Research On Acoustic Radiation From Elastic Structure By Combined Finite Element Method With Normal Mode

At present,most scholars focused on acoustic radiation from structures in shallow water use the simplified approximations of boundary conditions or sound source conditions.For example,the sea water is usually considered as a free field or half-space,and sound source is considered as a point source or a combination of several point sources.Considering the coupling effect of the elastic structure-fluid-waveguide boundary together,there is not an efficient and reliable prediction method to calculate the radiated sound field of any three-dimensional structure in a layered shallow water waveguide.A fast method for acoustic radiation prediction of elastic structures in shallow water waveguide is proposed in this paper: combined finite element method with normal mode(FEMNM).In this method,a cylindrical surface is taken at a certain distance so that it can completely cover the structural sound source,and the sound field is divided into an active region and a passive region.For the active region,a structure-fluid-boundary coupling model is established and sound field value is calculated using the finite element method.For the passive region,the field is written as the sum of a series of modes,and then the sound field is expanded at the depth direction and the azimuthal direction on the coupling cylinder.The orthogonality of the mode function is used to obtain the coefficients of each order.,And synthesizing each order mode again to obtain a complete sound field solution.The core of the FEM-NM is computing the coefficients of the normal modes.Factors such as the selection of the coupling distance,the truncation of the eigenfunctions,the waveguide environmental parameters,and the position and directivity of the sound source may affect the calculation of the coefficients and the calculated sound field.This paper analyzes and verifies these factors,and the simulation results show that the coupling range does not affect the result in most cases;although the orthogonality of the mode after truncated is affected,the far-field results are still accurate;and the method in this paper is effective for different channel parameters and different acoustic sources,therefore the adaptability of FEM-NM is very well;compared with the finite element method(FEM),the computational power and speed of this method have absolute advantages.Therefore,it can be said that the method has good accuracy,stability and high efficiency.Furthermore,the FEM-NM method is used to calculate the radiated sound field of the Benchmark standard submarine model in shallow water waveguides.The spatial distribution characteristics of the radiated sound field of the submarine model under different sea depths,different submarine depths and different excitations were studied.From the perspective of the excitation coefficient,the differences between point sound sources and structure sound sources are compared.