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<p style="margin-top:0;margin-bottom:0">How much time are you spending in your scalar fields?</p>
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<p style="margin-top:0;margin-bottom:0">Do you have projection turned on for all these fields?</p>
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<p style="margin-top:0;margin-bottom:0">grep tep logfile </p>
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<p style="margin-top:0;margin-bottom:0">will tell you how much time per step</p>
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<p style="margin-top:0;margin-bottom:0">grep gmr logfile will tell you how much time in the pressure on each step</p>
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<p style="margin-top:0;margin-bottom:0">What's left over is mostly passive scalar, unless you are using characteristics.</p>
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<p style="margin-top:0;margin-bottom:0">I would not recommend characteristics when running 10 scalar fields.</p>
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<p style="margin-top:0;margin-bottom:0">Paul</p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Nek5000-users <nek5000-users-bounces@lists.mcs.anl.gov> on behalf of nek5000-users@lists.mcs.anl.gov <nek5000-users@lists.mcs.anl.gov><br>
<b>Sent:</b> Saturday, September 1, 2018 9:50:38 AM<br>
<b>To:</b> nek5000-users@lists.mcs.anl.gov<br>
<b>Subject:</b> [Nek5000-users] Proper setup for AMG solver</font>
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<p>Dear Nek users & experts,<br>
<br>
I am currently running a turbulent pipe flow very similar to the simulations from El Khoury et al 2013.
<br>
Additionally, I solve for 10 thermal fields being treated as passive scalars.<br>
<br>
The Reynolds number is the same as the highest in El Khoury (2013), Re_b = 37700.
<br>
As I am using the relaxation term filtering (RT-Filter), I have a slightly lower resolution of about 250,000 elements at N=11 (5 times less than El Khoury).<br>
<br>
As the simulation is still very heavy, I have been looking into ways for speeding it up.<br>
I found some good suggestions here:<br>
http://nek5000.github.io/NekDoc/faq.html?highlight=amg#computational-speed<br>
<br>
and here (older version?)<br>
http://nek5000.github.io/NekDoc/large_scale.html<br>
<br>
<br>
However, I have some questions regarding these suggestions.<br>
1) Dealiasing:<br>
Usually I use lxd = 3/2*lx1. Can I lower that or even use lxd=lx1?<br>
<br>
2) Tolerances:<br>
I have tested to reduce the tolerance for pressure from 1e-8 to 5e-5 for a run at Re_b=5300 without any significant speedup. Would you consider 5e-5 for pressure accurate enough for evaluating statistics like turbulent kinetic energy budgets, Reynolds shear
stress budgets or budget of turbulent heat fluxes?<br>
<br>
3) Time discretisation: BDF2 and OIFS with Courant=2-5<br>
If I go from BDF3/EXT3 at C=0.5 to BDF2/OIFS at C=5.0, will I not miss high frequency fluctuations in time, since DT is much larger?<br>
<br>
4) AMG instead of XXT:<br>
I have tested AMG instead of XXT for both Re_b=5300 and Re_b=37700 without any speedup. Time/timestep is even higher with AMG. My workflow looks like this<br>
4.1) Set SEMG_AMG in the par file.<br>
4.2) Run the simulation once to dump amg files.<br>
4.3) Run amg_hypre (Here I do not know which options to choose, thus I have only uses default settings)<br>
4.4) Run the simulation.<br>
Maybe I should choose different options for amg_hypre, or should I rather use the amg_matlab2 tools? For the matlab tools I have not found an explanation on how to use them.<br>
<br>
<br>
I am grateful for any advice regarding these aspects.<br>
<br>
Best Regards,<br>
Steffen <br>
<br>
<br>
<br>
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