Bubble entrainment and underwater noise caused by a single water drop falling on the surface of freshwater and saltwater

Abstract

Bubble entrainment caused by and underwater sound produced by a single water drop falling on the surface of freshwater and saltwater were experimentally investigated in this study. A charge-coupled device camera was employed to photograph the drop's impact and the process of bubble formation. Underwater sound was received using a hydrophone connected to a charge amplifier and a data-acquisition system. To capture the temporal and spectral characteristics of the sound, the time series of the underwater sound was transformed into the time–frequency domain by using the Gabor transform. The diameter of the drops was varied from 2.6 to 5.66 mm, and the release heights investigated were 10–150 cm. The experimental results revealed three distinct bubble formation processes following the impact of a water drop. Type-I bubbling was the formation of a singular bubble at the apex of the contracting crater produced by the initial drop impact. Type-II bubbling was the formation of one or multiple bubbles entrained by the downward motion of the water column produced during the initial drop impact. Type-III bubbling was the formation of a singular bubble generated due to the impact of a secondary water drop above the water column. The effects of salinity on bubble entrainment and underwater noise were also studied. For the same experimental parameters, bubbles entrained in saltwater were discovered to be larger than those entrapped in freshwater; moreover, the saltwater bubbles collapsed into two bubbles.