Research On Forecasting Sound Scattering Field For Targets In Shallow Water

There is not a solution,which is mature and applicable to the problem on how to calculate the sound scattering field of an elastic structure in the waveguide.In the early years,scholars gave the analytical solution without taking the multiple scattering between the sound source and the target into the consideration,so as to achieve the calculation of the scattering sound field of the target in the waveguide.While,nowadays scholars are concerned about giving more accurate results with high computing efficiency.Aiming to predict the sound scattering field of the target in the shallow water at low frequency,a hybrid method,finite element method combined with normal mode theory(FEMNM),is presented in this paper,which can be of fast and accurate prediction of the sound field.According to the finite element method in coupling multi-physical fields and normal mode theory and its orthogonality,a theoretical model about the calculation on the sound scattering field of the target in the waveguide is established.Part of the calculation domain,which contains the structure and sound source,is first completed by the finite element method.The transition from the finite element theory to the normal mode theory is then realized by using the orthogonality of the eigen-function.The sound field at any point in the waveguide is then obtained by using the normal mode theory,and the sound scattering field is finally abstracted from the sound field.The key of this hybrid method is the parameter setting of the orthogonal decomposition surface and the processing of the incident sound field.Based on several numerical models and many factors considered such as geometry,material,multi physical field,accuracy,efficiency,etc.,the rules of the parameter setting are obtained through analysis: the calculation domain computed by FEM only needs to include the sound source and the target,and the number of points on the position of the orthogonal decomposition surface only needs to be the same as the rule of finite element mesh generation,i.e.one sixth of the wavelength.As for the procession of the incident sound field,there are three methods: subtraction method,the method based on boundary element method and the method based on Fourier transform method.For numerical simulation,the subtraction method is the most convenient,while for the actual sound field processing,the method based on boundary element method is more effective.Under the optimal configuration,the numerical calculation of the sound scattering field of the typical target in the typical waveguide is carried out.The results show that: The boundary has a great influence on the sound scattering field of the target;The sound scattering field of the target is greatly affected when the target is a shell structure,especially the air or water inside;The influence of the sound scattering field is different via the different distance between the target and the interface,however the influence can be less at a low frequency;The sound scattering field at each azimuth of the three-dimensional sound field is different,thus the simplification can be used less for a practical situation;Different seabed properties and different sound velocity gradients have different effects on the sound scattering field of the target.At last,the validity of numerical calculation and the law of the actual sound field distribution are illustrated by experiments.