## Using the Noise thorn

Denis Pollney

April 2002

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.

### 1 Initial data

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.

### 2 Boundary conditions

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.

### 3 Example

The following parameters can be used to apply a random adjustment of size
$A=\pm 0.0005$ to the
initial data and boundaries of the metric variables.