1,721,036 research outputs found
Active noise cancellation of a spherical multipole source using a radially vibrating spherical baffled piston
No full textThe creation of quiet zones in a diffuse sound field due to a multipole spherical primary source by means of a radially vibrating surface set in the side of a rigid sphere (secondary source) is investigated in this article. The formulation utilizes the appropriate wave field expansions along with the translational addition theorems for spherical wave functions to develop a closed-form solution in the form of an infinite series. The numerical results reveal that using a baffled spherical piston model as a secondary source instead of a monopole control source will obviously improve the sound minimization efficiency of such noise-control systems in all cases, especially for a dipolar primary source
Active control of sound radiated from a spherical source in an acoustic half-space by using a radially vibrating spherical baffled piston
No full text
Acoustic radiation from a shell-encapsulated baffled cylindrical cap
No full textAn exact study of radiation of an acoustic field due to radial/axial vibrations of a baffled cylindrical piston, eccentrically positioned within a fluid-filled thin cylindrical elastic shell, into an external fluid medium is presented. This configuration, which is a realistic idealization of a liquid-filled cylindrical acoustic lens with a focal point inside the lens when used as a sound projector, is of practical importance with a multitude of possible applications in underwater acoustics and ocean engineering. The formulation utilizes the appropriate wave field expansions along with the translational addition theorems for cylindrical wave functions to develop a closed-form solution in the form of an infinite series. Numerical results reveal the key effects of excitation frequency, cap angle, radiator position (eccentricity), dynamics of the elastic shell, and cap surface velocity distribution on sound radiation
Modal vibrations of a cylindrical radiator over an impedance plane
No full textThe problem of acoustic radiation from an infinite cylinder undergoing harmonic modal surface vibrations near a locally reacting planar boundary is considered. The formulation utilizes the appropriate wave field expansions, the classical method of images, and the translational addition theorem for cylindrical wave functions, along with a simple local surface reaction model involving a complex amplitude wave reflection coefficient applied to simulate the relevant boundary conditions for the given configuration. The analytical results are illustrated with a numerical example in which the cylindrical surface is immersed near a layer of fibrous material set on an impervious rigid wall. The numerical results reveal the important effects of interface local surface reaction and source position on the computed modal impedance component values and the radiated on-axis far-field pressure. The benchmark solution presented can lead to a better understanding of acoustic radiation from near-interface two-dimensional sources, which are commonly encountered problems in outdoor acoustics and noise control engineering. Eventually, it could be used to validate those found by numerical approximation techniques
Jet noise prediction using different turbulent scales
No full textThe turbulent energy dissipation rate time-scale and length-scale has been routinely used for the prediction of noise from turbulent flows, particularly jet streams. However, this is not the only possible choice. In general, scales evolving in a turbulent medium are threefold. First, those associated with the mean flow; second, those attributed to the turbulence and the mean flow interactions; and third, scales related to the turbulence-turbulence interactions. In this paper, special attention will be paid to further study of the underlying physics of aerodynamic noise by examining various time-scales. To do so, three time scales, namely, dissipation, production, and strain rate time scales, are defined and used in the source modelling to emphasis the effect of the turbulence structures at different jet regions on the jet noise production mechanism. The required mean value and turbulence parameters are obtained using a modified k ? ? turbulence model, and Lighthill’s Acoustic Analogy is used for the prediction of the emanated noise. <br/
Eccentricity effects on acoustic radiation from a spherical source suspended within a thermoviscous fluid sphere
No full textAcoustic radiation from a spherical source undergoing angularly periodic axisymmetric harmonic surface vibrations while eccentrically suspended within a thermoviscous fluid sphere, which is immersed in a viscous thermally conducting unbounded fluid medium, is analyzed in an exact fashion. The formulation uses the appropriate wave-harmonic field expansions along with the translational addition theorem for spherical wave functions and the relevant boundary conditions to develop a closed-form solution in form of infinite series. The analytical results are illustrated with a numerical example in which the vibrating source is eccentrically positioned within a chemical fluid sphere submerged in water. The modal acoustic radiation impedance load on the source and the radiated far-field pressure are evaluated and discussed for representative values of the parameters characterizing the system. The proposed model can lead to a better understanding of dynamic response of an underwater acoustic lens. It is equally applicable in miniature transducer analysis and design with applications in medical ultrasonics
Acoustic radiation from a pulsating spherical cap set on a spherical baffle near a hard/soft flat surface
No full textRadiation of sound from a spherical piston, set in the side of a rigid sphere, undergoing harmonic radial surface vibrations in an acoustic halfspace is analyzed in an exact fashion using the classical method of separation of variables. The method of images in combination with the translational addition theorems for spherical wave functions is employed to take the presence of the flat boundary into account. The analytical results are illustrated with numerical examples in which the piston is pulsating near the rigid/compliant boundary of a water-filled halfspace. Subsequently, the basic acoustic field quantities such as the acoustic radiation impedance load and the radiation intensity distribution are evaluated for representative values of the parameters characterizing the system. Numerical results reveal the important effects of excitation frequency, source position, and cap angle on the acoustic radiation impedance load and the radiation intensity distribution. The presented work can lead to a better understanding of dynamic response of near-surface underwater transducers
Theoretical and analytical comparison of stochastic noise generation and radiation (SNGR) method and acoustic analogy using a new timescale
No full textNoise prediction of a shortcowl jet using energy transfer rate time-scale
No full textFrequency dependency of the defining parameters in aero-acoustics has been a controversial
issue and many theoretical and experimental studies have been directed towards its
understanding and modelling. In this paper a new time scale based on the turbulent energy
transfer rate is introduced and used along with the MGBK method for a short-cowl nozzle. The
required CFD results for noise prediction are obtained using a modified k ? e turbulence model.
Numerical results are provided for two flow working conditions, bypass velocity ratio 1 and 0.9.
Besides the comparisons of the new time scale against the traditional time scale, namely
dissipation time scale, corresponding noise prediction results at ninety degrees as well as other
angles are compared and discussions are provides. Generally, results show that the new time
scale is capable of performing the anticipated frequency dependency and has significantly
improved noise prediction, particularly at low and high frequencies
- …
