Laboratory Measurements of Spontaneous Oscillations for Moderate-Size Raindrops

Abstract

The natural oscillations of moderate-size raindrops were studied in a seven-story fall column using a computer-controlled generator to produce isolated water drops at terminal speed. Instantaneous shapes were photographed to obtain oscillation sequences of single drops by a multiple-strobe technique. The oscillation frequencies were determined from fall-streak modulations that were photographed in backscattered light of the primary rainbow. Measurements were made at three levels for 2.0- and 2.5-mm diameter drops to assess the role of aerodynamic feedback as the source of drop oscillations. Variations as large as 15% in axis ratio were observed at the bottom of the fall column, even though the initial oscillations were predicted to die out by viscous decay theory. Practically all oscillations were at the fundamental and first harmonic frequencies. The oscillation modes deduced from the axis ratio scatter indicated that the axisymmetric modes died away slowly and that transverse modes persisted. The slow decay of the axisymmetric modes is postulated to be caused by positive feedback of shape-induced changes in pressure and drag from the initial oscillations. The transverse mode is believed to persist because of transverse pressure perturbations associated with eddy shedding. Various types of feedback are considered that could explain the broad coupling between eddy shedding and oscillations. The mean experimental axis ratios were higher than equilibrium values?an apparent consequence of shape changes from transverse modes. The deviation from equilibrium shape was generally consistent with previous field measurements of raindrop axis ratios. Use of empirical mean axis ratios in differential reflectivity calculations would change equilibrium values of ZDR by 20%?30%.