Structure Of Acoustic Field Research Articles

Structure of Acoustic Field in Infrared Tellurium Based X-Y Deflector

Structure of Acoustic Field in Infrared Tellurium Based X-Y Deflector

Characteristics of acoustic-controlled arc in ultrasonic wave-assisted arc

As a new welding method, ultrasound has been successfully introduced into the pool during ultrasonic wave-assisted arc welding process. However, the interaction mechanism between the ultrasound and the arc plasma is not clear, thus preflenting the new technique from engineering applications. In this paper, the characteristic of arc regulation by external ultrasonic field is investigated based on the experimental data and the corresponding theory. In order to figure out the characteristics of arc, the arc images obtained by high-speed camera are processed. Compared with the conventional welding arc, ultrasonic wave-assisted arc is more contracted and becomes brighter, the high-temperature region in an arc column greatly expands, and there are internal particle agglomerations shaking up and down at a constant frequency. The arc shape varies with ultrasound excitation current and the height of ultrasonic radiator. In the vicinity of the resonance point, the straight-degree of the arc is the strongest and the ripple frequency is also the largest. Results show that the purpose of using external ultrasound field to regulate the thermal plasma has basically achieved. Analyzing the acoustic pressure wave equation for the neutral component shows that the spatial distribution of acoustic wave can be generated in the arc and its intensity is proportional to the local amplitude of acoustic waves. Acoustic pressure field can be calculated based on the dependence of the electron temperature and density on time and space. In addition to the action of acoustic field within the arc, the arc plasma is also controlled by the acoustic field structure. A two-cylinder model incorporating boundary element method is developed, establishing a relationship between the binding capability and the geometric parameters of an ultrasonic radiator with reflerence to wavelength. This model is successful in predicting resonant modes of the acoustic field and explaining the influences of the ultrasonic radiator height on welding arc. Variation of arc shape is the result of the combined effect of axial and radial acoustic radiation forces on particles (electron, ion and neutral). The thermal efficiency will be significantly enhanced since the particle density increases in the ultrasonic wave-assisted arc. The acoustic propagation in the arc is the interacting process between acoustic and thermal plasmas. The mechanism of ultrasound acting on the arc can be reasonably explained in this study. And the results may provide a reflerence for plasma engineering applications. However, it also needs further reflearch on the impact of an arc on the acoustic field.

Influence of Acoustic Field Structure on Polarization Characteristics of Acousto-optic Interaction in Crystals

Influence of acoustic field structure on polarization characteristics of acousto-optic interaction is investigated. It is shown that inhomogeneity of acoustic field and mechanism of ultrasound excitation causes changes in values of acousto-optic figure of merit for ordinary and extraordinary light beams in comparison with theoretic values. The theoretic values were derived under assumption that acoustic wave is homogeneous. Experimental analysis was carried out in acousto-optic cell based on lithium niobate crystal where the acoustic wave propagates at the angle 13 degrees to Z axis of the crystal. We used three different methods of ultrasound generation in the crystal: by means of external piezotransducer, by interdigital transducer and by two sets of electrodes placed on top of the crystal surface. In the latter case, the first pair of the electrodes was directed along X crystal axis, while the second pair of the electrodes was directed orthogonally to X crystal axis and the direction of ultrasound. Obtained values for diffraction efficiencies for ordinary and extraordinary polarized optical beams were qualitatively different which may be caused by spatial inhomogeneity of the generated acoustic waves in the crystal. Structure of acoustic field generated by these sets of electrodes was examined by laser probing. We performed the analysis of the acoustic field intensity using acousto-optic method. A relation of diffraction efficiencies for ordinary and extraordinary light waves was measured during each iteration of the laser probing.

The interference structure of very low frequency vector acoustic field and its positioning application

Vector hydrophone has natural dipole beam-pattern, and can also simultaneously and colocatedly measure the scalar and vector information of ocean acoustic field, which makes it convenient to determine the direction of arrival (DOA) and represent the stable interference structure of vector acoustic field. Utilizing the DOA and interference structure information together acquired by single vector hydrophone, it is able to position the target of interest. Firstly, bearing-time course for passing-by target is obtained by using the line or continuous spectrum imbedded in the received signal of vector hydrophone. Then, CPA ratio between the CPA range and speed of target as well as CPA instant are estimated on the basis on LMS criteria. Finally, the speed of target can be determined through the theoretically predicted interference range and the real interference time interval to fulfill target positioning including DOA and range. The idea was effectively validated during the experiment that took place in October of 2010 in South China Sea nearly 100m depth. Moreover, the idea presented here can be easily extended for further application such as combining the waveguide invariant. [Work supported by the National 863 Project (No. 2011AA090502) and National Defense Foundation Project (B2420132004).]

