[Users] TwoPunctures and [xyz]_offset

[Ian Hinder]

On 24 Jun 2010, at 09:34, Ian Hinder wrote:

>
> 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.


Since the coordinate gridfunctions are reset on regridding, after the  
first regridding (probably within the first few M) Psi4 is corrected.   
This is visible as a discontinuity in the extracted Psi4 mode  
coinciding with the first regridding that causes the extraction level,  
or the level one finer, to change. See attached plot, where I extract  
at r = 10 M, and the dashed line at t = 1.08, represents the  
regridding.  If the extraction level, or the one finer than it, is not  
ever regridded, then you will have to wait for the first recovery for  
the coordinates to be reset (the coordinates are not checkpointed).

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

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