Internal wave turbulence in a stratified fluid with and without eigenmodes of the experimental domain

We present laboratory experiments on turbulence in a linearly stratified fluid driven by an ensemble of internal gravity waves which approaches statistical homogeneity and axisymmetry. In a way similar to several recent experimental works, nonlinearities develop through the establishment of a set of internal wave modes at discrete frequencies, when the forcing amplitude is increased. We show that the most energetic of these modes are resonant eigenmodes of the fluid domain. The discretization of the energy in frequency and wave number associated to the emergence of these modes prevents the flow from approaching a regime described by the weak/wave turbulence theory, in which a forward cascade carried by a statistical ensemble of weakly nonlinear waves in an infinite domain forms an energy continuum in the frequency and wave-number spaces. We then show that the introduction of slightly tilted panels at the top and at the bottom of the fluid domain allows to inhibit the emergence of the discrete wave modes. In this new configuration, the nonlinear regime results in a continuum of energy over one decade of frequencies which is mainly carried by internal gravity waves verifying the dispersion relation. We therefore achieved a turbulent flow approaching a three-dimensional internal wave turbulence regime with no discretization of the energy in the frequency and wave-number domains. These results constitute a significant step forward in the search of the laboratory observation of a fully developed weakly nonlinear internal-gravity-wave turbulence.

Automated summary

We conducted laboratory experiments in a linearly stratified fluid and found that turbulence driven by internal gravity waves can approach statistical homogeneity and axisymmetry. Nonlinearities develop through the establishment of discrete frequencies of internal wave modes when the forcing is increased. We also discovered that the most energetic modes are resonant eigenmodes of the fluid domain. By introducing slightly tilted panels, we were able to inhibit the emergence of discrete wave modes and achieve a turbulent flow with a continuum of energy in the frequency and wave-number domains.