Geometry induced flow disturbances
Yazdani, Saami Kaveh
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From clinical studies it is well known that atherosclerosis has preferred locations in the vascular system, primarily sited in the carotid arteries, coronary arteries, and in vessels supplying the lower extremities in the arterial system. In the vicinity of bifurcations flow tends to separate forming re-circulation regions. In addition, due to the pulsatile character of blood flow during the deceleration part of the cycle, the flow becomes unstable and transition to turbulence may occur. Vascular stents provide a novel method in treatment of atherosclerotic vessels. Although stents have dramatically decreased the re-stenosis rate of vessels compared to balloon-angioplasty, restenosis still occurs in 25-30% of coronary implanted stents. Understanding how stents influence flow patterns may lead to more hemodynamically compatible stent designs that alleviate thrombus formation and promote endothelialization. The first study employed time-resolved Digital Particle Image Velocimetry (DPIV) to compare the hemodynamic performance of two stents in a compliant vessel. The first stent was a rigid insert, representing an extreme compliance mismatch. The second stent was a commercially available nitinol stent with some flexural characteristics. DPIV showed that compliance mismatch promotes the formation of a ring vortex in the vicinity of the stent. Larger compliance mismatch increased both the size and residence time of the ring vortex, and introduced in-flow stagnation points. These results provide detailed quantitative evidence of the hemodynamic effect of stent mechanical properties. Better understanding of these characteristics will provide valuable information for modifying stent design in order to promote long-term In the second study, DPIV was utilized to explore the fluid dynamics phenomena in a symmetric compliant bifurcation. We studied the effects of the Womersley and the Reynolds numbers under pulsatile flow conditions. New insight of the fluid mechanics is revealed. The flow topology results indicate that the formation of coherent vortices in the vicinity of the bifurcation apex is governed by physical process that dictates the energy and strength of the formed vortices. This is manifested by the identification of a characteristic dimensionless time-scale that combines the impulsive vortex formation with the inertia of the unsteady flow.
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