Modelling nasal airflow coefficients: an insight into the nature of airflow

Abstract

Background: There has been considerable discussion and conflicting views regarding the presence of laminar or turbulent flow within the nose. The aim of this study was to investigate how the modelling of variable flow coefficients can assist in the evaluation of the characteristics of flow in the resistive segments of the nose. Methodology: A comparison was made between the flow coefficient for the nasal valve, obtained from a mathematical model, and resistive flow components such as a Venturi meter and orifice tube. Also, a variable loss coefficient was formulated for the whole (unilateral) nose which, by utilising the intersection of the laminar and turbulent asymptotes, provided an estimation for the critical Reynolds number (R crit). Results: The results show that the flow resistance of the nasal valve is considerably greater than that for both a Venturi meter and an orifice tube implying turbulent or turbulent-like flow for much of nasal inspiration. Regarding the loss coefficient for the whole (unilateral) nose, normal respiration flowrates are displaced well away from the laminar asymptote. The critical Reynolds number was estimated to be 450. Conclusions: A novel method of determining the flow characteristics of the nose, particularly the critical Reynolds number, is presented. The analysis indicates a higher degree of turbulence than is assumed from a simple traditional calculation using a hydraulic diameter and flow through straight tubes. There are implications for computational fluid dynamics (CFD) modelling where either the entire nasal airflow is assumed to be laminar or a low turbulence model implemented.