The airway geometry of the nasal cavity is manifestly complex, and the manner in which it controls the airflow to accomplish its various physiological functions is not fully understood. Since the complex morphology and inaccessibility of the nasal passageways precludes detailed in-vivo measurements, either computational simulation or in-vitro experiments are needed to determine how anatomical form and function are related. The fabrication of a replica model of the nasal cavity, of a high optical clarity and derived from in-vivo scan data is described here, together with characteristics of the flow field investigated using particle image velocimetry (PIV) and flow visualization. Flow visualization is shown to be a capable and convenient technique for identifying key phenomena. Specifically the emergence of the jet from the internal nasal valve into the main cavity, how it impacts on the middle turbinate, and the large enhancement of dispersion that accompanies the initial appearance of flow instability are revealed as particularly significant features. The findings from the visualization experiments are complemented by PIV imaging, which provides quantitative detail on the variations in velocity in different regions of the nasal cavity. These results demonstrate the effectiveness of the cavity geometry in partitioning the flow into high shear zones, which facilitate rapid heat transfer and humidification from the nasal mucosa, and slower zones affording greater residence times to facilitate olfactory sensing. The experimental results not only provide a basis for comparison with other computational modelling but also demonstrate an alternative and flexible means to investigate complex flows, relevant to studies in different parts of the respiratory or cardiovascular systems.

The behaviour of incompressible side-branching flows is examined theoretically at high Reynolds numbers and compared with direct numerical simulation at moderate Reynolds numbers. The theoretical model assumes the branching (daughter) tube is small compared to the main (mother) tube and that the branching angle is small. The theory is applicable to steady and unsteady flows in two or three dimensions, and to a broad range of flow splits between mother and daughter vessels. The first main result of the work is that, in the vicinity of the branch, the flow adjusts to the imposed downstream pressure in the daughter tube through a jump (a rapid change over a short length scale) in flow properties across the daughter entrance. It is shown that, for large pressure drops in the daughter tube, fluid is sucked in at high velocities from the mother and thereby provides a favourable upstream feedback. This counteracts the tendency of the flow to separate from what would otherwise be an adversely shaped upstream wall. Increased divergence of mother and daughter tubes can thus be achieved at high daughter flow rates without separation. The second main result of the work is that the direct numerical simulations confirm the very rapid variation in flow properties and show reasonable agreement with the theory at moderate Reynolds numbers.

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