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A method based on the wave superposition method (WSM) and the adjusted Green function for predicting the structural acoustic radiation in shallow water is proposed, which is adopted to construct the equivalent acoustic radiation of elastic structure. The local acoustic vibration model is established by the finite element method (FEM) in the near field, and the WSM is used to calculate the acoustic field in the far distance. In the wave superposition calculation, the virtual source intensity is obtained by using the Green function associated with the ray method, and the sound field is calculated by the Green function based on normal mode. And acoustic radiation of a pulsating sphere in the shallow water is served as an example to verify the accuracy of the method. Considering the fact that the ill-posed problems in the calculation are caused by the velocity noise, the performance of the Tikhonov regularization method on suppressing noise is investigated. It is concluded that the Tikhonov regularization method can greatly improve the stability and accuracy of structural radiation sound field prediction in shallow water, and lay the foundation for practical applications.

A hybrid computational method is proposed for three-dimensional structural-acoustic radiation in shallow water, which combines the wave superposition method (WSM) with the adjustable Green function and the multi-physical-coupled finite element method (FEM). A low-frequency acoustic model in the near field is established by the FEM to discretize the continuous and arbitrary-shaped radiator, and the normal vibration velocity on the wetted surface is used as the input in the WSM to calculate the strengths of simple sources enclosed within the radiator. Finally, the sound propagation in the far field of the elastic structure is efficiently computed by superimposing the sound field of each simple source. The numerical examples of the pulsating sphere and elastic spherical shell in the shallow water are developed to verify the high accuracy and efficiency of the presented method, respectively. Moreover, the regularization algorithms are employed as the complementary approaches to improve the stability of this method in engineering practices, it is found that the algorithms can significantly hold the calculation accuracy in case the velocity data contain noise, and the least-squares QR is an efficient and accurate method with a few iteration numbers and better stability effect as compared with other algorithms.

Superhydrophobic surface is a promising technology, but the effect of superhydrophobic surface on flow noise is still unclear. Therefore, we used alternating free-slip and no-slip boundary conditions to study the flow noise of superhydrophobic channel flows with streamwise strips. The numerical calculations of the flow and the sound field have been carried out by the methods of large eddy simulation (LES) and Lighthill analogy, respectively. Under a constant pressure gradient (CPG) condition, the average Reynolds number and the friction Reynolds number are approximately set to 4200 and 180, respectively. The influence on noise of different gas fractions (GF) and strip number in a spanwise period on channel flow have been studied. Our results show that the superhydrophobic surface has noise reduction effect in some cases. Under CPG conditions, the increase in GF increases the bulk velocity and weakens the noise reduction effect. Otherwise, the increase in strip number enhances the lateral energy exchange of the superhydrophobic surface, and results in more transverse vortices and attenuates the noise reduction effect. In our results, the best noise reduction effect is obtained as 10.7 dB under the scenario of the strip number is 4 and GF is 0.5. The best drag reduction effect is 32%, and the result is obtained under the scenario of GF is 0.8 and strip number is 1. In summary, the choice of GF and the number of strips is comprehensively considered to guarantee the performance of drag reduction and noise reduction in this work.

The Green’s function (GF) directly eases the efficient computation for acoustic radiation problems in shallow water with the use of the Helmholtz integral equation. The difficulty in solving the GF in shallow water lies in the need to consider the boundary effects. In this paper, a rigorous theoretical model of interactions between the spherical wave and the liquid boundary is established by Fourier transform. The accurate and adaptive GF for the acoustic problems in the Pekeris waveguide with lossy seabed is derived, which is based on the image source method (ISM) and wave acoustics. First, the spherical wave is decomposed into plane waves in different incident angles. Second, each plane wave is multiplied by the corresponding reflection coefficient to obtain the reflected sound field, and the field is superposed to obtain the reflected sound field of the spherical wave. Then, the sound field of all image sources and the physical source are summed to obtain the GF in the Pekeris waveguide. The results computed by this method are compared with the standard wavenumber integration method, which verifies the accuracy of the GF for the near- and far-field acoustic problems. The influence of seabed attenuation on modal interference patterns is analyzed.

The traditional free-space and half-space analysis method ignore the reflection of the upper and lower boundaries of shallow sea and are not suitable for analyzing shallow sea problems especially at high frequency. Hence, a method combining ray theory and wave superposition theory is proposed in this paper to predict the high-frequency radiated acoustic field in shallow water. The proposed method takes into account the effect of channel boundaries on the acoustic field and has good adaptability to complex channel environments and accuracy of the calculated acoustic field.

