| Sails are one of the most important echo sources of underwater submarines.By changing the structure material s of permeable light shell of sail from traditional steel to sandwich type composite structure,and applying suitable shell shape designing and composite designing,the TS(target strenth)of sails will be reduced,which will improve sound stealthy performance of submarines against active sonars.Based on the PEM / HL algorithm(plate element-highlight hybrid algorithm)and orthogonal anisotropic material's transfer matrix theory,the TS predicting model of the sails made by sound-reflecting composite are established.The influence of the two TS controlling techniques including optimal design of the shell shape and the optimal design of the acoustic material are studied by using this model.Firstly,the accuracy of geometric acoustic theory and planar element method is verified by rigid sphere model and Benchmark submarine model.The acoustic coefficients of the composites were calculated by using different theoretical methods respectively.Compared with the test results,it was found that the measured results were in agreement with the results of the transfer matrix theory and the acoustic finite element theory,while exhibiting great difference with the results of sandwich panel's sound transmission theory.After correcting the material parameters,the theoretical results obtained by the transfer matrix method are very high in agreement with the measured results.It is proved that the transfer matrix method is suitable for calculating the sound coefficients of the composites.Using the shell target strength prediction model,it is found that: as for steel sails,the typical Bragge scattering characteristics are generated due to echo interference of internal pressure vessels,and the pressure vessels can block each other so that the TS in the bow and stern is slightly reduced;As for sound-reflecting sails,the impact of the internal pressure vessels is very small.When the shell is in vertical shape,the TS characteristics of sound-reflecting sails and steel sails are analyzed and compared.It is found that the TS of the sound-reflecting shell is lower than that of the latter in the direction of the stern entrance.Near the direction of the vertical incidence,the TS of sound-reflecting sails target is higher than the latter.The airfoils shape of the cross section of the sail and the horizontal plane is changed,and the TS directivity distribution is basically the same as the thickness of the cross section increases,and the bow intensity is improved.The shape of the composite shell is changed from vertical to inclined form,which can significantly reduce the echo level in the direction of the transverse direction.When the frequency is 10 kHz and the inclination angle is 10 °,the average TS ranging from 90 ° ~ 105 ° angle of incidence drop of about 18dB;as for steel sail,TS reduction is relatively smaller,repectively.And TS near the direction of vertical incidence would drop significantly firstly,and then tend to be stable,as inclining angle of the sail increases.The TS of the sound-reflecting envelope is higher than that of the steel sail in a certain grazing angle.And the TS would significantly increase when the grazing angle of incidence is close to the incling angle of the sail.Finally,a quantitative mathematical model of the superposition of the TS is established.It is pointed out that the total echo energy is related to the square of the pressure's reflection coefficient of the light shell in quadratic function.Based on this mathematical model,the ideal sound pressure reflection coefficient of the light shell corresponding to the minimum strength of the enclosure is obtained.By using the genetic algorithm,the sound-reflecting composites' parameters are optimized.The materials with weak sound reflecting performance is used in the front of the sails,while and the one with strong sound reflecting performance is used in the stern area of the sails,which reduces the TS of the sail in a certain degree. |
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