Mucus transport and distribution by steady expiration in an idealized airway geometry

Mucus clearance from the airways is vital to reduce the risk of infection and to improve pulmonary function. The removal of mucus is propelled either by a rhythmic ciliary motion or shear-induced by turbulent expiratory airflow. However, in chronic airway diseases, the mucociliary motion is impaired due to mucus hypersecretion and altered biophysical properties. As a result, the ciliary motion is insufficient to remove mucus from the airways and expiratory airflow plays the more dominant role. In this work, the role of expiratory airflow in pathologic mucus clearance was investigated in a three-dimensional idealized bifurcating lung geometry. The two-phase air-mucus annular flow was investigated using a homogeneous flow approach and the complex interface (free-surface) was tracked by employing the volume-of-fluid (VOF) method. Flow turbulence was modeled using the k - ω shear stress transport (SST) model to examine the role of mucus viscosity, airflow rate, gravity, and airway branching on mucus distribution. It was observed that a gravity dominated eccentric core-annular flow developed in the daughter branches. The eccentricity varied with the angle of inclination, curvature, airflow rate, and mucus viscosity, which affected the merging location of airflow from the daughter branches. A mucus secondary flow developed due to the curvature in the airways and caused a local redistribution of mucus reducing the eccentricity. In addition, it was also observed that the thickness of the mucus layer was affected by the secondary airflow in the parent branch. These results emphasize the importance of accurate modeling of mucus-lined airways and indicate that an effective clearance therapy can be devised by analyzing the distribution of mucus in the airway tree.