Plumes of settling and dissolving sugar grains as a model of snow-driven flows in the lab*

We present a laboratory analog to model snow-driven flows in planetary interiors that are driven by the settling and remelting of snow flakes in a lighter ambient fluid. Our experiments consist in settling and dissolving sugar grains that are continuously sieved above water with various mass fluxes and grain sizes. Through drag and dissolution, these particles force a central plume whose inner structure is analysed in a laser sheet thanks to PIV and the use of home-made fluorescent sugar to track the negatively buoyant sugary water through LIF. The size of grains controls a wealth of behaviours, from a laminar plume that gains intensity over a long transient when forced by the rectilinear fall of large grains, to a turbulent lazy plume that emerges faster when forced by fast-dissolving small grains. This transition is determined by the enhanced forcing imposed by smaller grains, and their ability to nourish a Rayleigh-Taylor-like instability at the plume onset. The two-way interaction between settling and dilution of the buoyant material through advection controls the quasi-steady plume velocity, as well as the maximum depth of dissolution of sugar grains.