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The interactions between turbulent penetrative convection and gravity waves: from laboratory experiments to geo- and astrophysical applications

Final Report Summary - COWAVE (The interactions between turbulent penetrative convection and gravity waves: from laboratory experiments to geo- and astrophysical applications)

In many geo- and astrophysical situations, a turbulent convective fluid layer is separated from a stably stratified one by a relatively sharp but deformable interface. Examples include the atmospheric convective layer and overlying stratosphere, the convective and radiative zones in stars, the Earth’s core... In classical models of planetary and stellar fluid mechanics, motions in stratified zones are often neglected. They can nevertheless support oscillatory motions called gravito-inertial waves (GIW), which transport energy and momentum, can break and mix, generate mean motions, etc. Besides their direct observation as for instance in asteroseismology, GIW are essential for accurate models of global climate as well as of planetary and stellar dynamics. It is thus necessary to understand how the waves are excited, how and what they propagate, and how they influence their environment.

Global integrated models including length scales and time scales spanning many orders of magnitude are required to fully address motions in turbulent and stratified zones and to understand the details of the highly non-linear couplings between rotation, meridional circulation, turbulence and waves: this is clearly very challenging from both analytical and numerical points of view. Our approach is thus based on a non-conventional experimental approach. Two complementary laboratory set-ups have been developed, using water as a working fluid and salt or temperature to control the buoyancy. Those experiments allow addressing the whole range of relevant physical issues in simplified models. In the first set-up, we take benefit from the unusual property of water that its density has a maximum value near 4^oC to study its convective and oscillatory motions in a tank with a bottom boundary at about 0^oC and a hotter upper surface. In the second set-up, a turbulent jet generated by injection of water impinges upon the interface between a uniform density layer and a stratified one of salted water. Combining high precision local measurements of temperature fluctuations and non-intrusive global velocity measurements, we have investigated the time and space frequency spectra of waves excited by convection, as well as the energy and momentum fluxes at the stratified/convective zones interface. Numerical simulations and analytical approaches of our experiments have complemented our physical understanding of the ongoing processes. Our studies have demonstrated that the main mechanism for wave excitation is the deep excitation mechanism due to the Reynolds stresses over the whole convective domain. Our studies have also demonstrated that while the convective motions mostly excite low frequency waves that remain localized around the interface, larger frequency waves propagate deeper into the stratified zone because of a selective damping, depending on the rotation frequency and the buoyancy frequency of the ambiant fluid. Thanks to those validated results, knowing the turbulent convective field of a given system, we are now in a position to quantitatively predict the spatiotemporal characteristics of the excited wave field. Finally, an additional study has allowed to described the non-linear fate of the excited waves subject to the so-called Parametric Subharmonic Instability.

Our original experimental approach has thus allowed to establish and validate a set of rules and generic scaling laws that can now be either directly applied to natural systems or used to derived parameterization of wave processes in global dynamical models. In the longer term, it is expected that our results will participate in at least 2 scientific challenges, priorities at the European level. In atmospheric sciences, the inclusion of gravity waves in general circulation models is necessary for accurate predictions of global weather patterns; but their correct parameterization necessitates a perfect understanding of their physics, to which we have clearly contributed. In astrophysics, our results should help to interpret the increasing amount of data coming from helioseismology and asteroseismology (both priorities of the European Spatial Agency ESA), hence allowing a better understanding of the dynamics of our Sun and a better prediction of its further evolution.

In addition to this main project, two side projects – also focusing on experimental approaches of geophysical fluid mechanics problems - have arisen from the Marie-Curie fellowship and the collaborations that it has allowed to initiate. In the first project, the dynamics of sedimentation and fragmentation of iron blobs through the primitive magma ocean during the latest stages of Earth accretion have been characterized using a laboratory model. Our model has led to a better understanding of the early ages of our planet and of its initial thermochemical state. The second project focuses on the description of the turbulence generated by libration in planetary cores. It proposes an alternative route towards magnetic field generation in planets, that could for instance help understanding the puzzling dynamo of the Moon and Ganymede.