[FLASH-USERS] Avoiding runaway cooling

[Jesús Zavala Franco]

Hi John,

Thanks for your answer. I agree that the best would be to introduce a
feedback model. In fact, it would be best to have a whole SF and feedback
model, but then again I think this could be hard to implement since FLASH
doesn't have default units for this. Besides, for our purposes, we are
mostly interested to what happens to the hot gas in the clusters (actually
they are not in the cluster mass range but rather in the groups regime)
during their interaction, particularly to the region around the
"smaller-mass" group in its orbit through the more massive group and how the
properties of the gas change through mixing, so it is this mixing regions
where we want to concentrate. Cooling might be potentially important there
as well.

Regarding option 1), do you actually mean setting a floor to the temperature
or internal energy overall, such as the cooling time doesn't drop below
certain value? is this equivalent to setting a minimum time step?

Regarding option 2), I will check if there is a heat point source unit in
the supplementary units of FLASH.

Cheers and thanks again,
Jesus

2011/8/17 John ZuHone <jzuhone at milkyway.gsfc.nasa.gov>

> Jesus,
>
> Unfortunately feedback is the only "correct" way, physically speaking.
> However, depending on your needs, there are things you could do that would
> allow the simulation to proceed faster, though you won't be getting the core
> regions right. here are some ideas, in order of increasing code complexity:
>
> 1) you could simply impose a floor for the cooling function at some
> temperature/internal energy.
>
> 3) you could set up some kind of "fake feedback", such as heat deposition
> from a point source at the center.
>
> 3) you could require that dense, cold cells of gas be condensed into
> massive particles, thereby keeping the same amount of gravitating mass but
> prohibiting the high densities and low temperatures that cause the short
> cooling times.
>
> Using these ideas, you will not likely be reproducing core properties
> correctly. It depends on how much that matters to you. Incidentally, the
> cooling times outside the core will be very long, and they will likely not
> be very different with cooling on or not. Could you elaborate on the need
> for including radiative cooling in the simulation? Maybe then we could zero
> in on the best option.
>
> Best,
>
> John ZuHone
>
> Sent from John ZuHone's iPhone
>
> On Aug 17, 2011, at 7:08 PM, Jesús Zavala Franco <jzavalaf at uwaterloo.ca>
> wrote:
>
> > Dear all,
> >
> > I am trying to setup a cluster merger simulation (gas + dark matter)
> including radiative cooling. For that purpose I have used the
> > supplementary unit for Cooling for FASH 3.3 using the cooling tables of
> Sutherland & Dopita (1993).
> >
> > My main interest is in the evolution of the properties of ICM quite far
> from the center of the clusters.
> >
> > The inclusion of cooling seems to be working fine, but of course, the
> cooling time in the center of the clusters is very low (~1Myr)
> > after running the simulation for just a fraction of the merger time,
> after this, the time steps become too small to continue with the simulation.
> >
> > Has anyone tried to implement cooling only in selected regions of the
> simulation? or can you think of a way to avoid such small
> > cooling times in the center of the clusters? Of course feedback would
> prevent this, but I'm trying to keep the amount of physics
> > that goes into the code to the minimum.
> >
> > Any help will be appreciated,
> > Jesus Zavala
> > University of Waterloo
>
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