Effect of pulsatile blood flow on thrombosis potential with a step wall transition

It is well known that thrombus can be formed at stagnation regions in blood flow. However, studies of thrombus formation have typically focused on steady state flow. We hypothesize that pulsating flow may reduce persistent stagnation at the sites of low shear stress by decreasing exposure time. In this study, a step-wall transition, which is commonly found on implantable devices, is used as a test bed causing a recirculation vortex. Stagnation at such a step is considered using computational fluid dynamics studies and flow visualization experiments. Parametric studies were performed with varying step height, pulsatility, and velocity. The percentage of time along the wall with shear stresses below a threshold for thrombosis and the total length of wall that maintains contact with stagnant flow throughout the cardiac cycle are calculated. Persistent stagnation occurs at the corner of a step-wall transition in all cases and is observed to decrease with a decrease in step height, an increase in mean velocity, and an increase in pulsatility. Under steady flow conditions, a flow reattachment point resulting from recirculation is observed with expanding steps, whereas a flow separation point is observed with contracting steps. Pulsatility decreases persistent stagnation at the flow separation point with contracting steps, whereas it completely eliminates persistent stagnation at the flow reattachment point with expanding steps. The results of this work conclusively show that stagnation can be reduced by increasing pulsatility and flow velocity and by decreasing step height.