[Users] GW150914 example, noisy Psi4 waveforms

[Ian Hinder]

On 31 Mar 2017, at 16:38, Christian D. Ott <cott at tapir.caltech.edu> wrote:

> Hi Ian,
> 
> On 3/30/17 23:09, Ian Hinder wrote:
>> 
>> 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?
> 
> Yes, they are there in the 8th-order case, just much weaker than in the 4th-order case. See attached plot. Perhaps, understanding why lower order leads to worse oscillations can guide us to their root cause?

Hi Christian,

You are using the same dissipation parameter setting for the 4th and 8th order cases.  I chose the 8th order value as the maximum that gave me stable evolutions with 8th order.  It's possible that the effective dissipativity for a given epsdis is lower for the 5th order dissipation used in the 4th order case, than for the 9th order dissipation used in the 8th order case.  You might try increasing the dissipation parameters in the 4th order case. They are currently

SummationByParts::epsdis                             = 0.15
GlobalDerivative::epsdis_for_level               [0] = 0.075

and you would want to scale them by the same factor.  Maybe try increasing them by 30%. The runs might crash, but if not, I expect the oscillations will be reduced.  If so, then the difference in the noise between the two orders may just be due to the different interpretation of epsdis for the different schemes. If not, then the dissipation is not significantly damping these oscillations, and the difference must come from somewhere else.

-- 
Ian Hinder
http://members.aei.mpg.de/ianhin

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