A Wind Tunnel Study on the Shape, Oscillation, and Internal Circulation of Large Raindrops with Sizes between 2.5 and 7.5 mm

Abstract

Precipitation prediction using weather radars requires detailed knowledge of the shape parameters of raindrops falling at their terminal velocities in air. Because the raindrops undergo oscillation, the most important shape parameters from the radar prediction point of view are the equilibrium drop shape, the timeaveraged axis ratio, and the oscillation frequency. These parameters for individual water drops with equivalent diameter from 2.5 to 7.5 mm were investigated in a vertical wind tunnel using high-speed video imaging. A very good agreement was found between the measured and the theoretically determined raindrop shape calculated by a force balance model. A new method was developed to determine the equivalent drop diameter with the help of the oscillation frequency. The drop size determination by means of the frequency method was found to be three times more precise than by volumetric methods. The time-averaged axis ratio was found to be equal to the equilibrium axis ratio in the investigated raindrop size range. The analysis of the oscillation frequency of the raindrops revealed that the drops undergo multimode oscillations and are oscillating in a transverse mode in addition to an axisymmetric oblate-prolate mode. Experiments are included in which the internal circulation associated with drop oscillation was investigated and compared to theory.

Shape and oscillations of the water drops freely suspended in a horizontal electric field: A wind tunnel study

“…1. for the averaged data points show S-shaped trend as reported in our earlier results as well as the experimental and theoretical results of Szakáll et al (2009) and Coquillat et al (2003). Results show that the drop axis ratio decreases, i.e.…”

“…Various photographic techniques have been used in earlier experiments to study the shapes and oscillations of the drop (Nelson and Gokhale, 1972;Rasmussen et al, 1985;Kamra et al, 1993;Szakáll et al, 2009). For example, in our earlier experiments, series of photographs of water drops was captured with a still camera or a 16 mm movie camera with a maximum speed of only 48 frames per second to determine the mean axis ratio of oscillating drops.…”

“…Following these initial experiments, several experimental studies, notably by Blanchard (1950); Jones (1959); Pruppacher and Beard (1970); Richards and Dawson (1971); Rasmussen et al (1985); Kamra and Ahire (1989); ; Coquillat and Chauzy (1993); Bhalwankar and Kamra (2007) and Szakáll et al (2009). provided comprehensive information about the forces acting on the raindrop which determine the shape and stability of the falling drops.…”

A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques

“…The freezing temperature of INPs either immersed in or in contact with levitated supercooled water droplets suspended in the air can also be determined by the change in light scattering with a charge-coupled device (CCD) camera using an electrodynamic balance (EDB; Hoffmann et al, 2013), an acoustic levitator (Diehl et al, 2014) or in a vertical wind tunnel (Szakáll et al, 2009). The advantage of these methods is the ability to provide, via high-resolution images, substratefree information for statistically representative ice nucleation processes on a single droplet basis.…”

Free fall of water drops in laboratory rainfall simulations

“…Vertical wind tunnels have been used to study various raindrop characteristics, including raindrop shape, internal circulation, and drop growth through coalescence (e.g. Pruppacher and Beard, 1970;Vohl et al, 1999;Szakall et al, 2009;Müller et al, 2013). It is important to point out that while there are such problems (e.g.…”