An excision region enables the user to remove a singularity from the
computational domain. This idea is used most in numerical relativity simulations
evolving black holes, where the singularity is excised via a mask function. The parameters
controlling the mask usage are:
The size of the mask and the location of the mask is tied to the apparent horizon finder.
- use_mask = 0 [default] No mask is used in the simulation.
= 1 A mask may be needed during a simulation.
- mask_period : this parameter indicates how many fine mesh steps are taken
between masking. [default = 1]
- mask_usemin = 0 [default] turns off the initialization of the horizon finder
using the minimum of a user specified gridfunction and instead uses user inputs on the location
of the mask.
= 1 turns on initialization of the horizon finder using the
minimum of a user designated field (mask_minfield).
- mask_minfield = 1 [default] Number of the field to minimize ( see fields.inc
to related field number to field name )
- mask_type : this parameter is deprecated. In earlier versions, HAD supported
both spherical and cubical excision regions. It now only supports cubical excision regions.
- The maximum number of independent excisions supported is two.
- The mask has it's own grid function, mask. This can be output
into an sdf file by setting output_mask = 1. It has a value less than zero at
excised points and greater than zero elsewhere.
- If findhorizon = 1, the driver will determine the appropriate mask size
for the excision region. See apparent horizon finder.
- If findhorizon = 0, the user can directly control the mask size and location
through the mask_center and mask_radius arrays defined in had/include/mask.inc.
apparent horizon finder
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