Free fall of water drops in laboratory rainfall simulations

• We study free fall of water drops to provide guidance for laboratory rainfall simulations.
• Drop fall velocity and shape parameters are investigated using high-speed imaging.
• Three distinct fall zones are identified.
• Simulated raindrops achieve equilibrium shapes after fall of approximately 6 m.
• Simulated raindrops achieve terminal velocities after fall of approximately 12 m.

Motivated by various hydrological and meteorological applications, this paper investigates the free fall of water drops to provide guidance in laboratory simulations of natural rainfall and to elucidate drop morphodynamics. Drop fall velocity and shape parameters such as axis ratio (ratio of the maximum vertical and horizontal chords of the drop), chord ratio [ratio of the two orthogonal chords where one chord (cl) is the longest chord in the drop and the other one (cs) is the longest chord that is orthogonal to cl], canting angle (angle between the longest chord of the drop and the horizontal axis), and relative fluctuation of chords (difference between vertical and horizontal chord fluctuations) were investigated for three selected water drop sizes (2.6, 3.7, and 5.1 mm spherical volume equivalent diameter) using high speed imaging. Based upon experimental observations, three distinct fall zones were identified: Zone I, in which source-induced oscillations and shape adjustment take place; Zone II, in which equilibrium-shaped drops accelerate to achieve terminal velocity; and Zone III, in which equilibrium-shaped drops fall at terminal velocity. Our results revealed that the fall distance values of approximately 6 m and 12 m can be used as conservative reference values for rainfall experiments with oscillation-free fall of drops (i.e. end of Zone I and onset of Zone II) and with equilibrium-shaped drops falling at terminal velocities (i.e. end of Zone II and onset of Zone III), respectively, for the entire raindrop size spectrum in natural rainfall. These required fall distance values are smaller than the distances discussed in the literature. Methodology and results presented here will facilitate optimum experimental laboratory simulations of natural rainfall.