[Nek5000-users] Proper setup for AMG solver

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

Dear Nek experts,


I wanted to add some remarks on my performance tests conducted last week 
mostly for the turbulent pipe flow at Re_b=5300

- The bad performance of the AMG solver was indeed due to an old version 
of amg_hypre, as Stefan mentioned already.
For Re_b=5300 XXT and AMG showed the same performance.

- Additionally, I could increase the performance by about 20% using the 
following settings:
-- lxd=10 (instead of 12, for lx1=8)
-- lx2=lx1-0 (PN-PN)
-- lower tolerances for p=1e-5 (instead of 1e-8) and 1e-6 for velocity 
and passive scalars
-- turning of projection except for the fields of low Prandtl number

- Going from BDF3/EXT3 with a variable DT and targetCFL=0.5 to BDF2/OIFS 
with variable DT and targetCFL=0.5 resulted in a longer time / timestep 
(3x) but a also a larger average DT (6x).
Assuming the collected statistics require a similar averaging time in 
wash-outs, BDF2/OIFS is advantageous (even for 10 passive scalars).
Before this discussion, I was running at constant DT and collected 
statistics every 10th step. Now with a larger DT using characteristics, 
should I collect statistics for each step?

- Regarding the filterWeight and filterCutoffRatio, I found out (with 
Stefan's help) that the divergence error in L2 norm (when using PN-PN) 
is affected by filterWeight.
For the setup at Re_b=5300 a filterWeight of 27 seemed to be OK 
considering a divergence error in L2 norm of L2=3e-2, whereas a weight 
of 54 resulted in L2=1e-1.


How these settings will affect the statistics, I am testing now.

Thank you all for your help and suggestions.


Cheers,
Steffen