Nonstationary dynamics of turbulent axisymmetric jets in a stratified fluid: Part 2. Mechanism of excitation of axisymmetric oscillations in a submerged jet

It was shown based on laboratory experiments in a Large Thermally Stratified Tank (LTST) at the Institute of Applied Physics of the Russian Academy of Sciences that a turbulent axisymmetric jet in a stratified fluid with a sharp density drop (a pycnocline) intensively generates internal waves. An axisymmetric oscillation mode, for which a sufficient condition of stability in the parallel flow approximation is met, served as their source. This paper studies the stability of a nonparallel flow (with self-similar velocity profiles) that simulates a jet flow in the lower part of the pycnocline with respect to the axisymmetric mode. The estimates of the axisymmetric mode near the pycnocline are in agreement with the experimental data. The signs of the self-oscillating mode of the jet were experimentally revealed and the possibility of self-oscillations was theoretically proved: it was shown that the flow in the pycnocline vicinity is absolutely unstable.