浅海中孤立子内波引起的声能量起伏

Intensity fluctuations due to the motion of internal solitons in shallow water

当孤立子内波的波阵面与声传播路径所成角度较大时,简正波耦合是导致声信号起伏的主要因素。研究了浅海中孤立子内波引起的声能量起伏规律,给出声场起伏的耦合简正波表达式,并使用抛物方程模型进行仿真。数值分析表明,接收点声强随时间变化呈准周期性。在频谱图中能够得到声强起伏的主导频率,主导频率与孤立子内波沿声传播路径的移动速度成正比,与无扰动波导中简正波在距离上的干涉周期(对应于射线理论中临界声线的跨度)成反比,与孤立子内波的形状无关。此外,对声强频谱的垂直结构进行了分析,该结构与对声场起伏起主要作用简正波的本征函数相关。

Mode coupling plays a dominant role in the fluctuations of sound signals at the receiver for sufficiently large inclination angle of the internal soliton （IS） front relative to the acoustic path. The coupled-mode formulas for sound filed fluctuations are given in this situation. Theoretical analysis is carried out for intensity fluctuations due to the motion of ISs in shallow water, and numerical simulations are also performed using a parabolic equation model. The quasi-periodic variations of intensity can be observed from the numerical results. The peaks in the spectrum of intensity take place at the so called predominant frequency and its higher harmonics. The predominant frequency is proportional to the velocity of the IS motion along the acoustic path and inversely proportional to the scale of interference beating of the interacting modes, which corresponds to the cycle length of the critical ray in the ray theory. The predominant frequency value does not depend on the IS shape. Moreover, the vertical structures of the intensity spectrum are analyzed and shown to correspond the shapes of the modes which contribute significantly to intensity fluctuations.