A fast prediction method is proposed for calculating the sound scattering of targets in the deep-sea acoustic channel by equating the sound scattering field of a complex elastic target to the acoustic field excited by a directional point source. In deep-sea conditions, the effects of the sea surface on the impedance characteristics of the elastic target surface can be ignored. Through the finite element simulation of the acoustic scattering of the target in the free field, the sound scattering field is equated to the radiation field of a directional point source. Subsequently, the point source is placed in the channel, and the acoustic ray method is used to calculate the distribution of the scattering field. On the basis of theoretical modelling, the method of obtaining the directional point source and the influence of the sea surface on the impedance of the scattering field are analysed. Subsequently, the proposed method is compared with the finite element method in terms of computational efficiency. The result shows that the method considers the multiple complex coupling effects between the elastic structure and marine environment. The influence of the boundary is approximately negligible when the distance from the ocean boundary to the elastic structure is equal to the wavelength. The method only performs finite element coupling calculation in the free field, the amount of mesh size is greatly reduced and the calculation efficiency is significantly improved when compared with the finite element calculation in the entire channel, the. The calculation time in the example can be reduced by more than one order of magnitude. This method organically combines the near-field calculation with acoustic ray theory and it can realise the rapid calculation of the large-scale acoustic scattering field in complex marine environments.

A combined Finite Element Method with Normal Mode (FEM-NM) is proposed for calculation of the acoustic field radiated by a three-dimensional structural source in shallow water. The FEM is used to calculate the near range acoustic field, then the modes expansion at the vertical and azimuthal direction is performed at a certain coupling range. Hence, the true three-dimensional acoustic field at any range is obtained rapidly by the NM theory. The numerical examples show the efficiency and accuracy of this method. The coupling range and the truncation of the vertical modes hardly affect the far field results.

When underwater vehicles are sailing, high hydrodynamic noise will be generated through the opening cavities due to the interaction of the surface and the fluids. In the paper, we had tested different forms of back wall chamfer by numerical calculation based on the method of large eddy simulation, to control the impact of the eddy and break the flow at the trailing edge of the cavity, which is with the dimension of 100mm wide, 120mm high and 100mm long. The angle and shape of the trailing edge chamfer are changed to control the flow of the cavity. We had also investigated the effect of hydrodynamic noise suppression through the simulation, which is based on the method of Lighthill’s acoustic analogy. The results show that the change of back wall chamfer can stabilize the movement of the eddies inside the cavity and reduce the fluctuation pressure at the trailing edge of the cavity. The suppression of flow-induced noise can be up to 5 dB, if the back-wall chamfer is with the airfoil surface and the angle of back wall chamfer is properly designed.

The multibeam sonar is an important device for underwater survey, of which the measurement precision is a key parameter. We had designed an equipment to calibrate the depth and resolution precision for multibeam sonar in the anechoic pool in the laboratory. The main frame of the equipment is built up by circular and square steel pipes. The absorption materials are laid outside of the pipes, so that the multibeam sonar cannot receive the strong enough reflected acoustic waves from the boundary. We had made a two-dimensional scanning mechanism, which ascends and descends freely for evaluating the depth precision of multibeam sonar. The minimum precision of the depth is 0.1 mm. We had set up a series of iron blocks with the dimension of 0.1,0.2,0.4,0.6,0.8, and 1.0 square meters, respectively, as the standard objects for evaluating the resolution capability of multibeam sonar. In addition, we had placed an underwater camera system to record the iron blocks in time. The result of iron blocks measured by the calibrated multibeam sonar is compared to that recorded by the camera. Because the wavelength of acoustics is larger than that of optics, therefore, good precision of the resolution can be gotten.

A sound radiation calculation method by using dominant modes is proposed to predict the sound radiation from a cylindrical shell. This method can provide an effective way to quickly predict the sound radiation of the structure by using as few displacement monitoring points as possible on the structure surface. In this paper, modal analyses of a submerged cylindrical shell are carried out by taking the vibration mode of a cylindrical shell in a vacuum, as a set of orthogonal bases. The modal sound radiation efficiency and modal contributions to sound radiation power are presented, and comparison results show that a few modes dominantly contribute to the sound radiation power at low frequencies. These modes, called dominantly radiated structural modes in this paper, are applied to predict the sound radiation power of submerged cylindrical shells by obtaining the modal participant coefficients and sound radiation efficiency of these dominant modes. Aside from the orthogonal decomposition method, a method of solving displacement modal superposition equations is proposed to extract the modal participant coefficients, because few modes contribute to the vibration displacement near the resonant frequencies. Some simulations of cylindrical shells with different boundaries are conducted, and the number of measuring points required are examined. Results show that this method, based on dominant modes, can well predict the low-frequency sound radiation power of submerged cylindrical shells. In addition, compared with the boundary element method, this method can better reduce the number of required measuring points significantly. The data of these important modes can be saved, which can help to predict the low-frequency sound radiation of the same structure faster in the future.

In order to predict acoustic radiation from a structure in waveguide, a method based on wave superposition is proposed, in which the free-space Green’s function is used to match the strength of equivalent sources. In addition, in order to neglect the effect of sound reflection from boundaries, necessary treatment is conducted, which makes the method more efficient. Moreover, this method is combined with the sound propagation algorithms to predict the sound radiated from a cylindrical shell in waveguide. Numerical simulations show the effect of how reflections can be neglected if the distance between the structure and the boundary exceeds the maximum linear dimension of the structure. It also shows that the reflection from the bottom of the waveguide can be approximated by plane wave conditionally. The proposed method is more robust and efficient in computation, which can be used to predict the acoustic radiation in waveguide.

