Velocity & Vorticity Transport In 3D-Printed Idealized & Realistic Human Airways Using Magnetic Resonance Velocimetry (MRV) & Particle Image Velocimetry (PIV)

Abstract

Furthering our knowledge of respiratory fluid dynamics is greatly beneficial to understanding lung diseases and improving aerosol drug delivery and mechanical ventilatory techniques. To this end, we develop an in-vitro platform to study detailed flow features in human airways. Idealized and realistic replicas of the bronchial tree are inserted in a flow loop circulating aqueous fluid, and detailed information on the structure-function relationship is collected using Magnetic Resonance Velocimetry (MRV) and refractive-index-matched Particle Image Velocimetry (PIV). By ‘structure’ here we indicate the anatomical and morphological features, while ‘function’ refers to momentum transport and mixing. We extract and analyze velocity and vorticity fields, as well as flow descriptors that characterize the longitudinal and lateral dispersion along the bronchial tree. We consider regimes of steady inhalation, steady exhalation, and oscillatory ventilation for a range of physiologically relevant Reynolds (Re = 100 – 5000) and Womersley (Wo = 1.2 – 12) numbers. Longitudinal dispersion is found to be higher during inhalation, while lateral dispersion is higher during exhalation. Counter-rotating streamwise vortices are observed along the airway tree due to the local curvature of the branches (Dean mechanism) and constitute one of the main transport mechanisms. At the higher Re, however, inertia induces significant non-local effects, and the vortices are transported across successive generation of bronchial branching. Flow reversal, a phenomenon consequential for gas mixing, particle transport and mechano-transduction at the epithelium, is also identified in both idealized and realistic airway geometries during steady and oscillatory regimes. The net flow drift during the ventilation cycle (steady streaming) is experimentally evaluated for the first time, and found to be much smaller than the advective flow, although not insignificant for the realistic airway geometry. The instantaneous flow fields and Reynolds stresses measured in the idealized airway model indicate great sensitivity to the inflow conditions, and show that the flow at the bifurcation is prone to unsteadiness even at regimes sometimes treated as laminar in earlier numerical studies.