Changes of the acoustic modal phase velocity, group velocity, and interference distance in an eddy

Abstract

In this paper, by using the modal dispersion equation with the phase-integral approximation, and considering an eddy (or water mass) as a sound channel disturbance, the effects of the undisturbed channel, cold-core eddy, and warm-core eddy on the acoustic propagation characteristics are discussed. According to the solutions of the dispersion equation, the relation is obtained between the modal phase velocity (Cn), group velocity (Un), and interference distance (Ln), and the eddy intensity. When the plane wave (with an incident angle α) travels toward the center of a warm-core eddy (disturbed intensity BM), ‘‘double channel phenomenon’’ will take place in case of sin2<th>α<BM<2(1−cos<th>α), and then the modal phase velocity and interference distance will have anomalous changes. The interference distance of normal mode has a minimum. The larger the number of modes and the higher the source frequency, then the more obvious the ‘‘hollow case’’ for interference distance. However, for any cold-core eddy, the modal phase velocity or the interference distance will be monotonically changed.