Space‐time evolution of void fraction under a plunging breaking wave

Abstract

Bubble plumes generated by natural wave breaking in the ocean under moderate to strong wind are found to be important for acoustic reverberation near the air‐sea boundary layer. The most significant underlying physical parameter is the void fraction of the bubbly medium. So far, very little quantitative data of the void fraction due to wave breaking are available. In this paper, some results shall be presented on the spatial‐temporal evolution of void fraction under a plunging breaking wave. The experiment is conducted in a large wave tank (12 × 15 × 350 ft) with a programmable wave maker at the Oregon State University Wave Research Facility. The plunging breaking wave of a 3‐s wave period with a maximum wave height of about 5 ft is created within a wave packet. The void fraction is measured at three depths below the wave surface at six separate stations 10 ft apart. It is found that the maximum void fraction reach of about 40% to the depth of the maximum wave height over spatial distance is close to the wavelength of 60 ft. The e‐folding time for the decay of the void fraction of the initial bubble plume is less than the one‐half of the wave period, but a significant void fraction (70.1%), which causes large sound‐speed deficits persisting up to six wave periods, is also found. The void fraction is measured by a resistance gauge based on a theory originated by James Maxwell. [This project is funded by the Office of Naval Research Acoustic Reverberation program.]