Three-Dimensional Instabilities and Transition in Pulsatile Stenotic Flows

A straight tube with a smooth axisymmetric constriction is an idealised representation of a stenosed artery. We examine the three-dimensional instability of steady flow plus an oscillatory component in a tube with a smooth 75% stenosis using both linear stability analysis and direct numerical simulation. These flows become unstable through a subcritical period-doubling bifurcation involving alternating tilting of the vortex rings that are ejected from the throat with each pulse. These tilted vortex rings rapidly break down through a self-induction mechanism within the confines of the tube. While the linear instability modes for pulsatile flow have maximum energy well downstream of the stenosis, we have established using direct numerical simulation that breakdown can gradually propagate upstream until it occurs within a few tube diameters of the constriction, in agreement with previous experimental observations.