Rayleigh-Bénard Convection example

[OresteMarquis]

The video shown in the example illustrating the time evolution of the convective flow has the y-axis flipped for both fields (i.e., velocity and temperature).

I believe that the most interesting aspect of Rayleigh-Bénard instability is the formation of vortices. However, the choice of properties in this example causes the vortices to almost fully develop in the first half of the simulation, and the current output frequency makes it difficult to visualize this process. It might be worthwhile to redesign the example to better capture the transient behavior.