High hydrodynamic noise is a threat to the survival of underwater vehicles. We investigated a noise suppression mechanism by putting leading-edge serrations on the sail hull of a scaled SUBOFF model, through numerical calculation and an experimental test. We found that the cone shape of leading-edge serrations can decrease the intensity of the adverse pressure gradient and produce counter-rotation vortices, which destroy the formation of the horseshoe vortex and delay the tail vortex. To achieve the optimum hydrodynamic noise reduction, we summarized the parameters of leading-edge serrations. Then, two steel models were built, according to the simulation. We measured the hydrodynamic noise based on the reverberation method in a gravity water tunnel. The numerically calculated results were validated by the experimental test. The results show that leading-edge serrations with amplitudes of 0.025c and wavelengths of 0.05h can obtain hydrodynamic noise reduction of at least 6 dB, from 10 Hz to 2 kHz, where c is the chord length and h is the height of the sail hull. The results in our study suggest a new way to design underwater vehicles with low hydrodynamic noise at a high Reynolds number.

Acoustic radiation problem of elastic structures in shallow water has not been solved effectively. Finite Element Method (FEM) is not suitable for higher frequency and large range problems. In principle, the Combined Wave Superposition Method can deal with such problems. However, it is hard to optimize a large number of virtual sources. For low and middle frequency bands, we propose a new method combining FEM and normal modes. FEM is used to calculate the near range acoustic field of the structural source, then the eigen function expansion is performed on the field, and the coefficients of modes can be obtained by using the orthogonality of the eigen function. Therefore, we can calculate the acoustic field at any range. This method avoids the process of solving inverse matrix of a large complex matrix, which is an essential step in wave superposition method, and is very efficient to calculate the far field. To show the efficiency and accuracy of the method, the simulation results of various structural sources and waveguide models are compared with that of using FEM directly.Acoustic radiation problem of elastic structures in shallow water has not been solved effectively. Finite Element Method (FEM) is not suitable for higher frequency and large range problems. In principle, the Combined Wave Superposition Method can deal with such problems. However, it is hard to optimize a large number of virtual sources. For low and middle frequency bands, we propose a new method combining FEM and normal modes. FEM is used to calculate the near range acoustic field of the structural source, then the eigen function expansion is performed on the field, and the coefficients of modes can be obtained by using the orthogonality of the eigen function. Therefore, we can calculate the acoustic field at any range. This method avoids the process of solving inverse matrix of a large complex matrix, which is an essential step in wave superposition method, and is very efficient to calculate the far field. To show the efficiency and accuracy of the method, the simulation results of various structural sou...

A hybrid method to calculate the sound scattering field of the target in waveguide is presented, combining the theory of the normal mode with the finite element method (FEM). Although FEM used only can provide accurate results, it is deeply restricted by the property of the computers and does not have a satisfying computing speed, for its calculation is closely related to the size and complexity of the target. The hybrid method presented in this paper can calculate the sound scattering field of more arbitrary and more complicated target over farther ranges in oceanic waveguide at low frequencies. First, separate the sound field into two parts, domain A, containing the target in the nearer range, and domain B meaning the rest domain of the field. In domain A, the sound field was calculated via FEM. Then, apply the normal mode theory to forecast the sound scattering field over farther ranges, using the pressure data on the intersection area between domains A and B obtained by FEM. The method has been used to calculate the sound scattering field of a rigid sphere in waveguide and based on the results, it is demonstrated that this hybrid method is accurate and efficient.

The hydrodynamic noise from the horseshoe vortex can greatly destroy the acoustic stealth of underwater vehicles at low frequency. We investigated the flow-induced noise suppression mechanism by mechanical vortex generators (VGs) on a SUBOFF model. Based on the numerical simulation, we calculated the flow field and the sound field of the three shapes of mechanical VGs: triangular, semi-circular, and trapezoidal. The triangular VGs with an angle of 30&deg, to the flow direction achieved a better noise reduction. The optimum noise suppression is 8.93 dB, when the distance from the triangular VGs to the sail hull&rsquo, s leading edge is 0.1c, where c is the chord length. The noise reduction mechanism is such that the mechanical VGs can destroy the formation of the horseshoe vortex at the origin and produce counter-rotation vortices to weaken its intensity. We created two steel models according to the simulation, and the experimental measurement was carried out in a gravity water tunnel. The measured results showed that the formation of the horseshoe vortex could be effectively inhibited by the triangular VGs. The results in our study can provide a new method for hydrodynamic noise suppression by flow control.

