[Users] Instabilities with Summation by Parts 4th order operator

[Peter Diener]

Hi Miguel,

I have commented on the ticket you created about this. Please take a 
look at that comment and let me know what you think.

Cheers,

   Peter

On Thursday 2020-04-09 13:08, Miguel Zilhão wrote:

>Date: Thu, 9 Apr 2020 13:08:47
>From: Miguel Zilhão <miguel.zilhao.nogueira at tecnico.ulisboa.pt>
>To: Einstein Toolkit Users <users at einsteintoolkit.org>
>Subject: [Users] Instabilities with Summation by Parts 4th order operator
>
> hi all,
>
> i've been working on implementing the Summation by Parts (SBP) operators on 
> some evolution codes i have and i was seeing some instabilities when using the 
> 4th order accurate 2nd derivative operator.
>
> in order to try to understand the problem better, i've coded a very simple 
> thorn that evolves the wave equation in first-order in time, second-order in 
> space form. i can evolve the system just fine without any problems with the 
> SBP operators of order 2, 6 and 8. with order 4, however, there are 
> instabilities propagating from the boundary that crash the code very early on. 
> has anyone seen something like this before? could there be a bug in the 4th 
> order 2nd derivative SBP operators, or am i doing something wrong? i find it 
> very strange that it only crashes with the 4th order accurate 2nd derivative 
> SBP stencils (the code also seems to run fine with the parameter 
> SummationByParts::sbp_2nd_deriv = no)...
>
> i've uploaded this thorn (called IST_WaveToy) here: 
> https://github.com/mzilhao/IST_WaveToy
> i tried to keep everything as simple as possible. i'm attaching the thornlist 
> used to checkout and compile the code, and in the repository i've also added 
> the parameter files i tried with the different SBP orders (under ./par).
>
> any help or comments would be appreciated.
>
> thanks,
> Miguel
>