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Research Papers

Numerical Investigation of Turbulence in Abdominal Aortic Aneurysms

[+] Author and Article Information
Digvijay S. Rawat

Laboratory for Aero & Hydrodynamics,
Delft University of Technology,
Leeghwaterstraat 21,
Delft CA 2628, The Netherlands
e-mail: D.S.Rawat@student.tudelft.nl

Mathieu Pourquie

Laboratory for Aero & Hydrodynamics,
Delft University of Technology,
Leeghwaterstraat 21,
Delft CA 2628, The Netherlands
e-mail: M.J.B.M.Pourquie@tudelft.nl

Christian Poelma

Laboratory for Aero & Hydrodynamics,
Delft University of Technology,
Leeghwaterstraat 21,
Delft CA 2628, The Netherlands
e-mail: C.Poelma@tudelft.nl

1Corresponding author.

Manuscript received December 23, 2017; final manuscript received March 23, 2019; published online April 22, 2019. Assoc. Editor: Alison Marsden.

J Biomech Eng 141(6), 061001 (Apr 22, 2019) (9 pages) Paper No: BIO-17-1599; doi: 10.1115/1.4043289 History: Received December 23, 2017; Revised March 23, 2019

Computational fluid dynamics (CFD) is a powerful method to investigate aneurysms. The primary focus of most investigations has been to compute various hemodynamic parameters to assess the risk posed by an aneurysm. Despite the occurrence of transitional flow in aneurysms, turbulence has not received much attention. In this article, we investigate turbulence in the context of abdominal aortic aneurysms (AAA). Since the clinical practice is to diagnose an AAA on the basis of its size, hypothetical axisymmetric geometries of various sizes are constructed. In general, just after the peak systole, a vortex ring is shed from the expansion region of an AAA. As the ring advects downstream, an azimuthal instability sets in and grows in amplitude thereby destabilizing the ring. The eventual breakdown of the vortex ring into smaller vortices leads to turbulent fluctuations. A residence time study is also done to identify blood recirculation zones, as a recirculation region can lead to degradation of the arterial wall. In some of the geometries simulated, the enhanced local mixing due to turbulence does not allow a recirculation zone to form, whereas in other geometries, turbulence had no effect on them. The location and consequence of a recirculation zone suggest that it could develop into an intraluminal thrombus (ILT). Finally, the possible impact of turbulence on the oscillatory shear index (OSI), a hemodynamic parameter, is explored. To conclude, this study highlights how a small change in the geometric aspects of an AAA can lead to a vastly different flow field.

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Figures

Grahic Jump Location
Fig. 1

General geometric aspects used to characterize a 4A geometry. Roman numerals IV identify various sub parts of the geometry namely inlet, expansion region, streamwise length/aneurysm surface, contraction region, and outlet, respectively. The inlet length and outlet length of 5d0 and the total aneurysm length of 100 mm are common to all the 4A geometries.

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Fig. 2

Mills' time-dependent velocity waveform adapted for Rem = 600. The plot shows the variation of the bulk velocity against the phases of the cardiac cycle. The black markers are the phases at which data are visualized in this study. Phases 0.2–0.3 represent systolic acceleration and phases 0.35–0.5 represent systolic deceleration.

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Fig. 3

Phase-averaged z-vorticity contours in the XY plane (Z = 0) at selected phases of the geometry R2A30. The vortex ring can be seen as a counter-rotating vortex pair in the planar contours (between ϕ = 0.5 and ϕ = 0.8) that breaks down near the exit region of the aneurysm. Flow is from left to right.

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Fig. 4

Spatial distribution of turbulence intensity at ϕ = 0.3 across all the 4A geometries. The spatial localization of turbulence is clearly visible. The 4A geometries R1.5A15 and R2A15 are free of turbulent fluctuations. Flow is from left to right.

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Fig. 5

Area-weighted cycle averaged mass fraction of old blood at the flow outlet of R2A30 against number of cycles simulated. Even after 20 cycles, the marked fluid has not been washed out completely from the aneurysm. An exponential fit gives a reasonable approximation of the actual plot.

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Fig. 6

Instantaneous spatial distribution of old blood mass fraction over a period of 20 cardiac cycles for the geometry R2A30. Flow is from left to right.

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Fig. 7

Mean (over 60 cycles) OSI distribution over the aneurysm surface of R2A30. Flow is from left to right.

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Fig. 8

Surface-averaged mean OSI of all the 4A geometries. The dashed (- - -) line indicates the average OSI for a healthy aorta.

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Fig. 9

Normalized v in the neighborhood of the core center for R2A30 at ϕ = 0.5. The inset sketch depicts the counter rotating vortex pair and the axis along which velocity profile is plotted. The distance between the two extrema, highlighted by the two vertical dotted lines, is an indication of the vortex core diameter (a). The axial distance, from the center of the core (r =0), is normalized with the healthy aorta diameter. From the figure, a 0.26d0.

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Fig. 10

Normalized z-vorticity (ω) distribution along the normalized distance from the center of the core for R2A30 at ϕ = 0.5. The inset sketch depicts the counter rotating vortex pair and the axis along which z-vorticity profile is plotted. The vorticity peak corresponds to the core center. The vertical dotted lines indicate the core diameter as obtained from Fig. 9. From the figure, the ring radius r 0.497d0, which leads to a ring diameter d 0.993d0.

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Fig. 11

Growth of the azimuthal instability on the vortex ring formed in R2A30. From (i)–(v), the rearrangement of vorticity around the ring core due to the growth of the azimuthal instability can be seen. Figure (v) is a 3D view at ϕ = 1 to illustrate the large amplitude of the instability. Figure (vi) illustrates the core deformation and the shedding of streamwise vortices in the wake of the ring at a later stage of the instability.

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Fig. 12

Mean of area-weighted average old blood mass fraction in the expansion region with increasing number of cycles. With an increase in ER, the mass fraction of old blood stuck in a recirculation zone increases due to the size increase of the recirculation zones.

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