[Users] McLachlan BSSN

John Baker john.g.baker at nasa.gov
Mon Feb 18 11:19:46 CST 2013

Hi Peter,
     I can. I'd like to test it out a little bit though. Give me a day 
or two.

On 2/18/13 10:59 AM, Peter Diener wrote:
> Hi John,
> If you can give me a patch against the current release version of Jim's
> changes, I can attempt to generate a patch against the development version
> that can be applied separately from my patch regarding the gauge
> evolution. The patches could then be evaluated, tested and
> potenitally applied separately.
> Cheers,
>     Peter
> On Tue, 29 Jan 2013, John Baker wrote:
>> 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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