Non-Linear waves and turbulence in stratified and rotating fluids – ONLITUR

The present project aims to investigate experimentally several aspects of nonlinear waves and turbulence in rotating or stratified flows. The main objective is to develop simple and well-controlled experiments in order to explore and characterize some of the main physical mechanisms at play in geophysical flows involving transport, mixing and turbulence in the ocean. The idealized flow configurations proposed here include self-interaction of single inertial or internal wave beams, the ensuing generation of mean flows, the mixing induced by wave breaking, and the transition to wave turbulence. Mixing and transport of density and heat, and their role in the global thermal equilibrium of the ocean, as well as mixing and transport of pollutant and biochemical components are among the key issues addressed by the present project.

The project is driven by two partner laboratories, Fluides Automatique et Systèmes Thermiques - FAST (Université Paris-Sud, Orsay) and Laboratoire de Physique de l’Ecole Normale Supérieure de Lyon. Both laboratories are recognized for their expertise in rotating and stratified flows, respectively, as well as in the advanced experimental methods to investigate these flows, such as Particle Image Velocimetry (PIV), Laser Induced Fluorescence (LIF) and Synthetic Schlieren. A set of laboratory experiments, based on the rotating platform “Gyroflow” at Orsay, and the novel wave generator used in the stratified channels at Lyon, are proposed to address experimentally the elementary processes at work in some geophysical flows.

Waves propagating in rotating or stratified fluids (i.e. inertial or internal waves, respectively) possess similar unusual dispersion relations, and therefore share several properties, in their generation, propagation and reflection. Combined rotation and stratification effects are at play in most geophysical flows. However, these wave systems differ significantly regarding a number of nonlinear behaviors, such as mean transport and instabilities. These similarities and differences provide an original perspective to the combined study that we propose in this project, where the complementarity of our respective expertise will form the basis of our collaboration. Considering separately and comparing the rotating and stratified systems, we expect to gain more insight into the elementary processes at work in the atmosphere and the ocean.

The experimental approach presents several advantages for this project: (i) numerical simulations are very demanding due to the wide range of temporal scales, between the fast waves and the slow nonlinear effects; (ii) recent advances in optical measurements, including Particle Image Velocimetry (PIV), make laboratory experiments particularly interesting in terms of temporal and spatial resolution. In the case of rotating fluid, a stereoscopic PIV system will be developed, allowing us to probe the 3D features inherent to rotating systems. In the case of stratified fluid, the PIV/Schlieren or PIV/LIF combinations will give simultaneous access to velocity and density fields, whose interaction is at the heart of many geophysical and environmental physical processes. The spatial resolution of our devices will allow measurements of turbulent quantities, such as wave-induced turbulent viscosities and diffusivities and spatio-temporal spectra. Based on these tools, we will be able to study several aspects of the interaction between inertial/internal waves and turbulence, a subject largely unexplored quantitatively to date.