Theory And Experiment On Acoustic Radiation Prediction Of Elastic Structure In Shallow Water

Grounded in the practical situation of China’s coastal shallow sea and the strategic needs in underwater defense,the paper aims at the key issues of near-field acoustic measurement,acoustic stealth,and warfare environment assessment.A comprehensive prediction model,the combined wave superposition method,is established for lowfrequency coupled vibration,acoustic radiation,and acoustic propagation of elastic structures in shallow water,and the structural vibro-acoustic characteristics are studied from theoretical,numerical and experimental aspects,which are used to test the applicability of the method for practical acoustic radiation prediction.Firstly,the acoustic properties and boundary conditions of surface,seabed,and seawater in the shallow waveguide are respectively studied,and the wavenumber integration method,normal mode(NM),and rigorous image source method(ISM)for the acoustic transfer(Green’s)function are established.Thereinto,NM without the leakage mode in the near field is used as the far-field Green’s function to calculate the acoustic field.And the rigorous ISM based on wave acoustics and the mirror principle can be adopted as the near-field Green’s function to solve the source strength solution in the wave superposition method(WSM),which is derived based on the Helmholtz integral.And then,the combined wave superposition method(CWSM)for lowfrequency 3D acoustic radiation fields of elastic structures in shallow water is established by WSM combined with Green’s function and the Multiphysics coupled finite element method(FEM).A comparison with the theoretical model and FEM example shows that its computational efficiency is at least 10 times better than that in FEM.The proposed virtual source surface to structure surface size ratio is 3/5,the number of virtual sources is 4/5 of the vibration measurement points,and the discrete size of the surface is 1/5 of the experimental wavelength.The Tikhonov method and LSQR algorithm can suppress the perturbation of the measurement noise on the prediction accuracy,and LSQR can achieve better computational accuracy at a lower number of iterations.Then,the vibro-acoustic characteristics of the cylindrical shell in Pekeris waveguide were numerically analyzed,and it was observed that the structure close to the surface(or the seabed)would result in a higher(or lower)coupled frequency than that in the free field,and the boundary influence on the structural vibration could be neglected when the diving depth was greater than four times the radius from the surface and twice the radius from the liquid seabed.Due to the presence of shear waves in the seabed being analogous to the "softening" of the acoustic boundary,the relationship between the reflection strength of the seabed to the sound field is rigid boundary >liquid seabed > elastic bottom.The attenuation pattern of the sound field of the shallow submarine structure is in the order according to the near-field acoustic influence region,spherical wave expansion region,transition region,and cylindrical wave expansion region respectively,and the distance of each region is related to the diving depth and frequency.The experimental sound energy in shallow water is higher than that in other fluid environments because the upper and lower boundaries limit the expansion of the sound field.For the low-frequency acoustic stealth problem,the radiated sound power can be significantly reduced by choosing a soft substrate seabedFinally,the small-scale experiments in the underwater acoustic channel and the scaled-up model in the lake test are respectively established.Results show that the prediction accuracy of the presented method on the acoustic field can reach the practical engineering requirements.The influence of reflected sound on the structure vibration is relatively weak,but the effect on the sound field is much stronger.The far-field conditions of the acoustic radiation are related to the structure size and frequency.For example,it is necessary to increase the radial distance between the test hydrophone array and the source center in the case of the large-scale and low-frequency acoustic radiation test.For small-sized and high-frequency experiments,the hydrophone array can be deployed at least twice the maximum size of the structure,which will result in a high signal-to-noise ratio and reliable results.