Acousto-optic diffraction with allowance for the influence of the acousto-optic cell boundaries

The influence of the dimension of an acousto-optic cell in the plane orthogonal to the plane of acousto-optic diffraction on the acoustic field structure and, consequently, on acousto-optic diffraction is studied theoretically and experimentally. A method is suggested for approximate calculation of the acoustic field with regard to the acoustic column reflection from the crystal faces. Based on this method, the influence of the acousto-optic cell dimensions is analyzed in a wide range of ratios between the height of the piezoelectric transducer and the height of the crystal. A region of acoustic power concentration is predicted to arise both for parallel faces and for slight wedging. A number of statements are demonstrated with paratellurite used as an interaction medium. The results may be useful in designing acousto-optic devices.

Acoustic streaming field structure. Part II. Examples that include boundary-driven flow

In this paper three simple acoustic streaming problems are presented and solved. The purpose of the paper is to demonstrate the use of a previously published streaming model by Bradley [J. Acoust. Soc. Am. 100(3), 1399-1408 (1996)] and illustrate, with concrete examples, some of the features of streaming flows that were predicted by the general model. In particular, the problems are intended to demonstrate cases in which the streaming field boundary condition at the face of the radiator has a nontrivial lateral dc velocity component. Such a boundary condition drives a steady solenoidal flow just like a laterally translating boundary drives Couette flow.

INVESTIGATION OF INTERACTION BETWEEN ACOUSTIC FIELD AND NONHOMOGENEOUS BUILDING STRUCTURES

Two models of interaction between acoustic field and building structure are investigated. The first - two dimensional model, where interaction between partition wall and acoustic medium has been modelled (given different conditions of the partition wall). The second - three dimensional model, which is used to analyze two storey floor-columns structure, applying acoustic and mechanical excitation, and different nonhomogeneity degrees of the structure. Possible mechanical system defect (reduction of rigidity due to floor flaw) has been modelled during the latter.The results showed that the same size and frequency of the acoustic load at harmonic excitation may lead to higher deformation of the structure having a defect.http://dx.doi.org/10.5755/j01.mech.17.3.504

Investigation of the underwater noise produced by ships by means of intensity method

Investigation of the underwater noise produced by ships by means of intensity method This paper presents results of research on spectral structure of underwater noise acoustic field radiated into water by selected ships moving in shallow waters. Underwater acoustic field of ships in motion is associated with acoustic activity of ship mechanisms and equipment under work. Vibration energy radiated by the mechanisms and devices is transmitted by ship structural elements to surrounding water where it is propagated in the form of acoustic waves of a wide frequency band. In this paper are presented results of the research on propagation of energy of acoustic waves in the near fiel, obtained from acoustic pressure measurements by means of two sensors located close to each other.

Analysis of wavefronts for the piston source acoustic fields

The development of modern acoustic methods is based to a great extent on a thorough investigation of the structure of acoustic fields and on the refinement of methods of their analysis and calculation. The study of the local features of acoustic field and its space-time structure is an urgent problem of physical acoustics. This work is dedicated to analytical investigation of space-time structure of wavefronts corresponding to the acoustic fields which are radiated by the piston source into elastic media. It is assumed that the spatial distributions. Various spatial distributions of acoustic field over the surface of the transducer are taken into consideration.

Theoretical model of surface reverberation with allowance for interference of acoustic modes

We report on preliminary results of developing a theoretical model of surface reverberation with allowance for the interference structure of acoustic fields in oceanic waveguides. Basic expressions relating the intensity and two-dimensional frequency-time uncertainty function of the reverberation signals to the wind-wave parameters and the characteristics of sounding pulses are presented. Main physical effects determining the influence of intermode correlations on the reverberation are discussed. In this context, both narrow-and wideband emitted signals are analyzed.

