Modelling precipitation-driven flows in planetary interiors: effects of particle inertia and dissolution*

As an analog of settling snow flakes in planetary interiors which may drive large scale motions in liquid metal, we instantaneously release clouds of glass spheres in a water tank, which quickly evolve as turbulent thermals. Synchronous cameras analyse the two-way coupling between the settling spheres and the fluid : one camera performs a Lagrangian tracking of particles while the other one analyses turbulence with PIV or LIF. The size of particles is systematically varied in the range 5µm-1mm. We notably show that due to particle-turbulence interactions, particle clouds entrain more than salt water clouds of identical mass excess, with an optimum of entrainment for a finite inertia, as confirmed by our 3D DNS of the same clouds using Basilisk.

Subsequently, glass spheres are replaced with grains of custom-made sugar to model melting of the snow flakes by dissolution in water. Grains of controlled size are continuously sieved above water with a chosen mass rate. As grains fall and dissolve, they impart momentum and buoyancy to the fluid. We analyse the growth of the resulting mixing layer, which eventually collapses due to Rayleigh-Taylor instabilities, producing a macroscopic plume which carries grains to large depths and nourishes an inhomogeneous compositional convection.