[Users] GW150914 example, noisy Psi4 waveforms

[Christian D. Ott]

Hi Ian, All,

Coming back to the issue of noisy waveforms with McLachlan/Llama after a 
few months. Sorry that this got put on the back burner. The runs I 
discuss below are all 8th order and derived from the ET example parfile 
for GW150914.

Here are a couple of things:

(1) In a private thread with Ian and me, Christian Reisswig pointed out 
that increasing the resolution at which the transition between Cartesian 
and curvilinear grid happens helps with the oscillations.

Based on my experiment -- going from dx ~ 1.22 to dx = 0.64 while 
keeping the transition radius constant -- does not lend very strong 
support to this suggestion. See exhibit1.png, which compares the ET 
stock parfile 22 Psi4 waveform at the 167 detector with the waveform 
obtained with my dx=0.64 run.

That said, higher resolution should never hurt!

(2) Next, I tested keeping the resolution at dx=0.64 on the coarsest 
Cartesian mesh and pushing out the transition radius of the curvilinear 
grid from 51.2 to 76.8. exhibit2.png compares the ET stock parfile 22 
Psi4 waveform at the 167 detector with the waveform obtained with my 
dx=0.64 run, and the transition at 76.8 instead of 51.2.

The effect is quite obvious. So *larger* transition radius, which brings 
the r = 167 detector *closer* to the transition radius, makes things 
worse, even at high resolution. Note that the oscillations get better 
with increasing detector radius.

(3) Finally, I tested again dx=0.64 on the coarsest Cartesian mesh and 
moving *in* the transition radius to r=38.4. exhibit3.png shows the result.

So what really seems to matter is the radius at which we transition from 
Cartesian to curvilinear (and related to that, the distance of the 
location where the waves are extracted from the transition radius; see 
exhibit4.png). I don't claim to understand this, but this is what I've 
been able to infer thus far.

I also found that increasing the size of the fine AMR meshes on the 
Cartesian grid has only a very small effect on the oscillations.

Best,

  - Christian



On 3/31/17 14:10, Ian Hinder wrote:
>
> On 31 Mar 2017, at 16:38, Christian D. Ott <cott at tapir.caltech.edu
> <mailto: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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