The horizontal structure of the sound field in the ocean

Results of calculating the horizontal structure of acoustic field in the ocean with a canonical sound velocity profile are presented. The calculation is performed in the framework of geometrical acoustics by combining the fields of water-path rays at every point with allowance for their phases and amplitudes. The field distribution at a fixed depth is found to be not very informative: within a cycle length, it contains 1–3 maxima, which are mainly caused by the caustics present at the given depth. The width of caustics is ∼0.1–1 km. Between the caustics, as well as in their absence, the field amplitude is much smaller than that in the regions of caustics and varies depending on the phases and amplitudes of rays arriving at a given point. The comparison of the calculated horizontal field structure with that obtained from full-scale measurements for the regions between the caustics is difficult because of the possible fluctuations of sound propagation conditions.

The passive mode tomography of the ocean using data from short vertical arrays bent by the ocean currents

The possible realization of passive ocean tomography based on the widely discussed relation between the Green's function and ambient noise cross‐correlation is discussed. The problem is considered in the mode representation of acoustic field in adiabatic approximation. The estimated time of signal accumulation required to determine the Green's function with sufficient accuracy reveals the possibility of implementing the schemes of the mode tomography of the ocean based on the measuring the ambient noise field of the ocean. It is shown that the use of the vertical arrays with vector receivers allows a decrease in the accumulation time to one or several hours, depending on the conditions of experiment. The mode structure of acoustic field is determined from the cross‐correlation matrix of the noise field received by the hydrophones of short vertical arrays bent by the ocean currents and covering only the part of the sound channel. The proposed algorithm differs from the commonly used mode‐filtering procedur...

Influence of the benthic front on the mode structure of acoustic field in the ocean

The characteristic features of the mode structure of low-frequency sound field in the ocean are investigated for the ocean regions containing the benthic front, which represents the boundary between the Antarctic bottom water and the deep water. The hydrographic characteristics of the benthic front are described with the use of the data of the international WOCE experiment. On the basis of mathematical simulation, the changes introduced by the benthic front into the phase and group velocities and the vertical structure of modes are estimated for a frequency range of 5–20 Hz. Possible ways of reconstructing the hydrographic characteristics of the benthic front by the mode tomography methods are considered.

A hardware and software system for measuring the angular structure of the acoustic fields in acoustic tomography

A hardware and software system for measuring the angular structure of acoustic fields using the vector-phase methods for processing the complex M-sequence probing signals is described. The results of testing the system are given. A unique relationship between the change in the temperature condition on the stationary acoustic route and the times and angles of arrival of the acoustic energy is determined under controlled hydrological conditions. It is shown that, using vector receivers in tomographic schemes, it is possible to obtain an independent additional parameter of the pulse response of the diagnosed waveguide and increase thereby the efficiency of reconstruction of hydrophysical fields from acoustic probing data.

The use of low-frequency noise in passive tomography of the ocean

A possible design of the mode tomography of the ocean with the use of a scheme requiring no expensive low-frequency radiators is considered. The design is based on the widely discussed method of estimating the Green’s function from the cross-coherence function of noise field received in a great number of observation points. The relationship between the Green’s function and the noise coherence function is derived from the Helmholtz-Kirchhoff integral. The use of the vertical multielement arrays composed of vector receivers is suggested to decrease the duration of noise signal accumulation required for a reliable determination of the Green’s function. The solution of the tomographic problem is based on the determination of the mode structure of acoustic field from the eigenvectors and eigenvalues of the cross-coherence matrix of the received noise field.

Detection of microflaws in metals via investigation of acoustic fields

Distributions of acoustic fields on the surfaces of metal specimens with different degrees of microdamage are measured with a Doppler laser interferometer. Various methods are used for digital data processing for extracting useful information against a noise background. Specific features of the acoustic-field structure are analyzed as functions of the presence and parameters of microflaws in the specimens under study.

