Research On Multi-angle Acoustic Scattering Characteristics Of Small Underwater Targets

Small underwater targets have the characteristics of small size,strong flexibility and high intelligence,which play an important role in underwater military confrontation.For quiet small targets,they do not spontaneously radiate sound waves,that is,weak targets,so only active sonar can be used for detection.In underwater detection,affected by the shape and posture of the target,the echo received in different azimuths is different,that is,the scattering echo of the target changes dynamically with the incident angle of the sound wave.In addition,the Marine environment is complex and changeable,and the target may approach the free surface(water)or rigid wall(seabed)except in the general case of free field(infinite water).Due to the existence of such boundary,the fluid around the structure becomes acoustic half-space,which makes the acoustic scattering characteristics of the target different from that of the free field,thus affecting the scattering echo response of the target.The echo of the same target is different in different ocean environments and different incident directions,which makes detection and information acquisition of small underwater targets very difficult.Therefore,it is of practical significance to study the target acoustic scattering characteristics and fine characteristics.Taking the typical underwater shell target as the research object,this paper firstly introduces the typical shell acoustic scattering theory and acoustic scattering solution technology,which provides the basis for the following shell scattering research.For the hollow thin spherical shell target,the backscattering characteristics of the target in free field and above the interface are studied.Except for the geometric echo,the main energy component is the mid-to-high frequency subsonic anti-symmetric Lamb elastic wave,which contains the physical information of the target.In free field,the radius expression of hollow stainless steel shell is fitted by using the resonance characteristic of equal frequency formant interval,and the estimation method is verified by simulation and experiment.The estimation method is further applied to the target above the interface,and 3D-FEM-BEM modeling technology and 2D-FEM axisymmetric target acoustic scattering modeling technology based on variational principle are used to simulate the spherical shell scattering field above the rigid seabed and attenuated seabed respectively.The resonance characteristics of the backscattered Lamb elastic wave are less obvious than those of the free field condition.The probability density of formant interval was calculated,and the radius was estimated with the maximum probability of formant interval.The estimation results verified the effectiveness of the proposed radius estimation method.The finite element method is used to model the acoustic field of a limited length cylindrical shell and a hemisphere-head cylindrical shell,and the backscattering response under different incident angles is obtained.Based on the scattering theory of infinite cylindrical shell,the spatial dynamic characteristics of target acoustic scattering are studied,and the omnidirectional spatial variation law of each acoustic scattering component is determined.In a certain Angle range,the frequency interval of elastic component dimensionless formants increases with the increase of incident angle.This fine feature can be used to estimate the target radius at oblique incidence.Furthermore,the scattering field of hollow cylindrical shells with different sizes was simulated,and the relationship between the interval of dimensionless formant frequencies and the Angle of incident was fitted by the quadratic polynomial of one variable,and the simple relation between the radius of cylindrical shells and the incident Angle was obtained.So the radius of the shell can be estimated by using the scattering echo spectrum estimation under the condition that the Angle information is known.The simulation results are compared with the experimental data of the pool to verify the existence of the Angle variation law and the applicability of the feature.Finally,based on the hybrid FEM-BEM method,the acoustic scattering model of the submerged elastic cylinder on the rigid seabed is established,the scattered sound field distribution under different azimuth incident angles is analyzed,and the acoustic dispersion characteristics of the submerged elastic cylinder are preliminarily explored.This research provides the basis for the establishment of the characteristic model of the scattering signal of the omnidirectional target.