摘要:

Numerical solutions are given for the set of coupled differential equations describing the dynamic interaction of sound with an arbitrary number of gas phase reactions, for several systems, both near and far from equilibrium. Acoustic response is assumed linear and adiabatic. Absorption by relaxation is included. Results for simple combustion systems indicate that transient effects can result in appreciable amplification of low sound frequencies.

ISSN:0001-4966    

DOI:10.1121/1.381520

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The subject of this paper is the problem of diffraction of a time‐harmonic obliquely incident plane acoustic wave by a semicircular infinite soft strip. An integral‐equation technique is presented to solve this two‐dimensional boundary‐value problem by the standard perturbation method when the wavelength of the incident wave is much larger than the radius of the semicircular strip. Approximate expressions are obtained for the farfield amplitude and scattering cross section.

ISSN:0001-4966    

DOI:10.1121/1.381521

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

It is well known that geometrical acoustics (GA) approximations to reflected fields depend on the surface shape incident to the field in a neighborhood. However, because of the general complexity of these solutions certain canonical problems pertaining to regular shapes, such as cones and spheres, have been solved, but little attention has been given to the problem of approximating the essential geometric properties for a general surface in a neighborhood. Approximations to the reflected field (ka≫1) usually assume (although this is not a limitation of GA as pointed out by Fock [ElectroMagneticDiffractionandPropagationProblem(Perganom, New York, 1965), 1st ed. ] that the surface is a well known geometric shape, i.e., ellipse, cone, etc. This is done to make calculations of curvature and other geometric properties straightforward. This paper will discuss the problem of obtaining the needed geometric properties for more general geometric shapes. It will consider only a small neighborhood of the surface. Using differential geometry and finite differences, it will then show that all geometric properties in a neighborhood may be approximated by a small number of points from that neighborhood. This technique can with a few practical limitations geometrically describe any neighborhood on a surface. These neighborhoods form ’’areas’’ called patches. When the entire surface meets certain conditions, these patches when ’’tied’’ together actually describe the entire surface and all its geometric properties.

ISSN:0001-4966    

DOI:10.1121/1.381522

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

This study was motivated by the need to design the thermal shield in reactor internals and other system components to avoid detrimental flow‐induced vibrations. The system component is modeled as two coaxial shells separated by a viscous fluid. In the analysis, Flügge’s shell equations of motion and linearized Navier–Stokes equation for viscous fluid are employed. First, a traveling‐wave‐type solution is taken for shells and fluid. Then, from the interface conditions between the shells and fluid, the solution for the fluid medium is expressed in terms of shell displacements. Finally, using the shell equations of motion gives the frequency equation, from which the natural frequency, mode shape, and modal damping ratio of coupled modes can be calculated. The analytical results show a fairly good qualitative agreement with the published experimental data. With the presented analysis and results, the frequency and damping characteristics can be analyzed and design parameters can be related to frequency and damping.

ISSN:0001-4966    

DOI:10.1121/1.381523

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The effect of a pressure‐release reflector on the parametric interaction process is examined theoretically and experimentally. It is shown that the 180° phase change upon reflection instigates a reversal of the finite‐amplitude distortion process in the primary radiation. Although energy then passes from the harmonics to the fundamental components of the primaries, the difference frequency component suffers a phase interference effect that leads to its partial annihilation.

ISSN:0001-4966    

DOI:10.1121/1.381524

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Acoustic refraction effects in the atmosphere by wind‐speed and temperature gradients are investigated using a simple multilayer model for plotting the ray path of a signal transmitted by an acoustic echo sounder (AES) and scattered back at various heights to be received monostatically. It is found that while nonperfect backscattering has negligible effect on the scattering cross section for acoustic waves in a turbulent medium, the effect on the ’’angle of arrival’’ and the Doppler shift is important. A purely horizontal wind can produce a Doppler shift in a vertically pointing monostatic AES which for a wind of 20 m/sec at 1 km can simulate vertical wind speeds of 50 cm/sec. For nonvertical sounding, this effect is of little importance. The generally accepted first‐order approximation for the angle of arrival of the scattered signal appears, as a result of this more detailed analysis, to be incorrect. The problems of the height resolution of a horizontal scatter layer using a nonvertical AES are briefly considered in terms of the antenna characteristics as well as the length of the transmitted pulse.

ISSN:0001-4966    

DOI:10.1121/1.381525

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The underwater acoustic propagation channel is dispersive in time, frequency, and space. This paper investigates frequency coherence and time coherence for simple multipath channel models with an emphasis on situations where there are a few paths. The propagation channel is postulated to consist of a finite set of paths with random amplitudes and lengths. The fluctuation characteristics of the frequency selective fading in a random time spread channel are investigated in terms of the statistics of the channel transfer function. The effects of the fluctuations in path length are clearly separated from the effects of nominal multipath structure in the channel statistics. The conditions under which coherence bandwidth and coherence time provide reasonable characterizations for the multipath channel are examined. All of the results obtained for the random time spread channel can be applied to the mathematically dual problem of a random frequency spread channel.

ISSN:0001-4966    

DOI:10.1121/1.381526

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

In this paper we establish an integral transform relation between the solutions of the Helmholtz and parabolic equations for sound propagation in an arbitrary two‐dimensional waveguide. The sound speed is a function of both depth and range. For range‐independent sound speeds, the integral transform is proved to be exact by using a normal‐mode expansion. For range‐dependent sound speeds the stationary phase approximation of the transform is, in lowest order, equivalent to the usual parabolic approximation. Corrections to the parabolic approximation are also calculated using the stationary phase method.

ISSN:0001-4966    

DOI:10.1121/1.381527

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The equations of motion of linear viscoelasticity (Kelvin–Voigt Model) in cylindrical coordinates are reduced to an axisymmetric plane‐stress situation valid for short hollow cylinders. The analysis models the deformations and oscillations occurring in the annular region conceptually constructed around one of the holes in a multiperforated rubber sheet to be used as an underwater acoustic absorber. The eigenvalue problem for the ring’s natural frequencies is solved in two cases: first, when both rims of the ring are subjected to a stress boundary condition; second, when the inner rim (r=a) is stress free and the outer one (r=b) is fixed. For incompressible materials with Poisson’s ratio very close to 0.5, the first few eigen‐wave‐numbers are numerically computed and plotted in nondimensional form versus theb/aratio, in both cases, and the resulting sets of curves are shown to be interlaced, the ones from the first case being below those of the second. The transient vibration problem for the displacementuris solved by the Laplace‐transform method in the first case only. The finding that the frequencies at which absorption is maxima are the resonant frequencies of the cavity, is verified. We show a simple technique to estimate the amount of absorption present at these eigenfrequencies, and the estimates reasonably check with experimental meaurements within the limitations of this model (i.e., below critical damping).

ISSN:0001-4966    

DOI:10.1121/1.381528

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The attenuation of underwater sound at very low frequencies exceeds the values expected on the basis of the variation observed at higher frequencies, and this excess attenuation exhibits a regional dependence. Two theoretical models have been proposed to account for the effect observed. Temperature soundings made along a Tasmania‐Antartica section have revealed variations of fine structure which an be correlated with variations in acoustical transmission properties and provide a test for the two theoretical models.

ISSN:0001-4966    

DOI:10.1121/1.381517

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP