Rayleigh-Taylor instability in confined porous media: pore-scale simulations and experiments

We analyse the convective mixing due to Rayleigh-Taylor instability in homogeneous and isotropic porous media. Convection originates by solute-induced density differences. We investigate the flow at the scale of the pores using numerical simulations, experimental measurements, and physical modelling. Simulations and experiments have been designed to mimic the same flow properties in terms of medium porosity, Schmidt and Rayleigh numbers, and fluids density. We characterise the evolution of the flow via the mixing length, which grows linearly in time. The centre-line mean wavelength is observed to grow, in quantitative agreement with theoretical predictions. Finally, we analyse the mixing dynamics via the mean scalar dissipation: Three mixing regimes are observed. First, the evolution is controlled by diffusion. When the horizontal interfacial diffusive layer is sufficiently thick, it becomes unstable, forming finger-like structures and driving the system into a convection-dominated phase. Finally, when the fingers grow sufficiently to touch the horizontal boundaries of the domain, the mixing reduces dramatically due to the absence of fresh unmixed fluid. We further elucidated the physics of the observed phenomena with the aid of a simple physical model.