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Fluid-dynamic optimal design of helical vascular graft for stenotic disturbed flow.

Ha H, Hwang D, Choi WR, Baek J, Lee SJ - PLoS ONE (2014)

Bottom Line: Results showed that the swirling intensity and helicity of the swirling flow have a linear relation with a modified Germano number (Gn*) of the helical pipe.In addition, the swirling flow generated a beneficial flow structure at the stenosis by reducing the size of the recirculation flow under steady and pulsatile flow conditions.Therefore, the beneficial effects of a helical graft on the flow field can be estimated by using the magnitude of Gn*.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk, Republic of Korea.

ABSTRACT
Although a helical configuration of a prosthetic vascular graft appears to be clinically beneficial in suppressing thrombosis and intimal hyperplasia, an optimization of a helical design has yet to be achieved because of the lack of a detailed understanding on hemodynamic features in helical grafts and their fluid dynamic influences. In the present study, the swirling flow in a helical graft was hypothesized to have beneficial influences on a disturbed flow structure such as stenotic flow. The characteristics of swirling flows generated by helical tubes with various helical pitches and curvatures were investigated to prove the hypothesis. The fluid dynamic influences of these helical tubes on stenotic flow were quantitatively analysed by using a particle image velocimetry technique. Results showed that the swirling intensity and helicity of the swirling flow have a linear relation with a modified Germano number (Gn*) of the helical pipe. In addition, the swirling flow generated a beneficial flow structure at the stenosis by reducing the size of the recirculation flow under steady and pulsatile flow conditions. Therefore, the beneficial effects of a helical graft on the flow field can be estimated by using the magnitude of Gn*. Finally, an optimized helical design with a maximum Gn* was suggested for the future design of a vascular graft.

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Comparison of PIV and CFD results data at the outlet of a helical tube (H/R0 = 8, Rc/R0 = 1.0).(a) Axial velocity distribution and (b) normal-direction vorticity contours and corresponding streamlines.
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pone-0111047-g005: Comparison of PIV and CFD results data at the outlet of a helical tube (H/R0 = 8, Rc/R0 = 1.0).(a) Axial velocity distribution and (b) normal-direction vorticity contours and corresponding streamlines.

Mentions: The CFD results were validated by comparing the axial velocity distribution as well as the normal-direction vorticity field at the outlet of the helical tube (H/R0 = 8, Rc/R0 = 1) obtained by CFD with the PIV results in figure 5. The axial velocity distribution tends to be skewed toward the outer wall (figure 5a) because of the inertia of the flow at the helical tube. At Re = 213, the axial velocity distribution is slightly distorted from the Poiseuille flow profile. As Re increases, the skewing phenomenon significantly intensifies, thus causing the shift of the maximum velocity of the flow near the wall. Figure 5b shows the swirling secondary flow produced by the helical tube according to Re. The streamlines and vorticity fields indicate the clockwise direction of the swirling flow with a single rotating axis. As Re increases, the magnitude of the vorticity at the centre of the swirling flow increases, which implies strong swirling flow. The CFD results successfully demonstrate the skewing phenomena of the axial velocity distribution and the swirling characteristics of the flow. In addition, the magnitudes of the peak vorticity measured by PIV and CFD are compared quantitatively in figure 6. While the CFD data slightly underestimates the peak vorticity around 10% on average, it shows overall agreement with the PIV result. In addition, the CFD result successfully estimates the linear increase of the peak vorticity with respect to the Re within the PIV measurement uncertainty. Figure 7 shows the normal-direction vorticity field distribution of swirling flows at various helical curvatures and pitches. In the case of the helical tube with a small radius of curvature (Rc/R0 = 0.2), the magnitude of vorticity is highest at the short helical pitch (H/R0 = 4). By contrast, the most intensive swirling flow occurs at long pitches as Rc/R0 increases. The maximum vorticity is observed at H/R0 = 8 and 16 when Rc/R0 = 1.0 and 2.0, respectively. The swirling variations for a range of helical curvatures and pitches were quantitatively analysed using swirling intensity (S) (Figure 8). The swirling intensity shows the effect of the helical pitch on the swirling flow at a fixed radius of curvature. S is highest at the shortest helical pitch (H/R0 = 4) for small helical curvatures (Rc/R0 = 0.2 and 0.6). However, the maximum point shifts to H/R0 = 8 and 16 at Rc/R0 = 1.0 and 2.0, respectively. This is attributed to the variation of Gn*, which is a measure of the ratio of the twisting forces to the viscous forces, according to helical curvature. The Gn* has the maximum values at the larger helical pitches (H/R0 = 8 and 16) when the helical curvature is increased to Rc/R0 = 1.0 and 2.0, respectively. The variation of the swirling intensity with Re is shown in figure 9. For the fixed helical pitch (H/R0 = 8), the swirling intensity increases in proportion to Re, thus indicating that a high flow rate induces considerably intensive swirling flow.