When ships run at high speed-cavitation is formed in the bulge area. Because cavitation can make big noises-it will be a threat to the sonar. Cavitation can also destroy the blades of propellers. A method is proposed that underwater cavitation is suppressed through vortex generation. The effect of flow noise and radiated noise from vortex generator on sail hull model is investigated. Through the comparison of flow field-we find that vortex generator has changed the transition area of sail hull model. The results indicate that flow noise and radiated noise from underwater vortex generator is only a little-compared to that of sail hull model. The total level of sound radiation power from the model of sail hull and with vortex generator is less than that of sail hull model. Therefore-it is beneficial to suppress the cavitation by vortex generator-when ships run at high speed.

There usually exist rubber materials in the outside of underwater navigators. In one aspect, sound radiation from these navigators can be reduced by the materials, and it is beneficial to control the pollution of noises in ocean environments. In the other aspect, the resistance induced by the fluid can be decreased, and it is helpful to enhance the speed of these navigators. In the paper, a numerical model of sail hull in the software, ANSYS, is built up. Then, fluid field is calculated by the software, Fluent. After the data is dealt with by fast fourier transform, namely, FFT, sound radiation power of the model can be calculated by the software, ACTRAN. Then, viscous elastic materials have been put on the surface of the model. Sound radiation power of the model with viscous elastic materials can be calculated by the listed process. The results indicate that total level of sound radiation power from viscous elastic materials is proportional to 3–6 times of fluid velocity. After that, the model of sail hull has been designed, of which the length and height are 0.4m, 0.17m, respectively. The thickness of the model and viscous elastic materials is 0.002m, 0.005m, respectively. The experiment has been done in a water tunnel, which is induced by the gravity of water, so that the level of background noise is especially low. The velocities of fluid are 2.98m/s, 6.34m/s, 8.68m/s. Sound radiation power of the model and that with viscous elastic materials has been measured by reverberation method. The total level of sound radiation power decreased by viscous elastic materials is 5 dB. The results are coincided well with that calculated by the software. The results indicate that the method of calculating and measuring sound radiation power of viscous elastic materials is perfect.

Low-frequency acoustic scatterings from a finite cylindrical shell are numerically analyzed by FEM. The simulation results show that the acoustic-scattering field in waveguide has lots of frequency-related sidelobes, while no sidelobes exist in free space at low frequencies. The simulation also indicates that the module value in waveguide can be almost 20 dB larger than that in free space at low frequency, which is caused by the ocean boundaries. We also demonstrate that when the incident wave direction is normal to the target at low frequency, the target strength will be maximum and the distribution of the acoustic-scattering field is axisymmetric about the incident waving direction. Meanwhile, the acoustic-scattering field is also related to the impedance of the seabed, and the change of the impedance makes just a little contribution to the scattering field. Finally, the influence of different target locations is analyzed, including the targets near the sea surface, seabed and the middle region of the ocean waveguide, respectively. From simulation results, it is evident that the distribution of the acoustic-scattering field at low frequency has a little difference, which is smaller than 0.5 dB with various target locations, and the change is frequency and boundary-related.

Hydrodynamic noise is one of the three major noise sources of underwater vehicles. The sonar dome is a device placed in front of the ship and the submarine to absorb the flow fluctuation and to reduce the hydrodynamic noise, so that the sonar inside the dome is not affected by the external fluid. However, with the increase of the velocity of ships and submarines, cavitation can usually form in the bulge of the sonar domes, which will bring high level of noise to the sonar. The internal self-noise of the sonar dome mainly comes from two areas: the leading-edge stagnation point and the transition zone of boundary layer. In the paper, we designed the leading-edge serrations and dimples in the leading-edge and transition areas of the sonar dome respectively to reduce the movement resistance and prevent the separation of the boundary layer. The research on leading-edge serrations and dimple technology is carried out by using theoretical analysis, numerical calculations. The results show that the leading-edge serrations and dimples can add energy from the outer flow into the boundary layer; the cavitation phenomenon can be delayed. The hydrodynamic noise has been suppressed by about 20dB.

Viscoelastic structures with periodic cavity arrangement occupy an important position in acoustic stealth. Most studies focus on the absorption at low frequency, under large pressure and in a wide frequency band, however, the sound absorption properties of the anechoic coating in debonding states are seldom studied. The acoustic response of the composite structure is calculated by the FEM, and the incident wave and the reflected wave are separated based on the plane wave propagation principle. The sound absorption coefficient of the model can be solved with the pressure data of transmission wave, incident wave and reflected wave. Based on the finite element method, the absorption characteristics of anechoic coating in debonding states have been studied. Firstly, the FEM model is verified by comparing the solution of uniform non-cavity coatings with the corresponding analytical solution. The results indicate that the problem of acoustic pressure field can be simulated well by the symmetry condition and the absorbing boundary. Then, the Alberich anechoic coating models in different debonding ratio states and in different cavity water filling states are established by FEM, and then the corresponding absorption characteristics are calculated and analyzed. The results show that the effects of debonding on absorption characteristics are small when the cavities of coatings are filled with water, however the effects are much more when the cavities of coatings are filled with air. For the case of cavities filled with air, the main frequency band of sound absorption moves to low frequency with the debonding ratio increasing. When the frequency is very high, there are few impacts on the sound absorption coefficient no matter whether the packing medium is water or air, no matter whether the coating is debonding or not.

