Experimental investigation on Rayleigh-Taylor instability in confined porous media

Experiments in Hele-Shaw cells are used to mimic the problem of Rayleigh-Taylor instability in confined porous media. Rayleigh-Taylor instability phenomena arise when a layer of heavy fluid sits on top of a layer of a lighter fluid. The flow is initially controlled by diffusion, but rather quickly the action of gravity produces efficient fluid mixing in the entire domain. Small fluctuations of the interface separating the two fluid layers produce larger structures that eventually drive the flow into a nonlinear convective stage. The competition between buoyancy and diffusion is measured by the Rayleigh-Darcy number, the value of which controls the entire dynamics of the flow. We employ an optical method to obtain accurate measurements of the solute concentration and we perform experiments for a wide range of Rayleigh-Darcy numbers. The fluids consist of water and an aqueous solution of potassium permanganate. We characterize the flow evolution with the mixing length (a suitably defined extension of the mixing region) and with the degree of mixing of the two fluids. In this work, mixing is quantified by the mean scalar dissipation rate. The results obtained are compared against previous experimental and numerical works.