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Apr 1991

Volume 89, Issue 4B, pp. 1851-2015

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back to top Session 9UW: Underwater Acoustics: Bubbles and Ambient Noise
Contributed Papers
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Propagation theory of extended noise sources: Improvements and new developments (A)

C. David Rees

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2012-2013 (1991); (2 pages)

Online Publication Date: 14 Aug 2005

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Both understanding the source generation characteristics of ocean surface noise and other extended sources from the viewpoint of oceanographic acoustics, and achieving a useful understanding of the characteristics of a noise field at a sensor array to discriminate against noise, require accurate modeling of source to sensor propagation. Kuperman and Ingenito [J. Acoust. Soc. Am. 67, 1988–1996 (1980)] laid out a general Greens function technique for the totally range independent vertically stratified case, but treated in detail only a totally uncorrelated infinite sheet source. The author has developed and analyzed extensions to this approach, including specified source correlation functions and extended sources of finite extent. Development of a range‐dependent propagation theory seems to be required to achieve the observed noise peak at near‐horizontal angles. The parabolic equation formalism provides a natural basis for this: techniques and results developed by the author to date are discussed and presented.
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Three‐dimensional rough interface scattering of ambient noise (A)

Jin‐Yuan Liu and Henrik Schmidt

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2013-2013 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Based on the previously developed theory of perturbation approach to rough surface scattering [W. A. Kuperman and Henrik Schmidt, J. Acoust. Soc. Am. 86, 1511–1522 (1989)] and a model for the noise field generated by surface random sources in an occean waveguide [W. A. Kuperman and F. Ingenito, J. Acoust. Soc. Am. 67, 1988–1996 (1980)], a formulation for rough interface scattering of surface‐generated ambient noise in a horizontally stratified ocean is established. The result is then applied to study the three‐dimensional scattering in a shallow‐water waveguide environment bounded below by a viscoelastic medium. The three‐dimensional spatial correlation of the reverberated noise in the waveguide and the scattered noise in the elastic medium are examined in terms of the relation to frequency, water depth, and rough interface statistics. It is demonstrated that even in deep water, the scattering of surface generated noise into interface waves (Scholte waves) is important, showing these waves carriers of ambient noise energy consistent with experimental observation. [Work supported by NOARL.]
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Trends of wind and noise dependency at Lake Pend Oreille, Idaho (A)

Jonathan Cummings, II

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2013-2013 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Ambient noise and wind‐speed measurements were obtained at Lake Pend Oreille periodically from October 1985 through August 1989. Cyclic trends in noise levels were observed and are summarized. Variations in noise level with time of day and season of the year correlate well with wind speeds and trends in recreational boating use. Daily averaged data show the contribution of recreational boating to lake noise: 10–15 dB average level increase for midday as compared to night. Average seasonal trends are generally higher (up to 10 dB) during the summer months as compared to winter months. Noise dependencies on wind speed were empirically derived and are found to compare well with observations from ocean environments.
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Sound radiation from large raindrops: Dependence on salinity and temperature (A)

Peter W. Jacobus, Herman Medwin, and Jeffrey A. Nystuen

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2013-2013 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Raindrops are known to produce bubbles by at least two different mechanisms: type I, the pinch‐off of the bottom of the crater, and type II, air entrainment from a turbulent water jet formed as a part of the drop splash. Bubble energy is shown to dominate impact energy for the useful range of type II drops. Previous work [Snyder et al., J. Acoust. Soc. Am. Suppl. 1 88, S2 (1990)] has shown a relationship between drop size and bubble frequency of the type II mechanism. The onset of this mechanism is related to the drop kinetic energy at impact, rather than to velocity. As the drop kinetic energy increases above a threshold of 2 × 10−4 J, the likelihood of bubble entrainment increases to approximately 65%. Further work reveals the effects of temperature, salinity, and surface tension on the sound radiated from large raindrops. The relation between the drop diameter and the spectrum of the underwater acoustic energy is examined for terminal velocity drops. This relation makes possible the remote measurement of both drop size distribution and rainfall rate. [Work supported by ONR.]
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Bubble production by capillary waves (A)

