Self-similar concentration profiles in buoyant mixing of miscible fluids in a vertical tube

The influence of the density contrast (characterized by the Atwood number A t) on gravity-induced mixing between two miscible fluids in a long vertical tube has been studied experimentally. Cross-section averaged fluid concent ration profiles along the tube are measured optically: for large enough At values, they display a self-similar dependence in a broad range of times an d distances and verify a diffusion law with an effective diffusivity 10(5) times higher than for molecular diffusion. At lower At values, this diffusi ve domain is limited by a sharp front moving at a velocity increasing with At. Below a threshold At value the diffusive behavior disappears. (C) 2001 American Institute of Physics.