Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session G7: Biofluids: Aneurysms |
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Chair: Vitaliy Rayz, University of California, Berkeley Room: 3012 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G7.00001: Vascular growth and remodeling coupled with fluid simulation in patient specific geometry Jiacheng Wu, Shawn C. Shadden In this talk, we propose a computational framework to couple vascular growth and remodeling (G\&R) with fluid simulation in 3D patient specific geometry. Hyperelastic and anisotropic properties are considered for the vessel wall material. A constrained mixture model is used to represent multiple constituents in the vessel wall. The coupled simulation is divided into two time scales, the longer time scale for G\&R and the shorter time scale for fluid dynamics simulation. G\&R is simulated to determine the boundary of the fluid domain, the fluid simulation in turn generates wall shear stress and transmural pressure data that regulates G\&R. To minimize required computation cost, fluid is only simulated when G\&R causes significant vascular geometric change. This coupled model can be used to study the influence of the stress-mediated law parameters on the stability of the vascular tissue growth, and predict progression of vascular diseases such as aneurysm expansion. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G7.00002: A Parallel Monolithic Approach for Fluid-Structure Interaction in a Cerebral Aneurysm Mehmet Sahin, Ali Eken A parallel fully-coupled approach has been developed for the fluid-structure interaction problem in a cerebral artery with aneurysm. An Arbitrary Lagrangian-Eulerian formulation based on the side-centered unstructured finite volume method is employed for the governing incompressible Navier-Stokes equations and the classical Galerkin finite element formulation is used to discretize the constitutive law for the Saint Venant-Kirchhoff material in a Lagrangian frame for the solid domain. The time integration method for the structure domain is based on the energy conserving mid-point method while the second-order backward difference is used within the fluid domain. The resulting large-scale algebraic linear equations are solved using a one-level restricted additive Schwarz preconditioner with a block-incomplete factorization within each partitioned sub-domains. The parallel implementation of the present fully coupled unstructured fluid-structure solver is based on the PETSc library. The proposed numerical algorithm is initially validated for several classical benchmark problems and then applied to a more complicated problem involving unsteady pulsatile blood flow in a cerebral artery with aneurysm as a realistic fluid-structure interaction problem encountered in biomechanics. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G7.00003: Numerical predictions of hemodynamics following surgeries in cerebral aneurysms Vitaliy Rayz, Michael Lawton, Loic Boussel, Joseph Leach, Gabriel Acevedo, Van Halbach, David Saloner Large cerebral aneurysms present a danger of rupture or brain compression. In some cases, clinicians may attempt to change the pathological hemodynamics in order to inhibit disease progression. This can be achieved by changing the vascular geometry with an open surgery or by deploying a stent-like flow diverter device. Patient-specific CFD models can help evaluate treatment options by predicting flow regions that are likely to become occupied by thrombus (clot) following the procedure. In this study, alternative flow scenarios were modeled for several patients who underwent surgical treatment. Patient-specific geometries and flow boundary conditions were obtained from magnetic resonance angiography and velocimetry data. The Navier-Stokes equations were solved with a finite volume solver Fluent. A porous media approach was used to model flow-diverter devices. The advection-diffusion equation was solved in order to simulate contrast agent transport and the results were used to evaluate flow residence time changes. Thrombus layering was predicted in regions characterized by reduced velocities and shear stresses as well as increased flow residence time. The simulations indicated surgical options that could result in occlusion of vital arteries with thrombus. Numerical results were compared to experimental and clinical MRI data. The results demonstrate that image-based CFD models may help improve the outcome of surgeries in cerebral aneurysms. