Collisions of Small Cloud Droplets: Gas Kinetic Effects

Milford H. Davis National Center for Atmospheric Research, Boulder, Colo. 80302

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Abstract

Gas kinetic effects in the region between two small approaching cloud droplets were investigated by means of slip flow theory. The resulting correction to the force coefficients derived for creeping flow eliminates the “collision barrier” noted in previous work and thus the need for assuming an abitrary collision criterion. It was found that inclusion of the gas kinetic correction leads to collision efficiencies for droplets <20 μ in radius that are somewhat larger than those previously calculated. The effect is unimportant for droplets >30 μ in radius.

Abstract

Gas kinetic effects in the region between two small approaching cloud droplets were investigated by means of slip flow theory. The resulting correction to the force coefficients derived for creeping flow eliminates the “collision barrier” noted in previous work and thus the need for assuming an abitrary collision criterion. It was found that inclusion of the gas kinetic correction leads to collision efficiencies for droplets <20 μ in radius that are somewhat larger than those previously calculated. The effect is unimportant for droplets >30 μ in radius.

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