Raindrop Collisions Under Low Pressure, Experiments and Parameterizations.

A new experimental system was developed to simulate raindrop collision at a low pressure. This system permitted one pair of accurately sized water drops to collide one at a time at terminal velocities corresponding to the environmental pressure and represents the closest simulation of this specific aspect of the natural rain formation process. Statistics of the collision outcome were drawn from at least 100 collisions in more than 1250 attempts for each size pair. The five pairs studied at 50 kPa for this thesis were: 0.46;0.10 cm , 0.261;0.117 cm , 0.18;0.10 cm , 0.44;0.04 cm and 0.18;0.04 cm in diameter. These data were classified into three breakup types used by Low & List (1982): filament, sheet and disk. A new technique was developed to reanalyse part of Low & List's data. This revealed new trends in the growth and decay of the fragment after collision and a new set of equations was obtained to parameterize these results. Also, some of Low & List's equations were reparameterized resulting in increased accuracy. The extrapolations of these equations show that the large fragment can grow in size when the large and small drop are both relatively large and cannot coalesce. The data at 50 kPa were analysed using the new technique and then compared with the 100 kPa data. A set of parameterized equations cataloguing the pressure effect was obtained. In general, the large fragment of 50 kPa is larger than that at 100 kPa. The number of fragments decreases at large CKE (collision kinetic energy) and increases at small CKE for D(,L) close to D(,S) over that at 100 kPa. The percentage of occurrence did not change very drastically, while the coalescence equation of Low & List (1982) remains adequate for the present experiments. The 50 kPa results, compared with the 100 kPa, suggest that aerodynamics plays a major role in the breakup of a temporarily coalesced drop pair. The Reynolds number of the airflow around the drops at 50 kPa decreased by about 37% over that at 100 kPa. (Abstract shortened with permission of author.).