[FLASH-BUGS] Dual Energy and Poor Collapse Results

[Colin McNally]

Hi,
    Mike Zingale has told me about how the eint_switch can be dangerous 
at it's default seetting, we saw this in a case of a sedov explosion 
with very odd behaviour due to forced refinement. I have found  that on 
the Evrard collapse problem (a standard test for SPH codes) this same 
problem arises. The Evrard problem is a spherical distribution with 1/r 
density profile and a cold temperature which collapses with gravity. I 
have attached to this mail a tarball of the setup directory I used. The 
problem is described in 1988MNRAS.235..911E and 1993A&A...268..391S 
(those are adsabs bibcodes).

I ran this problem with eint_switch=1e-4 and have plotted the energy, as 
outputted in flash.dat here:
http://imp.mcmaster.ca/~colinm/FLASH/evrardenergy.png
Note the massive energy loss, and resulting crash of the code.

I also ran this problem, with eint_switch=0 and plotted the same, here:
http://imp.mcmaster.ca/~colinm/FLASH/evrardenergy2nde0.png
That's about a 7% gain.

The difference between these two suggest again that something is wrong 
with the dual energy formulation used in FLASH.  My collegue (James 
Wadsley) noted that it's worrying that a divergance is calculated 
seperatly in each direction of the PPM update in update_soln.F90.

Also, the results for this test are rather poor in their spherical 
symmetry. I have a scatter plot of the cell densities by radius here:
http://imp.mcmaster.ca/~colinm/FLASH/evrarddensscatter2nd08.png
Ideally, of course, this would be a thin curve instead of being so 
smeared. Do you have any recommendations that might improve the 
performance of this test, while not doing anything unrealistic for a 
cosmology-type simulation (i.e. can't use spherical coordinates). SPH 
codes can give much better resultsd than this for this problem, is there 
any particular reason why this test might be "unfair" towards AMR 
techniques?

-Colin McNally
McMaster University



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