Vorticity transport in human airway model

Many previous studies concerned with respiratory fluid mechanics have simplistically assumed steady and laminar flow. Above a certain ventilation frequency, the unsteady nature of the respiratory flow becomes apparent, and inhalation and exhalation cannot be approximated as quasi-stationary processes. Moreover, due to the geometrical structure of the bronchial tree, flow unsteadiness and transition to turbulence can incept even at Reynolds numbers usually considered laminar in parallel flows. Here we investigate the primary features of the oscillatory flow through a 3D printed double bifurcation model that reproduces, in an idealized manner, the self-similar branching of the human bronchial tree. We consider Reynolds and Womersley numbers relevant to physiological conditions between the trachea and the lobar bronchi. Three-component, three-dimensional velocity fields are acquired at multiple phases of the ventilation cycle using Magnetic Resonance Velocimetry (MRV). The phase-averaged volumetric data provide a detailed description of the flow topology, characterizing the main secondary flow structures and their spatio-temporal evolution. We also perform twodimensional by Particle Image Velocimetry (PIV) for the steady inhalation case at a Reynolds number Re = 2000. PIV is carried out by matching the refractive index of the 3D printing resin with a novel combination of anise oil and mineral oil. The instantaneous measurements reveal unsteadiness of the separating unsteady flow in the bifurcation, and the ensemble averages show a clear Reynolds stress pattern indicating that the flow is turbulent at the first bronchial bifurcation already at this relatively low Reynolds number.