A model study of flow dynamics in human central airways. Part I: axial velocity profiles.

We measured detailed steady inspiratory and expiratory velocity profiles in a 3:1 scale model of the human central airways. The model was constructed out of acrylic plastic, mounted vertically, and connected to a specially designed steady-flow system. Laterally introduced hot-wire anemoneter probes were used to record axial velocities along 4 diameters at each of the 12 pre-drilled stations of measurement; the flow distribution among the five lobar bronchi was controlled by distally positioned linear resistors. Whether with a flat entrance profile or entering as a narrow jet, the inspiratory flow velocity profiles in the frontal plane showed a high degree of asymmetry in all branches, with peak velocities near the inner wall of the bifurcation. In the sagittal plane the velocity profiles were nearly symmetric, exhibiting a single peak near the center in the frontal plane and almost flat in the sagittal plane. Overall, the velocity profiles were more sensitive to airway geometry than to flow rate. The only site of flow separation was observed in the right upper lobar bronchus. The most evident modification of axial velocity profiles in a single branch was found in the left main bronchus during expiratory flow.