[Users] McLachlan BSSN

[John Baker]

Hi,
     Erik mentioned changes that were suggested by Jim van Meter. The 
variant of the BSSN equations that our group has used for the last 
several years is slightly different than what is implemented in the 
trunk version of McLachlan.  I'll just copy Jim's description of his 
original changes:

  1. You are not taking full advantage of the chi=exp(-2phi) variable. 
There are several terms you divide by chi or chi2 in expressions with 
overall factors exp(-4phi). I rewrote the BSSN equations to make these 
cancellations before coding. So where you have an expression of the form 
chi2(A+B/chi2), I have chi2A+B. This gives a slight but noticeable 
advantage in both accuracy and performance.
  2. I added Hamiltonian-constraint-damping terms due to Duez et al. 
These terms don't seem to be well-known but they are effective.
  3. I added a Gamma-constraint-damping term due to Yo et al.
  4. I enforce det(g)=1.

Unfortunately McLachlan has evolved for a year since Jim produced his 
version.  Recently I have merged Jim's changes with the current 
(release) version, though I haven't yet verified the results in 
simulation tests.

John.

On 1/29/13 12:38 PM, Peter Diener wrote:
> Hi all,
>
> On Tue, 29 Jan 2013, Erik Schnetter wrote:
>
>> Please keep all discussion on the McLachlan BSSN code on this mailing list.
>> This ensures that everybody knows about everything that is going on, and
>> avoid duplicate work.
>>
>> At the moment, we seem to have approximately three different versions of the
>> BSSN code that seem to be incompatible:
>> - the (official) trunk version
>> - a version (potentially faster and more accurate) by Jim van Meter
>> - a more flexible version (regarding gauge conditions) by Peter Diener
>>
>> I would suggest that we discuss things on this list before we proceed.
>>
>> John, Peter, could you describe your changes here?
> Sure. In response to a long standing ticket #590, I split the gauge
> evolution routines out from the main RHS routines, in order to be able
> to schedule gauge evolution routines conditionally on the values of
> lapse_evolution_method and shift_evolution_method instead of
> unconditionally always evolve the gauges.  This should allow us to evolve
> spacetimes with static gauges or gauges set by other thorns (for example
> EinsteinExact). It also makes it easier to add new gauge evolution
> routines to McLachlan and have the code be more readable. In the process I
> also moved the advection terms into the new gauge evolution routines since
> these should only be added if McLachlan is actually evolving the gauge.
> The dissipation routine was handled similarly.
>
> Cheers,
>
>     Peter
>
>> -erik
>>
>> --
>> Erik Schnetter <schnetter at cct.lsu.edu>
>> http://www.perimeterinstitute.ca/personal/eschnetter/
>>
>>
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