Inertial capture of particles by obstacles in form of disks and stellar crystals

Abstract

The process of cloud droplet capture by falling plate-like ice crystals has been reproduced in the laboratory by impacting streams of monodisperse spherical particles (2 < R < 22 μm) onto disk-shaped and star-shaped fixed obstacles in the range between viscous and potential flow. the particles collected on the upwind surface of the obstacles are counted using an optical microscope. A collection efficiency is determined for the different planar shapes under the specific experimental conditions. For the disk shape the results have been compared with those from the potential and viscous flow theories. They show that collection efficiencies computed from the potential flow theory are not representative of the actual efficiencies. Comparison with the viscous flow theory shows that in the range 0.8 < Nstk < 2 the theory fits the experimental data well. Non-zero efficiencies have been found for Nstk lower than the theoretical cut-off, while for Nstk < 4 the collection efficiencies are lower than theoretically predicted. This difference has been interpreted in terms of the hydrodynamic interactions between the particle and the obstacle. For the planar shapes, the collection efficiencies regularly increase with the Stokes number and depend on the surface area distribution in the models. the values of the collection efficiencies are always higher than those of the disks of equal area.