A boundary condition remapping technique for computational fluid dynamics simulations of nasal exhalation

During exhalation, complex geometry in the larynx generates the pharyngeal jet, where higher velocity air is directed to the rear of the airway, influencing airflow downstream in the nasal passage. This study investigates the impact of boundary condition settings on the accuracy of airflow simulations in truncated airway geometries during exhalation, focusing on the nasopharynx and nasal passage. In addition to traditional inlet profiles, we tested a new method of remapping a profile from a complete airway to a truncated airway.

Using remapped velocity profiles extracted from simulations of complete airways, we compare their performance against traditional inlet profiles, including uniform, parabolic, and power-law profiles. The performance of inlet profiles was tested in a set of airway geometries varying the inlet extension length, angle, and cross section.

The results demonstrate that the remapped boundary condition provides the most accurate representation of downstream flow, particularly in replicating the pharyngeal jet and lateral asymmetry, with minimal error at bends and bifurcations in the nasal passage when applied to a truncated version of the airway it is sourced from. The study highlights the limitations of extended geometries, showing that shorter extensions (3×D) yield lower errors than longer ones (5×D). Angling the extensions further improves accuracy by smoothing abrupt transitions. The parabolic profile offers no significant advantage over uniform and power-law profiles, emphasising the importance of selecting an appropriate boundary condition in capturing the complex flow characteristics of the laryngeal jet.

The proposed remapping technique generalises irregular velocity profiles to circular cross-sections, enabling their application across varied geometries without requiring full airway simulations. This method improves computational efficiency and accuracy, making it adaptable for diverse respiratory, cardiovascular, and industrial applications. Future work must examine a larger sample of larynx and nasal passage geometries to further validate the use of this technique in applying profiles across geometries.