AN EXPERIMENTAL INVESTIGATION OF THE COLLECTIVE OSCILLATIONS OF BUBBLE PLUMES ENTRAINED BY BREAKING WAVES

Laboratory measurements of the sound produced by mechanically generate d two-dimensional (2-D) and three-dimensional (3-D) breaking waves are presented. In the 2-D breaking experiments it was observed that the m ean-square pressure at frequencies below 1 kHz correlated strongly wit h the fractional energy dissipated by breaking and the volume of air e ntrained. In addition, the volume of air entrained was found to be pro portional to the fractional energy dissipated. These results imply tha t measurements of the low-frequency sound may be useful for studying t he dynamics of breaking waves in the field. It was found that 2-D plun ging breakers produced significant increases in spectral levels at fre quencies below approximately 500 Hz but that spilling breakers did not . Large-amplitude low-frequency signals were observed to begin up to 1 /3 of a wave period after the start of active breaking in both the two - and three-dimensional experiments. It is believed that these signals were due to the collective oscillation of the entrained bubble plumes . A model of a cylindrically shaped plume. located immediately below t he free surface and the void-fraction measurements of Lamarre and Melv ille [J. Acoust. Soc. Am. 95, 1317-1328 (1994)] were used to compute t he eigenfrequencies of the volume mode of collective oscillation. The computed eigenfrequencies closely matched the frequencies of the obser ved signals. This agreement between the experimental observations and theory provides considerable support for the hypothesis that the obser ved bubble plumes were oscillating collectively in their volume mode. This leads to the conclusion that the collective volume oscillations o f bubble plumes entrained by breaking waves may be a source of low-fre quency sound in the ocean.