Guide to parameters

Although many of the parameters in the params.txt file are self explanatory and commented, this page aims to provide a description of the key parameters for GRChomboFixedBackground.

The GRChombo parameter guide can be found here, which will cover the majority of the variables.

Guide to specific parameters

For the purposes of this page, we will refer to the params.txt in the current Kerr black hole example.

Important parameters for fixed backgrounds

The most important parameters are the ones that control the regridding. If you want to fix the mesh hierarchy, you will need to set the regrid intervals to zero, i.e.

• regrid_interval = 0 0 0 0 0 0 so that the regridding only happens for the initial data. Note that the default for
• tag_buffer_size = 0 so that the tagged zones are not extended. This is because the fixed background code tags square boxes, and we want their size to be specific.

Initial Data parameters

• scalar_mass = $M \mu$ sets the ratio between the scalar field mass, $\mu$, and the mass of the BH $M$.
• scalar_amplitude is the initial the amplitude of the field
• bh_mass is the mass parameter of the BH, $M$, usually set to 1.
• bh_velocity in the case of a boosted BH, is the velocity of the boost, in units of c.
• bh_spin is the BH spin parameter a in the case of a Kerr metric - the dimensionless spin is then $a/M$.

Diagnostic extraction parameters

• Set activate_extraction = 1 if you wish the integrals to be computed while running your simulations, 0 otherwise (this can also be done in post-processing).
• Setting `write_extraction = 1' will output a file showing the integrand values across the spherical grid at each timestep. This is off by default as it creates a lot of output.

For volume integrals

The value of the diagnostics variables at radii smaller than

• inner_r

and larger than

• outer_r

are set to 0.

For flux (surface) extraction

• num_extraction_radii is the number of different radii at which you wish the surface integral to be calculated
• extraction_radii are the values at which the fluxes will be calculated, with the defaults set to inner_r and outer_r

See the guidance at Extraction and integration for more details on the integration of variables and how to change them depending on your problem.