[Nek5000-users] Projection technique's parameters

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

Hi Marco,

Here is the reference:


  1.  Fischer, P.F., "Projection techniques for iterative solution of Ax=b with successive right-hand sides." <http://www.mcs.anl.gov/~fischer/pubhtml/project.pdf> Comp. Meth. in Appl. Mech 163 (1998).


There is also a 1993 ICASE Report, but it's mostly a subset of this paper.

Paul

PS - these typical values are based on my experience...  but, it's easy to try a run for a few
hundred steps and see which parameters make things run faster.



________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Monday, June 27, 2016 8:01 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Projection technique's parameters

Paul,

is there a paper/report we can have access to that goes in the details of the implementation, please? Also, are the typical values based on your personal experience?

Thanks,
Marco
On Jun 27, 2016, at 8:55 AM, nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov> wrote:


Hi Andrew,

The projection routines are currently being overhauled, but the general strategy is:

   Number of projections controlled by p93 and mxprev in SIZE.

   p94 > 0:   start projecting velocity/temp after step p94

   p95 > 0:   start projecting pressure after step 95

I typically set p94 and p95 to 5

I set mxprev and p93 to 20, but sometimes to a higher value.

I used to occasionally to to much higher than 20 (say, 80), but in a recent
rewrite and effort to make projection more robust I slowed the projection
step down with too much work, so another rewrite is due soon.

Paul

PS - note that certain combinations of run parameters don't support
projection or potentially blow up.  Set p94=0 in this case and retry.
There was a bug that was fixed about 6 weeks back---it may have cured
some of the cases that users were running into.


________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov<mailto:nek5000-users-bounces at lists.mcs.anl.gov> [nek5000-users-bounces at lists.mcs.anl.gov<mailto:nek5000-users-bounces at lists.mcs.anl.gov>] on behalf of nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov> [nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov>]
Sent: Monday, June 27, 2016 3:17 AM
To: nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov>
Subject: [Nek5000-users] Projection technique's parameters

Hi Neks,

My question is about the parameters which manage the projection method in the NEK5000: p93, p94 and p95. How can the correct values be selected for these parameters to obtain the better performance depending on the nature of simulation: laminar / turbulent, convection / diffusion, time step?

Thank you for any suggestion,
Andrew
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