Presentation of the phenomenon

The Raleigh-Benard instability is encountered in horizontal fluid layers that are heated by the bottom.
It has been studied for quite a long time, since the result of this instability is a convection that is visually impressive. Some convection rolls are created due to the circulation of the fluid in the layer.
This movement is basically due to the buoyancy created by the difference of density (resulting from a difference of temperature) between the fluid particles.

The criteria that influence the particle are of two type:

- The destabilizing force that will create movement. It is called the buoyancy or Archimede force.
- The stabilizing process, that is of two type, the viscous stress and the thermal diffusion. If the particle is able to exchange all the heat that it receive, the intern energy will not change and the density will stay the same. In the other case, a buoyancy will result.

It is resumed in the following schema, with FA the buoyancy, FT the viscous stress.

The gravity is vertical and oriented to the bottom.

Here follows a small animation that illustrates simply the mechanism of the instabilities. The only goal is to give a visual example, not a strict description.