Research On Prediction Of Sound Radiated By Elastic Structure In Underwater Bounded Space

The prediction of sound radiated by underwater structures is one of the most importantresearches in the field of underwater acoustics. It is regarded as the theoretical foundation andthe important criterion for quantitative acoustic design and vibration-noise control. Practically,the structure is not always located in free-space, which brings problems, such as the coupledvibration, multipath propagation and scattering, etc. Thus investigations are needed for theprediction of sound radiated by elastic structure in underwater bounded space.In this dissertation, the methods of acoustic radiation prediction in free-space areinvestigated primarily. Then the methods are expounded and classified according to theirfundamentals. By comparing these methods, the wave superposition method (WSM) is chosenas the main method for acoustic radiation prediction in bounded space. However, the WSM(also known as equivalent source method: ESM) is not perfect, since the optimalconfiguration of the equivalent source and the measuring point has been considering as aproblem. Therefore, an equivalent source configuration method based on the least meansquare is proposed, in which a few reference points of sound pressure should be measuredahead to search the optimal position of equivalent sources. Moreover, the effects of thenumber, position of measuring points and amplitude distribution of vibration were studiednumerically. The principles of how to configurate the equivalent source and measuring pointappropriately are pointed out, in order to improve the accuracy of the WSM and make theWSM more suitable for acoustic radiation prediction.One reason of choosing WSM as the main method for acoustic radiation prediction inbounded space is that WSM simulates structure by an array of simple sources. The simplesources’ Green function can be adjusted conveniently according to the types and positions ofboundary. Furthermore, the existing algorithms of sound propagation in waveguide canhopefully combined with WSM to predict the sound radiated by elastic structure in waveguide.Unfortunately, there are no guildlines about how to reform the Green function in WSM forcombination. Therefore, the environment of half-spaces with a single boundary is investigatedfirstly, and a half-space wave superposition method is proposed for the fast prediction ofacoustic radiation from a complex structure, in which the free-space Green’s function is usedto match the strength of equivalent sources. As the submerged structure is coupled with water,the scattering between reflector and structure is considered. Moreover, two treatments fornormal velocity of a structure are proposed. One is called separation method which separates normal velocity by wave superposition method and the other is called fast method whichneglects the effect of reflection to the structural vibration, which makes the acoustic radiationprediction more efficiency. The finite element method (FEM) and boundary element method(BEM) are also combined with the proposed method to analyze the vibration and acousticradiation of the elastic structure in half-space.The proposed half-space wave superposition method avoids the three-dimensionalderivation of Green function on the surface of structure, which makes the possibility ofcombining WSM with the sound propagation algorithms in waveguide. In the dissertation, theWSM is combined with the normal-mode method and the ray method to predict the soundradiated by underwater structures in the plane-parallel waveguide. By analyzing theadvantages and applicability of the two proposed combination methods, it concludes that thecombined propagation algorithm should be chosen appropriately for specific cases. Then theFEM model in half-space is extended to analyze the coupled vibration and acoustic radiationof the elastic structure in waveguide. The applicability of the FEM model is also analyzed andcompared with two proposed combination methods. Actually, the proposed WSM cancombined with any sound propagation algorithm which calculates the sound radiated by amonopole effectively, thus it can combines with more sound propagation algorithms to predictthe sound radiated by elastic structure for cases where more complicated waveguides are to betreated.In order to prove the efficiency of the proposed methods and the numerical analysises, anexperiment for predicting sound radiated from a cylindrical shell with hemi-caps has beendone successfully in the half-space anechoic tank, and the experiment in the pool ofunderwater acoustic channel was accomplished afterwards.Since the analyzed structure in this dissertation is the scale model, more attentions havebeen paid to the frequency band above3kHz, and the proposed algorithms of the acousticradiation prediction has been extend to higher frequencies as far as possible. The simulationsand experiments on the scale model are greatly expected to provide useful working guidelinesto practical engineering.