The impact of a nearly cylindrical water mass on a water surface is studied both experimentally and theoretically. The experiments consist of the rapid release of water from the bottom of a cylindrical container suspended above a large water tank and of the recording of the free-surface shape of the resulting crater with a high-speed camera. A bubble with a diameter of about twice that of the initial cylinder remains entrapped at the bottom of the crater when the aspect ratio and the energy of the falling water mass are sufficiently large. Many of the salient features of the phenomenon are explained on the basis of simple physical arguments. Boundary-integral potential-flow simulations of the process are also described. These numerical results are in fair to good agreement with the observations.

A recent article [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. 96, 1755–1765 (1994)] reported the measurements of the ambient sound generated by laboratory breaking waves over the range 100–20 000 Hz in fresh water. Those observations from both spilling- and plunging-type breakers have been repeated in the same manner and wavemaker/anechoic tank with water that had 25‰ salt in its content. The observations in salt water, just like those in fresh water, reveal that the sources of sound in laboratory spilling breakers are due primarily to single bubble oscillations that can have frequencies lower than a few hundred Hertz. In the case of weak spilling breakers, the sound spectra level in fresh water was due primarily to single bubble oscillation, while the same breakers in salt water have introduced smaller size bubbles with large density. The relatively high-density populated bubble cloud generated by weak breakers shows the evidence of the onset of collective oscillation that was absent for the same breakers in the fresh water. In the case of moderate spilling and plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions in fresh and salt water. The average sound spectra reveal that the peak frequencies of the spectra shift from a few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker), and the high-frequency portions have slopes approximately 5–6 dB/oct, which are the slopes observed from the noise spectra of the ocean. Besides the high bubble density and smaller bubble sizes in salt water, all breakers experienced a significant increase in sound-pressure level in all observed frequency range. The ionic structure of the medium alters the sound radiation from bubbles. In this paper some of the observed acoustic signatures from breaking waves are discussed and a plausible explanation of how salt can effect the sound radiation from bubbles is given.

The results of an experiment to characterize the underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz are described. Waves are generated by a computer-controlled plunging-type wave maker and propagate along a 12.7-m-long channel where they are made to break at the mid-surface of a 3-×3-×2.5-m anechoic water tank. The individual bubbles and bubble clouds entrained by the breaking wave provide a mechanism for sound production. Using high-speed cinephotography, correlations were established between the hydrodynamic evolution of the cloud and the radiated acoustic emissions. The bubble size distributions inside the cloud were measured with the aid of a high-speed video camera and a fiber optic cable. These measurements indicate that single bubbles with radii as large as 7–8 mm may be entrained in this fresh-water system by moderate spilling breakers. Detailed measurements of the bubble size distribution of the bubble cloud enabled us also to obtain a measurement of the average void fraction in the cloud. These observations reveal that the sources of sound in laboratory spilling breakers is due mostly to single bubble oscillations that can have frequencies as low as 400 Hz; in the case of plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions. The acoustic radiation from bubble clouds is the result of collective oscillations of the bubbles, stimulated by large scale vortices created by the plunging breaker. The sound spectra, averaged over 100 breakers, reveal the following observations. First, the peak frequencies of the average sound spectra shifts from few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker). Second, the sound pressure levels increase in all frequency bands with increasing breaker severity. Lastly, the high-frequency portions of the sound spectra have slopes of about 5–6 dB/oct, which are the slopes observed for the noise spectra of the ocean. These results provide considerable insight into the likely source mechanisms for ocean ambient noise.

The impact of a jet of water onto a still-water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Results from an experimental study of the noise produced by this process is presented. Preliminary results of this study were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 89, 2452–2455 (1991)]. The densely populated bubble plumes were generated by dropping a fixed volume of water, held in a cylindrical container, onto a still-water surface. High-speed video images reveal the formation of a cylindrical bubble plume with a very high void fraction which grows in size until all the water is injected into the tank. As the leading end of the plume advances, a section of the plume separates near the crater region formed by the jet. After detachment, the separated plume, which is roughly spherical in shape, undergoes volume pulsations, and radiates relatively large-amplitude, low-frequency sound. The nature of the acoustic emissions from bubble plumes depends on the height of the water in the container, the container’s radius, and the velocity of the impacting jet. The natural frequency of oscillation of an individual bubble plume is inversely proportional to the radius of the plume and ranges from a few tens of Hz to over 100 Hz depending upon the void fraction of air contained within the plume. Results obtained with salt water as well as with rough jets are also discussed. The high-speed video observations reveal that immediately following the bubble plume detachment, there is evidence of an axial jet directed downward into the bubble plume and an opposing jet directed upward into the crater formed by the impact. This jet appears to be the physical mechanism that drives the cloud into oscillation. Measurements indicate that the acoustic intensity radiated from bubble plumes correlate with the total potential energy of the water jet.

In view of several practical ramifications of this problem, computational-analytical techniques for calculating waves induced by heaving arbitrary bodies in narrow tanks have been developed, including nonlinear wave groups produced near tank resonance. These feature computational near-field solutions matched with appropriate far-field solutions. In the linear case, the far field is provided by linear mode superposition. In the nonlinear case, the far field is described by a suitable nonlinear evolution equation of the cubic Schrödinger type. Matching techniques were developed. Calculations were successfully carried out and the results confirm the important effect of tank walls on added mass and damping.Results of computations have been compared with some data obtained with a conical wavemaker in a narrow tank. Pronounced nonlinear wave groups were obtained near resonance, and these are well reproduced in some detail by the nonlinear theory and computations, without considering any effects of dissipation.The related problem of resonant wave groups produced by a segmented paddle wavemaker has also been treated by analysis and subject to computation, with good general agreement with past experiments. The technique features matching near- and far-field computations using energy considerations.