A NUMERICAL-MODEL OF NASAL ODORANT TRANSPORT FOR THE ANALYSIS OF HUMAN OLFACTION

The transport and uptake of inspired odorant molecules in the human na sal cavity were determined using an anatomically correct three-dimensi onal finite element model. The steady-state equations of motion and co ntinuity were first solved to determine laminar flow patterns of odoro us air at quiet breathing flow rates. The air stream entering the vent ral tip of the naris traveled to the olfactory slit, and then passed t hrough the slit in nearly a straight path without forming separated re circulating zones. The fraction of volumetric flow passing through the olfactory airway was about 10%, and remained nearly constant with var iations in flow rate. The three-dimensional inspiratory velocity field was used in the solution of the uncoupled steady convective-diffusion equation to determine the concentration field in the airways and odor ant mass flux at the nasal walls. The mass-transfer boundary condition used at the nasal cavity wall included the effects of solubility and diffusivity of odorants in the mucosal lining, and the thickness of th e mucus layer. The total olfactory flux of odorants, that is highly co rrelated with perceived odor intensity, was determined as a function o f all transport parameters in our model. Increase in nasal flow rate a t a constant inlet concentration resulted in an increase in total olfa ctory uptake for all odorants. However, with increase in flow rate, th e fractional uptake, i.e., total olfactory flux normalized by convecti ve flux at the inlet, decreased for poorly soluble odorants, while it increased for highly soluble odorants. The pattern of flux (or imposed patterning) across the olfactory mucosa, that carries information con cerning odor identity, was also determined as a function of transport parameters. There was an overall decrease in odorant flux as the locat ion on the olfactory surface was varied from the anterior towards the posterior and from the inferior towards the superior ends. The flux pa ttern became more uniform, i.e., the steepness of the flux gradients a cross the olfactory surface decreased, as the mucus solubility of the odorants decreased. Different odorants generated discernibly different flux patterns across the olfactory mucosa that may contribute to the encoding of odor quality. Variation of total olfactory flux with time after cessation of airflow was determined by solving the unsteady diff usion equation in the air-phase. The flux decreased approximately expo nentially with time. The rate of decay decreased as solubility and dif fusivity decreased, but was very rapid over a wide range of the parame ters, with time constants of less than 0.5 s for most odorants, implyi ng a rapid decrease in perceived odor intensity with cessation of nasa l airflow. (C) 1997 Academic Press Limited.