摘要:

A pulsed spectrophone has been developed to study vibrational energy transfer processes in gases. CO2was chosen as the test gas to allow comparisons with other methods. The spectrophone proves to be a powerful tool to measure relaxation rates and study relaxation pathways when coupled with other measurement techniques. All the micro‐ and macroscopic processes which affect the spectrophone response must be considered simultaneously in order to interpret the measured temporal evolution of pressure following excitation. Formulas which include a generalized treatment of multilevel systems and important macroscopic processes are presented. The experimental data are represented in terms of characteristic times of the pressure waveform and amplitudes of pressure changes. At intermediate (20 Torr and above) pressures, collisional energy transfer rates and mechanisms have the greatest influence on computed waveforms. Spontaneous emission has an obvious effect on the zero pressure intercept of the characteristic times. Acoustic propagation and thermal conduction determine the rate at which the gas, once perturbed, returns to equilibrium; at intermediate and higher pressures, acoustic propagation is found to be most important. The assumed rates of energy transfer and the relaxation mechanisms are varied to give computed pressure waveforms which agree with the measured spectrophone response (characteristic times and amplitudes), and which are simultaneously consistent with measurements using other techniques. This process indicates that the path of energy transferred from CO2(001) to CO2(040) is consistent with all experimental observations.

ISSN:0001-4966    

DOI:10.1121/1.390268

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A wave propagating in a medium having random fluctuations in refractive index will suffer phase and amplitude perturbations. In the receiving plane, a random interference pattern will appear and this so‐called scintillation pattern will vary in time for two reasons: (1) the decay of the refractive‐index fluctuations producing the amplitude perturbation (eddy decay) and (2) advection of the eddies by the flow. In the case where eddy lifetimes are long compared with the scintillation period, we can derive estimates of flow from a statistical analysis of the scintillation pattern. In this paper, we discuss the propagation theory and report measurements of oceanic flows by analysis of the acoustic scintillation pattern produced by the density fluctuations in the ocean. By mounting a 214‐kHz source and two receivers on opposite sides of a barge such that the axis of propagation is perpendicular to the direction of travel, we induce a known flow rate equal to the barge velocity. We compute the slope of the time‐lagged covariance function of the logarithm of the amplitude at the two detectors. The slope is proportional to the path‐averaged flow transverse to the propagation path. Simultaneous measurements with a current meter provide sea truth. We have shown that such a technique will measure flow velocity with reasonable accuracy. An interesting result of the measurements is the demonstration that sound speed fluctuations in the spatial window 15–30 cm satisfy the Kolmogorov spectral slope for an inertial subrange, at the shallow depth (2.1 m) of the observation.

ISSN:0001-4966    

DOI:10.1121/1.390269

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

In the shallow water of the Yellow Sea, filtering of mode 1 and mode 2, by employing a vertical array of nine hydrophones, has been realized in the frequency range of 250–800 Hz with a near isovelocity condition. Eigenfunctions of the mode were calculated by two parameters (P,Q) to describe the characteristics of bottom reflection approximately at small grazing angles. The advantage of treating the bottom in terms ofPandQrather than using the familiar sound speed, density, and attenuation coefficient is that the bottom reflection loss due to the effect of bottom roughness can be incorporated. Results of mode filtering were quite favorable. Group delay measurement of mode 1 and mode 2 agrees well with theoretical values calculated by a fitting value of the bottom reflection phase shift parameterP. The parameterQof the bottom reflection loss can be extracted from the amplitude ratio of mode 1 and mode 2. The extracted values ofQwere near those ofQobtained by other approaches.

