<title>LDA measurements of merging flows in a symmetric curved junction model</title>

Abstract

A model symmetric junction is used to simulate steady expiratory flow in the upper part of human central airways. Velocity profiles were measured with a two color, two component laser Doppler anemometer at three Reynolds number levels of 518, 1036 and 2089. The test section is a symmetric junction of constant cross-sectional area with a branching angle of 70 degrees. The inlet flow rate into the two bronchi was the same for all Reynolds numbers studied. Results show that in the junction plane, velocity profiles in the bronchi are skewed towards the inner walls. In the trachea, the biconcave shape of the profile (i.e., a dip at the center of the profile) just downstream of the carina is rapidly transformed into a velocity spike. In a plane transverse to the junction plane, parabolic velocity distribution was conserved through the daughter branches. In the parent tube, following merging, the transverse profiles become flat as the flow proceeds downstream of the junction point. Further downstream towards the termination of the trunk of the junction the velocity profile develops a defect at the center. The velocity defect, nonetheless, is confined to a small region only in the vicinity of the centerline. Strong secondary helical motion is chiefly responsible for this type of velocity field in the junction.