Inhalation-exhalation asymmetry in pressure-driven airflow through a population-derived median unilateral human nasal cavity
This study investigates inhalation-exhalation asymmetry in population-derived median unilateral human nasal cavity under pressure-driven breathing using time-resolved computational fluid dynamics (CFD). A standardized geometry reconstructed from averaged CT data of 30 adults was subjected to a sinusoidal nasopharyngeal pressure waveform over a complete breathing cycle. Low, intermediate, and high pressure amplitudes were considered to quantify breathing-phase-dependent changes in flow rate, Reynolds number, wall shear stress, nasal resistance, and olfactory-region airflow. For identical imposed pressure magnitudes, inhalation produced a consistently larger flow rate and lower aerodynamic resistance than exhalation, demonstrating a direction-dependent pressure-flow response. Increasing breathing intensity increased Reynolds number and nasal resistance while largely preserving the spatial pattern of the flow. During inhalation, stronger airflow penetrated further into the superior nasal cavity and produced higher and more localized wall shear stress, whereas exhalation exhibited smoother airflow redistribution. The breathing-phase asymmetry also affected regional ventilation: the fraction of total airflow reaching the olfactory region increased with inspiratory intensity and reached approximately 6% under the high-pressure condition, while the expiratory fraction remained comparatively insensitive to breathing intensity. These results demonstrate that pressure-driven transient simulations naturally capture geometry-dependent inhalation-exhalation differences that are suppressed when identical flow rates are prescribed for the two breathing phases, with consequences for both global nasal resistance and regional airflow delivery.