885095971

2:45-3:00

Break

3:00

9UW8. Some nonlinear aspects of the frequency response of a strongly excited gas bubble oscillator. Dong H. Kim (Acoust. and Vibra!ion Lab., Korea Standards Res. Inst., P.O. Box 3, Daedok Sci. Town, Daejon, Korea)

A numericai 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 equa-tion for the analysis of the bubble interior. The numericai 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 dynamie eąuation has similar nonlinear aspects to the Duffing equation.

3:15

9LIW9. Multiple scattering from a randomly distributed cloud of bubbles. Lintao Wang and Kenneth E. Gilbert (Natl. Ctr. for Physical Acoust., University, MS 38677)

Exact multiple scattering calculations are presented for scattering from a randomly distributed cloud of bubbles. The conditions under which a single scattering (Bom) approximation holds are discussed. In addition, numericai calculations are presented that show the relative cnntributions of coherent and incoherent scatter. The multiple scattering calculations are compared to scattering from an‘⁴effcctivc fluid” model of a bubble cloud. It is shown that while the fluid model is adequate for predicting forward scatter, it cannot accurately prediet backscatter when the acoustic wavelcngth is comparable to the dimen-sions of the bubble cloud. Some a!temative approaches are discussed for accurately predicting backscatter from a randomly distributed cloud of bubbles. (Work supported by ONR.]

3:30

9UW10. Pulse length effects on the transmissivity of bubbly water.

H. R. Suiter (Naval Coastal Syst. Ctr., Codę 2120, Panama City, FL 32407)

The passage of sound through bubbly water is strongly attenuated by scattering and absorption. Such attenuation is most severe armmd 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 afłer the driving pressure ceases. Low backscatter for short puLse lengths has been obscrved in near-surface seawater (Akulichev et al., Sov. Phys. Acoust. 32, no. 3]. An experiment is described that looked for a corresponding enhancement in transmission. Comparisons were madę betwecn the attenuations of brief waveform bursts and longer bursts. The frequency rangę of this experiincnt was 50-200 kHz. The bubbles were madę by the electrolysis of fresh water in a smali laboratory tank. For bursts of 6-20 wavelengths in duration, no differ-cnce in the attenuations was discemed in comparison with a 2.7 wave-length duration burst. [Work supported by ONR.]

2014 J. Acoust Soc. Am., Vol. 89. No. 4, Pt 2, April 1991

3:45

9UW11. The release of air bubbles from an underwater nozzle. Michael Longuet-Higgins (La Jolla Inst., P.O. Box 1434, La Jolla, CA 92038 and Inst. for Nonlinear Sci., Univ. of Califomia, San Diego R-002, La Jolla, CA 92093), Bryan R. Kerman (Canada Center for Inland Waters, Burlington, Ontario, Canada), and Knud Lunde (Dept. of Appl. Math. and Theoretical Physics, Cambridge CB3 9EW, England)

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 theoreti-cally from a given nozzle, and it is shown that there is for each nozzle a bubble of maximum volume V_MX. Assuming that the bubble becomcs detached at its “neck,” and that the volume of the detached bubble equals the volume V* of the undctached bubble above its “neck,” it is determined for each nozzle diameter D an acoustic freąuencyy* corresponding to “slow" bubble releasc. Experiments show that the acoustic frequcncy, hence the bubble size, depends on the ratę of air flow to the bubble, but for slow rates of flow the frequency / is very close to the theoretical frequency./V 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 linc sink along the axis of symme-try. 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.

4:00

9UWI2. Numericai simulatkms of bubble release from underwater needles. Hasan N. Oguz and Andrea Prosperetti (Dept. of Mech. Eng., Johns Hopkins Univ., Baltimore, MD 21218)

The rclease of bubbles from a submerged needle is a noisy process that is difticult to stabilize. Longer needles are found to produce a much steadier stream of bubbles than short ones. A possible cxplanation for this behavior is given by mcans of a dynamie model that accounts for the pressure drop inside the needle. The flow field created by the in-jected 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 numericai results appear to be in good agreement with the availablc expcrimental measurements. The behavior of the bubble after detachment and the effect of the previously released bubble are also studied. (Work supported by ONR.]

4tl5

9UW13. Dynamics of air bubbles entrapped by capillary waves.

Hasan N. Oguz (Dept. of Mech. Eng., Johns Hopkins Univ., Baltimore, MD 21218) and Michael S. Longuet-Higgins (Ctr. for Studies of Nonlinear Dynamics, La Jolla Inst., La Jolla, CA 92038)

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 sim-ulate 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 ełongated bubble relatively far from the

121st Meeting: Acoustical Sodety of America 2014