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TECHNICAL PAPERS: Fluids/Heat/Transport

Inlet Conditions for Image-Based CFD Models of the Carotid Bifurcation: Is it Reasonable to Assume Fully Developed Flow?

[+] Author and Article Information
Keri R. Moyle

Imaging Research Laboratories,  Robarts Research Institute, London, Canada

Luca Antiga

Bioengineering Department,  The Mario Negri Institute for Pharmacological Research, Bergamo, Italy

David A. Steinman1

Imaging Research Laboratories,  Robarts Research Institute, London, Canada and Department of Medical Biophysics,  The University of Western Ontario, London, Canada

1

Corresponding author. Present address: Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8; e-mail: steinman@mie.utoronto.ca

J Biomech Eng 128(3), 371-379 (Jan 11, 2006) (9 pages) doi:10.1115/1.2187035 History: Received March 11, 2005; Revised January 11, 2006

Background: Computational fluid dynamics tools are useful for their ability to model patient specific data relevant to the genesis and progression of atherosclerosis, but unavailable to measurement tools. The sensitivity of the physiologically relevant parameters of wall shear stress (WSS) and the oscillatory shear index (OSI) to secondary flow in the inlet velocity profiles was investigated in three realistic models of the carotid bifurcation. Method of Approach: Secondary flow profiles were generated using sufficiently long entrance lengths, to which curvature and helical pitch were added. The differences observed were contextualized with respect to effect of the uncertainty of the models’ geometry on the same parameters. Results: The effects of secondary velocities in the inlet profile on WSS and OSI break down within a few diameters of the inlet. Overall, the effect of secondary inlet flow on these models was on average more than 3.5 times smaller than the effect of geometric variability, with 13% and 48% WSS variability induced by inlet secondary flow and geometric differences, respectively. Conclusions: The degree of variation is demonstrated to be within the range of the other computational assumptions, and we conclude that given a sufficient entrance length of realistic geometry, simplification to fully developed axial (i.e., Womersley) flow may be made without penalty. Thus, given a choice between measuring three components of inlet velocity or a greater geometric extent, we recommend effort be given to more accurate and detailed geometric reconstructions, as being of primary influence on physiologically significant indicators.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Inlet extensions shown for subject A. From left to right, (a) straight pipe, reference standard case, (b) planar curvature, (c) short pitch, (d) medium pitch, and (e) long pitch. All inlet extensions were 22.5 CCA inlet diameters long, and the position of the inlet and outlet extensions and CCA inlet are shown in (e). ICA: internal carotid artery, ECA: external carotid artery, CCA: common carotid artery.

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Figure 2

Schematic showing the relationship between effective curvature radius (R), cylinder radius (r), and pitch (p) for examples of the short and medium pitch cases

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Figure 3

Flow profiles resulting from the inlet extensions at midsystole. (a) Straight, reference case, (b) planar curvature, (c) short pitch, (d) medium pitch, and (e) long pitch. Inset: The axial profile for the planar curved case showing the M-shape reported in the in vivo measurements of Caro (Ref. 12) and Tortoli (Ref. 13).

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Figure 4

Qualitative comparison of time averaged WSS (dyn∕cm2): (a) reference standard, straight pipe, week 1 (b), planar curvature, (c) short pitch, (d) medium pitch, (e) long pitch, (f) week 2, and (g) week 3

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Figure 5

Qualitative comparison of OSI: (a) reference standard, straight pipe, week 1 (b) week 1, planar curvature, (c) week 1, short pitch, (d) week 1, medium pitch, (e) week 1, long pitch, (f) week 2, no extension, and (g) week 3, no extension

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Figure 6

Time averaged HAD in deg for subject A: (a) reference standard, straight pipe, week 1, (b) planar curvature, (c) short pitch, (d) medium pitch, (e) long pitch, (f) week 2, and (g) week 3.

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