Research On Acoustic Radiation Prediction Method For Cylindrical Shell In The Shallow Water

It can be a difficult problem to precisely predict the sound field radiated from a finite elastic structure in shallow water waveguide because of its strong coupling with up-down boundaries and the fluid media,whose sound field cannot be calculated directly by current methods,such as Ray theory,normal mode theory and other different methods,which are adaptable to sound fields from idealized point sources in waveguide.So far,there is no reliable prediction method for this kind of problems.A combined wave superposition method is proposed for such problems,which combines the traditional wave superposition method with the transfer function in shallow water waveguide and the multi-physics field coupling numerical model.This method mainly consists of three sections: firstly,obtaining the normal velocity on the elastic structure surface in shallow water waveguide by finite element method,whose FEM model includes the up-down boundaries and the completely absorbent sound boundaries in horizontal directions;secondly,getting the equivalent point sources strength by traditional wave superposition method;finally,calculating the total sound field by adding up every point source field which is obtained by normal mode method,Which solves the bottleneck problem effectively when encountered in calculating the remote acoustic radiation of large structure under shallow water waveguide.The key of the wave superposition method lies in the configuration of the virtual source,but the source configuration and data extraction process involve in many factors.It is necessary to analyze the influence of each factor on the calculation of the acoustic radiation distance in the shallow water,in order to improve the calculation accuracy of the wave superposition method separately.In this paper,we take the example of the acoustic field of the pulsation source in the shallow water,and the calculation program of the structural acoustic radiation in the shallow water is compiled and the influence of the factors such as geometry type,size,number of virtual sources and data error calculation on the accuracy of the wave superposition method for the structure acoustic radiation is studied.Comparing with the analytical solution,it is concluded that the source surface and the structural surface are best conformal in order to achieve higher accuracy over the entire distance,and the source surface-to-structure size reduction ratio is between 0 and 0.8,and the source surface and the structure The ratio of surface nodes is controlled in the range of 0.5-1;when the surface vibration information of structures contains errors,the stability and accuracy can be calculated by Tikhonov regularization and L-curve posterior parameter selection method.After verified by numerical method and analytical method,the necessity of the acoustic radiation investigated from elastic cylindrical shells is explained from the radiation source,the environmental impact and the radiation field measurement.The influence of shallow water boundaries on the coupled modal frequency is significant,the presence of free surface(seabed)will result in the increase(reduction)of coupled modal frequency.But the coupled modal frequency of the cylindrical shell in shallow water waveguide tend to be the same as that in the free field.The seabed types and acoustic parameters have less influence on the coupled modal frequency.The results show that the elastic structure can be equivalent to the point source only in low-frequency due to a significant coupling effect of up-down-direction boundaries on the sound field.The spatial field directivity distribution is especially obvious in high-frequency,the acoustic power results measured by vertical line array are greatly influenced by the acoustic waveguide boundary and the diving depth.