Abstract
The Noise thorn can be used to place random values on Cactus grid functions at initial data and
at the boundaries during evolution. This can be used to carry out “robust stability” tests, such as
those proposed by Jeff Winicour.
To apply a random perturbation to initial data, set noise::apply_id_noise="yes". Then each grid function listed in the parameter noise::id_vars will be adjusted by a random factor. The maximum size of the random perturbation is controlled by the parameter noise::amplitude. The perturbations are applied during the CCTK_POSTINITIAL time bin.
A random number will be added to each point on the boundary of grid functions listed in the noise::bc_vars parameter if the flag noise::apply_bc_noise="yes" is set. As with the initial data, the maximum size of the perturbation is given by the noise::amplitude parameter. The adjustments are applied at each CCTK_POSTSTEP.
The following parameters can be used to apply a random adjustment of size to the initial data and boundaries of the metric variables.