Assessing hemodynamic performances of small diameter helical grafts: Transient simulation

The present study numerically simulated the physiological pulsatile flow in helical grafts to increase understanding of its flow mechanism which may contribute to the design of better grafts. The wall-indices like time-averaged wall shear stress (WSS) and oscillatory shear index (OSI), joint with a quantitative index for helical flow by means of Lagrangian approach, were introduced as effective instruments to classify the hemodynamic performance of helical grafts. The simulation suggests that the helical geometry created amplified WSS magnitudes as well as elevated velocities near the wall. The calculated oscillatory shear index (OSI) values were never exceeded to 0.07 which is not considered physiologically significant. In addition, the strong secondary flow in helical graft helped the flow mixing between low-momentum fluid closer to the surface and high-momentum fluid at the center which brought the high-momentum fluid to the surface. Furthermore, Helicity analysis revealed that most of the fluid particles experienced counter-clockwise rotation during the whole cardiac cycle which helps to protect the graft wall from damage by reducing the laterally directed forces and keep flow stability. It concluded that a helical graft provides guaranties for the graft wall surface to get smooth and even washing by the blood and eliminates mechanical trauma to blood cells so that atherosclerotic plaques can hardly form in the graft wall.