The Radiation of Internal Waves by a Turbulent Fountain in a Stratified Fluid

The objective of the present paper is to study by large-eddy simulation (LES) the dynamics of a fountain created by a turbu-lent vertical jet flow penetrating a pycnocline (a sharp density interface) in a density-stratified fluid. A circular, turbulent jet flow of neutral buoyancy is considered which propagates vertically upwards towards the pycnocline level and penetrates a distance into the layer of lighter fluid. At a certain height the jet fluid stagnates and flows down under gravity around the up-flowing core thus creating a fountain. The LES results show that if the Froude number Fr (defined by the jet flow velocity and diameter at injection and the buoyancy frequency in the pycnocline) is large enough, the fountain top collapses periodically and generates circular internal waves. The dependence of the amplitude of the fountain-top oscillations on Fr is well described by a Landau model for an instability mode with soft self-excitation.