Oscillatory Flow Structure and Unidirectional Flow in Model Avian Bifurcation

Abstract

In the circular channel of avian lungs, there exists unidirectional flow, which is thought to be induced by the mechanism of 'aerodynamic valves'. To clarify flow phenomena in the avian lung, oscillatory flow in a right angle branched tube was experimentally studied. Periodical variation of flow rate was measured with Particle-Imaging velocimetry (PIV). Velocity fields near the bifurcation were visualized with illuminated tracers and measured by PIV technique. The main concern was focused on the separation vortex near the bifurcation and convective inertia, which were found to cause unidirectional net flow from a side-daughter tube to a daughter tube. Based on the experimental observation, a flow model was proposed. It was suggested that the periodical variation of pressure drop induced by the separation vortex near the bifurcation in the inspiratory phase should work as the 'aerodynamic valves', and cause the unidirectional flow. Due to convective inertia, the unidirectional flow persisted even in the expiratory phase, in which no vortex existed near the bifurcation.