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Statistics have been computed for the motion Of small particles settling under gravity in an ensemble of randomly oriented, periodic, cellular flow fields that are steady in time. The particles are small, spherical, and subject to a quasi-steady Stokes drag force from the flow. In the absence of particle inertia, a fraction of the particles may be suspended indefinitely, but inertia, however weak eventually causes all particles to settle out at a rate that over most parametric ranges is faster than in still fluid. More surprisingly, particles with small free fall velocity and weak inertia show a strong tendency to collect along isolated paths. Reducing inertia does not greatly alter this process, but only delays it.
Maxey et al. (Sun,) studied this question.