摘要:

This paper reviews published information on aerodynamically generated noise from large horizontal axis wind turbines operated for electric power generation. Methods are presented for predicting both the discrete frequency rotational noise components and the broadband noise components, and results are compared with measurements. Refraction effects that result in the formation of high-frequency shadow zones in the upwind direction and channeling effects for the low frequencies in the downwind direction are illustrated. Special topics such as distributed source effects in prediction and the role of building dynamics in perception are also included.

ISSN:0001-4966    

DOI:10.1121/1.401021

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Power flow is computed for sound incident on a solid aluminum sphere in water. First, power flow together with kinetic and potential energy distributions are investigated for the standing modes of free vibration of the sphere. The instantaneous power flow excited in the sphere by a continuous train of plane waves is examined next. Finally, time-averaged power flow is investigated. Little has been done so far to compute sound-power flow in structures immersed in a fluid. Computations of this kind provide a new insight into the relation between the elastic waves in the structure and the sound radiated into the fluid.

ISSN:0001-4966    

DOI:10.1121/1.400822

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A model for the scattering of wideband signals from three-dimensional rough surfaces is presented. This development is an extension of the scattering based on the Kirchhoff approximation method for a monochromatic wave source. The usual plane-wave approximation is relaxed and the Kirchhoff method is extended to include spherically diverging waves from the scattering surface. The scattered signal is expressed in terms of the Fourier transform of the acoustic channel. The frequency correlation function of the channel is then derived for a rough surface having the Gaussian spectrum. The coherent and incoherent components of the scattered signal intensity are determined from the frequency correlation function. The theory is applied to the signals produced by the Polaroid transducer operating in air and mounted on a mobile vehicle capable of automatic translation and rotation. Theoretical results are compared with experimental observations from two different rough surfaces. The analytic results are verified over the observable range of incidence angles from 0° to 60°.

ISSN:0001-4966    

DOI:10.1121/1.400692

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Two new methods are presented to determine the transient acoustic radiation loading on circular cylinders. The acoustic modal forces are expressed as a sum of the convolution of the modal radiation impulse responses and the modal velocities. In the wave-vector-time domain method, the modal radiation impulse responses are obtained by using the radiation impulse responses of an infinite cylinder and the modes in wave-vector space. The approach can thus be easily used for parametric studies on the effects of boundary conditions and normalized lengths on the modal radiation impulse responses. In the Kirchhoff integral equation method, space and time are discretized. The pressure and acceleration in each element are then expressed in the form of a Taylor-series expansion about points at which the pressure is obtained by marching forward in time. The radiation impulse responses of both infinite and simply supported finite cylinders are obtained by the two methods and compared.

ISSN:0001-4966    

DOI:10.1121/1.400693

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

An acoustic ray analysis is employed in synthesizing the form function for backscattering,f(θ=π,ka),from a fluid-loaded evacuated elastic spherical shell wherekis the wave number of the incident plane wave andais the outer radius of the shell. The synthesis contains a component associated with a specular reflection,fsp,and contributions from leaky Lamb waves. The contributionflof thelth leaky Lamb wave is expressible in a Fabry–Pérot resonator form [P. L. Marston, J. Acoust. Soc. Am.83, 25–37 (1988)]. A comparison of the ray synthesis forf(ka)with the exact partial-wave series representation for a 440c stainless-steel shell verifies the usefulness of the ray synthesis for the present case of a shell. The present synthesis is also new in that it includes the effects of longitudinal resonances onfsp.A novel ray synthesis offspindicates a significant resonance effect near the conditionkLh=nπ(n=1,2,…). The thickness of the shell ish, andkL=ω/cLis the longitudinal wave number wherecLis the longitudinal speed of sound in the elastic material. The ray synthesis demonstrates that the curvature of the shell is essential to the modeling of longitudinal resonances. Although acoustic ray modeling is generally a high-frequency technique, the ray synthesis off(ka)for a 440c stainless-steel shell appears to be applicable forkaas small as 7. Certain anomalies in the synthesis are investigated to better understand the limitations of the present ray model.

