Soutenance de Julie Deleuze

Stratified fluids are found in various geophysical settings such as the atmosphere, stellar interiors or the ocean. In such media, buoyancy driven oscillations can propagate through the fluid and are known as internal gravity waves. Theses waves can transfer energy from large-scale forcings like tides or wind toward smaller scales where they become unstable and break. This irreversible energy transfer alters the stratification and is referred as diapycnal mixing.

The ocean dynamics is highly sensitive to its stratification. Therefore, understanding the energy cascade toward small scales is key to properly parametrize mixing in global climate models. One of the main mechanisms for internal waves to transfer energy toward smaller scales is the triadic resonant instability (TRI), whereby a high amplitude wave generates two secondary waves of shorter wavelength. To observe this phenomenon, we generate highamplitude internal waves by exciting internal wave resonant modes in a rectangular domain. We demonstrate that the characteristics of the secondary waves are constrained both by nonlinear resonance conditions and by resonance conditions imposed by the geometry of the domain (box modes). We explore the combined influence of these two sets of constrains by varying the characteristics of the main wave and the geometry of the domain. The interplay between nonlinear resonance and box resonance leads to complex nonlinear dynamics, with the generation of internal waves deviating from their dispersion relation. For intermediate forcing, even if the dynamics is non linear, no mixing is observed in experiments, indicating that nonlinear internal wave dynamics do not necessarily lead to mixing.

To investigate internal wave-induced mixing, we perform other experiments with a higher forcing amplitude. Using simultaneous velocity and density field measurements, we observe nonlinear dynamics similar to those in the previous experiments, now associated with a slow evolution of the background stratification. We show that the evolving stratification affects the spatial structure of the internal waves, creating a feedback loop between the nonlinear dynamics and mixing. We identify several mixing regimes, each associated with different mixing efficiencies. Furthermore, we detect the signature of local mixing events and examine the nature of the instabilities leading to wave breaking. Finally, we consider an other mechanism for irreversible energy transfer in a stratified fluid: settling of particle clouds in a quiescent stratified fluid. We experimentally study the settling of particle clouds in a stratified fluid at rest. We observe the entrainment of lighter fluid in the wake of the settling cloud into regions of denser fluid. We study how the fluid returns to rest after this perturbation, and quantify the irreversible energy transfer from the particle cloud to the background fluid. We show that this mixing occurs only under specific flow regimes.