Ali Kolaini, Ronald A. Roy, and Lawrence A. Crum

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2013-2013 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Bubble‐producing capillary waves can be easily generated in a laboratory tank (8 cm × 22 cm × 150 cm) by blowing air over the water. The lowest wind speed required for the occurrence of this process is measured. The generation of the capillary waves depends solely on the surface tension, which can be changed by adding surfactants and other chemicals to the tank water. The effect of surface tension on the windspeed threshold for bubble production is discussed. Using a coincidence detector, the bubble production rates per unit area can be measured. Lowering the surface tension, surprisingly, increases the rate of bubble production. The dependence of the bubble production rate on wind speed and wind fetch is also discussed. The underwater acoustic emissions from these bubbles are measured. Average power spectrum for several wind speeds are obtained that exhibit a broad range of radiated frequencies (i.e., bubble sizes) and a weak dependence on the wind speed. [Work supported by ONR, ONT, and AEAS.]
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The underwater sound produced by the impact of solid objects (A)

Michael Nicholas, Ronald A. Roy, and Lawrence A. Crum

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2013-2013 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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There have been many studies that indicate that there is significant underwater sound produced by precipitation [e.g., J. A. Scrimger, Nature 318, 647 (1985)]. It has also been demonstrated that the major contribution to the underwater sound of rainfall is that due to the entrainment of gas bubbles by the impacting drop [Pumphrey et al., J. Acoust. Soc. Am. 31, 1080 (1989)]. In an attempt to understand the underwater sound produced by hail, the impact of solid objects with a plane water surface have been examined. With the aid of a high‐speed movie camera and underwater transducers, it has been determined that the air entrained by the impacting object plays a major role in the sound production. The details of this process as well as the implications concerning the underwater sound produced by hail will be presented. [Work supported by ONR.]
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The hydrodynamic and acoustic behavior of bubble plumes generated by an impacting water jet (A)

Ali Kolaini, Ronald A. Roy, and Lawrence A. Crum

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2013-2013 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The preliminary results of an experimental study of the underwater sound field emitted by a bubble plume generated by dropping fixed volume of water, held in a cylindrical container, onto a still water surface were previously presented [J. Acoust. Soc. Am. Suppl. 1 88, 514 (1990)]. Further studies of the acoustic and hydrodynamic characteristics of the bubble plume are presented. The high‐speed video images reveal the formation of a cylindrical plume that grows in length until all of the impacting water volume is injected into the still water. As the leading end of the plume advances, a “substructure” separates from the rest of the plume. The onset of the large‐amplitude, low‐frequency sound emission occurs at the instant the substructure detaches. The resonance frequencies of the densely populated substructures are inversely proportional to their radii and are highly dependent on the void fraction. Experimental results are presented which show that detached plumes undergo damped volume oscillations. The measured damping coefficients are found to be constant and are believed to be related to the thermal damping coefficient. The dependence of the detached plume diameters and undetached plume lengths on the containers radii, lengths, and heights above the still water level are also discussed. [Work supported by ONR and ONT.]
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Some nonlinear aspects of the frequency response of a strongly excited gas bubble oscillator (A)

Dong H. Kim

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2014-2014 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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A numerical analysis is carried out for the nonlinear phenomena of the bubble oscillator. The model is based on the Keller's formulation for the bubble dynamics. Interpretation of the bubble interior is based on the formulation by Prosperetti. His formulation adopts the energy equation for the analysis of the bubble interior. The numerical simulation shows typical nonlinear phenomena in its frequency response. Among such nonlinear aspects are the jump phenomenon, the hysterisis effect, the shift of natural frequency of the system, and the appearance of superharmonic resonances. It is deduced that the nonlinear frequency response is dependent upon the initial condition of the bubble oscillator, and some multivalued frequency region can appear in the response curve. Nonlinear phenomena that appeared in the bubble oscillator are compared with those of the Duffing equation and it may be said that the bubble dynamic equation has similar nonlinear aspects to the Duffing equation.
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Multiple scattering from a randomly distributed cloud of bubbles (A)

Lintao Wang and Kenneth E. Gilbert

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2014-2014 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Exact multiple scattering calculations are presented for scattering from a randomly distributed cloud of bubbles. The conditions under which a single scattering (Born) approximation holds are discussed. In addition, numerical calculations are presented that show the relative contributions of coherent and incoherent scatter. The multiple scattering calculations are compared to scattering from an “effective fluid” model of a bubble cloud. It is shown that while the fluid model is adequate for predicting forward scatter, it cannot accurately predict backscatter when the acoustic wavelength is comparable to the dimensions of the bubble cloud. Some alternative approaches are discussed for accurately predicting backscatter from a randomly distributed cloud of bubbles. [Work supported by ONR.]
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Pulse length effects on the transmissivity of bubbly water (A)

