A Numerical Determination of the Efficiency with which Electrically Charged Cloud Drops and Small Raindrops Collide with Electrically Charged Spherical Particles of Various Densities

Abstract

Numerical computations were carried out to determine the efficiency with which electrically charged cloud drops and small raindrops of diameters ranging from 84 to 866 µm collide with electrically charged particles of densities between 1 and 4 g cm⁻³ and diameters between 0.4 and 20 µm. For this purpose, numerical flow fields about rigid spheres of Reynolds numbers 1, 10, 20, 100 and 200 were used to integrate the trajectory of the particle relative to the collector drop. Two approximate methods were used to represent the electrostatic force between the drop and the particle, and the resulting collision efficiencies show that the representation of the drop and particle by two point charges is a sufficient approximation of the electrostatic force for the majority cf atmospheric charges, as long as the drop diameter is larger than 208 µm. For typical charges on drops and particles observed in the atmosphere, and for attractive electrostatic forces, the collision efficiency results for particles smaller than 2 µm in diameter show a significant increase over the collision efficiencies for the case of no electric charges.