On boundary-layer and free-shear resistances in the human airways

The airway resistance has been reported to be greater on expiration than inspiration. To understand the underlying mechanism, we perform large eddy simulation of airflow in the 3D CT-resolved 7-generation airways constrained by physiologically-consistent lobar ventilation. The dimensionless viscous pressure drops in all the airway segments exhibit a similarity behavior proportional to \textit{(ReD/L)}$^{n}$ with the average optimal values of 1.4 and 1.6 for inspiration and expiration, respectively, where \textit{Re} is the Reynolds number, and $D$ and $L$ are the respective average diameter and length of an airway segment. It is found that the dissipations in the boundary layer as well as the free-shear core flow contribute to the airway resistance, thus the n value. Flow is partitioned to examine the roles played by the boundary layer and the free-shear flow, respectively. A hypothesis is proposed to explain higher airway resistance on expiration.