Flow patterns in stenotic blood vessel models

A model study was undertaken to investigate disturbances of blood flow through stenotic blood vessels. Both axisymmetric and nonsymmetric models, having different diameter ratios of constriction, were used in the experiments. (1) Experiments with steady flow. A sudden decrease in the critical Reynolds number took place as the degree of axisymmetric constriction increased. Even at Reynolds numbers far below the critical value, a region of flow separation was clearly seen just behind the constriction. An eddying wake inside the separated region spread downstream as the Reynolds number increased. At the critical Reynolds number vortices were formed at the distal end of the wake and shed downstream in succession. Two symmetrical standing eddies were noticed within the separated region behind a hemispherical bulge projecting into the boundary layer. Further increase in Reynolds number resulted in the formation of secondary flow. The horse-shoe vortex, which passed round the front of the bulge in both directions and led to a vortex pair trailing downstream, is a secondary flow ensued from a radial pressure gradient generated by a bluff obstacle within the boundary layer. (2) Experiments with nonsteady flow. A striking feature of a pulsatile laminar flow through a circular cylinder was the appearance of reverse flow near the wall at the end of the decelerating phase. The presence of an axisymmetric constriction caused pulsatile disturbances. With the progress of acceleration, large vortices were formed near the constriction, shed downstream and broken down into turbulence. The turbulent flow thus formed did not diminish during the decelerating phase but spread upstream against the flow direction until the start of next accelerating phase. Pulsation seemed to facilitate not only the production of vortices but also the backward spread of turbulence formed downstream.