Excitation of internal waves and stratified turbulence by parametric instability

Standing internal waves have been studied in a tank where a stable density stratification has been initially introduced using a salt concentration gradient. A primary wave grows by parametric instability: a periodic modulation of the apparent gravity is produced by a vertical oscillation of the tank. Two aspects of the dynamics have been studied (1) the mechanism of breaking that leads to turbulence and (2) a type of turbulence forced by wave breaking in the permanent regime. A succession of instabilities leading to wave breaking has been clearly identified: the primary wave first excites a secondary wave at half its frequency by a mechanism of parametric instability. The density isolines associated with this secondary wave then locally overhang, and breaking occurs through a convective instability. In the turbulent regime, a specific inertial range with k(-3) density spectra is measured.