ISSN:0001-4966    

DOI:10.1121/1.390270

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A uniform asymptotic solution is presented for sound propagation from a constant frequency point source in shallow water whose depthH(r) decreases monotonically with cylindrical distancer. The water has constant sound speedc1and density ρ1; the bottom fluid extends indefinitely in depth and has sound speedc2and density ρ2, wherec2>c1. The interface depth has constant valueH0up to ranger0and thereafter decreases linearly to zero. The solution appears as a sum of modal terms, each such mode eventually encountering a critical depthHc(n) (at which modal phase velocity equalsc2) at a critical rangerc(n). A previously derived local solution for a modal term near its critical range is modified such that it automatically reduces to the adiabatic mode solution at nearer ranges and such that it is valid at arbitrary distances beyond the critical range. Bulk attenuation is incorporated into the model using an appropriate modal average over depth. Numerical results are compared with four parabolic equation computations supplied by Jensen. In these four cases only single mode propagation is considered andr0is 0. The two theories agree well, with discrepancies typically less than 1 dB. Additional comparisons with parabolic equation computations (simultaneous propagation of three and two modes) presented by Jensen and Kuperman [J. Acoust. Soc. Am.67, 1564–1566 (1980)] show comparable agreement, but for the higher modes there are marked discrepancies in the directions at which sound is beamed into the bottom fluid from regions encompassing those mode’s critical ranges. This is attributed to the progressive deterioration of the present theory with increasing mode number.

ISSN:0001-4966    

DOI:10.1121/1.390271

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

In this paper we describe a new wide‐angle parabolic equation based on an operator‐splitting that permits the use of a marching‐type Fourier transform solution method. The equation was first presented by Feit and Fleck [Appl. Opt.17, 3990–3998 (1978)] for studying propagation within optical fibers. Existing computer codes which numerically solve the standard parabolic equation of ocean acoustics by the split‐step algorithm of Tappert and Hardin are easily modified to accommodate the wide‐angle capability of the new equation. In addition, since the new wide‐angle equation is less sensitive to the value of the reference wavenumber, the effects of phase errors are greatly reduced. The results of a simple error analysis indicate that improved accuracy can be achieved by the new wide‐angle equation for propagation conditions typical of deep ocean environments. This is supported by our numerical experience, a summary of which is presented in the paper. For test cases, where the variation of the acoustic index of refraction was large, the new wide‐angle equation gave results superior to those of the standard parabolic equation. Moreover, even for conditions which support long range, low‐angle propagation in the deep ocean, the predictions based on the new equation are a significant improvement over those obtained with the standard equation.

ISSN:0001-4966    

DOI:10.1121/1.390272

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

We consider the question: Does sound fall under gravity?

ISSN:0001-4966    

DOI:10.1121/1.390273

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Detailed spectral analysis of sodar echoes can be utilized to yield information on the structure and vorticity of the wind field within the scattering volume. The analysis is applied to a case of unusually intense turbulence and wind shear recorded at the Capanna Margherita, Monte Rosa, Italy (4559 m.a.s.l.).

ISSN:0001-4966    

DOI:10.1121/1.390177

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A relation has been derived which gives the effective nonlinearity parameter of a system of immiscible liquids, given the individual nonlinearity parameters as well as the respective densities, compressibilities, and volume fractions in the mixture.

ISSN:0001-4966    

DOI:10.1121/1.390283

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The propagation of acoustic waves from a high‐frequency line source in a two‐dimensional parallel shear flow adjacent to a rigid wall is analyzed by a ray method. The leading term in the resulting expansion is equivalent to the geometrical acoustics theory of classical wave propagation. It is shown that energy from the source is radiated either directly to the farfield, or by first reflecting from the wall. In addition, energy is trapped in a channel adjacent to the wall and downstream from the source. The rays in this channel form an infinite sequence of caustics progressing downstream. Since the geometrical acoustics approximation is invalid on and near caustics, a boundary layer method is employed to determine the acoustic fields near the caustics. It is shown that the amplitude of the fields on and near the caustics isk2/3larger than the geometrical acoustics field for largek. Herekis a dimensionless wavenumber of the source. Moreover, the vorticity of the acoustic fields in the caustic regions isk7/6larger than the geometrical acoustics field. The possible significance of these results for vehicle self‐noise and the formation of turbulent spots in the sublayer of a turbulent boundary layer is discussed.

ISSN:0001-4966    

DOI:10.1121/1.390233

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The conventional formulations of the acoustic and electromagnetic scattering integral equations, which are valid only for surfaces with continuous and differentiable curvature, are generalized to surfaces containing curvature discontinuities. It is argued that these generalizations are essentially a generalization of the 1859 and 1882 Kirchhoff integration of the scalar and vector wave equations, respectively, as well as a generalization of the 1938 Stratton–Chu integration of Maxwell’s equations.

ISSN:0001-4966    

DOI:10.1121/1.390234

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP