Acoustic field propagation through a time-evolving buoyant jet

Abstract

Previous theoretical work regarding internal wave/acoustic wave interaction has shown that in the case of soliton (i.e., nonoverturning) propagation, significant effects on acoustic propagation occur via both amplitude and phase components. Acoustic flow visualization data obtained recently in the South China Sea [Orr and Mignerey, J. Geophys. Res. 108 (2003)] indicate the presence of internal bores with associated Kelvin–Helmholtz instabilities at the base of the mixed layer. These overturning bores are examples of nonhydrostatic dynamics associated with significant vertical acceleration of the fluid. In order to explore the effect of overturning on acoustic field propagation, we have used a nonhydrostatic hydrodynamic model to simulate the temporal evolution of internal bores. The initial condition is a stationary front separating two regions characterized by slightly different but homogeneous densities. The front is released at t=0 and the gravity-induced flow evolves into an internal bore with associated Kelvin–Helmholtz instabilities, or rotors, behind the leading edge of the bore. Results for transmission loss and signal gain degradation over a frequency range of 200–500 Hz will be presented. [Research sponsored by ONR.]