[Users] TwoPunctures and [xyz]_offset

[Ian Hinder]

On 24 Jun 2010, at 09:24, Erik Schnetter wrote:

> On Jun 24, 2010, at 1:23 , Ian Hinder wrote:
>
>> On 23 Jun 2010, at 14:25, Frank Loeffler wrote:
>>
>>> On Wed, Jun 23, 2010 at 02:06:13PM -0400, Ian Hinder wrote:
>>>> I'm confused about what parameters you are talking about.  In
>>>> EinsteinInitialData/TwoPunctures/param.ccl, there is a parameter
>>>> center_offset[i].  Is this the one you are referring to?
>>>
>>> That is right.
>>>
>>>> We do use this parameter for unequal mass BBH simulations.
>>>
>>> I suspected so.
>>>
>>>> If so, in what way is it broken?
>>>
>>> It changes the grid functions x, y and z by that offset. You might  
>>> be
>>> lucky and Carpet might overwrite that before the first time step
>>> though.
>>>
>>> Do you still get the same results of you comment out the lines  
>>> around
>>> 621 in TwoPunctures.c (the whole loop basically)?
>>
>> I computed initial data before and after this change and wrote the
>> BSSN conformal factor to HDF5.  I then used h5diff to compare the
>> resulting files.  There were no differences apart from the build-id
>> and simulation-id.  However, the coordinates themselves *do* show
>> differences, which I need to look into in more detail.  The only  
>> thing
>> I can think of in a BBH simulation which uses the coordinates
>> gridfunctions is wave extraction, as they are often used to compute
>> the tetrad.  Are they also used by the interpolator?
>
> No, the coordinate grid functions are only used by initial data.   
> The PUGH and Carpet interpolators assume coordinates that can be  
> expressed as "offset with delta", which is much more efficient than  
> looking at grid functions.

Good!

> Since one knows during wave extraction where the extraction sphere  
> is located, the coordinate grid functions are probably also not used  
> there (but this depends on the code).  I assume that one would not  
> interpolate coordinates (since this should be an identity),  
> therefore errors in the coordinate grid functions should not matter  
> there either.

Psi4 is a contraction of the Weyl tensor with a tetrad.  The  
components of this tetrad are usually based on the coordinates, and  
the coordinate gridfunctions are typically used for this (at least in  
WeylScal4) since this is a pointwise operation at each point in the  
grid.

-- 
Ian Hinder
ian.hinder at aei.mpg.de