Modeling nasal airflow and olfactory mass transport

An anatomically correct, 20X enlarged scale model of the right healthy human nasal cavity was constructed from CAT scans for the study of nasal airflow patterns. Detailed velocity profiles for inspiratory and expiratory flows through the model were measured using a hot-film anemometer probe with 1mm spatial resolution. Steady flow rates equivalent to 1260, 630, and 260 cc/sec through the real human nose were studied. From the turbulent velocity profiles, the flow distributions throughout the nasal cavity were determined, and iso-velocity contour maps were constructed for various cross sections. The iso-velocity contour maps and volume flow distributions are similar for all inspiratory flow rates. It was found that about 50% of the inhaled air passes through the inferior and middle airways, and 14% through the olfactory region for all examined physiologic flow rates. The average value of the intensity of turbulence was determined to be about 2.5% in the main nasal cavity and in the olfactory region for all flow rates. Higher values of the intensity of turbulence (3-5%) were measured near the nasal valve and in the nasopharynx. Local mass transfer coefficients throughout the nasal cavity were determined from the velocity profiles and intensities of turbulence by momentum-mass transport analogy. The average value of the mass transfer coefficient in the olfactory region was found to be around 5cm/sec for the normal breathing rate in good agreement with the few other measurements in the literature. A theoretical model of olfaction involving all the major mechanisms in mass transport of odorant molecules from inspired air to the olfactory receptors was developed. The model was solved to yield the olfactory response as a function of various physical and chemical variables that are currently available. It was found that the flow rate of the odorant carrier gas, length of the olfactory mucus surface, and the solubility of odorant molecules in the olfactory mucus play important roles in determining the odor intensity for these odorants. The theoretical results show good agreement with various experimental data obtained from both psychophysical and electrophysiological studies of olfaction in animals and human subjects.