|M.Sc Student||Loyevsky Amir|
|Subject||Time-Resolved, Tomographic Measurements of Oscillatory|
Respiratory Flows in an Upper Airway Model
|Department||Department of Mechanical Engineering||Supervisors||Professor Rene Van Hout|
|Professor Josue Sznitman|
|Full Thesis text|
All aerobic creatures, including humans, need oxygen in order to produce energy from food, in a process known as “breathing”. In this process, carbon dioxide is created as a waste product, and needs to be transported out of the cells. This is the main role of the respiratory system, which is responsible for the gas exchange between oxygen and carbon dioxide in the lungs. The severity of common pulmonary diseases such as asthma and acute respiratory distress syndrome, as well as the complexity of the lungs' structure, make respiratory fluid dynamics the second most explored type of biological flows. Both experiments as well as numerical simulations have been performed. However, most experiments have focused solely on steady inhalation without analyzing the exhalation part of the breathing cycle. In addition, very little is known about the actual breathing cycle that is oscillatory.
In this work, two experimental setups have been used to investigate flows in models of the upper airways. The first one was a stereoscopic micro particle image velocimetry setup, that was used to investigate flows in straight channels. This setup proved to be inadequate to perform detailed measurements of complex flows. The second setup was tomographic particle image velocimetry (tomo-PIV), which was used to investigate oscillatory respiratory flows in an Elastosil model of the upper airways with a double bifurcation, mimicking human airways. Common-practiced frequencies in high frequency ventilation protocols were applied, resulting in different flow phenomena, depending on the dimensionless Reynolds and Womersley numbers for each experiment, as well as the associated Dean number. The Reynolds number describes the ratio between the inertial effects and the viscous effects acting upon the flow, the Womersley number describes the “unsteadiness” of the flow (i.e. its tendency to change with respect to time) and the Dean number describes the effects of curved geometries through which the fluid flows. At inhalation, a strong jet generated reverse flows. The jet's penetration decreased with increasing Wo, and the impingement onto the first bifurcation led to high stresses. The results showed that with increasing Wo, Dean effects decrease. For Wo numbers smaller than 4.1 (Re = 450), Reynolds stresses were significant at the outer periphery of the first bifurcation during exhalation. In addition, helicity was used to study the spatial organization of helical vortices. Their spatial organization strongly depended on Re as well as Wo. Furthermore, we observed asymmetric flow patterns despite an overall symmetric bifurcation tree at Re = 450. Our experiments add to the general understanding of oscillatory flow phenomena in bifurcating networks.