires several simulations for a single case study, each of which requires releasing a significant number of particles, to obtain temporal and spatial convergence. In this work, we introduce a new non-discrete method, in which RT is calculated in an Eulerian non-discrete framework, using the advection-diffusion equation. Starting with an existing and a newly developed intuitive definition for the RT, the formulation for calculating RT in a region of interest is presented. The physical significance and sensitivity of each measure of RT is discussed and an extension of these definitions to a point-wise value is presented. Application to simulations of shunt insertion for single ventricle heart patients is demonstrated. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G16.00008: Computational Study of Non-Physiological Hemodynamics in the Cephalic Arch Kevin Cassel, Michael Boghosian, S.M. Javid Mahmoudzadeh, Mary Hammes Numerical simulations of the unsteady, two-dimensional, incompressible Navier-Stokes equations are performed for the flow in a two-dimensional geometry created from radiological images and Doppler flow measurements of the cephalic arch in dialysis patients with a brachiocephalic fistula (surgically placed direct arterial-venous connection). The simulations are performed before insertion of the fistula and at subsequent time intervals as the cephalic vein arterializes over a period of three to six months. A mature fistula, with increased diameter and flow rate, can exhibit Reynolds numbers that are more than one order of magnitude larger than that of the pre-fistula vein. We evaluate the effect of this increased (physiologically abnormal) Reynolds number on flow structures and wall shear stresses through the curved cephalic arch, which is a site prone to stenosis in fistula patients. The long-term goal is to investigate if the development of initimal hyperplasia and stenoses correlates with wall shear stresses or other hemodynamic variables obtained using computational hemodynamics. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G16.00009: Quantifying Turbulent Kinetic Energy in an Aortic Coarctation with Large Eddy Simulation and Magnetic Resonance Imaging Jonas Lantz, Tino Ebbers, Matts Karlsson In this study, turbulent kinetic energy (TKE) in an aortic coarctation was studied using both a numerical technique (large eddy simulation, LES) and in vivo measurements using magnetic resonance imaging (MRI). High levels of TKE are undesirable, as kinetic energy is extracted from the mean flow to feed the turbulent fluctuations. The patient underwent surgery to widen the coarctation, and the flow before and after surgery was computed and compared to MRI measurements. The resolution of the MRI was about 7x7 voxels in axial cross-section while 50x50 mesh cells with increased resolution near the walls was used in the LES simulation. In general, the numerical simulations and MRI measurements showed that the aortic arch had no or very low levels of TKE, while elevated values were found downstream the coarctation. It was also found that TKE levels after surgery were lowered, indicating that the diameter of the constriction was increased enough to decrease turbulence effects. In conclusion, both the numerical simulation and MRI measurements gave very similar results, thereby validating the simulations and suggesting that MRI measured TKE can be used as an initial estimation in clinical practice, while LES results can be used for detailed quantification and further research of aortic flows. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G16.00010: Multiscale Simulation for the Initial Stage of Thrombus Formation Shu Takagi, Satoshi Ii, Seiji Shiozaki, Norio Shimamoto, Kazuyasu Sugiyama, Yoichiro Matsumoto Thrombosis is regarded as one of the most important diseases, which cause the myocardial and cerebral infarctions. The thrombus formation is strongly related to the multiscale coupling phenomena from molecular scale protein-protein interaction to continuum scale in blood flow. Initially, platelets start aggregate at the injured vessel wall, where von Willebrand Factor (vWF) is attached. The Glycoprotein, GPIb-$\alpha $, on platelet membrane starts showing ligand-receptor interaction with this vWF and platelets start aggregating around this spot. In the present study, multiscale coupling method is developed to simulate the initial stage of thrombus formation. In this method, the molecular scale interactions between vWF and GPIb-$\alpha $ is solve using the stochastic Monte Carlo simulations and the binding force at each computational cell is calculated. Then the force is directly coupled with the continuum scale simulation using finite different method. The results illustrate that fluctuation given by the motion of red blood cells plays an important role for the platelets to adhere to the injured vessel walls. [Preview Abstract] |
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