Flow-induced noise from the sail hull, which is induced by the horseshoe vortex, the boundary layer separation and the tail vortex shedding, is a significant problem for the underwater vehicles, while has not been adequately studied. We have performed simulations and experiments to reveal the noise generation mechanism from these flows using the scaled sail hull with part of a submarine body. The large eddy simulation and the wavenumber&ndash, frequency spectrum are adopted for simulations. The frequency ranges from 10 Hz to 2000 Hz. The simulation results show that the flow-induced noise with the frequency less than 500 Hz is mainly generated by the horseshoe vortex, the flow-induced noise because of the tail vortex shedding is mainly within the frequency of shedding vortex, which is 595 Hz in the study, the flow-induced noise caused by the boundary layer separation lies in the whole frequency range. Moreover, we have conducted the experiments in a gravity water tunnel, and the experimental results are in good accordance with the simulation results. The results can lay the foundation for the design of flow control devices to suppress and reduce the flow-induced noise from the sail hull.

There is no reliable research method for the acoustic radiation prediction of elastic structures in shallow water at present. Based on the finite element method coupled with the parabolic equation, the theoretical model for structure acoustic radiation in shallow water in low frequency is established in this paper. The vibro-acoustic characteristics of elastic cylinder influenced by with upper and lower fluid boundaries is calculated and analyzed. The study shows that when the cylindrical shell approaches the sea surface or bottom, the coupled frequency is higher or lower respectively than that of the shell immersed in the free field. When the diving depth reaches a certain distance range, the coupled frequency tends to be the same as that in free field. The acoustic field radiated from an elastic shell in Pekeris waveguide is similar to that from a point source in low frequency, but significant differences exist between them in high frequency. The structural source can’t be equivalent to a point source. The research results also show that the method proposed in this paper has good adaptability to the type of structure geometry and shallow water waveguide, and the calculation efficient and accurate are also very high.There is no reliable research method for the acoustic radiation prediction of elastic structures in shallow water at present. Based on the finite element method coupled with the parabolic equation, the theoretical model for structure acoustic radiation in shallow water in low frequency is established in this paper. The vibro-acoustic characteristics of elastic cylinder influenced by with upper and lower fluid boundaries is calculated and analyzed. The study shows that when the cylindrical shell approaches the sea surface or bottom, the coupled frequency is higher or lower respectively than that of the shell immersed in the free field. When the diving depth reaches a certain distance range, the coupled frequency tends to be the same as that in free field. The acoustic field radiated from an elastic shell in Pekeris waveguide is similar to that from a point source in low frequency, but significant differences exist between them in high frequency. The structural source can’t be equivalent to a point source. ...

An underwater open cavity, such as a submarine's open weapons firing tube, is a typical kind of structure that can induce fluid noise. To study the fluid noise generated by the submarine's open tube, greater attention is paid when the cavity's opening is vertical to the flow direction. This paper focuses on flow noise induced by an underwater cavity having openings under small attack angles, and simulations were conducted for different small attack angles. The Large Eddy Simulation method combined with Lighthill's acoustic analogy theory is used by software FLUENT and ACTRAN to simulate the underwater flow noise. The submarine cavity model is simplified as a cylinder with a hemisphere on one end, and there are openings on the top of the hemisphere. The flow field and the underwater fluid noise from this model have been numerically simulated for three cases. Case 1: the cavity has no opening; case 2: the cavity has four openings; case 3: the cavity has six openings. The inflow direction is assumed to be in parallel with the cylinder and against the hemisphere, and the flow speed is assumed to be 5 m/s and 10 m/s in all cases. First, the open cavity transition zone is calculated and analyzed by use of FLUENT. Then, the flow noise frequency characteristics and its far field pattern are calculated and analyzed for different flow velocities by use of ACTRAN. The numerical results show that the frequency spectrum curve of those cases with different flow speeds are very similar. However, the flow noise level will get higher with the increase of the flow speed and number of openings. The far field will tend to have obvious space directivity for the cases with more openings or at higher flow velocity, and the main lobes of the far field pattern coincided with the openings' direction. Finally, the radiated sound power level from the cavities was compared and analyzed for three different attack angles. Simulations show that the flow noise will get higher with the increase of the attack angle in the case of small attack angles. So, the opening of the model should be closed completely, or the flow velocity and attack angle should be lower, in order to decrease the fluid noise level.

