Chaotic mixing of alveolated duct flow in rhythmically expanding pulmonary acinus

Abstract

We examined the effects of rhythmic expansion of alveolar walls on fluid mechanics in the pulmonary acinus. We generated a realistic geometric model of an alveolated duct that expanded and contracted in a geometrically similar fashion to simulate tidal breathing. Time-dependent volumetric flow was generated by adjusting the proximal and distal boundary conditions. The low Reynolds number velocity field was solved numerically over the physiological range. We found that for a given geometry, the ratio of the alveolar flow (QA) to the ductal flow (QD) played a major role in determining the flow pattern. For larger QA/QD (as in the distal region in the acinus), the flow in the alveolus was largely radial. For small QA/QD (as in the proximal region in the acinus), the flow in the alveolus was slowly rotating and the velocity field near the alveolar opening was complex with a stagnation saddle point typical of chaotic flow structures. Performing Lagrangian fluid particle tracking, we demonstrated that in such a flow structure the motion of fluid could be highly complex, irreversible, and unpredictable even though it was governed by simple deterministic equations. These are the characteristics of chaotic flow behavior. We conclude that because of the unique geometry of alveolated duct and its time-dependent motion associated with tidal breathing, chaotic flow and chaotic mixing can occur in the lung periphery. Based on these novel observations, we suggest a new approach for studying acinar fluid mechanics and aerosol kinetics.

Kinematically irreversible acinar flow: a departure from classical dispersive aerosol transport theories

“…In a previous investigation (40), we used the single-alveolus model to explore the basic physics operating in a viscous flow subjected to cyclic alveolar wall motion. The alveolus model used in the present investigation is also axisymmetric but comprises a central circular channel around which are placed nine tori, equispaced in the axial direction (Fig.…”

“…One is the flow in an isolated alveolus model (Fig. 1B), similar to that used in Tsuda et al (40), and the other is the flow in a nonalveolated, rhythmically expanding straight tube (Fig. 1C).…”

“…Because, under normal breathing conditions or during moderate exercise, the value of Q A /Q D is usually Ͻ0.05 in the majority of alveoli along the acinar tree (from the respiratory bronchioles to the last few generations) (37), we expect that most of the alveoli are likely to possess recirculation in their flow field. Importantly, the presence of alveolar recirculation in the cyclically expanding alveoli is topologically associated with the existence of a stagnation saddle point in each alveolar flow field (40), implying that chaotic mixing could originate in most of the alveoli (see DISCUSSION).…”

“…Our laboratory's recent findings (37)(38)(39)(40)(41), as well as those of others (8,18), however, have questioned the validity of the basic assumptions that formed the basis of the dispersive theories. We have demonstrated that, because of the peculiar geometry of the alveolated duct and its time-dependent motion associated with tidal breathing, under certain conditions, alveolar flow can be chaotic (16,40).…”

“…We have demonstrated that, because of the peculiar geometry of the alveolated duct and its time-dependent motion associated with tidal breathing, under certain conditions, alveolar flow can be chaotic (16,40). As a consequence, acinar flow can be kinematically irreversible, even though it is a lowReynolds number viscous flow, and lung expansion and contraction are approximately self-similar and reversible (1,13,14,24,46).…”