Particle flow behaviour in models of branching vessels: I. Vortices in 90° T-junctions

Fluid mechanical factors are probably involved in the localization of atherosclerotic lesions and the deposition of platelet thrombi at arterial branches where secondary flows and vortices develop. Hence, we have studied the flow patterns in glass models of 3 mm diameter right angled T-junctions with square or rounded corners. With flow entering through the main tube, cine films of the paths of microspheres in dilute suspensions were taken at inflow Reynolds number Reo from 15 to 420 and flow ratios Q₁/Q₂ main: side tube from 0.05 to 4.0. In the square T-junction, paired vortices symmetrical about the common median plane formed at the entrances of the main and side daughter tubes over a wide range of Reo and Q₁/Q₂. Particles spiralled in open streamlines through the large main vortex; some then crossed above or below the mainstream to the side tube and through the side vortex, downstream of which there was a double helical flow. At high Q₁/Q₂, only the side vortex was present. At Q₁/Q₂ < 0.1 and Re₀ > 100, a third vortex formed downstream of the main vortex. In the rounded T-junction, the main vortex was formed at a lower, and the side vortex at a higher Reo than in the square T-junction. When flow entered through the side tube, paired connected vortices were also formed, but only when one daughter tube was severely occluded.