Interaction of laminar airflow with viscoelastic airway mucus.

Basic wave interaction mechanism between the laminar airflow and viscoelastic layer in a rigid tube is investigated numerically. The purpose is to explore the effect of mucus viscoelasticity on the stability of the coupled airflow-mucus system in pulmonary airways under clinical conditions where the serous layer is absent. The results indicate that the onset flow speed, for the initiation of unstable surface waves, is very sensitive to mucus viscosity and it may be as high as 35 times the elastic case for a very viscous mucus with the same elasticity. While the onset speed and wavelength increases, wave speed decreases with increasing mucus viscosity, reducing from about 40% of the flow speed for elastic mucus to less than 1% for a very viscous mucus. Also, a case study for a patient with chronic bronchitis shows that large amplitude waves may form on the mucus surface during forced expiration.