[Users] ReflectionSymmetry, tensor tags, ML_BSSN, etc.

Tue Nov 23 16:09:08 CST 2010

Hi Erik.

The following statement seemed strange to me:

"In our counting, a tensor density of weight 1 is a regular tensor,
so the definition of K should be fine."

The only density-weight convention I've seen has w = 0 for
(non-densitized) tensors. Is the Cactus convention just a uniform shift of
1 from what I'm used to, or something more complicated? And where did it
come from?

Also, looking at the CoreDoc PDFJohn linked to, the "tensorweight"
description states:

"If omitted, most code will use a default weight of 0."

Presumably this statement is incorrect now, as you'd want the default
behaviour to be that of a normal (non-densitized) tensor, right?

Bernard

On 11/23/10 4:52 PM, "Erik Schnetter" <schnetter at cct.lsu.edu> wrote:

>John
>
>
>Most symmetries are rigid rotations, and tensortypealias is the only
>tag you need to declare. The other tags are for general coordinate
>transformations such as applied e.g. in our multiblock infrastructure
>to interpolate between patches. In our counting, a tensor density of
>weight 1 is a regular tensor, so the definition of K should be fine --
>but as you note, most symmetry conditions ignore the tensor weight
>since it does not influence rigid rotations. The McLachlan arrangement
>(see <http://einsteintoolkit.org>) is a good example since it declares
>tensor properties and is used in production runs, so is likely to be
>correct.
>
>The tensor parity may be useful for magnetic fields, depending on how
>you declare them -- I hope this tag is taken into account as well.
>
>-erik
>
>PS: If there is documentation missing, feel free to help us out, since
>you are now in the unique position to judge what information would be
>helpful for a first-time user of these symmetries. Alternatively, open
>a ticket at <https://trac.einsteintoolkit.org/>.
>
>On Tue, Nov 23, 2010 at 4:29 PM, John Baker <john.g.baker at nasa.gov> wrote:
>> Hi,
>>     We've run into some problems recently with our evolution
>> code, which boil done to the fact that we'd developed the
>> code to support the sort of reflection symmetry defined
>> in cartgrid3d, but recently started working with other thorns
>> which are expecting the use of ReflectionSymmetry to define
>> reflection symmetries.
>>      I assume that the motivation for this transition was to
>> support more general grids, multipatch codes, etc, but I
>> haven't found any documentation for the ReflectionSymmetry
>> thorn, or any explanation of its use.
>>     One likely problem we are having is that we haven't applied
>> any tensor-related tags in our interface.ccl, and rumor has it
>> around here that ReflectionSymmetry relies on these tags.
>> Presumably there are other EinsteinToolkit thorns which rely on
>> tensor tags as well.
>>     I did find one document giving some explanation of tensor
>> tags
>>
>> http://cactuscode.org/documentation/CoreDoc.pdf
>>
>> (thanks Erik).  Erik outlines a number of potential tags there, but
>> I would like to know which of these are required to support
>> interaction which the various thorns in the toolkit?
>>
>> Looking at various other interface.ccl files, there didn't seem to be
>> a high degree of consistency about which tags are set.  Some
>> also naively appear to be set incorrectly, in ML_BSSN.  For instance:
>>
>> CCTK_REAL ML_trace_curv type=GF timelevels=3
>> tags='tensortypealias="Scalar" tensorweight=1.0000000000000000000'
>> {
>>   trK
>> } "ML_trace_curv"
>>
>> Which seems to declare the trK to be a tensor density.  If this is
>> indeed wrong,
>> and no one has noticed, then I infer that tensorweight may not really be
>> used
>> by anything important.
>>
>> If anyone can clarify these issues for me I would appreciate it.
>> Thanks,
>> John.
>>
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>>
>
>
>
>-- 
>Erik Schnetter <schnetter at cct.lsu.edu>   http://www.cct.lsu.edu/~eschnett/
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