[Nek5000-users] Proper setup for AMG solver

[nek5000-users at lists.mcs.anl.gov]

Dear Nek users & experts,

I am currently running a turbulent pipe flow very similar to the simulations from El Khoury et al 2013.
Additionally, I solve for 10 thermal fields being treated as passive scalars.

The Reynolds number is the same as the highest in El Khoury (2013), Re_b = 37700.
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).

As the simulation is still very heavy, I have been looking into ways for speeding it up.
I found some good suggestions here:
http://nek5000.github.io/NekDoc/faq.html?highlight=amg#computational-speed

and here (older version?)
http://nek5000.github.io/NekDoc/large_scale.html


However, I have some questions regarding these suggestions.
1) Dealiasing:
Usually I use lxd = 3/2*lx1. Can I lower that or even use lxd=lx1?

2) Tolerances:
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?

3) Time discretisation: BDF2 and OIFS with Courant=2-5
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?

4) AMG instead of XXT:
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
4.1) Set SEMG_AMG in the par file.
4.2) Run the simulation once to dump amg files.
4.3) Run amg_hypre (Here I do not know which options to choose, thus I have only uses default settings)
4.4) Run the simulation.
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.


I am grateful for any advice regarding these aspects.

Best Regards,
Steffen



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