The role of fluctuations across a density interface

A statistical mechanics theory for a fluid stratified in density is presented. The predicted statistical equilibrium state is the most probable outcome of turbulent stirring. The slow temporal evolution of the vertical density profile is related to the presence of irreversible mixing, which alters the global distribution of density levels. We propose a model in which the vertical density profile evolves through a sequence of statistical equilibrium states. The theory is then tested with laboratory experiments in a two-layer stably stratified fluid forced from below by an oscillating grid. Quantitative measurements of density fluctuations across the interface are made by planar laser induced fluorescence. These fluctuations are splitted in a "wave" part and a "turbulent" part. The wave part of the density field is well described by a previous theory due to Phillips. We argue that statistical mechanics predictions apply for the turbulent part of the density field sufficiently close to the interface. However inside the mixed layer density fluctuations are instead controlled by a balance between eddy flux downward and dissipation by cascade to small scales. We report exponential tails for the density pdf in this region

A statistical mechanics approach to mixing in stratified fluids

Predicting how much mixing occurs when a given amount of energy is injected into a Boussinesq fluid is a longstanding problem in stratified turbulence. The huge number of degrees of freedom involved in those processes renders extremely difficult a deterministic approach to the problem. Here we present a statistical mechanics approach yielding prediction for a cumulative, global mixing efficiency as a function of a global Richardson number and the background buoyancy profile.Comment: Accepted in Journal of Fluid Mechanic

A novel internal waves generator

We present a new kind of generator of internal waves which has been designed for three purposes. First, the oscillating boundary conditions force the fluid particles to travel in the preferred direction of the wave ray, hence reducing the mixing due to forcing. Secondly, only one ray tube is produced so that all of the energy is in the beam of interest. Thirdly, temporal and spatial frequency studies emphasize the high quality for temporal and spatial monochromaticity of the emitted beam. The greatest strength of this technique is therefore the ability to produce a large monochromatic and unidirectional beam

Laboratory experiments on the generation of internal tidal beams over steep slopes

We designed a simple laboratory experiment to study internal tides generation. We consider a steep continental shelf, for which the internal tide is shown to be emitted from the critical point, which is clearly amphidromic. We also discuss the dependence of the width of the emitted beam on the local curvature of topography and on viscosity. Finally we derive the form of the resulting internal tidal beam by drawing an analogy with an oscillating cylinder in a static fluid

Internal gravity waves breaking above Great Meteor Seamount

We present a 19 days, high frequency record of temperature profiles above the eastern shelfbreak of the Great Meteor Seamount. Seamounts are known to be efficient regions for internal tides generation, but also participate in mixing and transport through nonlinear processes and wave breaking. We measured extreme overtuning events associated with the upgoing tidal flux pushing cold water up the slope. Strong shear also lead to small scale instabilities that are also resolved by our sensors

Étude expérimentale des ondes de gravité internes en présence de topographie. Émission, propagation, réflexion.

The dynamics of atmosphere and ocean is strongly influenced by their stratification, that allows the propagation of internal gravity waves. Those waves interact with topography, leading both to their generation and dissipation, and play a key role in the global energy budget of the Earth.We designed a syntehic schlieren setup to visualize those waves, and led large scale experiments on internal waves critical reflection. We also designed a novel internal waves generator.With those experiments, we verified analytical predictions on internal waves emission by oscillating bodies and applied it to tidal conversion at a continental shelf. We also analyzed nonlinear effects during critical reflection. Last, we led the first experiments on gravito-inertial waves propagation.L'étude physique des enveloppes externes de la Terre est fortement influencée par leur stratification. Cette dernière permet la propagation d'ondes anisotropes appelées ondes de gravité internes. L'interaction de ces ondes avec la topographie gouverne la génération et la dissipation de ces ondes, et joue un rôle non négligeable dans le bilan énergétique global de la planète.Nous avons mis au point une technique de visualisation des ondes internes par strioscopie synthétique, et mené parallèlement des expériences à grande échelle sur leur réflexion critique. Nous avons également élaboré un nouveau type d'excitateur d'ondes internes planes.Ces expériences nous ont permis de valider une théorie analytique d'émission par des corps oscillants et de l'appliquer à la génération d'ondes par un talus continental. Nous avons également confirmé la présence d'effets nonlinéaires lors de la réflexion critique. Enfin, nous avons étudié l'influence de la force de Coriolis sur ces phénomènes

Internal gravity waves and topography: emission, propagation and reflection

We study the interplay between internal gravity waves and topography. When internal waves are impinging onto a sloping bed, striking phenomena are expected to occur close to the slope due to the unusual reflection properties. We have designed several internal wave generators to study experimentally this reflection. We have also designed a simple laboratory experiment to study the internal tide generation. We consider a steep continental shelf in an oscillating flow, for which an internal tide is shown to be emitted from the critical point. We discuss the dependence of the width of the emitted beam on the local curvature of topography and on the viscosity, by drawing an analogy with an oscillating cylinder in a static fluid