Modelling of Flow Dynamics and Nasal Function in Simplified Nasal Airways

This work comprises an investigation of the fluid mechanics of nasal airflow, primarily using computational fluid dynamics simulations, though with some flow visualisation experiments. The objective of the work is to provide a basic understanding of the flow phenomena that in turn govern transport and exchange processes in the nasal airways. In keeping with the goal of elucidating the basic fluid mechanics, simplified models of the nasal cavity airway were considered together with two representative realistic models. Computations were performed using a commercially available 3D laminar finite-volume solver (Fluent 6.3.26, ANSYS), for steady and unsteady flow conditions. The reduced models replicate the steady pressure loss vs. flow curve found in realistic geometries, and in the unsteady case, confirm the validity of simple inertance modelling to deduce the form of the pressure-flow loop. By selective inclusion/ exclusion of a simplified middle turbinate, the simulations reveal how the turbinate redirects and controls inspired air. In the absence of the middle turbinate, large scale instabilities were observed in the cavity flow and their strength and distribution were seen to increase concomitant with increasing flow rate. It was further identified that the inclusion of this turbinate reduced large scale flow instability and that flow partitioning was predominantly determined by the impingement of the inspiratory jet on its surface. The consequence of the narrow mean passage width characteristic of the nasal airways is explored by comparing an idealised 2D model with 3D geometries of increasing calibre. A strong interrelationship was found to exist between geometric and flow characteristics. In particular, the narrow width of the normal nasal airways is shown to exert a strongly stabilising effect on the mean flow during inspiration. In 3D, whereas increasing calibre width is associated with a progressive destabilisation of the flow, for the same inspiratory flow rate, there would be a concomitant drop in maximum velocity. The computational results moreover detail the evolution of the time-dependent flow within the simplified anatomies and the instability at the margins of the inspiratory jet are shown to compare well with those found in flow visualisation experiments. In the last part of the thesis, steady modelling of the flow in the anatomically realistic geometries is used to investigate their heat and water exchange capacity as well as the characteristics of particle transport and deposition. These results are related to those found in the idealised models.