Diffusive Convection - Layer formation
Purpose:
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.
![](../pics/Layers.gif)
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.
![](../pics/NegativeK.gif)
References: Turner, J.S., Buoyancy Effects in Fluids, Cambridge
University Press, Cambridge, U.K.368 pp.,1973.
Credits:
Movie and text - Barry Ruddick
Digitization of movie - Dave Hebert
Load and run
layer formation movie