[Users] GW150914 example, noisy Psi4 waveforms

Thu Mar 30 17:36:41 CDT 2017

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 <cott at tapir.caltech.edu
> <mailto:cott at 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:     cott at 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:  joann at caltech.edu; +1 626 395-4280
==================================================
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fig3.png
Type: image/png
Size: 66223 bytes
Desc: not available
Url : http://lists.einsteintoolkit.org/pipermail/users/attachments/20170330/8508079f/attachment-0001.png 