Browsing by Author "Lu, N.Q."

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Active and Passive Acoustic Behavior of Bubble Clouds at the Ocean’s Surface
(The Journal of the Acoustical Society of America, 1993-06) Prosperetti, Andrea; Lu, N.Q.; Kim, Hyun S.
The emission and scattering of sound from bubble clouds is studied theoretically. It is shown that clouds having a size and air content similar to what might be expected as a consequence of the breaking of ocean waves can oscillate at frequencies as low as 100 Hz and below. Thus cloud oscillations may furnish an explanation of the substantial amount of low?frequency wind?dependent oceanic ambient noise observed experimentally. Detailed results for the backscattering from bubble clouds—particularly at low grazing angles—are also presented and shown to be largely compatible with oceanic data. Although the cloud model used here is idealized (a uniform hemispherical cloud under a plane water free?surface), it is shown that the results are relatively robust in terms of bubble size, distribution, and total air content. A similar insensitivity to cloud shape is found in a companion paper [Sarkar and Prosperetti, J. Acoust. Soc. Am. 93, XXX (1993)].
An Investigation of the Collective Oscillations of a Bubble Cloud
(The Journal of the Acoustical Society of America, 1991-02) Yoon, S.W.; Crum, Lawrence A.; Prosperetti, Andrea; Lu, N.Q.
It is well known that ocean ambient noise levels in the frequency range from a few hundred hertz to several tens of kilohertz are well correlated with wind speed. A physical mechanism that could account for some of this sound generation is the production of bubble clouds by breaking waves. A simple laboratory study of the sound generated by a column of bubbles is reported here. From measurements of the various characteristics of this column, good evidence is obtained that the bubbles within the column are vibrating in a collective mode of oscillation. Based upon an assumption of collective oscillations, analytical calculations of the predicted frequency of vibration of this column as well as the dependence of this frequency on such parameters as bubble population and column geometry agree closely with the measured values. These results give evidence that the bubble plumes generated by breaking waves can be a strong source of relatively low frequency (< 1 kHz) ambient noise.
Cavitation and Bubble Bursting as Sources of Oceanic Ambient Noise
(The Journal of the Acoustical Society of America, 1988-09) Prosperetti, Andrea; Lu, N.Q.
Cavitationlike bubble collapses and the bursting of floating bubbles have been proposed in the literature as sources of oceanic ambient noise at kilohertz frequencies. The first process is shown to be physically impossible in the oceanic environment. The noise produced by the second mechanism is estimated and shown to be too weak to be of any significance.
The oscillations of a small floating bubble
(The Physics of Fluids, 1989-02) Lu, N.Q.; Oguz, H.N.; Prosperetti, Andrea
A simple model of a small bubble floating at the surface of a liquid before bursting is considered. The oscillations of this system are studied by means of a Lagrangian method. It is found that two fundamentally different modes exist. The surface mode has low frequency and does not change appreciably the volume of the immersed part of the bubble: As a consequence, its efficiency as a source of sound in the water is very limited. The volume mode has a much higher frequency and is a more efficient radiator in the water, although it may be hard to excite. Both modes behave as monopole sources in the air. It is therefore predicted that an oscillating floating bubble is a much more intense source of sound in the air than in the liquid. This conclusion seems to be supported by experimental observations.