two gas mixing
 

Hopefully someone can give me some help on what is happening:

I'm modeling gas mixing in 2-D. I have a heavy cold fluid in a volume (area) above a lighter hot fluid, they are connected by a small opening. I'm doing a time-dependent calculation to see how the fluid mixes due to the buoyancy difference. I'm using incompressible ideal gas, and my gravity is on.

When I run it the concentration contours looks as you'd expect. But the temperatures are strange. At later time steps I get some cells (just a few) with a temperautre much higher than the initial hot temp, and I get some temps lower than the initial low temp.

If I lower my time step, or lower under-relax for energy, or run more inner iterations/ time step it does better.

But to me it seems like this should be a relatively easy problem, so I'm wondering if there are tips for modeling, and not so much just the solving controls. I'm using PISO and Presto for the pressure, and node based for the gradient.

Also: I have the same problem in Star CCM+.

Thank you much