Numerical Collision Efficiencies for Small Raindrops Colliding with Micron Size Particles

Numerical calculations were carried out to determine collision efficiencies for small raindrops with micron size particles based on a numerical description of the axisymmetric steady-state flow about a rigid sphere for drop Reynolds numbers of 1, 10, 20, 100, 200 and 400. The particle was assumed to move with a Stokes-Cunningham resistance to motion in the unperturbed flow of the drop. The results-for drop radii 40 ≲ A ≲ 600 µm and particle radii a ≳ 1 µm, as long as a/A ≲ 0.1, are intermediate to previously calculated collision efficiencies for the potential and viscous flow limits but do not follow the Langmuir interpolation formula. Comparison with experimental results shows good agreement when electric forces are unimportant. A new interpolation scheme is presented for the present results as well as the potential flow case for larger rain-drops which may be evaluated either graphically or by computer for a range of atmospheric conditions of 800–1100 mb and 0–30C.