Research On Very Low Frequency Sound Propagation Characteristics Based On Spectral Element Method

At present,in order to develop the marine economy and build a maritime power,China is committed to efficient development,utilization and reasonable control of marine resources.Therefore,theoretical and technical research in the field of underwater acoustic has become a general trend.With the research of sonar system gradually develops to low frequency,the problem of very low frequency(below 100Hz)sound propagation needs to be concerned by scholars at home and abroad.The study of VLF sound propagation characteristics will have important reference significance for sound field prediction,target detection and national defense construction.In this thesis,based on spectral element method,2D and 3D sound field modeling in different marine environments is carried out to analyze sound propagation characteristics under very low frequency conditions.The main research contents and innovations are as follows:In the analysis of the 2D ocean sound propagation characteristics,SPECFEM2 D was firstly used to model the sound field of the horizontal layered waveguide and the wedge-shaped seabed terrain.SCOOTER,RAM,RAMS and COMSOL were used to verify the accuracy of spectral element method,proving that spectral element method can deal with the sound field modeling problems related to fluid,elastic seafloor and distance-dependent sound field modeling problems.According to the obtained time-domain solution,the wave components and arrival structure of VLF sound propagation were analyzed and it is confirmed that the interface waves appearing in the elastic seafloor can propagate over long distances.On this basis,the spectral element method is applied to the sound field modeling of the sea trial,and the propagation characteristics and physical mechanism of various wave types are analyzed.The simulation analysis and numerical calculation are carried out by using the measured sound velocity profile and seabed topography.The calculated results are in good agreement with the experimental data,which can be successfully applied to the simulation and prediction of ocean sound field.In the analysis of the 3D ocean sound propagation characteristics,SPECFEM3 D was used to model the sound field of the three-dimensional wedge waveguide,trench topography,and Kermadec Trench topography,and the calculation results of the three-dimensional spectral element method were compared with the traditional finite element method,which verified the reliability of the three-dimensional spectral element method to solve complex ocean acoustic problems.Firstly,the sound propagation characteristics of the 3D wedge model at different horizontal azimuth angles are analyzed.The results show that the 3D effect of the sound field becomes more obvious with the increase of the horizontal azimuth Angle,which is of great significance to the study of the sound characteristics of the 3D ocean sound field modeling.Next,The acoustic field modeling analysis of trench topography,which is less researched at present,is carried out.By comparing with the flat bottom,the sound propagation characteristics,wave components and 3D effects in the trench terrain are analyzed.The results show that the trench terrain will generate a series of back-propagating sound waves.Due to the continuous reflection of the sound waves in the trench,sound waves will propagate to the left side of the trench,making the time-domain waveform in the trench terrain more complex.Finally,this study is extended to the Kermadec Trench,which is more complicated,and the sound field modeling calculation is carried out.The analysis results provide a theoretical basis for the study of marine earthquake disaster warning.