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Two aspects of fluid dynamics in planetary cores

Abstract : This manuscript presents two independent studies on the fluid dynamics of planetary interiors. The first part of this manuscript is a numerical study of thermal convection and magnetic field generation driven by internal heating in rotating spheres; a configuration appropriate for planetary cores prior to inner-core nucleation. For sufficiently vigorous convection, we find that the flow becomes strongly asymmetric with respect to the equator; this result contrasts with previously published studies of convection in spherical shells (i.e. with an inner core) where the flow is essentially symmetric. An antisymmetric and axisymmetric (EAA) mode then strongly influences the total flow and conflicts with the Taylor-Proudman theorem. We show that this spontaneous emergence of antisymmetric flow components induces localized magnetic fields with up to 90% of the total magnetic energy contained in a single hemisphere. Our results suggest a parsimonious scenario to explain the hemispherical crustal magnetic field of Mars. In the second part of this manuscript, we present experiments on the instability and fragmentation of blobs of a heavy liquid released into a lighter immiscible liquid. These processes likely occurred on a massive scale during the formation of the Earth and its core, when dense liquid metal blobs were released within deep molten silicate magma oceans. During the fragmentation process, we observe deformation of the released fluid, formation of filamentary structures, capillary instability, and eventually drop formation. We find that, at low and intermediate Weber number (which measures the importance of inertia versus surface tension), the fragmentation regime results from the competition between a Rayleigh-Taylor instability and the roll-up of a vortex ring. At sufficiently high Weber number (the relevant regime for core formation), the large-scale flow behaves as a turbulent vortex ring or a turbulent thermal: it forms a coherent structure with self-similar shape during the fall and grows by turbulent entrainment of ambient fluid. An integral model based on the entrainment assumption, and adapted to buoyant vortex rings with initial momentum, is consistent with our experimental data. Such results provide the relevant framework for the development of geochemical core formation models that incorporate fluid dynamic constraints.
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Submitted on : Monday, February 20, 2017 - 6:30:16 PM
Last modification on : Saturday, September 4, 2021 - 3:02:02 PM
Long-term archiving on: : Sunday, May 21, 2017 - 3:44:30 PM

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  • HAL Id : tel-01472508, version 1

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Maylis Landeau. Two aspects of fluid dynamics in planetary cores. Geophysics [physics.geo-ph]. Institut de Physique du Globe de Paris (IPGP), France, 2013. English. ⟨tel-01472508⟩

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