Efforts to learn the FLASH hydrocode continue with an attempt to setup a simple, stable gas disc in a gravitational potential. The dense, cold gas sits inside a very low-density hot medium. The dense gas is not self-gravitating; gravity is only modelled by a fixed potential that simulates a dark matter halo, the gas itself, and the stars. That should be fine as long as the gas is stable.
... but for some reason, it isn't. The top picture shows a complete gas disc which pretty rapidly starts to expand in the center, and then the whole thing collapses. The resident FLASH expert thinks it might be due to the low resolution of the simulation causing artificial viscosity (drag) in the center, so the density builds up and so does the temperature and pressure.
Solution ? Take out a chunk of the gas in the center (second picture down). That should remove the densest gas where the problem is worse, and wouldn't be unrealistic since many spiral galaxies don't have much gas in their centers. But it didn't work.
Oh well. Maybe the hole wasn't big enough. Try a bigger hole (middle) ? Nope. Same problem.
Hmm. Perhaps if we go for broke and try just using a thin ring of gas (fourth one down). Nope. Looks like the hot thin medium (sounds like a dodgy late-night TV channel) is collapsing to very high densities due to the gravitational potential. Why this means the dense gas ring collapses I'm not sure.
In for a penny, in for a pound.. how about removing the dense gas entirely (last one) ? Nope. Same runaway density problem. Well, it's only been a day. And at least they're pretty.
The initial conditions were modelled by my officemate. Different colour schemes were used for each rendering, because I wanted too. I believe the last two use a logarithmic stretch, that's probably why they looks so much worse than the others at the end. Simulations were rendered in Blender.
