Axis Ratios of Water Drops Levitated in a Vertical Wind Tunnel

Abstract

The shapes of falling raindrops are often significantly altered by drop oscillations, complicating dual-polarization radar methods that rely on a predictable, monotonic variation of drop axis ratio α with equivolume drop diameter d. This oscillation behavior varies with d so that time-averaged shapes, which are determined by oscillation mode, sometimes deviate from the d-dependent quiescent shape. The literature identifies a predominance of particular oscillation modes at discrete d, as well as the onset of oscillations at d ≈ 1 mm; however, the specific mechanisms of this phenomenon are unknown. In the present work, measurements of drop axis ratio α were obtained from observations of drops levitated in a vertical wind tunnel. Discordance of the present data with the literature suggests a correlation between oscillation mode and fall trajectory, as well as a steady-state mechanism for the excitation of specific modes for d = 1.3?3-mm drops.