COLLECTIONS

Documents published in Scipedia

• New methods for computational fluid dynamics modeling of carotid artery from magnetic resonance angiography

  J. Cebral, P. Yim, R. Lohner, O. Soto, H. Marcos, P. Choyke

  (2001). Proceedings Volume 4321, Medical Imaging 2001: Physiology and Function from Multidimensional

  
  

  Abstract

  Computational fluid dynamics (CFD) models of the carotid artery are constructed from contrast-enhanced magnetic resonance angiography (MRA) using a deformable model and a surface-merging algorithm. Physiologic flow conditions are obtained from cine phase-contrast MRA at two slice locations below and above the carotid bifurcation. The methodology was tested on image data from a rigid flow-through phantom of a carotid artery with 65% degree stenosis. Predicted flow patterns are in good agreement with MR flow measurements at intermediate slice locations. Our results show that flow in a rigid flow-through phantom of the carotid bifurcation with stenosis can be simulated accurately with CFD. The methodology was then tested on flow and anatomical data from a normal human subject. The sum of the instantaneous flows measured at the internal and external carotids differs from that at the common carotid, indicating that wall compliance must be modeled. Coupled fluid-structure calculations were able to reproduce the significant dampening of the velocity waveform observed between different slices along the common carotid artery. Visualizations of the blood flow in a compliant model of the carotid bifurcation were produced. A comparison between compliant and rigid models shows significant differences in the time-dependent wall shear stress at selected locations. Our results confirm that image-based CFD techniques can be applied to the modeling of hemodynamics in compliant carotid arteries. These capabilities may eventually allow physicians to enhance current image-based diagnosis, and to predict and evaluate the outcome of interventional procedures non- invasively.

  Abstract
  Computational fluid dynamics (CFD) models of the carotid artery are constructed from contrast-enhanced magnetic resonance angiography (MRA) using a deformable model and a [...]

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• Interpretation of arterial velocity waveforms

  P. Yim, J. Cebral, R. Lohner, P. Choyke, H. Marcos, O. Soto

  (2001). Proceedings Volume 4321, Medical Imaging 2001: Physiology and Function from Multidimensional

  
  

  Abstract

  Blood flow temporal waveforms change with position along an artery. The change in the flow waveforms can be accounted for by a transmission line model of flow. According to this model, pulse waves propagate at a finite velocity in both directions along the artery. In principle, given flow waveforms measured at three locations along an artery, the pulse-wave velocity, (c) can be determined from the wave equation (d2Q/dt2 equals c2d2Q/dz2, Q is flow, t is time, z is position). Given the vessel diameter, the vessel-wall compliance can be derived from pulse-wave velocity. However, direct solution of the wave equation for pulse-wave velocity is highly susceptible to flow-measurement error. Thus, we propose a new method for estimating pulse-wave velocity from arterial flow waveforms. In our method, ideal flow waveforms are reconstructed from three measured flow waveforms. The ideal waveforms are reconstructed by minimization of the total error between the ideal and measured waveforms subject to constraints of the wave equation. Ideal flow waveforms are reconstructed for a range of assumed pulse-wave velocities. The true pulse-wave velocity is considered to be that which produces the minimum total error. The method applied to blood flow measurements made with phase-contrast magnetic resonance imaging.

  Abstract
  Blood flow temporal waveforms change with position along an artery. The change in the flow waveforms can be accounted for by a transmission line model of flow. According to [...]

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• Unstructured Navier–Stokes grid generation at corners and ridges

  D. Sharov, H. Luo, J. Baum, R. Lohner

  Int. J. Numer. Meth. Fluids (2003). Vol. 43 (6-7), pp. 717-728

  
  

  Abstract

  Problems related to automatic generation of highly stretched unstructured grids suitable for 3‐D Reynolds‐averaged Navier–Stokes computations are addressed. Special attention is given to treatment of such geometrical irregularities as convex and concave ridges as well as corners where the ridges meet. The existing unstructured grid generation approaches may fail or produce poor quality meshes in such geometrical regions. The proposed solution is based on special meshing of non‐slip body surfaces resulting in smooth and robust volume meshing and high overall quality of generated grids. Several examples demonstrate the efficiency of the method for complex 3‐D geometries.

