Axial and Secondary Flow Study in a 90 Deg Bifurcation Under Pulsating Conditions Using PIV

Abstract

Both axial and secondary flow were experimentally studied in a 90-degree bifurcation of square tubes, under pulsating conditions and equal branch flow rates. The examined peak Reynolds number (Re) was in the range 330–830, and the Womersley parameter (α) between 5 and 15. In the extension of the mother tube during deceleration, axial flow separated from the wall opposite of the 90-degree branch. The formed shear layer rolled up, for higher values of Re and low α, generating discrete vortices. However, for the highest value of α, the shear layer was limited close to the wall, without rolling up, for all the examined Re. During acceleration, all recirculation zones were washed out and the flow became attached before flow peak. In the 90-degree branch during acceleration, a vortex was formed at the bifurcation apex and moved streamwise towards the center of the cross-section where it degenerated. The secondary flow of this branch had the following characteristics: during deceleration two Dean vortices were located close to the outer bend side of the cross-section, and their strength was progressively reduced. A little before flow peak any Dean vortices of the previous phase were destroyed and two new ones were born. Maximum secondary velocities were on the order of 20% higher than the bulk velocity of the branch.