Mechanisms of unidirectional flow generation and gas transport enhancement in oscillatory flow of avian lungs were numerically studied with bifurcation models of avian trachea. Model systems of right-angled branched tubes with single and multi-bifurcating tubes were adopted for the numerical simulation. The effect of constriction near the bifurcation was also investigated. Main concern was placed on the effects of bifurcation structure on unidirectional flow generation and axial gas transport enhancement. Oscillatory flow fields as well as concentration fields were numerically analyzed with a developed code based on SIMPLE and CIP methods. Similar to the previous experiment by the authors, unidirectional net flow was observed from side-daughter tube to daughter tube. Strongly affecting on the oscillatory flow fields and flow distribution, the constriction and the multi-bifurcation structure were found to improve unidirectional net flow generation. In particular, multi-bifurcation structure was found not only to improve the unidirectional flow generation and gas transport but also to reduce the total pressure drop of the system.
