# In-Vitro Experimental Modeling of Oscillatory Respiratory Flow in a CT-Scanned OSAHS Tract
## 4 Results and Discussion
### Reversal Phases  
At phase one, the respiratory flows are undergoing transition from expiration to inspiration (Figure 6a). The flow patterns presented are irregular both at the A-A section and the B-B section. The reversal flow and jet flow structures can be clearly observed at the A-A section. The interaction between the expiration and inspiration flows leads to strong flow reversal at the oral cavity and oropharynx region, which is hardly considered as either expiration or inspiration pattern. However, the local flow structure at the B-B section mainly corresponds to the expiration pattern with a large portion of expiration flow at the mainstream region, except only a small portion of reversal flow at the near wall region. The flow pattern at the expiration-inspiration phase is more complex than that observed in [PERSON] and [PERSON] [22], who claimed that the flow at this phase could be regarded as expiration flow. This difference may be caused by more realistic oscillatory flow rate boundary conditions used in this research rather than the simplified sine-law flow rate boundary conditions used in [PERSON] and [PERSON] [22]. Such jet-reversal flow structure is an indicator of complex  
Figure 7: Measured main-stream velocities at four cross-sections at four representative phases: (**a**) cross-section 1; (**b**) cross-section 2; (**c**) cross-section 3; (**d**) cross-section 4. Locations of these four cross-sections are defined in Figure 2d.  
expiration-inspiration transition at the quasi-static conditions where the net respiratory flow rate is zero while the unsteady inertial force and thus the respiratory flow acceleration and velocity are not zero.  
We can also see that compared with the quasi-static condition at phase one, the jet-reversal flow structure occurs even more obviously at phase three (Figure 6b), due to the milder transition from inspiration to expiration (Figure 4). Neither an inspiration nor an expiration pattern is the dominating flow pattern at phase three. The jet-reversal flow structure extends into the oral cavity, oropharynx, laryngopharynx and trachea (Figure 6b, A-A section), and the widely spread reversal flow vortexes at the obstructed region (Figure 6b, B-B section). The inspiration-expiration pattern is very different from that in [PERSON] and [PERSON] (2020), who considered the flow at this phase to can be regarded as inspiration flow.  
From the reversal transients, we could conclude that the realistic oscillatory respiratory boundary condition and the realistic model have a significant effect on respiration pattern, and associated flow structures. This observation can be clearly confirmed in Figure 7. Although the main-stream velocity profiles at reversal phases one and three are relatively small, their patterns are not similar to those at inspiration phase two and expiration phase four. Although quite few numerical and experimental studies focused on the reversal transients, the present PIV measurements confirm that the respiration patterns at the reversal phases can neither be regarded as the pattern of inspiration nor that of expiration due to the occurrence of jet-reversal flow and quasi-static condition.