Raindrop Oscillations: Evaluation of a Potential Flow Model with Gravity

Abstract

Potential flow oscillations about an equilibrium raindrop distortion were modeled for ellipsoidal variations driven by changes in surface and gravitational potential energy with linear dissipation of kinetic energy. The model was found to be quantitatively similar to the Navier-Stokes results of G. B. Foote for axisymmetric oscillations without gravity. Computed frequencies and average axis ratios for vertical and horizontal oscillations with gravity were compared to wind tunnel observations of oscillating water drops and raindrop camera data. Simple formulas with good accuracy were developed for the time variation and time average axis ratios as a function of oscillation amplitude.