ISSN:0001-4966    

DOI:10.1121/1.400694

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

To recognize a target submerged in water, a spectroscopy based on the backscattering spectrum or on the resonance spectrum can be used. These spectra are experimentally obtained from the quasiharmonic or short pulse method of isolation and identification of resonances (MIIR). They show sharp amplitude variations that are related to the scatterer’s resonances. These resonances are related to the surface waves that propagate around the target. The backscattering spectrum and the resonance spectrum are power spectra. They are obtained from the computation of the modulus of the Fourier transform of the signal scattered by the target. In this paper, the computation of the phase tangent of the Fourier transform is presented. The spectra obtained are called the Im/Re spectra. The experimental results are compared with the computation of the ratio of the imaginary part over the real part of the complex pressure observed at a large distance from the target center. It is shown that the Im/Re spectra are highly useful for distinguishing resonances in scatterers with large absorptivity, where conventional backscattering or resonance spectra prove insensitive.

ISSN:0001-4966    

DOI:10.1121/1.400684

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper shows how an inversion formula can be derived for reconstructing the magnitude and direction of stratified fluid flow from two scattering measurements. The measurements are recorded by transmitting an acoustic pulse from a source below the region of stratified flow and then recording the scattered wave at another point horizontally displaced from the source. A second measurement is performed with the source and receiver interchanged and the result is subtracted from the first scattering measurement. This difference is, within the Born approximation, independent of scattering that arises from stationary inhomogeneities and thus solely sensitive to scattering due to flow. As a by-product, the sum of the two scattering measurements with source and receiver interchanged yields a quantity sensitive only to scattering from stationary inhomogeneities. This allows the stationary inhomogeneities to be recovered independently of flow.

ISSN:0001-4966    

DOI:10.1121/1.400695

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A hybrid method combining the finite-element and eigenfunction expansions is proposed to study the guided waves in composite cylinders. This method is shown to be flexible in modeling generally anisotropic cylinders and computationally efficient. However, most of the analysis is confined to the case of a transversely isotropic cylindrical core surrounded by coaxial isotropic cylinders. The object is to model uniaxial fiber-reinforced materials with interface zones between the fiber (which is anisotropic) and the matrix (which is isotropic). Numerical results are presented for the axisymmetric case for a graphite fiber in magnesium and epoxy matrices. In particular, the measurable and analyzable effects of imperfect bonding are investigated. The reduction of bond stiffness is found to have significant effect on the leaky waves.

ISSN:0001-4966    

DOI:10.1121/1.400696

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The dispersion equation, for Love wave propagation in a slow elastic layer overlying a liquid saturated porous solid half-space, is derived. Both the media are assumed to be under initial stress. The effect of porosity and initial stress on the phase velocity of Love waves has been discussed. Dispersion curves for the phase velocity have been plotted, both with the presence and absence of prestress, for different values of the sandiness parameter. Some special cases have also been deduced.

ISSN:0001-4966    

DOI:10.1121/1.400697

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

At low wave numbers, according to recent indications, fluctuating wallshear stressgenerated by a turbulent boundary layer may be comparable with fluctuating wallpressure. Consideration is given here to the fluctuatingnormalstress generated within an elastic layer by both shear-stress and pressure excitation on its outer face. The transfer level from surface shear to normal stress is shown to be comparable with or exceed that from surface pressure in a significant range of low to intermediate wave numbers. This transfer from surface shear stress is enhanced by the outer fluid loading and by the imposition of vanishing displacement at an inner rigid surface. Results of computations are presented and analyzed in numerical examples based on a configuration where an elastomer layer is bounded by a fluid half-space at its excited surface and by a thin, damped plate at its inner surface, with a similar fluid half-space below the plate. Results are given also for the limiting cases of a rigid inner surface and of an elastic half-space with and without fluid loading. The transfer level from shear stress is computed also where a concomitant wall pressure is present and related to shear stress as recently determined from a theoretical treatment of the viscous boundary condition at low wave numbers in turbulent boundary-layer flow. It is thus suggested that the contribution of turbulent wall-shear stress to flow noise on recessed hydrophones in the absence of a plate-like element between these and the flow is likely an important one.

ISSN:0001-4966    

DOI:10.1121/1.400698

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP