The Effect of Waveform Shape on Dynamics and Kinematics of the Flow in Endovascular Stents

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Date
2012-06-11
Authors
Rouhi, Amirreza
Keyword
Atherosclerosis , Computational Fluid Dynamics , Stenosis , Arteries , Fluid Dynamics , Stents , Biology
Abstract
The effect of waveform shape and extension of negative flowrate are studied at two Reynolds numbers (Re = 80, Re = 200) on stented channels by looking at the kinematics and dynamics of the flow. The waveforms are reconstructed by Fourier decomposition of a waveform corresponding to left anterior descending (LAD) of coronary artery. The stents are modeled by an immersed boundary method. Two stent geometries are created which are the idealizations of two clinical ones. The first geometry is an idealization of XIENCE V stent which is called Lambda stent and the second one is the idealization of Endeavor stent and is called X stent. The former has larger inter-strut spacing and smaller thickness than the latter one. The use of immersed boundary method for creating the stents is validated, and the spatial resolution requirements are determined. The shape of the waveform is changed by systematically filtering out the higher modes of Fourier decomposition and the negative flowrate extension is reduced by shifting the waveform to the positive flowrate. The presence of the stent causes vortical structures to be created between stent struts. These vortices are migrating to the centre of the channel and disappear. It is observed that the confined geometric feature of X stent and its larger thickness, leads to larger areas of flow recirculation which causes smaller wall-shear-stress parameters with respect to Lambda stent and more deviation of the flow from a healthy vessel. The importance of the convective terms of the Navier-Stokes equations was studied at the two Reynolds numbers for both stents. It is observed that at high Reynolds number (Re = 200), the convective terms play significant role throughout the waveform cycle while at low Reynolds number (Re = 80), the effect of convective terms become negligible during negative flowrate. Moreover the convective terms become more significant for flow in a channel with Lambda stent than X stent due to the specific shape and size of the stents. The kinematics of the flow corresponds to the study of vortex timing. It was found that this timing is mainly affected by the waveform and Reynolds number rather than the stent geometry. The time at which vortex creation occurs is coincident with the time at which wall shear stress changes its sign in an unstented channel. Therefore the analytical solution of unsteady channel flow can be used as a tool for analysing the kinematics of the flow,
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