A prediction method of submerged structure sound radiation is presented, in which the measured structure vibration data is processed to obtain sound radiation by a transfer method. For an underwater vehicle, its own vibration and sound radiation usually needs to be known accurately. Therefore, accurate prediction of sound radiation from submerged structures is of great significance. As we know, the generation of radiated sound by a submerged structure is a very complicated process. Consequently, it is very difficult to predict radiated noise by monitoring submerged structure vibration. Traditionally, BEM methodology is suitable for solving the problem of the acoustic field calculation by known structure vibration. However, the required number of measuring vibration points is very large; and, vibration phase information needs to be measured accurately - but we know that these requirements are difficult to be satisfied in practice. Therefore, this paper proposes a method of sound radiation prediction of submerged structures by using a transfer method. As we know, under the action of an excitation force sound radiation of a submerged structure is generally a linear system, so the radiation field is proportional to the structure vibration. For an experimental structure model, the structural vibration and the sound radiation field can be calculated by theoretical modeling, and their transfer relationship can be derived easily. Then the sound radiation field can be predicted according to the structure vibration data obtained by experiment. In this paper, the structure vibration is described by the mean square velocity, and sound radiated power is concerned with sound radiated field. For the underwater cylindrical shell model, the simulation calculation is carried out with LMS Virtual Lab software; and, the transfer relationship between the structure mean square velocity and sound radiated power is investigated for different force excitation cases. Then, a measuring experiment is carried out for a cylindrical shell model. By measuring vibration acceleration of the cylindrical shell, the sound radiated power of the model is predicted by a transfer method. The predicted result has a good agreement with the directly measured sound radiated power. The experiment result shows that this paper's prediction method is feasible. In this paper, the prediction method of sound radiation is suitable for engineering application, which has the advantages of needing fewer measuring points and also no need to measure the phase information.

For underwater acoustic measurement experiment, if the scale of underwater elastic structure is large, it is difficult to achieve the requirement of the classic far-field testing criterion. This paper has researched the measuring distance criterion for underwater structure radiated noise from the perspective of energy measurement. A concept of measuring distance criteria is derived responding to the far-field criterion of traditional acoustic theory. A numerical simulation is presented for some finite elastic cylindrical shells excited by a point force in the water. The radiation sound field at points on the acoustic axis of the shell is simulated by the finite element method (FEM) and boundary element method (BEM). The pressure and velocity are numerical calculated at the annular field points in the three coordinate planes. The directivity and the sound pressure amplitude attenuation characteristic on the field points are analyzed to research the most suitable distance for measuring the radiated sound power. Then, a calculation method for the measuring distance has been preliminarily summarized. The simulation results show that the radiated sound power from the cylindrical shell can be relatively precisely measured at the distance determined by the numerical simulation results. And this distance is much less than the classic criterion of far-field for sound sources in general. The experimental data of an underwater cylindrical shell is processed and analyzed. The experimental results demonstrate that: the attenuation law of sound pressure amplitude at the criterion distance determined by the calculation method proposed in this paper is very similar to the attenuation law of spherical waves. This means that if the radiated sound power is calculated by the mean squared sound pressure method, the measurement distance could be nearer to the underwater elastic structure. It is very valuable for sound source level measurements when the signal-to-noise ratio (SNR) is low. And more, because the phase does not need to be measured, the problem caused by the poor accuracy of phase measurement can be avoided.

The far-field distance of underwater structure sound sources is usually considered, especially in underwater acoustic combat, target detection, vibration reduction and noise control. There exist two formulas for the determination of far-field distance. One is derived from a uniformly vibrating piston in a rigid boundary. The other formula is usually used in finite element software. However, underwater structure sound sources, such as submarines and torpedoes, are not in rigid boundaries. These sources do not radiate sound uniformly. Thus, the far-field distance of actual underwater structure sound sources may not be correctly calculated by the former formula. The latter formula in finite element software provides that sound waves are completely absorbed at the boundary, so as to fulfill the precise calculation of sound field. Hence, the distance calculated by that formula is not a real far-field distance. In the paper, the simulation of radiated sound pressure from underwater piston, plane array and cylindrical shell has been done. The piston is a uniformly vibrating plate, and the diameter is 0.2m. The plane array consists of several pistons, and the outer diameter is 0.75m. The cylindrical shell is a single layer steel shell, which is excited by a point force. The diameter and length of the shell is 1m, 2m, respectively. The determination process of far-field distance is as follows. If the decay of sound pressure in the distance coincides with the law of spherical wave expansion, then the minimum distance between the model and field point is the far-field distance. From the simulation, we find that the far-field distance of underwater structure sound sources is less than that calculated by the first formula. Meanwhile, the far-field distance of underwater structure sources is different from that calculated by the second formula in most frequency ranges. If the distance between field point and underwater structure sound sources is 1.5D, where, D is the maximum dimension of underwater structure sound sources, sound pressure in the distance decays as the law of spherical wave expansion, in most frequency ranges. This may be a simple determination of far-field distance of underwater structure sound sources. The decay of radiated sound pressure by distance has been validated in the experiment. The underwater sound sources are piston, plane array, single layer cylindrical shell. The experimental results are compared with the simulations.

