Gravity induced mixing of miscible fluids in a confined geometry

International audienceGravity induced mixing of miscible fluids initially separated in the two halves of a long vertical tube has been studied as a function of the tube diameter d, of the density contrast characterized by the Atwood number At and of the viscosity ν of the fluids. At higher At values, macroscopically diffusive mixing characterized by a coefficient D is observed while, at lower ones, a stable counterflow regime is observed in which the light and heavy fluids flow up and down in different parts of the tube section. In the diffusive regime, an unexpected increase of both D and the internal fluid velocity Vf is observed as viscosity increases. Experimental values of D/ν at different d and ν follow a master curve when plotted as a function of the Reynolds number Re = (At g d^3/ν^2)^1/2. At high At, minute deviation angles θ of the tube from vertical induce the transition from turbulent diffusive mixing towards the counterflow regime : the transition occurs for θ ˜= 5^o for d=8 mm and At = 10-2