CFD analyses

All CFD models were created using Nagare (iCFD, Tokyo, Japan). We calculated the unsteady-state of the almost laminar flow between the nostrils and epipharynx using DNS, without approximate calculation models, (time for inspiration = 2.5 s, time for exhalation = 2.5 s, flow rate [tidal volume] = 630 mL, non-slip wall condition). The flow rate mimics the human respiratory curve using a sine curve. The site of inspiratory outflow is the lower nasopharynx, while the nostrils are the sites of expiratory outflow. We visualized and calculated the pressure drop, velocity, and flow rate within the olfactory cleft bilaterally in the same interval used for cut-off point examination using ParaView (Los Alamos National Laboratory et al, New Mexico, USA). The pressure drop—indicating energy dissipation within the measurement interval—was defined as the pressure difference between the plane containing nasal columella and the lower plane of the epipharynx at maximum flow, or nnPD.