Velocity measurements were made in the downstream branch of a symmetric rigid glass model of an artery bifurcation for both steady and pulsatile flows. A laser Doppler anemometer was used to make the measurements. The symmetric model had a bifurcation angle of 90° and an area ratio (ratio of daughter tubes cross sectional area to mother tube cross sectional area) of 1.2. The Reynolds numbers based upon the upstream flow investigated for steady flow were 600, 1200, 1800, 2400 and 3000 with flow division percentages of 70%, 60%, 50%, 40%, 30% in the branch being studied. For pulsatile flow only a 50/50 division between daughter tubes was employed and the mother tube average Reynolds numbers (Reynolds number based on velocity averaged in time and space) used were 900 and 450. The Womersley parameter for the pulsatile flow studies was 33. Measurements of velocity versus time were also made for steady flow to allow for a better understanding of the nature of the unsteadiness in the flow just after the bifurcation.
This study shows the existence of secondary motions that play a very important role in the development of the flow and in the magnitude of shear stresses at the wall near the outside of the branch. The magnitude of these secondary velocities could reach up to 50% of average mother tube velocities. A small separation region near the outside wall is evident, from the steady flow data, that varies in size depending on the flow conditions present at the time. Helical motions are seen within this area. The velocity versus time plots for steady flows show very large fluctuations in the value of axial and vertical velocities near the outside part of the flow. These fluctuations can be as large as 500% of the value of the local average velocities, and thus show that even for steady flows very large fluctuations in the shear stress are present at the outside wall. No evidence was obtained to show that these fluctuations were periodic in nature. In pulsatile flow, the separation region plays a very dominant role during diastole and is not present during systole. The shear stresses for pulsatile flow at the inside wall are very high and unidirectional while at the outside wall they are oscillatory and low in absolute value.
