Rayleigh-Taylor induced diffusive mixing in vertical tubes
Résumé
Rayleigh-Taylor induced mixing is studied experimentally in long vertical tubes (4 m length, 20 mm diameter) for a broad range of Atwood numbers (2 × 10-5 <= At <= 10-1). The axial concentration profiles follow an accurately diffusive macroscopic spreading law over several decades of At values (2 × 10^4 <= At <= 10-1). The corresponding diffusion coefficient D is the product of the velocity v of the internal fluid motions and of a characteristic length l. At low increasing At values, the decrease of l balances the increase of the velocity v to achieve a constant D value. At high At values (At > 10-2) l reaches a constant value scaling with the tube diameter and D increases linearly with At. Increasing the viscosity μ of the fluids and reducing the tube diameter d both narrow the domain of diffusive behaviour. D decreases only by 30% when d is varied from 20 to 8 mm and increases with μ. At very low At values one observe a transition towards a stable non-diffusive counterflow of the two fluids.