Tidal time-scale variations of sound propagation in the Hudson River Estuary

The nowcast and forecast of sound propagation in an urban estuary is important for the development of acoustic surveillance systems. Sound propagation in an estuary is highly affected by the temporal and spatial variability of salinity and temperature due to tides, freshwater inflows, winds, etc. Estuarine processes are analyzed here focusing on the formation and breakdown of the salinity and temperature stratification and their influence on sound attenuation. Transmission loss (TL) variability was determined first using real‐time data collected in the Hudson River near Hoboken, New Jersey. For TL forecasting, 24‐h forecasts of salinity and temperature distributions from a high‐resolution New York harbor observing and prediction system (NYHOPS) developed at Stevens were used (www.stevens.edu/maritimeforecast/). These forecasts provided the basis for calculating sound speed profiles. A parabolic equation based acoustic model was implemented to simulate the acoustic field structure. Real‐time TL measurements were made by transmitting sweep frequency signals (1–100 kHz) to a distance of 175 m. The observed tidal time‐scale TL variation of 10 dB was in good agreement with the model calculations. The results indicate that TL can be predicted within the context of the NYHOPS system. [Work supported by ONR Project No. N00014‐05‐1‐0632: Navy Force Protection Technology Assessment Project.]

Parabolic equation for nonlinear acoustic wave propagation in inhomogeneous moving media

A new parabolic equation is derived to describe the propagation of nonlinear sound waves in inhomogeneous moving media. The equation accounts for diffraction, nonlinearity, absorption, scalar inhomogeneities (density and sound speed), and vectorial inhomogeneities (flow). A numerical algorithm employed earlier to solve the KZK equation is adapted to this more general case. A two-dimensional version of the algorithm is used to investigate the propagation of nonlinear periodic waves in media with random inhomogeneities. For the case of scalar inhomogeneities, including the case of a flow parallel to the wave propagation direction, a complex acoustic field structure with multiple caustics is obtained. Inclusion of the transverse component of vectorial random inhomogeneities has little effect on the acoustic field. However, when a uniform transverse flow is present, the field structure is shifted without changing its morphology. The impact of nonlinearity is twofold: it produces strong shock waves in focal regions, while, outside the caustics, it produces higher harmonics without any shocks. When the intensity is averaged across the beam propagating through a random medium, it evolves similarly to the intensity of a plane nonlinear wave, indicating that the transverse redistribution of acoustic energy gives no considerable contribution to nonlinear absorption.

Parabolic approximation versus geometrical acoustics for describing nonlinear acoustic waves in inhomogeneous media

Propagation of intense periodic acoustic waves in inhomogeneous media is studied in the nonlinear geometrical acoustics (NGA) approximation and using nonlinear parabolic equation (NPE). Various types of 2-D inhomogeneities are considered, such as a phase screen, single Gaussian inhomogeneities, and random inhomogeneous media. Distributions of acoustic rays are obtained by numerical solution of the eikonal equation. Pressure field patterns are calculated numerically with account for nonlinear effects and diffraction using a frequency-domain algorithm for the NPE. The location of caustics and shadow zones in the ray patterns are compared with the results of the parabolic model for the areas of increased and decreased sound pressure. Both linear and nonlinear propagation is investigated in order to reveal the validity of NGA in predicting the acoustic field structure and to better understand how the combined effects of inhomogeneities, diffraction, and nonlinearity determine the overall peak and average parameters of the acoustic field. It is shown that NGA does not accurately represent all the locations of enhanced or reduced acoustic pressure even for single scattering inhomogeneities, and the discrepancies become larger for smaller size inhomogeneities and at longer distances in random inhomogeneous medium. [Work supported by CNRS, RFBR, and NIH Fogarty.]

Acoustic near-field characteristics of a conical, premixed flame

The occurrence of self-excited pressure oscillations routinely plagues the development of combustion systems. These oscillations are often driven by interactions between the flame and acoustic perturbations. This study was performed to characterize the structure of the acoustic field in the near field of the flame and the manner in which it is influenced by oscillation frequency, combustor geometry, flame length and temperature ratio. The results of these calculations indicate that the acoustic velocity has primarily one- and two-dimensional features near the flame tip and base, respectively. The magnitude of the radial velocity components increases with temperature ratio across the flame, while their axial extent increases with frequency. However, the acoustic pressure has primarily one-dimensional characteristics. They also show that the acoustic field structure exhibits only moderate dependencies upon area expansion and flame temperature ratio for values typical of practical systems. Finally, they show that the local characteristics of the acoustic field, as well as the overall plane-wave reflection coefficient, exhibit a decreasing dependence upon the flame length as the area expansion ratio increases.