Generation of weakly nonlinear turbulence of internal gravity waves in the Coriolis facility

TL;DR

This study experimentally investigates weakly nonlinear internal gravity wave turbulence in stratified fluids within a large-scale Coriolis facility, revealing wave-dominated spectra and supporting wave turbulence theory in stratified turbulence.

Contribution

First experimental demonstration of weakly nonlinear internal gravity wave turbulence in a large stratified domain, with detailed spectral analysis and observation of wave-dominated turbulence regimes.

Findings

Spectrum consistent with wave turbulence but differs from Garrett & Munk spectrum

Discrete modes and continuum part observed in velocity field

Turbulence transitions from wave-dominated to nonlinear at small scales

Abstract

We investigate experimentally stratified turbulence forced by waves. Stratified turbulence is present in oceans and it is expected to be dominated by nonlinear interaction of internal gravity waves as described by the Garrett & Munk spectrum. In order to reach turbulent regimes dominated by stratification we use the Coriolis facility in Grenoble (France) which large size enables us to reach regimes with both low Froude number and large Reynolds number. Stratification is obtained by using vertically linearly varying salt concentration and we force large scale waves in a $6\times6\times 1$ m$^3$ domain. We perform time-resolved PIV to probe the space-time structure of the velocity field. We observe a wide band spectrum which is made of waves. Discrete modes are observed due to the square shape of the flow container as well as a continuum part which appears consistent with an axisymmetric superposition of random weakly nonlinear waves. Our observations support the interpretation of turbulence of a strongly stratified fluid as wave turbulence of internal waves although our spectrum is quite different from the Garrett & Munk spectrum. Weak turbulence proceeds down to a small cutoff length scale (the buoyancy wavelength) at which a transition to more strongly nonlinear turbulence is expected.