Based on the combination of large eddy simulation theory and Lighthill's acoustic analogy equation, numerical simulation of underwater flow noise is investigated. The calculation model of nozzle and fluid is built up by finite element software, ANSYS. The property of fluid field is calculated by the software, Fluent, which is in the ANSYS. The fluid property changing with time is gotten and dealt with by FFT, then, introduced into ACTRAN. Therefore, sound field of flow noise can be calculated. The flow noise of a circular nozzle is carefully investigated. The diameter of the nozzle is 20 mm, and the velocity of the fluid is 10 m/s. The frequency is in the range from 20 to 5000 Hz. The results demonstrate that sound radiation from flow noise is mainly in transition area. The distance between the nozzle and transition area is in the range from 8 to 10 D. Here, D is maximum dimension of the nozzle. Meanwhile, the directivity of radiated noise in transition area is fourth polar. Based on the principle of reverberation method, a measurement system of underwater flow noise is built up. One kind of nozzle is a steel pipe, and inner diameter is 200 mm. The pipe is placed in a reverberation water pool. In the pool, there are two parts: one part is flow area; the other part is test area. In test area, there are 32 hydrophones, which are disposed at different depth. The other kind of nozzle is a circular nozzle with coverage formation. The diameter of the nozzles is 10 mm, 20 mm, 30 mm, respectively. The nozzle is placed in a reverberation water tank, which is made by steel plates and supported by separated points at the bottom. In the top, there are three ducts. The ducts can hinder sound propagation from the tank to the outside, and vice versa. The tank is also separated into two parts: one part is flow area; the other part is test area. In test area, there are four hydrophones, which are treated as a line array to receive the signal caused by the flow. All the designs can protect hydrophones from the knock by the flow. The flow noise source is a tank in high place. And the flow is generated by the gravity. The total level of sound radiation power from flow noise is proportional to the eighth power of the velocity. The results demonstrate that total level of flow noise from the nozzle of uniform type is only decided by the pressure of flow noise source, and not related to the section area of nozzle. The results are coincided with the simulation.

Wine has played an important role in people's daily lives. The method of accelerating the aging of wine by different ultrasonic wave is reported. A system has been built up. Three kinds of composite rod transducers are mounted at the bottom of three cylindrical steel barrel. The diameter of cylindrical steel barrel is 300 mm, 400 mm, and 500 mm, respectively. The resonant frequency of transducers is 25 kHz, 40 kHz, 60 kHz, respectively. The barrel is filled with water and the temperature of water is 20°C. The sound pressure level in the barrel is 170 dB, 180 dB, 200 dB, respectively. When sound pressure level is 200 dB, acoustic cavitation is observed. The wine, which is brewed by sorghum and wheat, is contained in five glass bottles, and considered to be one sample. Then, each sample is put into the barrel. The alcohol content in the wine is 70.0. At a certain sound pressure level, the working time is 5 minutes, 10 minutes, 20 minutes, respectively. In total, there are 27 samples of wine by the treatment of ultrasonic wave. Besides, an origin sample without ultrasonic wave treatment is selected as the comparison. The sound field distribution of the barrel is simulated by the software of VIRTUAL LAB. In the simulation, sound field distribution of the barrel is not uniform and each sample should be spatially averaged. The total acid, total ester, ethyl lactate, ethyl caproate, and alcohol content of each sample have been tested. The test period is 1 hour, 2 days, 4 days, 8 days, 30 days, respectively, after the process of ultrasonic wave treatment. The effect of ultrasonic wave on alcohol content, total acid, total ester, ethyl lactate, ethyl caproate has been discussed.

A pre-processing method for the holography measurement data is proposed to solve the problem caused by the multi-path interferences in finite space. In order to verify the effectiveness of this method, a standard steel plate is measured in an anechoic tank. The sound reflection coefficient of the steel plate is reconstructed by near-field acoustic holography (NAH) technology using the pre-processed data. The experimental results showed that: using the pre-processed data to reconstruct the sound reflection coefficient by NAH can effectively suppress the effect of multi-path interferences, and improve the measurement accuracy. And more, the requirements for the measurement environment are not very strict. Therefore, this pre-processing method will have broad application prospects.

A method for calculating the sound radiation of underwater cylindrical shells in low frequency band by using the equivalent beams is investigated in this paper. The vibrations of cylindrical shells with large length-to-radius ratio L/a>10 are similar to those of beams in very low frequency band, in which underwater sound radiation characteristics of cylindrical shells are mainly dominated by beam-type modes. If the structural parameters and boundary conditions of a beam model are same as those of a cylindrical shell, the radiated sound power levels will be correspondence in low frequency band. However, One-dimensional beam theoretical model only considers about the transverse displacement, the coupling effects of displacements in other directions are neglected. Only the first order beam-type modal frequency, not the other higher orders, of cylindrical shells can be predicted precisely by one-dimensional beam theoretical model. To solve this problem, an equivalent beam theoretical model is established, in which different Young's modulus values of the equivalent beam model are searched to make the beam-type modal frequencies identical to those obtained by cylindrical shell theoretical model. For convenience, the interaction between the structures and water is approximated to added mass. The results show that the equivalent Young's modulus values are mainly dominated by the length-to-radius ratio for cylindrical shells, and the effect of the shell thickness on those can be neglected. The efficient and accurate numerical solutions are presented to validate the proposed method.

