Abstract

Experimental Study of Sound Scattering by Vortex Rings

Y.D.Chashechkin, V.E.Prokhorov (Institute for Problems in Mechanics RAS, Moscow, Russia)

e-mail: chakin@ipmnet.ru

The axially symmetric structures such as vortex rings are often observed in stratified flows. They impact a transfer of the additives, creation of the fine structure in ambient liquid, internal waves generation, and scattering of the sound. A convenient model to study the phenomena mentioned is a vortex ring caused by pulse impact onto the fluid. The question of a great interest is an acoustic sounding of the ring that allows to investigate scattering properties and to detail a scattering model in stratified liquid. The paper presented deals with scattering of sound by vortex ring moving horizontally in a stratified liquid over the flow regimes from laminar to turbulent ones. The rings were sounded by monostatic scheme operating at frequency 1 MHz, ping duration 40 us, repetition period 0.16 s. Simultaneously, optical visualization of the rings was performed, so the geometry of the scattering ring itself as well as the wake past it was completely determined. According to experimental data, the ring is identified as an inhomogeneity with cross section of volume backscattering governed by internal microscale which is of an order of sound wave length. The dependence of cross section on ring\'s velocity U is approximated by U in power 5. Dimensionless scattering cross sections exceed those obtained in the sea scattering experiments, so the vortex contribution to the sound scattering in the ocean is comparable with one caused by biota.

Section : 7