Hi Ian,
Thanks for your reply. Good to know that others have the same problem.
I've done a 4th order run (with appropriately reduced # of ghost zones
etc.) with the Cartesian/Curvilinear boundary out at ~86. This indeed
reduces the oscillations, but only slightly. I was hoping for a bigger
effect. See attached plot comparing the 4th order run with
sphere_inner_radius at 51.4 with the new run at 85.66.
Hmm. That's a bit disappointing. In your original plot, I couldn't see any oscillations at all in the 8th order case, though I have seen them in my runs when plotting the frequency (which is yet another derivative). Can you see oscillations in psi4 itself in the 8th order case?