A model study of flow dynamics in human central airways. Part II: secondary flow velocities.

Secondary velocity components perpendicular to the tube axis were measured in a 3 : 1 scale model of the human central airways. Slanted hot-wire probes were introduced axially in order to measure the secondary velocities at about 12 points for each of the 7 stations investigated. Secondary velocities in the inspiratory direction never exceeded a mean value of 18% of the mean axial velocity. Secondary velocities in the expiratory direction reached a mean value of 21.5% of the mean axial velocity. In the inspiratory direction, two unequal eddies were formed in the left main bronchus and in the right upper lobe. Moreover, maximum velocities were observed near the wall and the decay of secondary velocities in the left main bronchus was observed. The secondary flow patterns observed in the left upper and lower lobes after the secondary bifurcation were difficult to recognize, although they seemed to be more influenced by the second bifurcation. The complexity of the flow pattern was reinforced by viscous effects acting near the wall. In the expiratory direction, only two stations in the trachea were measured; four uneven eddies seemed to have existed, with the ventral eddies appearing to be predominant. Overall, the secondary velocity magnitudes as well as the patterns of eddies were very dependent on the geometry of the model used.