Particles transported in a low-density medium do not necessarily have to follow the flow. They may be trapped permanantly, even in open flows. Such is the conclusion drawn by Rafael Vilela and Adilson Motter from their theoretical study (Phys. Rev. Lett. in the press; arXiv:0706.1336, 2007).
When fluid particles are injected into an open domain, one would expect that, sooner or later, any individual particle is able to escape the flow. For the case of so-called open chaotic advection (where the open domain contains a saddle point around which the particles move chaotically) this intuition has been proved correct. However, the situation is different for 'bubbles', that is, for finite-size objects that are less dense than the medium in which they are transported. They do not simply follow the local velocity of the flow, but can be drawn into vortices; for example, an empty tin can floating on a stream of water can be trapped in the wake of an obstacle.
The same mechanism, however, fails for particles that are more dense than the fluid. Such objects — aerosols, for instance — are pushed out of a vortex in the flow field, and off they go. But not necessarily, argue Vilela and Motter. They show that in the presence of several vortices, aerosols can be pushed around from one vortex to another, forcing them into bounded stable orbits. In their simulations, Vilela and Motter see such behaviour for a wide range of scenarios and parameters. In the case pictured here, where four vortices (black dots) move with the flow, the attractors (red crosses) for aerosols describe closed orbits, enabling their trapping.
The potential impact of research into heavy particles in open flows is significant, according to the authors, reaching from the transport of pollutants and cloud droplets in the atmosphere to planet formation, for which there are theories predicting that long-lived vortices in a turbulent protoplanetary nebula can capture large amounts of solid particles and in such a way initiate the formation of planetesimals.
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Trabesinger, A. Stuck with the flow. Nature Phys 4, 15 (2008). https://doi.org/10.1038/nphys829