[Nek5000-users] Proper setup for AMG solver

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

Try to use

lx1=8/lxd=10 with a (potentially) finer mesh
BDF2 + OIFS with a targetCFL=3.5
set dt = 0 (this will adjust dt to targetCFL) 
pressure tol = 1e-5 (residual projection turned on) 1e-6 for velocity and scalars (residual projection turned off)

Note, a mesh with more than 350k elements requires AMG. The default parameters are fine.

What version of Nek5000 are you using?

Cheers,
Stefan


-----Original message-----
> From:nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
> Sent: Saturday 1st September 2018 16:50
> To: nek5000-users at lists.mcs.anl.gov
> Subject: [Nek5000-users] Proper setup for AMG solver
> 
> 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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