The tracheal flow field shapes particle transport into the lower airways and thus influences both the spread of inhaled pathogens and the effectiveness of aerosol-based therapies. Identifying how different inhalation routes modify the flow field is therefore crucial for understanding lower-airway disease transmission and for guiding targeted drug delivery. To gain a detailed understanding of the influence of the inhalation route on the flow structures in the human trachea, the flow field in the trachea is investigated in vitro in a non-compliant, refractive-index matched silicone model of the human respiratory tract. The investigations comprise steady inhalation, and oscillatory flow to simulate calm breathing. A realistic breathing pattern is approximated by a sinusoidal waveform for two Reynolds numbers of $Re_{Tr} = [400, 1200]$, based on the bulk velocity at maximum volume flux and the hydraulic diameter of the trachea and two Womersley numbers of $Wo = [3, 4.5]$, representing the oscillation time scales. To capture the inherently three-dimensional and asymmetric nature of the flow field, 3D particle-tracking velocimetry measurements are performed using the Shake-The-Box algorithm. Using a refractive-index matched fluid consisting of water and glycerin, the complex flow structures inside the trachea are fully resolved. The PTV measurements confirm that the nasal and/or oral cavity must be considered when analyzing the flow field in the lower respiratory tract. In particular, we find that the presence of both cavities significantly alters the flow field compared to idealised, fully developed inflow conditions. However, velocity profiles in the sagittal and coronal plane in the trachea as well as contour plots of the of the normalized velocity magnitude evidence nearly identical flow structures for oral and nasal inhalation, indicating minimal influence of the inhalation route.
Recent years showed a significant increase in deaths from widespread respiratory diseases, such as chronic lower respiratory diseases, pneumonia, and influenza, with these diseases being responsible for approximately 7.2 % of deaths in 2019 [1]. The COVID-19 pandemic has since highlighted even more sharply how respiratory pathogens can spread through inhaled aerosols, drawing renewed attention to the mechanisms by which particles are transported and deposited within the airways. SARS-CoV-2 has demonstrated that small aerosol particles can remain suspended, travel through indoor air, and reach deeper regions of the respiratory tract, where the transmission and infection is strongly governed by the underlying flow structures. While airborne transmission is generally not considered the most efficient route of contagion [2], it nevertheless represents a critical pathway for numerous clinically significant pathogens, including Respiratory Syncytial Virus (RSV) [3], varicella (chickenpox) [4], and variola (smallpox) [5]. Consequently, a detailed analysis of airflow dynamics and mass transport within the respiratory tract is a prerequisite for understanding the spreading mechanisms of respiratory infections. In this context, the intricate and highly three-dimensional geometry of the airways possesses a distinct influence on particle and aerosol transport and deposition in the airways, emphasizing the need for a thorough investigation of flow structures in these regions. Usually, earlier few studies that included the upper airways is the methodological approach by Tauwald et al. [15], which analyzed the flow field inside a patient-specific human nasal cavity model based on a clinical CT scan. The flow field was investigated for physiological breathing cycles using Tomographic Particle-Image Velocimetry (Tomo PIV) with a phased-locked approach and repeated measurements to analyze unsteady flow phenomena during different phases of the breathing cycle. By providing high-spatial resolution flow data in different regions of the nasopharynx, the study emphasizes the relevance of the resulting flow structures induced by the different flow rates and the nasal geometry during the breathing cycle. It was shown that a small spiral-shaped structure affects airflow throughout the entire nasal geometry and surgical procedures in this area could disrupt airflow through the lower nasal turbinates and cause lasting problems.
Although these studies indicate that the geometry of the upper airways, i.e., the nasal and oral cavity significantly influence the flow features in the lower airways, systematic experimental studies quantifying this effect are scarce. Thus, this study focuses on the analysis of the influence of the route of inhalation, i.e. oral or nasal and the flow structures in the lower trachea. To explore this question, the present study employs time-resolved three-dimensional velocity measurements using the Shake-The-Box algorithm in a refractive-index matched patient-specific airway model from the oral and nasal inlets down to the lower trachea, capturing the unsteady oscillatory flow structures that propagate into the primary bifurcation. In a resting state, the tidal volume is about 500 mL per breathing cycle. With a quiet breathing rate of around 0.1 Hz, the average airflow is approximately 300 L h -1 . These conditions result in a tracheal Reynolds number of Re T r = 400. Moderate ventilation, and thus increased tidal volume and breathing frequency, raises the average airway speed, leading to a tracheal Reynolds number of Re T r = 1200. Therefore, steady inhalation with Re T r = [400, 1200] and oscillatory actuation (Re T r = [400, 1200] and W o = [3, 4.5]) are used in this study. By keeping the boundary conditions identical for both inhalation routes, the study isolates the impact of oral versus nasal breathing, enabling a direct comparison of velocity distributions and flow structures in the lower trachea.
The airway geometry used in this study [16,10] is based on a lung model developed at the Brno University of Technology and was reconstructed from a three-dimensional CT scan of an adult Caucasian male [17]. This baseline model includes the oral cavity, the laryngeal region, and the tracheobronchial tree down to the 7th generation. To enable a comprehensive assessment of airflow and particle transport during nasal, oral, and combined breathing, the geometry was expanded by incorporating a nasal cavity provided by the University of California, Davis [18]. The nasal model, derived from CT data of a healthy 25-year-old male, was further refined using Rhinoceros 3D [19] and Star-CCM+, where paranasal sinuses were removed, artefacts corrected, and the surface smoothed to ensure numerical and experimental robustness. Finally, the nasal and oral cavities were anatomically aligned and merged following established morphological references [20]. The geometric data was provided by Dr. František Lízal at the Brno Universit
This content is AI-processed based on open access ArXiv data.