  Abstract
  Problems related to automatic generation of highly stretched unstructured grids suitable for 3‐D Reynolds‐averaged Navier–Stokes computations are addressed. Special [...]

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• CFD shape optimization using an incomplete-gradient adjoint formulation

  O. Soto, R. Lohner

  Int. J. Numer. Meth. Engng. (2001). Vol. 51 (6), pp. 735-753

  
  

  Abstract

  A design methodology based on the adjoint approach for flow problems governed by the incompressible Euler equations is presented. The main feature of the algorithm is that it avoids solving the adjoint equations, which saves an important amount of CPU time. Furthermore, the methodology is general in the sense it does not depend on the geometry representation. All the grid points on the surface to be optimized can be chosen as design parameters. In addition, the methodology can be applied to any type of mesh. The partial derivatives of the Row equations with respect to the design parameters are computed by finite differences. In this way, this computation is independent of the numerical scheme employed to obtain the Row solution. Once the design parameters have been updated, the new solid surface is obtained with a pseudo-shell approach in such a way that local singularities, which can degrade or inhibit the convergence to the optimal solution, are avoided. Some 2D and 3D numerical examples are shown to demonstrate the proposed methodology. 

  Abstract
  A design methodology based on the adjoint approach for flow problems governed by the incompressible Euler equations is presented. The main feature of the algorithm is that [...]

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• A parallel advancing front grid generation scheme

  R. Lohner

  Int. J. Numer. Meth. Engng. (2001). Vol. 51 (6), pp. 663-678

  
  

  Abstract

  A parallel advancing front scheme has been developed. The domain to be gridded is first subdivided spatially using a relatively coarse octree. Boxes are then identified and gridded in parallel. A scheme that resembles closely the advancing front technique on scalar machines is recovered by only considering the boxes of the active front that generate small elements. The procedure has been implemented on the SGI origin class of machines using the shared memory paradigm. Timings for a variety of cases show speedups similar to those obtained for flow codes. The procedure has been used to generate grids with tens of millions of elements. 

  Abstract
  A parallel advancing front scheme has been developed. The domain to be gridded is first subdivided spatially using a relatively coarse octree. Boxes are then identified and [...]

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• Merging of intersecting triangulations for finite element modeling

  J. Cebral, R. Lohner, P. Choyke, P. Yim

  Journal of Biomechanics (2001). Vol. 34 (6), pp. 815-819

  
  

  Abstract

  Surface mesh generation over intersecting triangulations is a problem common to many branches of biomechanics. A new strategy for merging intersecting triangulations is described. The basis of the method is that object surfaces are represented as the zero-level iso-surface of the distance-to-surface function defined on a background grid. Thus, the triangulation of intersecting objects reduces to the extraction of an iso-surface from an unstructured grid. In a first step, a regular background mesh is constructed. For each point of the background grid, the closest distance to the surface of each object is computed. Background points are then classified as external or internal by checking the direction of the surface normal at the closest location and assigned a positive or negative distance, respectively. Finally, the zero-level iso-surface is constructed. This is the final triangulation of the intersecting objects. The overall accuracy is enhanced by adaptive refinement of the background grid elements. The resulting surface models are used as support surfaces to generate three-dimensional grids for finite element analysis. The algorithms are demonstrated by merging arterial branches independently reconstructed from contrast-enhanced magnetic resonance images and by adding extra features such as vascular stents. Although the methodology is presented in the context of finite element analysis of blood flow, the algorithms are general and can be applied in other areas as well.

  Abstract
  Surface mesh generation over intersecting triangulations is a problem common to many branches of biomechanics. A new strategy for merging intersecting triangulations is described. [...]

