**3. Conclusions**

A complete evaluation is required to fully understand OSA's UA behavior and make more accurate pretreatment predictions. The simplified model is insufficient for effectively predicting the flow behavior of the human UA. It is also agreeable that the inhalation airflow shows higher airflow, which defines the air inlet's location in the upper airway model. The nasal cavity adds more complexity to the investigation; thus, removing it is necessary. The highlighted technique of removing some details of voids and surface smoothing in the modelling of the upper airway reduces the complexity of the CAD model. Complex geometry limits excellent meshing capabilities and results in weak CFD convergence. This technique is a good reference; however, it needs more published data on sensitivity or specificity. Additionally, this article emphasized the importance of a precise geometry construction technique for the UA geometry-accurate model, which is critical for the CFD application's reliability and accuracy in comprehending OSAS. It is undeniable that good quality meshing is required for solution convergence, which has increased the demand for detailed advanced meshing approaches.

A comparable 1:1 scale physical model is necessary to determine the appropriate cell number or element size for CFD modelling. Through grid sensitivity analysis, a grid size agreeable with accurate experimental data is the ideal mesh size reference for modelling the UA. However, the data reading in the physical validation should be pressure-based, as it shows good agreemen<sup>t</sup> with the CFD validation.

The k– ω SST model is the most economical, as it has advantages in solving complex transitional flow and provides an enhanced description of flows involving adverse pressure gradients and curved boundary layers. However, it is vital to have an excellent comparison with the physical model to justify the ideal turbulence model for the UA study.

An idealized geometry focused on the UA from the pharynx until the larynx was greatly needed. It should have a better shape agreemen<sup>t</sup> than the average-person UA and not be as oversimplified as the Weibel model. The idealized model should have detailed geometry that another researcher efficiently replicates. Thus, it is concluded that the idealized geometry is inadequate at predicting the features of the flow of human UA.

**Author Contributions:** Conceptualization, M.F.A.L, N.N.N.G. and M.F.A.; methodology, M.F.A.L, N.N.N.G., M.F.A., N.B.I. and R.M.R.; software, I.A.B., S.K., M.H., N.A.A. and A.K.; validation, I.A.B., S.K., M.H., N.A.A. and A.K.; formal analysis, I.A.B., S.K., M.H., N.A.A. and A.K.; investigation, M.F.A.L., N.N.N.G., M.F.A., N.B.I. and R.M.R.; resources, I.A.B., S.K., M.H., N.A.A. and A.K.; data curation, I.A.B., S.K., M.H., N.A.A. and A.K.; writing—original draft preparation, M.F.A.L, N.N.N.G. and M.F.A.; writing—review and editing, N.B.I., R.M.R., I.A.B., S.K. and M.H.; visualization, N.N.N.G., N.B.I. and R.M.R.; supervision, N.N.N.G. and N.B.I.; project administration, N.B.I. and R.M.R.; funding acquisition, N.B.I., S.K. and M.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** King Khalid University under gran<sup>t</sup> number RGP 2/101/43.

**Data Availability Statement:** Data available in manuscript itself.

**Acknowledgments:** The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through the Large Groups Project under gran<sup>t</sup> number RGP 2/101/43.

#### **Conflicts of Interest:** The authors declare no conflict of interest.
