The horseshoe vortex: a secondary flow generated in arteries with stenosis, bifurcation, and branchings

In order to elucidate the fluid dynamic feature of arterial blood flow, the present flow visualization study was carried out with various transparent blood vessel models having a protuberance, a bifurcation, or branchings. The observed flow patterns could be understood in terms of occurrence of a secondary flow, named the horseshoe vortex. The mode of generation of the horseshoe vortex in a tube with a protuberance projecting into the boundary layer was explained as follows. A radial pressure gradient toward the tube wall was produced along the upstream surface of the protuberance because of the interaction between the viscous sheared flow and the wall. This pressure gradient made fluid particles turn round downward directly before the obstacle. Then they curled round on themselves and formed a bound vortex tube, the horseshoe vortex, which in turn passed round the front of the protuberance in both directions. In a tube with a Y-shaped bifurcation or rectangular side branch, the flow divider at the branching site acted in place of the protuberance to produce a vortex tube similar in pattern to the horseshoe vortex. The vortex tube extended from the high pressure region, i. e. the apex of the flow divider, to the low pressure region, i. e. the lateral margin of the branch orifice, and generated swirling secondary flows in the main and branched tubes. These results suggested that the following mechanical factors might initiate or facilitate athero- and thrombogenesis: collision of blood cells captured by the horseshoe vortex with blood vessel walls, the interaction of the walls and blood cells due to turbulence, and the occurrence of localized high wall shear stresses.