Dispersion of large particles in the near wake of a disc in a turbulent free stream

The dispersion of particles in the near wake of a flat disc placed perpendicular to a turbulent free stream was studied by optically measuring the dimensionless near-wake residence time of particles, H (= tr U/D, where tr is the time constant of the decay of the near-wake concentration following removal of the source, U the free-stream air velocity, and D the disc diameter). The dependence of H on the Stokes number (St, expressed in terms of U and D) was investigated for St up to about 8. The experiments were conducted for aerosols which were generated from narrowly graded fused alumina dust and glass beads, whose surface median aerodynamic diameter (SMAD) ranged from 13 to 96 μm, introduced directly into the near-wake region at the base of the disc. The results showed H decreasing steadily and uniquely with increasing St, leveling off for St > 1. This behavior is well explained by simple physical models which take account of the centrifugal forces exerted on a particle during its motion inside the recirculating body of fluid enclosed in the near-wake region, supplementing the turbulent exchange dispersion processes which occur for particles of all sizes.