A laboratory investigation of raindrop oscillations
Kubesh, Rodney Joseph
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https://hdl.handle.net/2142/22807
Description
Title
A laboratory investigation of raindrop oscillations
Author(s)
Kubesh, Rodney Joseph
Issue Date
1991
Doctoral Committee Chair(s)
Beard, Kenneth V.
Department of Study
Atmospheric Sciences
Discipline
Atmospheric Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Atmospheric Science
Physics, Fluid and Plasma
Language
eng
Abstract
Knowledge of raindrop shape is crucial to analyses of microwave scattering from precipitation, including the use of polarization radar in inferring the rainfall rate. The equilibrium shape of a water drop falling in air is nearly a sphere for millimetric drops, while larger drops are increasingly oblate with size; oscillations of the drop cause its shape to vary in time about the equilibrium shape. Oscillations are excited by collisions with other drops, or may be self-excited by aerodynamic effects arising from the airflow around the drop. The latter mechanism was studied with two experiments, one for water drops 1.1 to 1.5 mm diameter, and the other for 2.0 and 2.5 mm drops. Drops of the desired size were produced using a computer-controlled drop generator, and stroboscopic photographs of the drops a distance below the generator yielded instantaneous axis ratios. Measurements of the oscillation frequency for 2.0 and 2.5 mm drops were made from photographs of interruptions in the backscattered light of the primary rainbow. Significant variations in axis ratio were observed (as large as 15%, with standard deviations of up to 3% of the mean axis ratio), indicating oscillations were present for each drop size. In addition, the axis ratios were shifted higher than the equilibrium value by as much as 3.5%, making oscillating drops more spherical on average than quiescent drops. A large fraction of the drops did not possess symmetry about the vertical axis, due to forcing caused by the detachment of eddies that form in the drop wake. Drops around 1.5 mm diameter were the exception--they displayed a strong axisymmetric component as well. The oscillation measurements showed the presence of both the fundamental frequency and the first harmonic. The research revealed that equilibrium axis ratios should not be used in differential reflectivity calculations, as the observed axis ratio shift translates into $Z\sb{DR}$ changes of up to 30%.
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