Abstract
Although blood flows are mostly laminar, transition to turbulence and flow separations are observed at curved vessels, bifurcations, or constrictions. It is known that wall-shear stress plays an important role in the development of atherosclerosis as well as in arteriovenous grafts. In order to help understand the behavior of flow separation and transition to turbulence in post-stenotic blood flows, an experimental study of transitional pulsatile flow with stenosis was carried out using time-resolved particle image velocimetry and a microelectromechanical systems wall-shear stress sensor at the mean Reynolds number of 1750 with the Womersley number of 6.15. At the start of the pulsatile cycle, a strong shear layer develops from the tip of the stenosis, increasing the flow separation region. The flow at the throat of the stenosis is always laminar due to acceleration, which quickly becomes turbulent through a shear-layer instability under a strong adverse pressure gradient. At the same time, a recirculation region appears over the wall opposite to the stenosis, moving downstream in sync with the movement of the reattachment point. These flow behaviors observed in a two-dimensional channel flow are very similar to the results obtained previously in a pipe flow. We also found that the behavior in a pulsating channel flow during the acceleration phase of both 25% and 50% stenosis cases is similar to that of the steady flow, including the location and size of post-stenotic flow separation regions. This is because the peak Reynolds number of the pulsatile flow is similar to that of the steady flow that is investigated. The transition to turbulence is more dominant for the 50% stenosis as compared to the 25% stenosis, as the wavelet spectra show a greater broadening of turbulence energy. With an increase in stenosis to 75%, the accelerating flow is directed toward the opposite wall, creating a wall jet. The shear layer from the stenosis bifurcates as a result of this, one moving with the flow separation region toward the upper wall and the other with the wall jet toward the bottom wall. Low wall-shear stress fluctuations are found at two post-stenotic locations in the channel flow – one immediately downstream of the stenosis over the top wall (stenosis side) inside the flow separation region, and the other in the recirculation region on the bottom wall (opposite side of the stenosis).
Highlights
Blood flows are mostly laminar, which helps maintain stable hemodynamic environments and normal physiological activities
In order to help understand the behavior of flow separation and transition to turbulence in post-stenotic blood flows, an experimental study of transitional pulsatile flow with stenosis was carried out using time-resolved particle image velocimetry and a microelectromechanical systems wall-shear stress sensor at the mean Reynolds number of 1750 with the Womersley number of 6.15
In order to establish the accuracy of our Particle image velocimetry (PIV) measurements in a two-dimensional channel flow, we examine the results of velocity profiles and flow patterns under steady flow conditions with and without stenosis
Summary
Blood flows are mostly laminar, which helps maintain stable hemodynamic environments and normal physiological activities. The diseases are localized to specific sites where fluid mechanical parameters deviate from their normal patterns (Tarbell et al, 2014) This may be due to the fact that turbulent wall-shear stress fluctuations greatly contribute to the endothelial cell turnover (Davies et al, 1986). Xu et al (2017) carried out experiments in a sinusoidalshaped pulsatile pipe flow to observe that the critical Reynolds number increased with an increase in the Womersley number until Wo 1⁄4 2.5. These results were confirmed by Xu and Avila (2018) who carried out a numerical simulation with a low pulsation amplitude. The effect of the pulsatile waveform in a pulsatile pipe flow was investigated by Brindise and Vlachos (2018), who demonstrated that the waveform with a longer deceleration induced an earlier onset of transition, while a longer acceleration delayed the transition
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