Filters

Results

**1**-**10**of**62** Results

**1**-**10**of**62**. Search took:**0.013**secondsSort by: date | relevance |

AbstractAbstract

[en] Numerical hydrodynamic simulations of the growth and collapse of a 10-μm air bubble in water were performed. Both the air and the water are treated as compressible fluids. The calculations show that the collapse is nearly isentropic until the final 10 ns, after which a strong spherically converging shock wave evolves and creates enormous temperatures and pressures in the inner 0.02 μm of the bubble. The reflection of the shock from the center of the bubble produces a diverging shock wave that quenches the high temperatures (>30 eV) and pressures in less than 10 ps (FWHM). The picosecond pulse widths are due primarily to spherical convergence/divergence and nonlinear stiffening of the air equation of state that occurs at high pressures. The peak temperature at the center of the bubble is affected strongly by the ionization model used for the air. The results are consistent with recent measurements of sonoluminescence that had optical pulse widths less than 50 ps and 30-mW peak radiated power in the visible. [This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.]

Primary Subject

Record Type

Journal Article

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] A finite reverberant system having a discontinuity in some physical parameter will exhibit the splitting of ray trajectories in the high-frequency limit. It is known that this ray splitting can increase the amount of chaos in the ray trajectories [Couchman et al., Phys. Rev. A 46, 6193 (1992)]. This increase of chaos is expected to reveal itself in the eigenfrequency spectrum as a shift away from Poisson statistics and toward the Gaussian orthogonal ensemble (GOE) statistics of random matrix theory. Numerical results are presented that confirm the predicted shift in the spectral statistics. [Work supported by ONR and DOE.]

Primary Subject

Record Type

Journal Article

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] The establishment of stable sonoluminescence from a single trapped bubble of air in water requires more than 5 s. During this time the bubble goes through a transition period (about 1 s long) that is characterized by an emitted intensity which is over ten times smaller than the steady state. Pure noble gas bubbles turn on to their steady state values on a much shorter time scale (say less than 0.2 s). During the transient period light from an air bubble is weaker than light from an Argon bubble but in the steady state the air bubble is brighter. In view of the long time scale required for the establishment of sonoluminescence from a single bubble of air it is concluded that this is a fundamentally different phenomenon from the transient multibubble sonoluminescence that has been studied since its discovery in 1934. [Work supported by the U.S. DOE Division of Advanced Energy Projects.]

Primary Subject

Record Type

Journal Article

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] Plasma-induced bubbles respond to time-dependent injection of energy which modifies the acoustic output relative to ''classical'' gas bubbles. Additionally, they consist primarily of hot vapor which complicates the thermodynamics, elevates the importance of some traditionally untreated variables, and requires additional physical process treatments beyond the basic hydrodynamics. A code that is based upon fundamental physical principles was developed to study the importance of many of these variables. The code is capable of treating the actual driving circuit, resultant plasma behavior, transition from cylindrical to spherical geometry early in the discharge, radiation production, plasma chemistry, thermal transport, and hydrodynamics in a modified Flynn formulation. Primary output consists of bubble wall acceleration, radius, etc., energy balance, and far field pressure. The code has been validated with experimental data and will be compared with ongoing hydrocode model development as well. Experimental and theoretical results are presented for large vapor bubbles. [Work supported by the U.S. Navy.]

Primary Subject

Record Type

Journal Article

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] The experimentally observed peak in the backward radiation from a liquid--solid interface at the Rayleigh angle is caused by a sharp increase in the otherwise relatively weak incoherent scattering from the inherent inhomogeneities of the solid. The increase of the incoherent backscattering is essentially a specular effect in contrast with the nonspecular, finite-beam effect predicted to cause the much weaker coherent reflection. The incoherent scattering can be caused by either geometrical irregularity or material inhomogeneity (e.g., surface roughness or polycrystalline grain structure). This paper presents analytical results showing that, regardless of the physical nature of the scattering inhomogeneity, there is a distinct peak in the backscattered intensity around the Rayleigh angle. The angular dependence of the incoherent backscattering is determined by the average properties of the liquid and the solid. The peak always occurs at the Rayleigh angle and the width of the peak is determined by the density ratio between the solid and the liquid. The maximum backscattering at the peak is essentially independent of the average material properties and is determined solely by the scattering inhomogeneity

Primary Subject

Record Type

Journal Article

Journal

Journal of the Acoustical Society of America; ISSN 0001-4966; ; CODEN JASMAN; v. 96(4); p. 2537-2545

