Olfaction begins when an animal draws odorant-laden air into its nasal cavity by sniffing, thus transporting odorant molecules from the external environment to olfactory receptor neurons (ORNs) in the sensory region of the nose. The dog and other macrosmatic (keen-scented) mammals have evolved a complex nasal anatomy that facilitates the efficient aerodynamic sampling of inspired odorant molecules. Here, airflow and odorant transport patterns in the canine nasal cavity are studied through a set of flow visualization experiments and computational fluid dynamics (CFD) simulations.

An anatomically-correct experimental model of the canine nasal cavity, based on high-resolution magnetic resonance imaging scans, was designed and fabricated using a rapid prototyping technique. Dye-injection flow visualization experiments were performed using this model to characterize the canine’s internal nasal airflow patterns. The results from these experiments illustrated the complex three dimensional flow patterns throughout the nasal cavity. The experimental results also confirmed the existence of distinct olfactory and respiratory airflow paths and were used to study the transition between laminar and turbulent flow domains. Moreover, these experiments were used to both qualitatively and quantitatively validate previous and current CFD simulations of canine nasal airflow.

Steady and unsteady CFD simulations of odorant transport and deposition in the nasal cavity were performed. The simulations modeled the transport of odorant from the external environment, through the nasal airways, across the olfactory mucus layer, and to ORNs sites. Steady simulations were performed to study the relationship between odorant solubility and odorant deposition patterns (i.e. odorant flux patterns) across the canine’s olfactory region. Highly-soluble odorants were deposited mainly along the entrance to the olfactory region. Moderately-soluble and insoluble odorant fluxes to the olfactory epithelium are significantly lower than those for highly-soluble odorants. However, these less soluble odorants are deposited more uniformly across the olfactory epithelium. The canine’s nose apparently utilizes odorant absorption over a large surface area to compensate for the lower absorption rate of insoluble odorants.

Physiologically-realistic sniffing was simulated to examine the effects of unsteady airflow on odorant transport. The unsteady simulations showed that the airflow is unidirectional in the olfactory region during inspiration and stagnant on expiration. Unsteady odorant transport patterns for highly-soluble and moderately soluble odorants were observed to become temporally fully-developed after a single sniff. Accordingly, the steady and unsteady odorant deposition patterns are qualitatively similar for these odorants. Insoluble odorant transport and deposition patterns however continue to develop over the course of many sniff cycles.

This work shows that the canine has evolved a complex nasal anatomy that creates odorant-specific deposition patterns. These patterns allow the canine’s nose to separate the components of a complex scent, as in chromatography, in order to possibly improve neurological olfactory pattern recognition. The lessons learned on efficient aerodynamic sampling techniques are also used to suggest biomimetic design principles to improve artificial olfaction devices.