H. R. Suiter

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2014-2014 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The passage of sound through bubbly water is strongly attenuated by scattering and absorption. Such attenuation is most severe around the frequency of resonance of individual bubbles. A bubble takes a finite time to ring up to steady‐state conditions and continues to oscillate for a finite time after the driving pressure erases. Low backscatter for short pulse lengths has been observed in near‐surface seawater [Akulichev et al., Soc. Phys. Acoust. 32, no. 3]. An experiment is described that looked for a corresponding enhancement in transmission. Comparisons were made between the attenuations of brief waveform bursts and longer bursts. The frequency range of this experiment was 50–200 kHz. The bubbles were made by the electrolysis of fresh water in a small laboratory tank. For bursts of 6–20 wavelengths in duration, no difference in the attenuations was discerned in comparison with a 2.7 wavelength duration burst. [Work supported by ONR.]
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The release of air bubbles from an underwater nozzle (A)

Michael Longuet‐Higgins, Bryan R. Kerman, and Knud Lunde

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2014-2014 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Air bubbles released from an underwater nozzle emit an acoustical pulse that is of interest both for the study of bubble detachment and for elucidating the mechanism of sound generation by a newly formed bubble. In this paper, the sequence of bubble shapes is calculated theoretically from a given nozzle, and it is shown that there is for each nozzle a bubble of maximum volume Vmax. Assuming that the bubble becomes detached at its “neck,” and that the volume of the detached bubble equals the volume V∗ of the undetached bubble above its “neck,” it is determined for each nozzle diameter D an acoustic frequency f∗ corresponding to “slow” bubble release. Experiments show that the acoustic frequency, hence the bubble size, depends on the rate of air flow to the bubble, but for slow rates of flow the frequency f is very close to the theoretical frequency f∗. High‐speed photographs suggest that when the bubble pinches off, the limiting form of the surface is almost a cone. This is accounted for by assuming a line sink along the axis of symmetry. Immediately following pinch off, there is evidence of the formation of an axial jet going upward into the bubble. This may play a part in stimulating the emission of sound.
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Numerical simulations of bubble release from underwater needles (A)

Hasan N. Oguz and Andrea Prosperetti

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2014-2014 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The release of bubbles from a submerged needle is a noisy process that is difficult to stabilize. Longer needles are found to produce a much steadier stream of bubbles than short ones. A possible explanation for this behavior is given by means of a dynamic model that accounts for the pressure drop inside the needle. The flow field created by the injected bubble is assumed to be irrotational and a boundary integral formulation is used to calculate the evolution of the bubble surface. Realistic bubble formation histories are obtained and the numerical results appear to be in good agreement with the available experimental measurements. The behavior of the bubble after detachment and the effect of the previously released bubble are also studied. [Work supported by ONR.]
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Dynamics of air bubbles entrapped by capillary waves (A)

Hasan N. Oguz and Michael S. Longuet‐Higgins

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2014-2015 (1991); (2 pages)

Online Publication Date: 14 Aug 2005

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It is well known that entrapment of air bubbles near the sea surface contributes substantially to the underwater noise levels. The collapse of air pockets created by steep capillary waves is a possible mechanism of bubble formation. A boundary integral formulation is employed to simulate the bubble behavior after detachment from these air pockets. The initial profile of the trapped bubble, approximated from the form of the capillary waves, suggests a high elongated bubble relatively far from the sea surface. A sequence of initial bubble shapes is generated from successively closer approximations to the bubble shape. Depending on the initial curvature of the detachment point the bubble may break up into two or oscillate as a single bubble in a rather violent manner. Volume oscillations that are responsible for the radiation of sound are found to be affected by the shape oscillations. It is found that a traveling capillary wave on the bubble surface can cause a pressure pulse upon reaching the axis. As a result, the pressure signal may deviate substantially from the simple damped sinusoid of a spherical bubble. [Work supported by ONR.]
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Direct numerical simulation of bubble behavior in oscillatory flow (A)

D. Z. Zhang, A. Prosperetti, and A. S. Sangani

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2015-2015 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The motion of spheres (bubbles) in an incompressible liquid undergoing a small‐amplitude oscillatory motion is calculated by a multipole expansion method. In the limit of small viscosity, the Stokes layer is confined to the vicinity of the surface of the bubbles, which therefore interact approximately only through the pressure field. The motion of the spheres is parametrized in terms of added mass, Basset, and drag forces; the coefficients of which are obtained from the simulation. To obtain results useful for the study of pressure wave propagation in bubbly liquids, several bubble configurations are studied for different (finite) volume fractions and the results then averaged. The effects of surface tension and bubble density are also considered. [Work supported by DOE.]
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