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] Most acoustic sources found in the ocean environment are spatially complex and broadband. In the case of shallow water propagation, these source characteristics complicate the analysis of received acoustic data considerably. A common approach to the broadband problem is to decompose the received signal into a set of narrow-band lines. This then allows the problem to be treated as a multiplicity of narrow-band problems. Here a model-based approach is developed for the processing of data received on a vertical array from a broadband source where it is assumed that the propagation is governed by the normal-mode model. The goal of the processor is to provide an enhanced (filtered) version of the pressure at the array and the modal functions. Thus a pre-processor is actually developed, since one could think of several applications for these enhanced quantities such as localization, modal estimation, etc. It is well-known that in normal-mode theory a different modal structure evolves for each temporal frequency; thus it is not surprising that the model-based solution to this problem results in a scheme that requires a 'bank' of narrow-band model-based processors each with its own underlying modal structure for the narrow frequency band it operates over. The 'optimal' Bayesian solution to the broadband pressure field enhancement and modal function extraction problem is developed. It is shown how this broadband processor can be implemented (using a suboptimal scheme) in pseudo real time due to its inherent parallel structure. A set of noisy broadband data is synthesized to demonstrate how to construct the processor and achieve a minimum variance (optimal Bayesian) design. It is shown that both broadband pressure-field and modal function estimates can be extracted illustrating the feasibility of this approach. copyright 1998 Acoustical Society of America

Primary Subject

Secondary Subject

Record Type

Journal Article

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] A prominent feature predicted for the backscattering of tone bursts by thin spherical shells is an enhancement of a guided wave contribution in the midfrequency range. This feature may be useful for certain inverse problems and is associated with a strongly coupled slightly subsonic wave denoted by some authors as the a

_{0-}wave. The present research gives a comparison between a ray approximation and experiments in which tone bursts having carrier frequencies in the range 35 < ka < 70 were incident on an empty stainless steel shell in water of radius a. The sphere's radius to thickness ratio is a/h = 43.8. Time records of echoes provided a means for measuring the guided wave contribution distinct from that of the specular reflection. As predicted, the guided wave echo can be over three times the amplitude of the specular echo and the ka dependence generally follows the predicted hump. Echoes manifest structures similar to those calculated by a time domain Fourier synthesis from the partial wave series. 16 refs., 9 figs., 1 tabPrimary Subject

Record Type

Journal Article

Literature Type

Numerical Data

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] Oscillating thermal diffusion in a sound wave in a mixture of two gases is remarkably effective for separating the components of the mixture. We consider this separation process in boundary-layer approximation, with zero temperature gradient and zero concentration gradient along the direction of sound propagation. In the boundary layer, the combination of thermal diffusion with the oscillating temperature gradient and oscillating velocity gradient leads to second-order time-averaged fluxes of the two components of the mixture in opposite directions, parallel to the wave-propagation direction. The oscillating thermal diffusion also adds to the dissipation of acoustic power in the boundary layer, modifying thermal-relaxation dissipation but leaving viscous dissipation unchanged. copyright 1999 Acoustical Society of America.< exclamation point--[HEB] -->

Primary Subject

Record Type

Journal Article

Journal

Journal of the Acoustical Society of America; ISSN 0001-4966; ; CODEN JASMAN; v. 106(4); p. 1794-1800

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] The form of the relative operating characteristic (ROC) describing auditory detection typically differs from that expected based on the assumption that the observer's responses reflect underlying distributions that are normal and of equal variance [e.g., Green and Swets, Signal Detection Theory and Psychophysics]. Specifically, it is found that binormal ROCs often have slopes less than one, and are not strictly linear [Watson et al., 283--288 (1964)]. It has been suggested that nonlinear binormal ROCs may be the result of extreme criteria being more variable than those more centrally positioned [Emmerich and Binder, J. Acoust. Soc. Am. Suppl. 1 65, S59 (1979)]. The forms of rating ROCs obtained in an experiment designed to reveal the effects of such variability were consistent with the proposition that criteria associated with ''no'' responses are more variable than those associated with ''yes'' responses. Criterion operating characteristics [Wickelgren, J. Math. Psychol. 5, 102--122], which reflect the relative locations and variances of the boundaries of confidence categories, favored the same interpretation. It is concluded that the variability of criteria defining categories of rated confidence is sizable and not necessarily constant across criteria, and that such variability significantly influences the forms of empirical ROCs determined using confidence ratings in auditory signal detection

Primary Subject

Record Type

Journal Article

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

AbstractAbstract

[en] The dynamic sound field pressure P

_{a}required to generate sonoluminescence (SL) from a single trapped bubble is a little higher than the ambient pressure P_{0}(e.g., P_{a}∼1.2P_{0}). Since the acoustic energy density is proportional to the square of P_{a}observation of SL at lower drive levels would imply that even greater degrees of energy concentration accompany the transduction of sound into light. Motivated by this perspective, the dependence of SL on ambient pressure is being measured. Light emission at P_{0}=0.3 Atm has already been achieved. Pressures higher than an atmosphere are also being investigated, especially with attempts to find single bubble SL in liquids other than water. [Work supported by the U.S. DOE Division of Advanced Energy Projects.]Primary Subject

Record Type

Journal Article

Journal

Country of publication

Publication YearPublication Year

Reference NumberReference Number

INIS VolumeINIS Volume

INIS IssueINIS Issue

1 | 2 | 3 | Next |