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Double-diffusive thermochemical convection in the liquid layers of planetary interiors : a first numerical exploration with a particle- in-cell method

Soutenance de thèse
Soutenance de thèse de M. Mathieu BOUFFARD du Laboratoire de Géologie de Lyon, sous la direction de M. Stéphane LABROSSE et, sous la codirection de M. Gaël CHOBLET du Laboratoire de Planétologie et Géodynamique de Nantes
20 septembre 2017

Lieu(x) :

Salle de conférence de la BU sciences - Campus de la Doua- Lyon 1
Numerous planetary bodies contain internal liquid layers in which convective currents are generated by the combination of buoyancy sources of thermal and compositional origin.
The strong difference between the thermal and chemical molecular diffusivities and the possibility of thermochemical coupling at melting or freezing boundaries create a convective regime that is much more complex than pure thermal convection, partly due to the potential occurrence of double-diffusive instabilities. Traditionally, numerical simulations have modeled the dynamics of the liquid part of planetary cores in a more simplistic way by neglecting the diffusivity difference and combining both fields into one single variable, an approximation that is convenient but maybe not relevant.

However, distinguishing both fields and dealing with a large or infinite diffusivity ratio makes it compulsory to use numerical methods that minimize numerical diffusion. In this thesis, I adapted a semi-Lagrangian particle-in-cell (PIC) method into a pre-existing dynamo code to describe the weakly diffusive compositional field. I optimized and validated this code on two benchmarks. I compared the PIC method to Eulerian schemes and showed that its advantages extend beyond its lower numerical dissipation.

I performed first numerical simulations of pure compositional convection and showed that the stratification below the Earth's core mantle boundary could be of chemical origin. In the case of a thermally stratified layer, I performed a scaling analysis and ran a few simulations of rotating fingering convection. The potential effects of the magnetic field and thermochemical coupling are finally discussed.

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