Turbulence strongly influenced by a stable density stratification is studied experimentally, numerically and theoretically. This type of turbulence is encountered in the atmosphere and Oceans in an intermediate range of scales for which the Coriolis force is negligible. In a first part, the transition to turbulence of a simple flow: a pair of columnar contra-rotating vortices is studied. Direct Numerical Simulations (DNS) show that when the dissipation is sufficiently weak, two secondary instabilities, the shear and gravitational instabilities, develop after the zigzag instability. The characteristic length scale of the Kelvin-Helmholtz billows is of the order of the buoyancy scale. Both instabilities lead to a transition to turbulence which exhibits anisotropic spectra similar to those associated to fully developed strongly stratified turbulence. For the first time, a return to isotropy is observed for scales smaller than the Ozmidov length scale. In a second part, a fully developed turbulent flow forced with several vortex generators is studied. The experiments at the larger buoyancy Reynolds numbers have enabled for the first time to nearly reach the strongly stratified turbulent regime. These experimental results have been reproduced and extended to larger Reynolds numbers with numerical simulations forced in physical space with the same type of forcing. They reveal that the larger scale of the overturnings is of the order of the buoyancy scale. Finally, a generalisation of the 4/5s Kolmogorov law is proposed for stratified turbulence.
