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Pulsatile Flow In The Aortic Arch

A Thomson, Swift CFD Solutions, Dundee, UK
JG Houston, Ninewells Hospital, Dundee, UK

Swift CFD Solutions in conjunction with Tayside Institute of Cardiovascular Research have recently completed a study in the comparison of spiral and non-spiral flow in a human aortic arch. Spiral flow has been observed in the human aortic arch: Healthy patients are known to have high prevalence of spiral flow while patients with vascular disease commonly lack spiral flow1. It is accepted that low wall shear stress is associated with sites of development and progression of arterial disease.

 

 

Magnetic Resonance Image (MRI) scans of a healthy subject’s arotic arch were non-invasively acquired and used to construct the geometry of the arch in the form of an stereolithographic (stl) surface mesh. The trimmed cell meshing technique in STAR-CD, was used to repair the surface and a trimmed cell volume mesh was constructed with approximately 400,000 cells. The methodology used was ideal for this type of highly irregular surface as it was possible to have an extrusion layer next to the surface boundaries, comprising of two layers of hexahedral and prismatic cells. This was essential for the accurate prediction of wall shear stress.

A transient analysis for both spiral and non-spiral flow was carried out over two cardiac cycles with a time step of 0.001 s. Second order space (MARS), and first order temporal discritization was adopted. These parameters were based on past experience and are known to give satisfactory results. The model had one inlet and thirteen outlets. Each vessel with an outlet boundary had a flow split based on measurements. Figure 1 shows a surface plot of the arotic arch used in this study. 

Turbulence was simulated using the quadratic high Reynolds number k - omega model with hybrid wall functions. The value of Y+ can be as low as 1 towards the end of the cardiac cycle. Each simulation took approximately 20 hours on a personal computer powered by a single 2.7 GHz Athlon XP processor with 1.5 GB of RAM.

The percentage increase of wall shear stress of the spiral flow over the non-spiral flow at the phase of the cardiac cycle with lowest shear stress is shown in Figure 2. It should also be noted at this point that high wall shear stress will also mean a lower wall normal stress, therefore cyclic loading may be reduced.

The use of MRI derived vessel geometry and flow parameters and the STAR CD analysis has proved a useful tool in analysing the relevance of blood flow patterns to disease. This qualitative wall shear stress prediction agrees well with observations and has improved the understanding of the relationship between hemodynamics and vascular disease. 

1. 2-Dimensional Flow Quantitative MRI of Aortic Arch Blood Flow Patterns: Effect of Age, Gender and Presence of Carotid Atheromatous Disease on the Prevalence of Spiral Blood Flow. Houston JG, Gandy SJ, Sheppard DG, Dick JBC, Belch JJF, Stonebridge PA. J. Magn. Reson. Imaging (2003); 18(2):169-74

For further information, please contact:
Allan Thomson: athomson@swiftcfd.co.uk
Graeme Houston: graeme.houston@tuht.scot.nhs.uk

 

 

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