Dynamics of liquid sheet breakup in the presence of acoustic excitation

We present the results of experimental investigations on the effect of acoustic excitation on the breakup of the liquid sheet formed by oblique impingement of two liquid jets for varying Weber number over a wide range of excitation frequencies (100-2000 Hz) and sound pressure levels (50-120 dB). Visualization studies reveal violent sheet flapping, reduction in the sheet breakup length and width, impact wave amplification and rise in droplet density shed from the sheet. Overall, increase in excitation frequency increases the threshold sound pressure level at which the sheet responds. Reduction in the sheet breakup length and width with frequency is observed at some selected frequencies, which rules out the possibility of resonance. For smooth sheet regime, visual observations show that the dominant sheet breakup mechanism shifts from Rayleigh-Plateau (R-P) to Kelvin-Helmholtz (K-H) instability. It is shown using temporal analysis that the reduction in the sheet breakup length may be attributed to periodic sheet collapse and reformation. Drop shedding frequencies in flapping liquid sheet are seen to synchronize with those excitation frequencies which are lower than or nearly equal to the fundamental frequency without excitation. Acoustic excitation reduces the mean droplet size significantly without altering the drop size distribution pattern that follows a universal Gamma distribution.