Steady and oscillatory flow in the human bronchial tree

In this study, we investigate the steady inhalation, steady exhalation, and oscillatory flow in a realistic airway geometry for physiologically relevant regimes ranging from quiet breathing to respiration under high frequency ventilation (HFV). We use magnetic resonance velocimetry to characterize and quantify three-dimensional (3D) velocity fields in 3D printed replicas of realistic bronchial trees. Expanding on previous studies [Jalal et al., Exp. Fluids 57, 148 (2016)10.1007/s00348-016-2234-5; Jalal et al., Phys. Rev. Fluids 3, 103101 (2018)10.1103/PhysRevFluids.3.103101] which focused on respiration in planar double bifurcation geometries, we compare levels of axial and lateral dispersion, and find that they exceed those found in the idealized models. Furthermore, we find that the secondary flows in realistic airways propagate deep in the bronchial tree and are stronger during exhalation as compared to inhalation, while the mean flow topology does not vary significantly between the two steady regimes. Under HFV, we note significant regions of flow reversal during the inhalation-exhalation and exhalation-inhalation transitions. This is found to be due to a difference in impedance (dominated by inertance) in the different regions of the lung and results in an asynchronous ventilation between the upper and lower lobes. This phenomenon, also known as pendulluft is demonstrated experimentally for the first time, using both Eulerian velocity fields and Lagrangian pathlines. Secondary flows are stronger in exhalation compared to inhalation and at the peak of the ventilation cycle, match the steady cases although the flow topology can be significantly different. Finally, the cycle-averaged drift velocity suggests that steady streaming, while not negligible, is not the main transport mechanism during high-frequency ventilation.