Effects of glottis motion on airflow and energy expenditure in a human upper airway model

The periodic movement of the glottal aperture during tidal breathing has been long recognized as a physiological factor in regulating the respiratory airflow dynamics. However, studies of the dynamic glottis and its functions are scarce due to the complex respiratory anatomy, lack of glottis kinematics, and challenges in simulating moving structures. The objective of this study is to numerically investigate the influences of the glottis motion on airflow features and energy expenditure in an image-based human upper airway model. To examine the relative importance of glottal motion and tidal breathing, both static and dynamic glottal apertures were considered using large eddy simulation under either constant or sinusoidal breathing profiles. The glottis was specified to move in phase with the inhalation profile, which widens and contracts periodically at an amplitude typical of a human adult. Results show highly oscillating features of the instantaneous main and secondary flows for all breathing scenarios considered, indicating the inadequacy of time-averaged steady simulations in dynamic respiratory studies. The glottal aperture and cyclic flow both modified the laryngeal jet instability and vortex generation by varying the main flow speed. However, the cyclic flow has a greater impact on the main flow instability (streamwise direction), while the glottal motion has a greater impact on secondary flows (transverse direction) associated with swirling flows, flow separation, and vortex shedding. A widening glottis during inhalation was observed to significantly postpone the development of vortices, flow oscillation, and intra-glottal pressure drop, which might have key biological implications in alleviating the diaphragm muscle effort and reducing the risk of pharyngeal wall collapse.