Collision-induced drop oscillations from wind-tunnel experiments

Our understanding of drop shapes for drop diameters larger than 2 mm is now on a firm footing from both precise wind-tunnel measurements (Szakall et al. 2010) and from 2D-video disdrometer (2DVD) measurements (Thurai et al. 2009). A thorough examination of the 2DVD camera data from several locations have shown that in the vast majority of cases, the most ‘probable' shapes conform to those arising from the axisymmetric (2,0) mode (Beard et al. 2010).

There have been a few exceptions, however. A recent study (Thurai et al., 2011) of an intense organized line convection using two collocated 2D video disdrometers (2DVD) and a C-band polarimetric radar, it was possible to infer the occurrence of mixed mode drop oscillations, probably sustained by collisional forcing. Additionally, the mixed mode oscillations seemed to be coupled with reduced fall velocities for moderate-to-large sized drops. These inferences were based on the fact that (a) a significant fraction of the larger drops (e.g. drop diameters of 3 mm and larger) from the 2DVD measurements did not possess rotational symmetry – when the line convection had passed over the 2DVD location - and (b) that the fall velocity distributions showed a negative skewness towards lower values. Polarimetric radar information such as rho_hv had also supported the notion of mixed mode oscillations because of their lower values than those expected from equilibrium shapes (together with the wide DSDs measured during the passage of the line).

To investigate collision-induced drop oscillations further, we have made use of the vertical wind-tunnel facility, based in Mainz, Germany. More than 130 cases were recorded with a high speed digital video camera. Many of the recorded collision events had to be abandoned because the drops were not in the focal plane of the camera, however, it was possible to analyze around 40 collision events. Among these were 27 which resulted in drop coalescence, and the remaining in non-coalescence collision.

The sizes of the colliding drop pairs were chosen to be typical for real-atmospheric conditions, i.e. for the collector drops, they were in the 2.4 mm - 3 mm drop diameter range, while the small droplets had sizes of around 500 microns. In each case, the collector drop was freely-floated inside the wind-tunnel until a small droplet (injected from below) coming from the upstream side of the larger drop collided with it. Data analysis clearly shows that the larger drop - upon collision - undergoes mixed-mode oscillations, with (2,1) and (2,2) modes dramatically increasing in oscillation amplitudes. The perturbation caused by the collision lasts for over several hundred milliseconds before effectively getting damped out and reverting to its usual (2,0) oscillation mode as the dominant mode. The coalescence cases show somewhat more pronounced mixed mode oscillations compared with the non-coalescence cases. The drop vertical velocities of the larger drops were also calculated, with the center of mass as being the reference. These were determined as time series for, before, during, and after collision.

Results of the drop collision experiments (including videos) will be shown and discussed in conjunction with the 2DVD measurements during the aforementioned rain event.

Reference:

Beard, K. V., V. N. Bringi, and M. Thurai, 2010: ‘A new understanding of raindrop shape,' Atmos. Res., 97, 396-415.

Szakáll, M., K. Diehl, S. K. Mitra, and S. Borrmann, 2010: ‘Shapes and oscillations of falling raindrops — A review', Atmos. Res., 97, 416-425.

Thurai, M., V. N. Bringi, M. Szakáll, S. K. Mitra, K. V. Beard, and S. Borrmann, 2009: ‘Drop shapes and axis ratio distributions: Comparison between 2D video disdrometer and wind-tunnel measurements'. J. Atmos. Oceanic Technol., 26, 1427–1432.

Thurai, M., V.N. Bringi, W.A. Petersen, L.D. Carey, P.N. Gatlin, and A. Tokay, ‘Drop shapes versus fall velocities in rain: 2 contrasting examples', the 35th Conference on Radar Meteorology, Pittsburgh, PA, September 2011.