I’ve noticed that instabilities develop when using the 4th order accurate 2nd derivative Summation by Parts operator, from the SummationByParts thorn. I’ve coded a simple wave equation evolution thorn that demonstrates the problem, which can be found here: https://github.com/mzilhao/IST_WaveToy. The thornlist used to checkout and compile this thorn is attached.
2nd derivative SBP operators of order 2, 6 and 8 work fine. With order 4, however, there are instabilities propagating from the boundary that crash the code very early on. The code also seems to run fine with the parameter “SummationByParts::sbp_2nd_deriv = no”.
In the repository I've also added the parameter files tried with the different SBP orders (under ./par).