CHAOTIC MOTION OF FLUID AND SOLID PARTICLES IN PLANE WAKES

Adaptive algorithms are used to simulate mixing of point and solid par ticles in the time-dependent wake behind a cylinder for Re = 100. Mixi ng phenomena qualitatively similar to those observed in externally-for ced time-dependent flows take place in this simulated flow. Chaotic re gions evolve as a result of tangles of manifolds arising from a downst ream hyperbolic periodic point. The unstable manifold of this point cr eates high stretching regions in the now. Statistical analysis of the stretching experienced by populations of point particles reveals log-n ormal distributions that are characteristic of chaotic mixing processe s observed in other flows. Viscous wakes generate efficient mixing of fluid particles, but they also generate efficient separation of solid particles. An initially homogeneous mixture of particles with a distri bution of Stokes numbers becomes considerably segregated as it is carr ied downstream the now. This separation process is governed by the str ucture of the flow. Small particles with low Stokes numbers accumulate in the center of eddies in the wake; particles with Stokes numbers ne ar unity approach the unstable manifold of the fluid point particles; and large particles with high Stokes number migrate to the center of t he wake and move backwards, colliding with the downstream surface of t he cylinder. Copyright (C) 1997 Elsevier Science Ltd.