Flow characteristics in a nasal to tracheo-bronchial airway using a scale resolving simulation

Background and Objective Airflow dynamics are fundamental to understanding respiratory function, yet the complexity of flow regimes within the nasal and tracheo-bronchial pathways remains poorly characterised. This study employed a patient-specific respiratory model, extending from the nasal cavity to the 7^th-generation bronchioles, to investigate airflow characteristics at a steady inhalation rate of 30 L/min. Methods A hybrid Stress-Blended Eddy Simulation (SBES) model was used to capture unsteady flow structures, while Dynamic Mode Decomposition (DMD) and spectral analysis were applied to identify dominant flow dynamics. The computational model, derived from CT scans, used a poly-hexcore mesh with ∼ 30 million cells. This study provides a unified, scale-resolving characterisation of airflow from the nasal cavity to distal bronchioles, linking flow behaviour across regions using modal and spectral diagnostics. Results The results revealed laminar-dominant flow in the nasal cavity, with velocities increasing to 6.2-8.4 m/s, and high-velocity jet formation in the laryngeal region with peak velocities of up to 14.9 m/s. This region exhibited enhanced unsteadiness and strong oscillatory behaviour, with resolved turbulence kinetic energy reaching up to 35 m²/s². Downstream, flow progressively transitioned toward laminar-dominant conditions in distal bronchioles, with reduced velocity (0.8-3.6 m/s) and low energy content. Coherent structures identified via the q-criterion and DMD highlighted regions of energy redistribution and high-frequency oscillations. Flow partitioning between left and right bronchial branches showed asymmetry, with total distributions of 44.65% and 55.35%, respectively. Conclusions This study provides a unified characterisation of airflow dynamics across the upper and lower airways, identifying key regions of enhanced unsteadiness and downstream energy dissipation, with implications for respiratory modelling and device design.