Rayleigh Taylor instability

[einatlev]

Hi all
I am trying to construct what I thought would be a very simple model -- 1 2D 1x1 box, two layers of fluids, high density on top. Obviously, and unstable situation (assuming some random noise at the interface).
Sadly, it is not working!!!
What I did, which might be the wrong thing to do, is:
define two bodies and two materials, and assign the high density to the "body" on top.
What i get is -- total blow-up of the solution!!!

Attach is the SIF file and mesh.elements file. The other mesh files are the same as the ones in fem/examples/tutorial_files/RayleighBenard/Mesh

Thanks!!