# In-Vitro Experimental Modeling of Oscillatory Respiratory Flow in a CT-Scanned OSAHS Tract
## 4 Results and Discussion
### Peak Phases  
At peak inspiratory transient (phase two), the inspiration flows form an obvious jet-wake structure and a strong flow reversal (Figures 6c and 7b). Jet-wake flow structure, which often occurs in the human trachea and bronchi due to wall curvature [7], also appears at this phase in the ETA model with OSAHS. High-speed jet fluids accumulate at the dorsal side of the laryngopharynx region, while the anterior side of the trachea (Figure 6c, A-A section, Figure 7a,b). The narrowed tract does have a significant impact on inspiration flow at this phase with a clear injection phenomenon. As shown in the enlarged vector diagrams at the phase of peak inspiration, the jet-wake structure together with the flow reversal enhances fluid mixing and heat transfer, which benefits the velocity, temperature and humidity conditioning of the inhaled air as confirmed by [PERSON] et al. (2019). At the peak expiration phase (phase four), however, the expiration flow shows very different flow patterns (Figure 6d). The high speed jet fluids accumulate at the anterior side of the laryngopharyngeal region and the dorsal side of the trachea (Figure 6d, A-A section, Figure 7a,b), which are at opposite sides of the tract at the peak inspiration phase (Figure 6c, A-A section). Such differences may be due to the combined effect of convection and viscous motions in the curved tract and complex wall attachments. The injection phenomenon caused by the narrowed region resembles the numerical observation in [PERSON] et al. (2019). It should be noted that we used the oropharyngeal model in our research rather than the nasopharyngeal model used in [PERSON]'s study. The flow patterns at the trachea region are very different with the results in [PERSON] et al. (2019) using a simplified model and the results in [PERSON] et al. (2019) using an idealized model. In those studies, the expiration flow patterns at the trachea region were quite regular and presented straight tube-like flow. The differences may be caused by the more realistic model used in this research. From the results above, we can clearly conclude that the inspiration and expiration flow patterns are quite different from each other. High speed jet fluids appeared at the opposite sides of the oropharynx, laryngopharynx and trachea regions, which was not highlighted in the previous similar numerical studies [6; 14; 19; 26; 37]. Expiration process is not a simple reversal process of inspiration. Considering that the patient's nasal passageway was completely blocked, we believe that the collapse of the passageway most likely occurs at the peak inspiration phase at the obstructed region, thus resulting in apnea. There are two main reasons for this: firstly, the effect of obstructed tract on oscillatory respiratory flow fields at peak phases is much more apparent than that at the reversal phases. The occurrence of injection phenomenon at the narrowing obstructed area at the peak inspiration phase indicates very large pressure differences over this short, obstructed distance [13]. Secondly, at inspiration phase, the driving force induced by the movements of the thoracic cavity and abdominal cavity is a negative pressure. The combination of large pressure difference across the obstructed area and negative pressure at the peak inspiration phase would more easily induce collapse of passageway wall and eventual blockage of the tract, thus resulting in apnea. This condition worsens and becomes more dangerous for the deep-sleeping OSAHS patient, whose passageway tissue walls become very loose. Overall, we presented a temporal and spatial evolution of the oscillatory respiratory flow patterns within a CT-scanned model with OSAHS using PIV measurements. It should be recognized that the calibrated CFD (computational fluid dynamics) method can provide even more flow details that are hardly, if not impossible, captured with PIV techniques [39], such as pressure drop and airway resistance in the CT-scanned complex geometry.