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.
Best, - Christian On 3/27/17 12:37 AM, Ian Hinder wrote: > > On 27 Mar 2017, at 04:06, Christian D. Ott <c...@tapir.caltech.edu > <mailto:c...@tapir.caltech.edu>> wrote: > >> Hi All, >> >> I have been experimenting with the GW150914 par file (Thanks for putting >> it together!), which uses Llama multipatch (MP) with McLachlan (ML) and, >> of course, Carpet AMR in the interior Cartesian patch, tracking the >> punctures. >> >> I'm noticing three things when looking at Psi4 l2,m2 waveforms (and this >> is largely independent of extraction radius): >> >> (1) The waveforms have high-frequency wiggles that similar pure-AMR >> simulations do not show. I don't have a pure AMR simulation for >> GW150914, but am comparing with an 8-orbit, equal mass, non-spinnning >> case. I can run GW150914 with pure AMR if need be. > >> (2) The high-frequency wiggles get *worse* if I decrease the finite >> differencing order from 8th to 4th order. >> >> (3) This appears to be a wave amplitude-dependent 'feature', since the >> wiggles are much less pronounced once the waves reach higher amplitudes. >> >> I'm attaching two gnuplot screenshots: fig1.png shows the first ~800 M >> of Psi4 l2,m2 real at r=136 M with the stock par file and with a >> modified par file for 4th order. fig2.png is a zoom-in. >> >> I'm also attaching the par file that I'm using for the 4th-order run. >> >> Now my questions: Does anybody have an idea where the high-f noise is >> coming from and why it's getting worth for lower FD order? Any >> suggestions on how to mitigate it? > > Hi, > > Yes, I have observed this as well. I saw it originally with CTGamma > when I first started using Llama, and now see it with McLachlan. I > believe I have looked at 2D movies of Psi4r and seen the junk radiation > reflecting off the interpatch boundary at r ~ 45 M, hitting the BHs, and > causing them to emit high frequency noise in the waves. > > Note that the boundary condition "generating" the reflections in this > case would be the Cartesian boundary at x^i = 45 M, which takes its data > from the angular grid. I'm not sure why this is worse than in a pure > Cartesian run, but it might be because the high frequencies are > dissipated away by being under-resolved in the wave zone before they get > to the extraction spheres in the Cartesian case. > > I think this can be improved by moving the spherical inner radius from > 40 M to something larger, e.g. 80 M. I have also tried adjusting the > angular resolution, but this doesn't seem to help very much. Another > option is to switch to using constant Courant factor, which will give > lower time resolution in the wave zone, and hence reduce the highest > frequency oscillations. > > There are two possible explanations for why the wiggles get worse once > the waves reach higher amplitude. One is that the noise is sourced only > once, initially, from the junk radiation, and the effects simply > diminish with time, so by the time the wave amplitude is large, you > cannot see it any more. However, I think I have observed that the noise > is much worse in longer runs, indicating that instead, perhaps the > reason is that the noise has the same amplitude a certain time after the > start of the run, independent of the separation, but for longer runs, > the real waves are weaker, so the noise is relatively stronger. The > noise amplitude being related to the junk amplitude would fit with this. > > I don't know why the noise would get worse with 4th order. A 4th order > run is not only different by the finite differencing order. It also has > fewer ghost (and hence buffer) zones, and is usually run with lower > order dissipation. It may be interesting to run the 4th order case with > everything else the same as in the 8th order case (i.e. with 5 ghost > zones, and 9th order dissipation) to see if it is really the finite > differencing order, or something else. By changing the number of buffer > zones, the grid structure can change dramatically in some cases. > > In your plot, I can't actually see any wiggles in the 8th order case. > > It would be really good to find a solution to this. > > -- > Ian Hinder > http://members.aei.mpg.de/ianhin > -- ================================================== Christian D. Ott: c...@tapir.caltech.edu TAPIR 350-17 Caltech, Pasadena, CA 91125 http://www.tapir.caltech.edu/~cott Phone: +1 626 395-8410; Administrative Assistant -- JoAnn Boyd: jo...@caltech.edu; +1 626 395-4280 ==================================================
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