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This dissertation, Mechanisms of Expiratory Flow and Its Role in Droplet Spread by Jianjian, Wei, 魏健健, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: An abstract of this thesis titled Mechanisms of expiratory flow and its role in droplet spread Submitted by Jianjian Wei For the Degree of Doctor of Philosophy Department of Mechanical Engineering at The University of Hong Kong in October 2015 Respiratory droplet spread by expiratory flow has been confirmed as significant vectors for transmitting respiratory diseases. This study aims to understand the mechanisms of expiratory flow, with a focus on cough, and its roles in the spread of expiratory droplets. My understanding was applied into an innovative design of an apparatus for trasmisison studies using animals. I first focused on the effect of turbulence fluctuation on the dispersion of particles and/or droplets, and the cough was modelled as a steady turbulent jet. The concept of reach probability was proposed to characterise the travel distance. My study showed that jet-like cough airflow turbulence greatly enhances the wide spread of particles and expiratory droplets, and that the effect of evaporation on medium droplets (50 m) is most significant especially under humid conditions ( RH 80%). All small particles can travel over 4 m, and only 1% of large particles travel over 2 m (initial U = 10 m/s, mouth diameter D = 2 cm). The intermittent cough jet was subsequently characterized as a two-stage jet, e.g. starting-jet stage and interrupted-jet stage, and both experimental and theoretical investigations were carried out. It showed larger stroke ratios or larger Reynolds i numbers result in further spread of the cough flow. The maximum penetration distance of all cases tested was in the range of 50.6-85.5D. Besides, a protocol was developed to scale particle experiments between the prototype in air and the model in water. Experiments showed that particles of different sizes spread to a similar distance of 38D when the cough is stopped. The leading vortex plays an important role in droplet transport. The travel distance is greatly overestimated by a steady jet, particularly for small particles. Further, as vortex rings are readily produced in a starting jet, experiments were performed to study cough vortex rings. Three types of openings, e.g. orifice, nozzle and nozzle superimposed with a lip, were investigated. The formation number lies in the range between 2.2 and 4.0. The vortex ring reaches a steady transitional velocity after pinch-off, which is 0.54 to 0.72 times of the exit velocity. The superimposition of a lip slightly decreases the vorticity flux and the ring transitional velocity, while the vorticity flux out of the orifice is 1.5 times that of the nozzle due to the vena contracta effect. To estimate the fraction of the exhaled airflow that is re-inhaled during normal nasal breathing, experiments were carried out in a water tank with an anatomically accurate respiratory tract model. It showed that a negligible fraction (ii DOI: 10.5353/th_b5699890

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