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• From medical images to anatomically accurate finite element grids

  J. Cebral, R. Lohner

  Int. J. Numer. Meth. Engng. (2001). Vol. 51 (8), pp. 985-1008

  
  

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• Practical hydrodynamic optimization of a trimaran

  C. Yang, O. Soto, R. Lohner, F. Noblesse

  Ship Technology Research (2002). Vol. 49 (2), pp. 70-92

  
  

  Abstract

  An illustrative application of practical CFD tools to a simple ship design problem is presented. Four methods of analysis – a nonlinear method based on Euler’s equations and three linear potential flow methods – are used to determine the optimal location of the outer hulls of a trimaran. The main center hull and the outer hulls of the trimaran are shown in the frontispiece, together with the parameters a and b that define the arrangement of the hulls. The three potential flow methods correspond to a hierarchy of simple approximations based on the Fourier-Kochin representation of ship waves and the slender-ship approximation.

  Abstract
  An illustrative application of practical CFD tools to a simple ship design problem is presented. Four methods of analysis – a nonlinear method based on Euler’s [...]

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• Optimization of a Wave Cancellation Multihull Ship Using CFD Tools

  C. Yang, R. Lohner, O. Soto

  (2001). Proceedings of the Eighth International Symposium on Practical Design of Ships and Other Floating Structures 16, Vol. 1, pp. 43-50

  
  

  Abstract

  A simple CFD tool, coupled to a discrete surface representation and a gradient-based optimization procedure, is applied to the design of optimal hull forms and optimal arrangement of hulls for a wave cancellation multihull ship. The CFD tool, which is used to estimate the wave drag, is based on the zeroth-order slender ship approximation. The hull surface is represented by a triangulation, and almost every grid point on the surface can be used as a design variable. A smooth surface is obtained via a simplified pseudo-shell problem. The optimal design process consists of two steps. The optimal center and outer hull forms are determined independently in the first step, where each hull keeps the same displacement as the original design while the wave drag is minimized. The optimal outer-hull arrangement is determined in the second step for the optimal center and outer hull forms obtained in the first step. Results indicate that the new design can achieve a large wave drag reduction in comparison to the original design configuration.

  Abstract
  A simple CFD tool, coupled to a discrete surface representation and a gradient-based optimization procedure, is applied to the design of optimal hull forms and optimal arrangement [...]

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• Image-based finite element modeling of hemodynamics in stenosed carotid artery

  J. Cebral, R. Lohner, O. Soto, P. Choyke, P. Yim

  (2002). Proceedings Volume 4683, Medical Imaging 2002: Physiology and Function from Multidimensional

  
  

  Abstract

  A methodology to construct patient-specific, anatomically and physiologically realistic finite element models of blood flows in stenosed carotid arteries is presented. Anatomical models of carotid arteries with stenosis are reconstructed from contrast-enhanced magnetic resonance angiography (MRA) images using a tubular deformable model along each arterial branch. A surface-merging algorithm is used to create a watertight model of the carotid bifurcation for subsequent finite element grid generation. A fully implicit scheme is used to solve the incompressible Navier-Stokes equations on unstructured grids in three-dimensions. Physiologic boundary conditions are derived from cine phase-contrast MRA flow velocity measurements at two locations below and above the bifurcation. The methodology was tested on image data of a patient with carotid artery stenosis. A finite element grid was successfully generated from contrast-enhanced MRA images, and pulsatile blood flow visualizations were produced. Visualizations of the wall shear stress distribution and of changes in both its magnitude and direction were produced. These quantities may become important in order to characterize healthy and diseased flow and wall shear stress patterns. We conclude that MRA can be used to obtain all the anatomical and physiologic data necessary for realistic modeling of blood flows in carotid arteries with stenosis. Our results confirm that image-based computational fluid dynamics techniques can be applied to the modeling of hemodynamics in carotid arteries with stenosis. These capabilities may be used to advance our understanding of the generation and progression of vascular disease, and may eventually allow physicians to enhance current image-based diagnosis, and to predict and evaluate the outcome of interventional procedures non-invasively.

  Abstract
  A methodology to construct patient-specific, anatomically and physiologically realistic finite element models of blood flows in stenosed carotid arteries is presented. Anatomical [...]

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