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G7.00004: Influence of transport on thrombogenic potential in cardiovascular flows Kirk B. Hansen, Shawn C. Shadden Intraluminal thrombus is a common complication inside aneurysms, as well as inside hearts with pumping or rhythmic deficiencies. The mechanisms of thrombus formation in these scenarios remain unclear. As opposed to stenotic thrombosis, where shear-induced platelet activation likely plays a major role, the shear stresses in aneurysmal flows are typically lower than in the surrounding vasculature. This suggests an alternative mechanism more akin to the stasis-driven coagulation typically seen on the venous side of the cardiovascular system. In this work, we investigate how transport properties of the complex flow features inherent to aneurysmal (or intracardiac) flows may affect the potential for thrombus formation. Patient-specific three-dimensional velocity fields are obtained using image-based computational fluid dynamics. These velocity fields are then coupled to a computational thrombosis model based on a system of reaction-advection-diffusion equations. This continuum-based model accounts for the transport and activation of platelets, as well as the transport and reaction of other chemical species involved in the coagulation cascade. Near-wall values of thrombin concentration are used as a metric for localized thrombogenic potential. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G7.00005: Hemodynamic Changes in Treated Cerebral Aneurysms and Correlations with Long-Term Outcomes Patrick McGah, Michael Barbour, Michael Levitt, Louis Kim, Alberto Aliseda The hemodynamic conditions in patients with cerebral aneurysms undergoing treatment, e.g. flow diverting stents or coil embolization, are investigated via computational simulations. Patient-specific 3D models of the vasculature are derived from rotational angiography. Patient-specific flow and pressure boundary conditions are prescribed utilizing intravascular pressure and velocity measurements. Pre-treatment and immediate post-treatment hemodynamics are studied in eight cases so as to ascertain the effect of the treatment on the intra-aneurysmal flow and wall shear stress. We hypothesize that larger reductions in intra-aneurysmal inflow and wall shear stress after treatment are correlated with an increased likelihood of aneurysmal occlusion and treatment success. Results indicate reductions of the intra-aneurysmal inflow and wall shear stress in all cases. Preliminary clinical six-month follow-up data, assessing if the treatment has been successful, shows that the cases with a persistent aneurysm had a smaller reduction in inflow and wall shear stress magnitude in the immediate post-treatment conditions. This suggests that CFD can be used to quantify a treatment's probability of success by computing the change in pre- and- post-treatment hemodynamics in cerebral aneurysms. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G7.00006: Experimental Study of Fluid Flow in an Aneurysm for Varying Shape Indices Paulo Yu, Cyrus Choi, Vibhav Durgesh An aneurysm is an abnormal bulging of a blood vessel wall. A ruptured aneurysm can be severely debilitating or fatal. There is a lack of understanding of fluid flow parameters leading to aneurysm growth and rupture. Clinical studies have shown that certain aneurysm shape indices are strongly correlated to rupture. The overall goal of this study is to comprehensively characterize fluid dynamics parameters inside an aneurysm sac, for varying shape indices. As part of this work, two different idealized aneurysm glass models are used, and an in-house flow loop system has been developed to simulate constant and physiological pressure gradients. Index of refraction matching techniques have been used for accurate estimation of fluid flow parameters. Laser Doppler Velocimetry measurements are conducted for Reynolds number values from 10-200 to understand impact of inflow conditions on flow structures and parameters inside aneurysm sac. Particle Image Velocimetry measurements are performed on several horizontal and vertical planes inside aneurysm sac and show the presence of secondary fluid structures inside the sac, not observed in mid-plane measurements from earlier studies. The results show dependence of flow parameters/structures on aneurysm shape and inflow conditions. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G7.00007: FSI and CFD Modeling of Cerebral Aneurysm Model and Comparing to PIV Experiments Zhaopeng Wang, Qing Hao Wall shear stress or strain is considered as an important factor for cerebral aneurysm growth and even rupture. The objective of present study is to evaluate wall shear stress in aneurysm sac and neck by Fluid Structure Interaction (FSI) and solid wall Computational Fluid Dynamics (CFD) approaches and compare the simulation results against Particle Image Velocimetry (PIV) experimental data from an elastic in vitro aneurysm model. The FSI and CFD simulation results showed that both approaches captured the flow patterns inside the aneurysm sac under pulsatile flow, that in diastole time period the flow inside the aneurysm sac was a stable circular clock-wise flow; when higher velocity entered into the aneurysm sac during systole and in a short diastole time period an anti-clock circular flow pattern emerged near the distal neck. Both approaches showed that the shear stress near the proximal neck experienced higher shear stress than the distal neck, while in the aneurysm dome the shear stress was always the lowest. In this study, we also showed that shear stress values at proximal neck and distal neck from FSI approach were lower than solid wall CFD approach. