Near-limit drop deformation and secondary breakup

Abstract

The properties of drop deformation and secondary breakup were observed for shock wave initiated disturbances in air at normal temperature and pressure. Test liquids included water, glycerol solutions, n-heptane, ethyl alcohol and mercury to yield Weber numbers (We) of 0.5-1000, Ohnesorge numbers (Oh) of 0.0006-4, liquid/gas density ratios of 580-12,000 and Reynolds numbers (Re) of 300-16,000. Measurements included pulsed shadowgraphy and holography to find drop deformation properties prior to breakup, as well as drop size distributions after breakup. Drop deformation and breakup regimes were identified in terms of We and Oh: regimes at low Oh include no deformation, nonoscillatory deformation, oscillatory deformation, bag breakup, multimode breakup and shear breakup as We is increased. However, most of these regimes occur at higher We when Oh values are increased, with no breakup observed for Oh > 4 over the present test range. Unified temporal scaling of deformation and breakup processes was observed in terms of a characteristic breakup time that largely was a function of Oh. Prior to breakup, the drag coefficient evolved from the properties of spheres to those of thin disks as drop deformation progressed. The drop size distribution after breakup satisfied Simmons' universal root normal distribution function for the bag and multimode breakup regimes and could be characterized by the Sauter mean diameter (SMD) alone. Drop sizes after shear breakup, however, did not satisfy this distribution function due to the distorting effect of the core or drop-generating drop. Nevertheless, the SMD after secondary breakup could be correlated in terms of a characteristic liquid boundary layer thickness for all breakup regimes, similar to recent results for nonturbulent primary breakup. Drop properties after secondary breakup suggest that both reduced drop sizes and reduced relative velocities play a role in ending the secondary breakup process.