Carousel effect in alveolar models.

Experimental work over the past decade has shown that recirculation in alveoli substantially increases the transport of particles. We have previously shown that, for nondiffusing passive particles, this can be understood with the aid of Moffatt's famous corner flow model. Without wall motion, passive particles recirculate in a regular fashion and no chaos exists; however, wall motion produces extensive chaotic flow. Aerosols typically do not follow this flow as they are fundamentally different from fluid particles. Here, we construct a simple model to study diffusing particles in the presence of recirculation. We assume that all particles are passive, that is to say that they do not significantly alter the underlying flow. In particular, we consider particles with high Peclet number and neglect inertial effects. We modify the Lagrangian system for corner eddies to accommodate diffusing particles. Particle transport is governed by Langevin equations. Ensembles of diffusing particles are tracked by numerical integration. We show that transport of diffusing particles is enhanced by sufficiently strong underlying recirculation through a mechanism that we call the "carousel effect." However, as the corner is approached, the recirculation rapidly decreases in intensity, favoring motion by diffusion. Far from the corner's apex, recirculation dominates. For real alveoli, the model indicates that sufficiently strong recirculation can enhance transport of diffusing particles through the carousel effect.