The Influence of Salinity on the Spectra of Bubbles Formed in Breaking Wave Simulations

A "tipping bucket" experiment was conducted to investigate the influence of salinity on the characteristics of a bubble plume in the UCONN Whitecap Simulation Tank IV. A submersible video microscope system was deployed 100/w/w beneath the water surface and at the center of the bubble plume, and the size distribution of bubbles with radii from 180UTW to 5mm was measured. The mechanical action of the tipping bucket remained constant throughout the experiment while the water in the tank was diluted several times to salinities of 20%o to 0%o (fresh water). The experimental results indicate that the bubble concentration is a few orders of magnitude higher in brackish water at a salinity of 20%o than in fresh water, and that the bubble plume in brackish water has a longer lifetime than the freshwater bubble plume. Void fraction at the top of the bubble plume was estimated by integrating over the bubble spectrum. The maximum void fractions are about 20% for both the fresh water and brackish water cases, but the void fraction in brackish water has a much slower decay rate than that in fresh water, due to the presence of many tiny bubbles. From this void fraction estimation, the calculated temporal variations of sound velocity show significant differences between fresh water and brackish water.