Based on wave superposition method (WSM), radiated noise can be predicted quickly by monitoring the vibration data on machine surface. However, this method may produce great error for complex vibration distribution. The main cause is the improper configuration of equivalent sources. In order to solve this problem, an equivalent source configuration based on the least mean square method (LMS) is put forward for typical structure in this paper, in which a few reference points of sound field pressure should be measured ahead to search the optimal equivalent source position automatically. Since the structure has been equivalent to an array of monopole sources by WSM, the prediction for mechanical noise with boundaries can be realized by changing Green function. The result shows that the radiated noise of structure, for different excitations, can be predicted precisely from the surface velocity by WSM once the optimal equivalent source position is determined by LMS using the reference points of sound pressure field.

Nearfield planar acoustic holography method based on BEM is often used in experiments because it is very convenient to scan the sound field on planes. However, the sound field reconstruction error on the sound source surface is usually very big due to the dimension limitation of measuring planes. The method proposed here, by measuring some extra far field points out of the planes, which are combined with those on the planes to reconstruct the sound field on the source surface, can reduce this sort of error. The numerical simulation has been carried out by this method for a spherical sound source with radius of 1.0 m. The double planes are located at x=±0.6 m, assumed that the sound source is at (0,0,0) in Cartesian coordinate. The max reconstructing error of sound pressure on the sound source surface can be reduced from 6 dB to less than 2 dB at frequency 500 Hz when we add two extra far field points at (0,±10 m,0). Obviously, this method is very meaningful in practical experimental tests. More numerical simulation by this method for more complex sound source field reconstruction will be shown in the full paper.

Shallow coastal waters are characterized by high levels of suspended sediment particles relative to the open ocean. This kind of seawater is also characterized as turbid seawater. An experimental investigation on volume reverberation has been done in the sea area outside the Yangtze River Estuary, where the seawater contains a lot of suspended sediment particles. Sound scattering data at 8 sites in the frequency range from 10 to 40 kHz, in steps of 1 kHz, has been obtained. The results demonstrate that volume backscattering intensity in turbid seawater may be expected to fall in the range from ‐60 to ‐100 dB re m−1. Because sound absorption caused by suspended sediment particles in turbid seawater is much greater than that in clear seawater, the volume backscattering intensity does not change with the pulse length of the transmitted signal, even when the pulse length is less than 0.5 ms. Salinity in the sea area has little effect on volume backscattering intensity. However, suspended sediment particles ma...

The seawater, which contains suspended fine clay particles, is called turbid seawater. The research on sound absorption caused by suspended particles in turbid seawater has little been done. A reverberation time technique has been developed to measure the coefficient of sound absorption at 20~60 kHz in turbid seawater. In the test volumes employed, the effect is small. It is therefore measured by taking the difference in reverberation times of a volume of water with and without fine clay particles. Results demonstrate that if the concentration of fine clay particles in turbid seawater is below 110 mg/L, the fine clay particles don't cause additional sound absorption. However, if the concentration of fine clay particles in turbid seawater is above 140 mg/L, the coefficient of sound absorption in turbid seawater is as twice at least as that in clear seawater. The effect of temperature on sound absorption in turbid seawater has also been investigated.

A new method using SFT (Spatial Fourier Transforms) has been investigated to measure reflection coefficient at arbitrary angles of incidence by NAH(Near‐field Acoustical holography ) in semi‐space. The paper gives an experimental verification of this method. The experimental frequency is from 3 kHz to 10 kHz. The size of test steel plate is 1.6m×1.5m,the size of hologram surfaces is 1.32m×1.14m, the distance between two hologram surface and the test material is about 2cm, the distance between sound source and water surface is about 4cm. The measured results are generally in good agreement with the theoretical value; the mean‐square error of data is below 0.06. The experimental results show that this method eliminates the effect of the scattered sound field of the water surface completely. Because the sound source and the test material are set near the water surface during the experiment, it is very easy to set the measuring system, and this is very useful in practice.

The near field acoustic holography (NAH) theory model in semi‐space has been constructed on the foundation of NAH theory in full‐space. The technique of semi‐space acoustic field reconstruction by NAH has been developed on Fast Fourier Transform Algorithm. This method has considered the water surface as soft boundary condition. The equivalent free sound field can be acquired if the measured sound field has been handled dissymmetrically by the principle of the mirror. This method can overcome the disadvantages of the boundary element method which can be used to resolve the problems of sound source identification in semi‐space, such as the using of the semi‐space Green function, complex transformation matrix calculation, more time consummation and singularity solution existence. This method has been verified by numerical calculation and by experiment respectively. The experimental frequency is between 3 and 10 kHz. The distance between the hologram plane and source plane is 3 centimeter. The results show th...