12 research outputs found
Comparison of two stents in modifying cerebral aneurysm hemodynamics
There is a general lack of quantitative understanding about how specific design features of
endovascular stents (struts and mesh design, porosity) affect the hemodynamics in intracranial
aneurysms. To shed light on this issue, we studied two commercial high-porosity stents (Tristar
stent™ and Wallstent®) in aneurysm models of varying vessel curvature as well as in a patientspecific model using Computational Fluid Dynamics. We investigated how these stents modify
hemodynamic parameters such as aneurysmal inflow rate, stasis, and wall shear stress, and how such
changes are related to the specific designs. We found that the flow damping effect of stents and
resulting aneurysmal stasis and wall shear stress are strongly influenced by stent porosity, strut
design, and mesh hole shape. We also confirmed that the damping effect is significantly reduced at
higher vessel curvatures, which indicates limited usefulness of high-porosity stents as a stand-alone
treatment. Finally, we showed that the stasis-inducing performance of stents in 3D geometries can
be predicted from the hydraulic resistance of their flat mesh screens. From this, we propose a
methodology to cost-effectively compare different stent designs before running a full 3D simulation
LES, DNS and RANS for the analysis of high-speed turbulent reacting flows
The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Sep. 1993 - 1 Sep. 1994, we have focused our efforts on two research problems: (1) developments of 'algebraic' moment closures for statistical descriptions of nonpremixed reacting systems, and (2) assessments of the Dirichlet frequency in presumed scalar probability density function (PDF) methods in stochastic description of turbulent reacting flows. This report provides a complete description of our efforts during this past year as supported by the NASA Langley Research Center under Grant NAG1-1122
Validation of CFD simulations of cerebral aneurysms with implication of geometric variations
Background. Computational fluid dynamics (CFD) simulations using medical-image-based anatomical vascular geometry are now gaining clinical relevance. This study aimed at validating the CFD methodology for studying cerebral aneurysms by using particle image velocimetry (PIV) measurements, with a focus on the effects of small geometric variations in aneurysm models on the flow dynamics obtained with CFD. Method of Approach. An experimental phantom was fabricated out of silicone elastomer to best mimic a spherical aneurysm model. PIV measurements were obtained from the phantom and compared with the CFD results from an ideal spherical aneurysm model (S1). These measurements were also compared with CFD results, based on the geometry reconstructed from three-dimensional images of the experimental phantom. We further performed CFD analysis on two geometric variations, S2 and S3, of the phantom to investigate the effects of small geometric variations on the aneurysmal flow field. Results. We found poor agreement between the CFD results from the ideal spherical aneurysm model and the PIV measurements from the phantom, including inconsistent secondary flow patterns. The CFD results based on the actual phantom geometry, however, matched well with the PIV measurements. CFD of models S2 and S3 produced qualitatively similar flow fields to that of the phantom but quantitatively significant changes in key hemodynamic parameters such as vorticity, positive circulation, and wall shear stress. Conclusion. CFD simulation results can closely match experimental measurements as long as both are performed on the same model geometry. Small geometric variations on the aneurysm model can significantly alter the flow-field and key hemodynamic parameters. Since medical images are subjected to geometric uncertainties, image-based patient-specific CFD results must be carefully scrutinized before providing clinical feedback
Similarity Solutions to Some Nonlinear Impact Problems
Impact and wave propagation problems are considered for nonlinearly viscous and nonlinearly elastic materials. The governing partial differential equations are reduced to ordinary differential equations by means of similarity transformations. The resulting non-linear two point boundary value problems are then, in general, integrated numerically, although some closed form solutions are presented