Buoyancy driven mixing in confined geometries

We study the spontaneous mixing of two fluids with a density difference characterized by the Atwood number At, and occupying initially (in a gravitationally unstable configuration) half of the length of a tube at an angle θ from vertical. At low θ and high At values, one observes a turbulent flow inducing an efficient mixing across the tube section; at high θ's and low At's, a stratified laminar flow with three layers of different densities appears or, even, a fully separated viscous counter flow. At intermediate values, intermittent flows are observed with sequences of accelerating laminar flows during a fraction of the time increasing with θ. The local structures of the flow and of the concentration distribution have been studied by PIV and LIF techniques. In the inertial regimes, the flow velocity is determined by the concentration contrast at the front. In the turbulent regime, the velocity varies linearly on the tube diameter with a maximum of momentum transfer at the axis. Increasing θ and decreasing At increases the concentration gradient at the tube axis and channels of concentrated solutions of each fluid appear.