ERC project THEIA

Why building experiments?

In the frame of the ERC project THEIA, we aim at combining theory, numerics and experiments. They are indeed complementary to investigate the coupling between a solid boundary and a fluid flow, such as the one at the seafloor or at the Core-Mantle Boundary (CMB) :

• Theory gives physical insights and can directly probe the relevant regime of weak diffusivities and large magnetic fields. But it remains limited to laminar or weak turbulence, and dynamo studies remain a challenge.
• Numerics can handle spherical-like geometries, with all the physical ingredients, giving the flow in the whole volume. But it is limited to laminar or weak turbulence, and diffusive effects are over-estimated. Moreover, it remains challenging to include topographic effects in simulations.
• Experiments can easily probe the turbulent regime, a key ingredient which remains intractable with theory or numerics. It can also tackle any kind of topography. But the weakly diffusive magnetic effects or the spherical-like geometry remain out-of-reach.

Two experiments

To investigate turbulent boundary couplings in presence of rotation, density variations and topography, we are developing an experimental setup at two different scales: a small, but versatile and easy to modify, setup called mini-THEIA, and a large-scale experiment, less flexible but capable of probing other regimes (e.g. waves interactions):

Mini-THEIA will be able to tackle various experiments in rotating flows. It will allow a continuous filling and draining of the experimental tank, while in rotation (notably to avoid the mixing due to the fluid spin-up). Typical planned experiments are

• Linear spin-up (or spin-down) of a stratified fluid in presence of topography, in the continuity of Burmann & Noir (2018).
• Coupling of floating bodies (in order to get rid off unwanted solid friction)
• Filling box experiments, with plume injections and draining while in rotation (collaboration with Prof. Renaud Deguen)
• Slichter modes measurements, with a rotating flow
• Libration Driven square instabilities (LDSI) will be excited and compared with theory (collaboration with Jérémie Vidal)

The large-scale experiment THEIA requires severe modifications of the experimental room to host the rotating 18T experiment of salty water. This long-term goal experiment will allow to study the boundary wave drag when rotation, topography and turbulence are simultaneously present. It is still under development, and mini-THEIA will help to design it.