Diffusive Convection - Bobbing Henry


To demonstrate the growth of diffusive convection

What Happens:

A tank of water is stratified with crystals of salt on the bottom, and ice cubes near the top. The tank is heated from below, making a stratification that is cool and fresh near the top and warm and salty near the bottom. A small container is weighted to float upside-down, and filled with just enough air to remain close to neutrally buoyant. After a few seconds the float rises, then falls, only to repeat the cycle indefinitely.

Physics of the Phenomenon:

This is an example of a double-diffusive instability (Turner, 1973) called diffusive convection. Starting near the bottom, the float picks up heat from the warm surroundings, which causes the air to expand and the float to rise. When the float nears the top, it gives up the heat, the air contracts, and the float sinks, only to begin the cycle again. As it shuttles up and down, the float transfers heat upwards.

The same instability occurs with water parcels in such a stratification. The key to the instability is the fact that heat diffuses much more rapidly than salt (hence the term double-diffusion). The water parcel picks up heat, rises, loses heat, and falls, so transferring heat upwards just as the float does. Theoretical calculations for this situation (Turner, 1973) show that water parcels undergo a growing overstable buoyancy oscillation (see Internal waves 1), driven by heat diffusion.

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!

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

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