Diffusive Convection - Layer formation


To demonstrate the growth of diffusive layering

What Happens:

A tank of water is stably stratified with salt solution. The tank is heated from below, and a series of convecting layers separated by sharp diffusive interfaces forms.

Physics of the Phenomenon:

The heating at the bottome of the tank causes a convecting layer to form that is warmer than the water above. Molecular heat diffusion in the region above the layer makes a warm-salty under cool-fresh gradient region, and this eventually goes unstable and breaks down, forming a new convecting layer above the original one. Then molecular heat conduction continues into the region beyond, causing the system to repeat the cycle indefinitely.

The heat flux is large and upwards, while the salt flux is quite small because of the slow diffusion of salt. The net result is a downwards density flux, dominated by the heat flux. In terms of eddy diffusivities, the effective salt and heat diffusivities are positive (i.e., downgradient), but the density diffusivity is negative -- an upgradient flux! Consider the diffusion equation for density, with a negative diffusion coefficient. This is equivalent to ordinary diffusion with time running backwards; instead of perturbations "diffusing away", they grow instead, causing the stratification to develop layers.

References: Turner, J.S., Buoyancy Effects in Fluids, Cambridge University Press, Cambridge, U.K.368 pp.,1973.


Movie and text - Barry Ruddick
Digitization of movie - Dave Hebert

Load and run layer formation movie