[Nek5000-users] Error in Hmholtz solving

Thu Jan 29 10:00:45 CST 2015

Tony,

It looks like your Helmholtz tolerance is too tight.

If your problem is scaled in convective time units with characteristic length scale 1, timescale 1, and visc ~ 1/Re,
then typical Helmholtz tolerances are around 1.e-7.

You might also try turning on projection ---

set p93=20 (and lmxprev=20 in SIZE), p94=5 and p95=5 in the rea file.

No guarantees at this point because there seems to be something flaky in the projection routines
these days under certain runtime combinations (e.g., I think splitting + heat transfer +
velocity projection does not work)...  It's on my todo list to clean these up.

Note that your helmholtz solves are pretty well converged... residual is 1.7e-9, etc.
Projection should easily put you over the top

Paul

________________________________
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: Thursday, January 29, 2015 9:50 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: [Nek5000-users] Error in Hmholtz solving

Dear Neks,

I'm getting the error message of '**ERROR**: Failed in HMHOLTZ: ' when running my simulation. I've checked the subroutine 'cggo' in hmholtz.f where the error message is printed. I understand that when the tolerance is not satisfied, the code would output this error message. But what I'm wondering is why the maximum iteration number is only 100. In the subroutine 'cggo', niter is equal to min(maxit,maxcg) where maxit=200 and maxcg=900. I can't see other places where maxit is restricted to only 100. So why the maximum iteration number is 100 when solving the Hmholtz equation?

Also, is it right that if I increase the maximum number of iterations to certain point where the tolerance is satisfied, I won't get the error message?

Hope anyone can help me on this. Thank you very much in advance.

Best regards,

Tony

Example of the error message:

Step     52, t= 2.5600000E+01, DT= 5.0000000E-02, C=  2.689 5.6747E+02 1.0237E+01
 Solving for fluid T T T
       52 PRES gmres:   29  9.7057E-07  1.0000E-06  1.5219E-03  1.0926E+00  2.1480E+00    F
         52    Hmholtz VELX:     99   8.5954E-10   6.0898E-03   1.0000E-09
    52   100 **ERROR**: Failed in HMHOLTZ: VELY   1.7088E-09   2.5049E-02   1.0000E-09
    52   100 **ERROR**: Failed in HMHOLTZ: VELZ   1.4665E-09   2.5165E-02   1.0000E-09
               L1/L2 DIV(V)    :   2.7573E-18   9.1661E-04
               L1/L2 QTL       :   0.0000E+00   0.0000E+00
               L1/L2 DIV(V)-QTL:   2.7573E-18   9.1661E-04
         52   2.5600E+01  1.0094E+01 Fluid done
Step     53, t= 2.5650000E+01, DT= 5.0000000E-02, C=  2.697 5.7766E+02 1.0186E+01
 Solving for fluid T T T
       53 PRES gmres:   28  9.6076E-07  1.0000E-06  1.1322E-03  1.0558E+00  2.0542E+00    F
         53    Hmholtz VELX:     98   9.4891E-10   5.8783E-03   1.0000E-09
    53   100 **ERROR**: Failed in HMHOLTZ: VELY   1.6450E-09   2.4941E-02   1.0000E-09
    53   100 **ERROR**: Failed in HMHOLTZ: VELZ   1.4160E-09   2.5049E-02   1.0000E-09
               L1/L2 DIV(V)    :   2.4695E-18   1.0845E-03
               L1/L2 QTL       :   0.0000E+00   0.0000E+00
               L1/L2 DIV(V)-QTL:   2.4695E-18   1.0845E-03
         53   2.5650E+01  9.9794E+00 Fluid done
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20150129/b14c95c6/attachment.html>