Computational modelling of airflow and transport in the upper respiratory tract

Abstract

This thesis comprises an investigation of the dynamics of airflow in the upper respiratory tract from the nose to the carina using computational modelling. The work primarily considers inspiratory flow and comprises separate detailed studies of airflow in three distinct regions: the nose, the larynx and the trachea.

In the nose, both airflow dynamics and the associated processes of heat and water exchange are studied. The geometry of the nasal cavity is highly variable both inter- and intra-subjectively. The consequences of inter- and intra-subject variability for both airflow and exchange processes during inspiration is investigated using 10 subject geometries obtained by MRI scans in the decongested and congested states. Non-dimensional scalings are applied to quantify the effects of flowrate and geometry on wall shear stress, heat and water transport. It is found that the anterior nasal cavity contributes most to the exchange process and that decongestion generally affects the distribution of inspiratory flow, promoting a switch towards more inferior pathways.

Flow in the larynx is controlled by the position of the vocal folds, which determine the glottis aperture. Even with the vocal folds fully abducted, inspiratory airflow experiences a degree of constriction and exits the glottis as a jet which dissipates in the trachea. The effect of glottis aperture on pressure loss and flow structure is examined, and the results compared with those from a separate experimental investigation.

In pathological tracheal geometries, characterized by severe constriction and deviation as caused by compressive retrosternal goitre, far more severe pressure losses may occur than in the normal trachea. The third investigation performed in this work describes a computational approach to assess pathological tracheal resistance. Direct and large eddy simulation results are applied to determine the profile of energy dissipation in the case of a progressive tracheal compression. The study examines the effects of inflow disturbances on the breakup of the jet downstream of the constriction and the consequent loss characteristics. The effect of domain truncation, associated with the typical field of view of clinical imaging, is an important consideration and is investigated.