Research On Prediction Of Noise Radiated By Large Underwater Structures Via Surface Vibration Monitoring

The prediction of noise radiated by underwater structures is one of the most important research contents in the field of underwater acoustics. It is regarded as the theoretical foundation and the important criterion for quantitative acoustic design and vibration-noise control. For the large underwater structures such as submarines, fast prediction or real-time monitoring of noise is crucial in improving the fighting and survival capacities. Therefore, the research on prediction of noise radiated by underwater structures has important theoretical value and wide engineering application. In this dissertation, fast prediction of noise radiated by large underwater structures is focused on. Theoretical and experimental investigations are implemented following the principle of theoretical research combined with engineering application.The physical essence of transfer law between the surface vibration and the acoustic field has been analyzed deeply. The acoustic pressure equals the inner product of two vectors, namely, surface velocity vector and Acoustic Transfer Vector (ATV). The element of ATV equals the acoustic pressure radiated by the corresponding piston vibrating in unit speed on the rigid baffle coinciding with the vibrating surface. The discussion is focused on the rationality of the acoustic pressure radiated by the piston on the actual baffle approximated by the analytical solution of that on the regular baffle. In the case of the dimension of the actual baffle far larger or less than the acoustic wavelength, the radiation of the piston can be regarded as that of a point source or a planar wave, respectively. Although the regular baffle does not fit the actual baffle well, the difference of the acoustic pressure radiated by the piston before and after fitting can be ignored. In the case of the dimension of the actual baffle comparable with the acoustic wavelength, the acoustic pressure is mainly determined by the shape of the neighboring baffle on the acoustic wavelength scale. The error is mainly determined by the fitting degree between the regular baffle and the actual baffle. As long as the elements of ATV dominating the whole acoustic pressure are accurate, a satisfying prediction can be obtained. Finally, a method named Element Radiation Superposition Method (ERSM) is established which has profound theoretical foundation and can be applied to the fast prediction of noise radiated by large underwater structures. It succeeds to avoid the problems of non-uniqueness, singularity integral and high-dimensional matrix inverse. Consequently, it has a larger advantage in calculating speed than the numerical integration methods. A series of numerical analysis prove that the results of ERSM are in satisfactory agreement with those of analytical solutions or BEM either in low frequency or in high frequency.The relationships between the prediction precision of acoustic pressure and the spatial sampling interval on surface vibration are analyzed theoretically for two typical surfaces, namely, plane and cylinder. The distributions of surface vibration are transformed to two-dimensional wavenumber domain or axial circumferential wavenumber domain respectively. The mappings between the transformed spectrums and the acoustic pressures are established. The prediction error is totally determined by the degree of aliasing in the corresponding transformed domains resulted from spatial sampling. For the fixed observed bearing and analyzing frequency, the prediction precision of acoustic pressure radiated by a vibrating plane is determined by the ratio of the two-dimensional wavenumber spectrums at the specific wavenumber before and after spatial sampling. The prediction precision of acoustic pressure radiated by a vibrating cylinder is mainly decided by the ratio of the axial circumferential wavenumber spectrums at the specific axial wavenumber and below the specific circumferential wavenumber before and after spatial sampling. The spatial sampling interval required by the designated acoustic pressure prediction precision on the simply supported rectangular plate and that on the simply supported cylindrical shell are analyzed numerically. Some conclusions are extended to the cases of spatial sampling on the generally supported rectangular plate and that on the generally supported cylindrical shell.Experimental research on vibration and radiation of underwater multi-compartmented shell structure is performed together with the related Institute. An engineering method of noise prediction is discussed to solve the problem of vibration phase missing in this experiment. According to the superposition principle of acoustic field, the acoustic pressure level can be expressed as the sum of the acoustic pressure level radiated by the surface elements vibrating independently and a modifying factor. Therein, the modifying factor reflects the correlation between the acoustic pressures arising from the phase inconsistency of the surface vibration. Through the theoretical and experimental studies, the influences of the observation distance and the surface vibration interpolation degree on the modifying factor are analyzed. For the far observers in the same bearing, the modifying factor has nothing with the observation distance and increases with the number of surface elements in the interpolation process. On the basis, an engineering method is put forward to predicting the noise with the modifying factor obtained in the specific case. If the vibration equipments work in different frequency or in different intensity, the noise can be predicted by the modifying factor obtained in the case of all vibration equipments working. If not, the noise can be predicted in some frequency band by the modifying factor obtained in the specific case.The study in this dissertation provides some further understanding on the rule between vibration and radiation. A new method is put forward to predicting the noise radiated by large underwater structures and providing theoretical foundation for quantitative acoustic design and vibration-noise control. In view of the sampling and reconstruction of spatial signal, the relationships between the spatial sampling on surface vibration and the prediction error of acoustic pressure are analyzed. Some conclusions are of certain guiding significance in designing and evaluating the laying scheme of surface vibration measurements. The prediction method in the case of vibration phase missing has been discussed and provides a new way to solve the actual problems in engineering noise prediction.