[Users] AHFinderDirect?

[Eloisa Bentivegna]

On Jul 18, 2012, at 6:19 PM, Petr Tsatsin wrote:

> Hello, 
> I'm trying to use AHFinderDirect in the simulation of the NSs merger and was wondering if somebody can suggest how to set up the parameters for this thorn?
> More precisely I simulate two neutron stars tracked by moving grids. I can evolve this until merger and simulation fail once BH horizon has formed. I would like to use AHFinderDirect to look for a AH.

Hi Petr,

first off: is the simulation failing due to AHFinderDirect?

> Particularly I have a several questions:
> 1) Do I need 3 centres of refinement in order for AHFinder to work? By 3 centres I meant 2 moving grids and one stationary at the origin. (I just looked at qc0 parameter file as an example) 

If you're moving your grids based on the maximum-density tracker, then AHFinderDirect doesn't have anything to do with them. You can choose how many horizons to look for, where, and when in a completely independent way. You can look at AHFinderDirect's param.ccl to find out about all the different options you can use, but in most cases what's in qc0-mclachlan.par should be all you need:

AHFinderDirect::origin_x                             [1] = +1.168642873
AHFinderDirect::initial_guess__coord_sphere__x_center[1] = +1.168642873
AHFinderDirect::initial_guess__coord_sphere__radius  [1] =  0.25
AHFinderDirect::which_surface_to_store_info          [1] = 0
AHFinderDirect::reset_horizon_after_not_finding      [1] = no
AHFinderDirect::dont_find_after_individual_time      [1] = 30.0

Naturally, depending on where you expect your initial horizon to be, you may also need to use origin_y and origin_z (and initial_guess__coord_sphere__y_center and initial_guess__coord_sphere__z_center). The same holds for the "find_after_individual_time" parameter. Notice the important parameter "which_surface_to_store_info": this tells AHFinderDirect which surface you have set up (through SphericalSurface) to store the horizon's properties. Make sure you request a surface exclusively for this purpose in your parameter file.

> 2) What's going to happen with a 2 moving grids when apparent horizon was formed? Since I use maximum density tracking mechanism to track the positions of the NSs I want to switch the moving grids off after the formation of the apparent horizon and have only one centre of refinement at the origin.

This behavior is also not controlled by AHFinderDirect, but by CarpetRegrid2. The only way I know to trigger grid changes like you describe is to write your own thorn which inherits from CarpetRegrid2 and changes its variables num_levels[i] as desired. Perhaps someone else can give better advice here.

Eloisa