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G7.00008: Experimental Study of a Thoracic Aortic Aneurysm Prior to and After Surgical Repair Hemodynamics Anna-Elodie Kerlo, Steven Frankel, Jun Chen, Pavlos Vlachos Once a Thoracic Aortic Aneurysm (TAA) is detected, the risk of rupture is estimated based on the TAA diameter compared to the normal aortic diameter and its expansion rate. However, there are no reliable predictors that can provide accurate prognosis, and each aneurysm may progress differently. This work aims to assess the hemodynamic characteristics and flow structures associated with TAAs. The flow in a patient specific thoracic aortic aneurysm is compared to the same patient after treatment, in order to quantify the differences in the hydrodynamic forces acting on the aneurysm. Flow visualization with dye and Particle Image Velocimetry (PIV) are used to study flow features within both geometries. Local flow patterns are visualized to predict potential areas of recirculation and low shear stresses as they are associated with thrombogenicity. Understanding the differences in flow features between a thoracic aortic aneurysm and a normal aorta (or a TAA after surgical repair) may lead to a better understanding of disease mechanisms that will enable clinicians to better estimate the risk of rupture. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G7.00009: Transluminal Attenuation Gradient for Thrombotic Risk Assessment in Kawasaki Disease Patients with Coronary Artery Aneurysms Noelia Grande Gutierrez, Andrew Kahn, Jane Burns, Alison Marsden Kawasaki Disease (KD) can result in coronary aneurysms in up to 25{\%} of patients if not treated early putting patients at risk of thrombus formation, myocardial infarction and sudden death. Clinical guidelines for administering anti-coagulation therapy currently rely on anatomy alone. Previous studies including patient specific modeling and computer simulations in KD patients have suggested that hemodynamic data can predict regions susceptible to thrombus formation. In particular, high Particle Residence Time gradient (PRTg) regions have shown to correlate with regions of thrombus formation. Transluminal Attenuation Gradient (TAG) is determined from the change in radiological attenuation per vessel length. TAG has been used for characterizing coronary artery stenoses, however this approach has not yet been used in aneurysmal vessels. The aim of this study is to analyze the correlation between TAG and PRTg in KD patients with aneurysms and evaluate the use of TAG as an index to quantify thrombotic risk. Patient specific anatomic models for fluids simulations were constructed from CT angiographic image data from 3 KD aneurysm patients and one normal control. TAG values for the aneurysm patients were markedly lower than for the non-aneurysmal patient (mean -18.38 vs. -2). In addition, TAG values were compared to PRTg obtained for each patient. Thrombotic risk stratification for KD aneurysms may be improved by incorporating TAG and should be evaluated in future prospective studies. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G7.00010: Fractional Modeling of Viscoelasticity in Brain Aneurysms Yue Yu, George Karniadakis We develop fundamental new numerical methods for fractional order PDEs, and investigate corresponding models for arterial walls. Specifically, the arterial wall is a heterogeneous soft tissue with complex biomechanical properties, and its constitutive laws are typically derived using integer-order differential equations. However, recent simulations on 1D model have indicated that fractional order models may offer a more powerful alternative for describing arterial wall mechanics, because they are less sensitive to the parameter estimation compared with the integer-calculus-based models. We study the specific fractional PDEs that better model the properties of the 3D arterial walls, and for the first time employ them in simulating flow structure interactions for patient-specific brain aneurysms. A comparison study indicates that for the integer order models, the viscous behavior strongly depends on the relaxation parameters while the fractional order models are less sensitive. This finding is consistent with what is observed in the 1D models for arterial networks (Perdikaris {\&} Karniadakis, 2014), except that when the fractional order is small, the 3D fractional-order models are more sensitive to the fractional order compared to the 1D models. [Preview Abstract] |
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