Fluid-dynamic optimal design of helical vascular graft for stenotic disturbed flow.

Ha H, Hwang D, Choi WR, Baek J, Lee SJ - PLoS ONE (2014)

Comparison of PIV and CFD results data at the outlet of a helical tube (H/R0 = 8, Rc/R0 = 1.0).(a) Axial velocity distribution and (b) normal-direction vorticity contours and corresponding streamlines.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4215892&req=5

pone-0111047-g005: Comparison of PIV and CFD results data at the outlet of a helical tube (H/R0 = 8, Rc/R0 = 1.0).(a) Axial velocity distribution and (b) normal-direction vorticity contours and corresponding streamlines.
Mentions: The CFD results were validated by comparing the axial velocity distribution as well as the normal-direction vorticity field at the outlet of the helical tube (H/R0 = 8, Rc/R0 = 1) obtained by CFD with the PIV results in figure 5. The axial velocity distribution tends to be skewed toward the outer wall (figure 5a) because of the inertia of the flow at the helical tube. At Re = 213, the axial velocity distribution is slightly distorted from the Poiseuille flow profile. As Re increases, the skewing phenomenon significantly intensifies, thus causing the shift of the maximum velocity of the flow near the wall. Figure 5b shows the swirling secondary flow produced by the helical tube according to Re. The streamlines and vorticity fields indicate the clockwise direction of the swirling flow with a single rotating axis. As Re increases, the magnitude of the vorticity at the centre of the swirling flow increases, which implies strong swirling flow. The CFD results successfully demonstrate the skewing phenomena of the axial velocity distribution and the swirling characteristics of the flow. In addition, the magnitudes of the peak vorticity measured by PIV and CFD are compared quantitatively in figure 6. While the CFD data slightly underestimates the peak vorticity around 10% on average, it shows overall agreement with the PIV result. In addition, the CFD result successfully estimates the linear increase of the peak vorticity with respect to the Re within the PIV measurement uncertainty. Figure 7 shows the normal-direction vorticity field distribution of swirling flows at various helical curvatures and pitches. In the case of the helical tube with a small radius of curvature (Rc/R0 = 0.2), the magnitude of vorticity is highest at the short helical pitch (H/R0 = 4). By contrast, the most intensive swirling flow occurs at long pitches as Rc/R0 increases. The maximum vorticity is observed at H/R0 = 8 and 16 when Rc/R0 = 1.0 and 2.0, respectively. The swirling variations for a range of helical curvatures and pitches were quantitatively analysed using swirling intensity (S) (Figure 8). The swirling intensity shows the effect of the helical pitch on the swirling flow at a fixed radius of curvature. S is highest at the shortest helical pitch (H/R0 = 4) for small helical curvatures (Rc/R0 = 0.2 and 0.6). However, the maximum point shifts to H/R0 = 8 and 16 at Rc/R0 = 1.0 and 2.0, respectively. This is attributed to the variation of Gn*, which is a measure of the ratio of the twisting forces to the viscous forces, according to helical curvature. The Gn* has the maximum values at the larger helical pitches (H/R0 = 8 and 16) when the helical curvature is increased to Rc/R0 = 1.0 and 2.0, respectively. The variation of the swirling intensity with Re is shown in figure 9. For the fixed helical pitch (H/R0 = 8), the swirling intensity increases in proportion to Re, thus indicating that a high flow rate induces considerably intensive swirling flow.

Bottom Line: Results showed that the swirling intensity and helicity of the swirling flow have a linear relation with a modified Germano number (Gn*) of the helical pipe.In addition, the swirling flow generated a beneficial flow structure at the stenosis by reducing the size of the recirculation flow under steady and pulsatile flow conditions.Therefore, the beneficial effects of a helical graft on the flow field can be estimated by using the magnitude of Gn*.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk, Republic of Korea.

ABSTRACT
Although a helical configuration of a prosthetic vascular graft appears to be clinically beneficial in suppressing thrombosis and intimal hyperplasia, an optimization of a helical design has yet to be achieved because of the lack of a detailed understanding on hemodynamic features in helical grafts and their fluid dynamic influences. In the present study, the swirling flow in a helical graft was hypothesized to have beneficial influences on a disturbed flow structure such as stenotic flow. The characteristics of swirling flows generated by helical tubes with various helical pitches and curvatures were investigated to prove the hypothesis. The fluid dynamic influences of these helical tubes on stenotic flow were quantitatively analysed by using a particle image velocimetry technique. Results showed that the swirling intensity and helicity of the swirling flow have a linear relation with a modified Germano number (Gn*) of the helical pipe. In addition, the swirling flow generated a beneficial flow structure at the stenosis by reducing the size of the recirculation flow under steady and pulsatile flow conditions. Therefore, the beneficial effects of a helical graft on the flow field can be estimated by using the magnitude of Gn*. Finally, an optimized helical design with a maximum Gn* was suggested for the future design of a vascular graft.

Show MeSH
Related in: MedlinePlus