Numerical study of the airflow structures in an idealized mouth-throat under light and heavy breathing intensities using large eddy simulation.

An excellent understanding of the airflow structures is critical to enhance the efficiency of drug delivery via the human oral airway. The present paper investigates the characteristics of both steady and unsteady airflow structures in an idealized mouth-throat using large eddy simulation (LES). Representative inhalation flow rates of 15L/min at rest and 60L/min in exercise are considered. The study shows that there are more secondary vortices in the pharynx and the laryngeal jet is much longer and more concave in the steady flow field at 15L/min compared to the higher inspiration rate, which decreases the possibility of drug impinging on the wall. In contrast, the laryngeal jet is much more unsteady at heavy breathing and its strong interaction with the recirculation zone in the trachea leads to a enlarged mixing zone, increasing the possibility for carrying the particles from the laryngeal jet into the recirculation zone, which will lead to a longer residence time of the particles in the trachea and this increases the possibility of drug deposition in this area. In addition, the recirculation zone size is larger, the separation region is far away from glottis, and the reversed flow is slower at light compared to heavy breathing. In conclusion, these airflow structures show distinct properties at light and heavy breathing conditions, particularly in the unsteady flow field. The study provides evidence about the physical processes leading to both enlarged mixing zones and recirculation zones. It is known that stronger secondary vortices, a stronger laryngeal jet and enlarged recirculation zones definitely increase the particle deposition in the upper airway. The present paper aims to uncover the physical properties of the airflow for different breathing conditions, and their detailed effect on particle deposition will be studied in future.