We investigated the flow characteristics and heat transfer during nasal breathing in the complete human upper airway through the respiratory cycle using transient numerical simulations. We postulate the complete airway from the nasal cavity to the trachea most accurately represents dynamic airflow patterns during inhalation and exhalation as they are likely to be affected by downstream anatomical structures. We constructed a 3-Dimensional model from a healthy adult computed tomography (CT) scan. Computational Fluid Dynamics simulations were performed with Ansys Fluent software using the Stress-Blended Eddy Simulation (SBES) turbulence model, looking at airflow patterns, velocity, mucosal temperature and humidity (H₂O fraction). We simulated one-and-a-half breathing cycles (5.65 seconds) and discarded the first inhalation cycle to avoid start-up effects. The results demonstrated that airway geometry structures, including the turbinates, the soft palate and the glottic region, affect the flow patterns differently during inspiration and expiration. It also demonstrated phenomena not seen in steady flow simulations or those without the lower respiratory tract geometry, including the nasopharyngeal temperature imprint during inhalation, the nasopharyngeal jet during exhalation and the flow structures of the larynx and laryngeal jet. The inclusion of the exhalation phase demonstrates airflow pre-conditioning before inhalation, which we postulate contributes to achieving alveolar conditions. Alveolar temperature and humidity conditions are not achieved by the nasal cavity alone, and we demonstrate the contribution of the nasopharynx and larynx to air conditioning. Including the complete airway with realistic anatomy and using transient airflow modelling provided new insights into the physiology of the respiratory cycle.

The movement of the respiratory walls has a significant impact on airflow through the respiratory tract. The majority of computational fluid dynamics (CFD) studies assume a static geometry which may not provide a realistic flow field. Furthermore, many studies use Reynolds Averaged Navier-Stokes (RANS) turbulence models that do not resolve turbulence structure. Combining the application of advanced scale-resolving turbulence models with moving respiratory walls using CFD will provide detailed insights into respiratory flow structures. This study simulated a complete breathing cycle involving inhalation and exhalation in a nasal cavity to trachea geometry that incorporated moving glottis walls. A second breathing cycle was simulated with static glottis walls for comparison. A recently developed hybrid RANS-LES turbulence model, the Stress-Blended Eddy Simulation (SBES), was incorporated to resolve turbulent flow structures in fine detail for both transient simulations. Transient results were compared with steady-state RANS simulations for the same respiratory geometry. Glottis motion caused substantial effects on flow structure through the complete breathing cycle. Significant flow structure and velocity variations were observed due to glottal motion, primarily in the larynx and trachea. Resolved turbulence structures using SBES showed an intense mixing section in the glottis region during inhalation and in the nasopharynx during expiration, which was not present in the RANS simulations. Transient simulations of a realistic breathing cycle uncovered flow structures absent in simulations with a constant flow rate. Furthermore, the incorporation of glottis motion impacted airflow characteristics that suggest rigid respiratory walls do not accurately describe respiratory flow. Future research in respiratory airflow should be conducted using transient scale-resolving models in conjunction with moving respiratory walls to capture flow structures in detail.

Stenosis geometries are constrictions of a biological tube that can be found in many forms in the human body. Capturing the flow field in such geometries is important. For this purpose, simulations were performed using the generalised k-$\omega$ (GEKO) turbulence model to study flow through stenosis geometries with throat constrictions of 75, 50 and 25% area reduction. Laminar flow conditions of Re = 2000 and 1000 were applied and the results were compared with experimental data. The effect of four GEKO parameters ($C_{SEP}$, $C_{NW}$, $C_{JET}$ and $C_{MIX}$) on flow in the post-stenotic region was investigated by simulating a wide range of parameter values. Results showed that the $C_{MIX}$ parameter, combined with a modified GEKO blending function, had the greatest effect on axial velocity, velocity fluctuations and the location of the jet breakdown region. A $C_{MIX}$ value of 0.4 closely matched the experimental results for a 75% area reduction stenosis at $Re=2000$ and showed significant improvements over existing Reynolds-averaged Navier–Stokes models. The GEKO model was also able to closely match the axial velocity results predicted by previously published large-eddy simulation models under the same flow conditions. Furthermore, the GEKO model was applied to a realistic oral-to-tracheal airway model for a Reynolds number of 2000 and produced results consistent with the idealised stenotic tube.

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.

The flow characteristics and heat transfer during nasal breathing in the complete human upper airway were investigated through the respiratory cycle using transient numerical simulations. We postulate that the complete airway from the nasal cavity to the trachea most accurately represents dynamic airflow patterns during inhalation and exhalation as they are likely to be affected by downstream anatomical structures. A 3D model was constructed from a healthy adult computed tomography scan. Computational fluid dynamics simulations were performed with Ansys Fluent software [ANSYS Fluent, R1 User's Guide (ANSYS, Inc., 2020)] using the stress-blended eddy simulation turbulence model looking at airflow patterns, velocity, mucosal temperature, and humidity (H2O fraction). One and a half breathing cycles were simulated for a total of 5.65 s, where the first inhalation cycle was discarded to avoid start-up effects. The results demonstrated that airway geometry structures, including the turbinates, the soft palate, and the glottic region, affect the flow patterns differently during inspiration and expiration. It also demonstrated phenomena not seen in steady flow simulations or in those without the lower respiratory tract geometry, including the nasopharyngeal temperature imprint during inhalation, the nasopharyngeal jet during exhalation, and the flow structures of the larynx and laryngeal jet. The inclusion of the exhalation phase demonstrates airflow preconditioning before inhalation, which we postulate contributes to achieving alveolar conditions. Alveolar temperature and humidity conditions are not achieved by the nasal cavity alone, and we demonstrate the contribution of the nasopharynx and larynx to air conditioning. Including the complete airway with realistic anatomy and using transient airflow modeling provided new insights into the physiology of the respiratory cycle.

The flow characteristics and heat transfer during nasal breathing in the complete human upper airway were investigated through the respiratory cycle using transient numerical simulations. We postulate that the complete airway from the nasal cavity to the trachea most accurately represents dynamic airflow patterns during inhalation and exhalation as they are likely to be affected by downstream anatomical structures. A 3D model was constructed from a healthy adult computed tomography scan. Computational fluid dynamics simulations were performed with Ansys Fluent software [ANSYS Fluent, R1 User's Guide (ANSYS, Inc., 2020)] using the stress-blended eddy simulation turbulence model looking at airflow patterns, velocity, mucosal temperature, and humidity (H2O fraction). One and a half breathing cycles were simulated for a total of 5.65 s, where the first inhalation cycle was discarded to avoid start-up effects. The results demonstrated that airway geometry structures, including the turbinates, the soft palate, and the glottic region, affect the flow patterns differently during inspiration and expiration. It also demonstrated phenomena not seen in steady flow simulations or in those without the lower respiratory tract geometry, including the nasopharyngeal temperature imprint during inhalation, the nasopharyngeal jet during exhalation, and the flow structures of the larynx and laryngeal jet. The inclusion of the exhalation phase demonstrates airflow preconditioning before inhalation, which we postulate contributes to achieving alveolar conditions. Alveolar temperature and humidity conditions are not achieved by the nasal cavity alone, and we demonstrate the contribution of the nasopharynx and larynx to air conditioning. Including the complete airway with realistic anatomy and using transient airflow modeling provided new insights into the physiology of the respiratory cycle.