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

Laboratory measurements of the sound produced by mechanically generated two-dimensional (2-D) and three-dimensional (3-D) breaking waves are presented. In the 2-D breaking experiments it was observed that the mean-square pressure at frequencies below 1 kHz correlated strongly with the fractional energy dissipated by breaking and the volume of air entrained. In addition, the volume of air entrained was found to be proportional to the fractional energy dissipated. These results imply that measurements of the low-frequency sound may be useful for studying the dynamics of breaking waves in the field. It was found that 2-D plunging breakers produced significant increases in spectral levels at frequencies 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 the free surface and the void-fraction measurements of Lamarre and Melville [J. Acoust. Soc. Am. 95, 1317-1328 (1994)] were used to compute the eigenfrequencies of the volume mode of collective oscillation. The computed eigenfrequencies closely matched the frequencies of the observed signals. This agreement between the experimental observations and theory provides considerable support for the hypothesis that the observed bubble plumes were oscillating collectively in their volume mode. This leads to the conclusion that the collective volume oscillations of bubble plumes entrained by breaking waves may be a source of low-frequency sound in the ocean.