[FLASH-USERS] Domain issues in the core-collapse problem

[James Guillochon]

Hi Sean and Cole,

Correct me if I'm wrong, but I thought there was a hacky way to achieve
homologous expansion of the grid through the Cosmology unit, where one sets
all the lambdas to zero? I've never tried it myself, but I had heard that
this was possible.

On Fri, Nov 7, 2014 at 10:53 AM, Sean Couch <smc at flash.uchicago.edu> wrote:

> Hi Cole,
>
> 1. Excise an inner region with radius less than some value, and simply
> compute the gravity from the excised mass as a point source. This would
> seem to involve both modifying the gravity solver to allow xmin > 0
> (perhaps not possible with the multipole solver), and creating a hybrid
> between the poisson gravity solver and the point mass gravity
> implementation. My main interest will be in the outer regions of the star,
> so I don’t want my timestep to be limited by the inner regions.
>
>
> The most direct way of doing this is to, as you say, set xmin > 0 in
> spherical coordinates.  The New Multipole solver (of Couch, Graziani, &
> Flock 2013) doesn’t currently support this behavior but the old Multipole
> solver does.  And since you are doing 1D there really isn’t any benefit in
> using the new solver.  The old Multipole solver also already includes the
> addition of a point mass potential on top of the self-gravity of the fluid
> on the grid.  Just set the runtime parameter point_mass to the mass, in
> grams, of your mass cut and you should be good to go.
>
> There are other approaches to doing what you want, but this is the most
> direct.  If you’re happy using spherical coordinates in 3D, then the
> transition from 1D to 3D should be relatively painless.
>
> 2. Have a moving grid, so that the domain can expand over time. I haven’t
> seen any mention of this capability in FLASH, so I’m guessing it isn’t
> possible, but I thought I’d ask anyway.
>
>
> This had been tried long ago in the split PPM solver with little success.
> My solution to this (Couch et al. 2009, ApJ, 696, 953) was to periodically
> interpolate the solution on to an expanded domain.  Matching the solution
> to the new initial conditions for the regions that are added in this can be
> tricky, if you’re not careful.  Another approach (Couch et al. 2011, ApJ,
> 727, 104) is to simply brute-force it and include the entire domain of
> interest from the very start and allow the AMR to help you.  You can limit
> the max refinement level as a function of radius and you can also reduce
> the global max refinement level as a function of time, allowing you to
> derefine the inner core and increase your time step size.  Check out the
> runtime parameter descriptions for the Grid unit here:
>
>
> http://flash.uchicago.edu/site/flashcode/user_support/rpDoc_4p22.py?submit=rp_Grid.txt
>
> And pay particularly attention to gr_lrefineMaxRedDoByTime,
> gr_lrefineMaxRedDoByLogR, and friends.  Using this, I was able to follow a
> developing supernova explosion all the way from the core to shock breakout
> and beyond in 2D.  Anecdotally, I found that each decade in radius included
> in the sim was roughly the same cost as the previous decade (i.e., the
> expense scaled logarithmically, not linearly).
>
> Good luck,
> Sean
>
>
>
> -----------------------------------------------------------
> Sean M. Couch
> Sr. Postdoctoral Scholar
> Theoretical Astrophysics Including Relativity
> California Institute of Technology
> Mail Code 350-17
> Pasadena, CA  91125
> (626) 395-4282
> www.flash.uchicago.edu/~smc
>
>


-- 
James Guillochon
Einstein Fellow at the Harvard-Smithsonian CfA
jguillochon at cfa.harvard.edu
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