Laboratory observation of internal gravity wave turbulence in a three-dimensional, large-scale facility
The search for solutions to the theory of weakly non-linear internal gravity wave turbulence is an active research topic. It is notably stimulated by the fact that this regime could drive fine-scale ocean dynamics for which the identification of a physical model could yield improved parametrizations in global oceanic models. In this context, analytical works lead to diverse predictions and the experimental observation of a regime of developed weakly-non-linear internal wave turbulence constitutes a major, still unachieved, objective of experimentalists in the field. In this study, building on recent experimental developments, we present laboratory observations of internal gravity wave turbulence in a linearly stratified fluid, performed in a large-scale, three-dimensional facility allowing the forcing of long-wavelength internal waves. Our setup allows to access large Reynolds numbers favoring the development of turbulent power-law spectra while keeping the Froude number relatively low in order to remain weakly non-linear. As the forcing amplitude increases, the flow seems to approach a wave turbulence regime: we indeed observe the progressive construction of a continuous distribution of energy both in the frequency and wave number spaces, whereas the spatio-temporal spectra indicate that the energy remains almost exclusively carried by internal gravity waves verifying the dispersion relation. We finally show that, as the transition to turbulence proceeds, the bicoherence spectrum of the velocity field becomes smooth over the internal wave frequency domain, taking values of the order of the Froude number. While these observations are in line with the phenomenology of weakly non-linear wave turbulence, the power laws in $k^{-3}$ we report over about a decade for the horizontal and vertical spatial energy spectra agree with the prediction that can be made from raw [...]