Temporal properties of secondary drop breakup in the bag breakup regime

Abstract

The temporal properties of secondary drop breakup in the bag breakup regime were measured as a function of time for shock-wave-initiated disturbances in air at normal temperature and pressure. The test liquids included water, ethyl alcohol and various glycerol mixtures to yield liquid/gas density ratios of 633–893, Weber numbers of 13–20, Ohnesorge numbers of 0.0043–0.0427 and Reynolds numbers of 1550–2150. Single- and double-pulse shadowgraphy and holography were used to measure the structure, size and velocity of the parent drop, and the sizes and velocities of drops produced by secondary breakup. The parent drop undergoes significant deformation and lateral growth during breakup before forming a thin bag having a basal ring that is characteristic of the bag breakup regime. The basal ring contains roughly 56% of the initial drop volume (mass) and eventually yields drops having mean diameters of roughly 30% of the initial drop diameter by a Rayleigh breakup process; the size variations of drops formed from the basal ring increases with increasing Weber number due to the appearance of large ‘node’ drops that are characteristic of the onset of the multimode breakup regime. Breakup of the bag yields nearly monodisperse drops having diameters of roughly 4% of the initial drop diameter. The velocity distributions of the drops formed from breakup of the basal ring and the bag were individually independent of drop size but varied as a function of time and differed between the two groups. Many features of these phenomena were successfully correlated using phenomenological analyses. Finally, bag breakup requires considerable time (5–6 characteristic secondary drop breakup times) and extends over considerable streamwise distances (50–100 initial drop diameters) by the end of breakup, which suggests that bag breakup should be treated as a rate process, rather than by jump conditions, in some instances.