[FLASH-USERS] hydrostatic models in flash

[Philip Chang]

Hi everyone,

I'm working with the new flash 4 in 2.5 d simulating a white dwarf at 
sits in hydrostatic balance; the white dwarf has significant rotation 
and hence the 2.5 d.  However, I am finding that when I put in a 
hydrostatic white dwarf, the white dwarf initially oscillates a little, 
but soon these oscillations grow to be very violent over a few hundred 
seconds.  Obviously there is a source of energy that is being tapped.

Looking at Zingale et al (2002, ApJ 143, 539), it seems that one thing 
might be that these oscillations arise from the imperfect cancellation 
between pressure and gravity.  What he did was to subtract out \rho g 
from pressure gradient before calculating the state.  This was 
implemented in the PPM code and can be turned on with the 
ppm_modifystates = .true. flag.

As I am using 2.5 d (and hence, the unsplit solvers), I don't have this 
same luxury.  However, as I am reading FLASH, it seems that this 
cancellation may be built in.  In particular, for the unsplit solvers, 
it seem that gravity is used to calculate the various fluxes.  But I am 
not sure.  Is there some combination of use_gravHalfUpdate, 
use_gravConsv, or use_GravPotUpdate in the unsplit solver that would 
have the same effect as ppm_modifystates = .true. in the split PPM solver?

Cheers,
Phil