声衍射相移对深海会聚区焦散结构的影响

Effect of acoustic diffraction phase shift on caustic structure in deep sea convergence zone

本文从射线-简正波理论出发,推导了计及衍射相移时声线跨距、传播时延与群速度的表达式,在此基础上给出了一种包含衍射相移影响的深海会聚区焦散结构计算模型.对典型深海声道中第一个上会聚区的仿真研究表明:高频条件下纯折射(refracted-refracted,RR)型会聚区有3条焦散线,海面反射(refracted surfacereflected,RSR)型会聚区有4条焦散线.通过与高频结果对比,低频条件下计及衍射相移后发现,界面反射相移引起的水平位移使RR型焦散线向靠近声源的方向水平偏移,使RSR型声线额外多产生数条焦散线,而声波以非均匀波形式传播时产生的位移使RR型焦散线向远离声源的方向水平偏移.频率升高后,声衍射效应减小,焦散结构趋于经典射线理论的计算结果.本文给出的模型物理意义清晰,计算简便准确,弥补了经典射线理论在低频条件下适用性不强的缺陷.

The constant phase shift-π/2 generated by a turning point is an ideal approximation under high frequency conditions.In fact,when sound waves pass through a turning point,it will travel horizontally for a certain distance in the form of inhomogeneous plane wave and then be refracted back.At this time,a functional phase shift should also be included at the turning point.In addition,a refracted ray will bring about reflection phase shift when the turning point is close to the waveguide edge.These two kinds of phase shifts which are associated with the diffraction phenomenon are called diffraction phase shifts in this paper and are more notable when the frequency goes down.In order to accurately obtain the caustic structure at low frequency,the diffraction phase shift is used to correct the classical ray theory in calculating ray skip distance,traveling time,and group velocity in this work.On this basis,a simple and explicit analytical model is proposed which is suitable for calculating the low frequency deep water caustics.The numerical study of the first upper convergence zone in the complete deep water sound channel shows that there are three caustic lines in the refracted-refracted(RR)convergence zone and four caustic lines in the refracted surface-reflected(RSR)convergence zone under the condition of high frequency hypothesis.When the diffraction phase shifts at low frequency are included,and compares with high frequency results,it is found that the horizontal beam displacement caused by the reflection phase shift will make the RR caustics horizontally move towards the source and also lead the RSR rays to generate several new caustics,while the beam displacement caused by sound propagating in the form of inhomogeneous plan wave will make the RR caustics horizontally move away from the sound source.Accompanied with the increased frequency,the diffraction effect will decrease,and the caustic structure gradually tends to the classical ray theory results.The reliability of the correction results is verified by the normal mode theory.The model proposed in this work makes up for the deficiency of classical ray theory.