Research On The Coherent Acoustic Field Of The Bubbly Ship Wake And Its Detection Using Multibeam Sonar

The acoustical characteristics of the bubbly ship wake and its detection method are one of the current key technical and scientific issues.This paper studies the coherent acoustic field of the bubbly ship wake and its multi-beam sonar detection,aiming to explore the coherent acoustic field of the oscillating bubbles under the sea surface reverberation and discuss the bubbly ship wake detection using multi-beam sonar.The main research contents are as follows:First of all,the mechanism of the acoustic scattering of the bubbly ship wake is studied.Bubbles inside the bubbly ship wake oscillate under the flucation of the pressure field surrounding them,and radiate pressure waves.When bubbles interact with each other,their bubble dyanmics will be further influenced.The differential equations dscribing coupled bubble dynamics are derived based on the wave equation and the Bernoulli equation.These differential equations can be treated as the delayed-differential equations and solved accordingly numerically.The numerical results of the proposed equation show that,the expansion,collapse and the rebound phase can be captured by the present model.To further reduce the computational cost,the bubbles are studied in the linear oscillation regime,and the the consistence between the delayed-differential equations and the multiple scattering theory is given.Second,the acoustic scattering of the bubble cluster is dicussed.For the coherent pressure field,using Dyson equation,the effective medium theory of the bubbly liquid is derived and the effective wavenumber considering the bubble distribution up to the second-order statistical moments is proposed.For the averaged intensity field,using the Bethe-Salpeter equation,the radiative transfer equation is derived,and the analysis about the influence of the multiple scattering among bubbles on the intensity scattered by the bubbly cloud is given.Due to the weak localization of the bubble cloud,the scattered intensity will concentrate on the backscattering direction,which is called coherent backscattering enhancement.The multiple scattering among bubbles is further studied numerically using the self-consistent approach in the frequency domain,the relation between the bubble distribution and the self-consistent approach is built,and both the coherent pressure field and the averaged intensity field are both focused.The spherical bubble cloud isolatedly and beneath the flat sea surface are further studied.For the situation of the bubble cloud under the sea surface,the self-consistent approach is modified to consider the multiple scattering between bubbles and the sea surface.Besides,using the effective medium theoy and the translational addition theorem for the spherical functions,the influence of the sea surface on the bubble cloud is also studied theoretically.Third,the bubbly ship wake under the rough sea surface is constructed acoustically in the temporal domain.The Pierson-Moskowitz spectrum is used to generate the rough sea surface in 3D space,and the Helmholtz-Kirchhoff integral is used to describe the acoustic scattering of such a rough sea surface in the half-infinite space.The probability density function of the bubble radius and the bubble spatially distribution in the bubbly ship wake is given,and the self-consistent approach are modified using Helmholtz-Kirchhoff integral to describe the acoustical scattering of the bubbly ship wake in the temporal domain.When the rough sea surface appears,the reverberation echo may cover the echo of the bubbly ship wake.Fourth,the detection of bubbly ship wake using multi-beam sonar based on coherent backscattering enhancement is proposed.The detections of bubbly ship wake using multi-beam sonar based on coherent backscattering enhancement,based on the conventional beamforming and spatial correlation coefficient are discussed.By comparing different methods,the advantage of the detection method based on the coherent backscattering enhancement is clarified.A qualitative discussion is given with regard to the porosity of the bubble wake,the polydispersity of the bubble and the spatial position of the array in the simulation environment.According to the theoretical and the numerical studies above,experimental studies are carried out using the multibeam sonar as the detector,and the bubbly ship wake as the acoustic target.The experimental data agrees with theoretical and the numerical results qualitatively.