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-   -   desperate Fatal overflow in linear solver - transient (http://www.cfd-online.com/Forums/cfx/71468-desperate-fatal-overflow-linear-solver-transient.html)

kingjewel1 January 4, 2010 08:13

desperate Fatal overflow in linear solver - transient
 
1 Attachment(s)
Hi there,

I'm a bit desperate here, tried everything i can think of :(

I'm running a simulation of a ventilated room (with k-e) under transient conditions and the solver has a fatal error after the first iteration. Steady state runs nicely even under high resolution scheme. Any Ideas? please:)

+----------------------+------+---------+---------+------------------+
| K-TurbKE | 2.00 | 4.1E-02 | 8.9E-01 | 6.3 2.2E-02 OK|
| E-Diss.K | 2.26 | 5.4E-02 | 1.9E+00 | 7.1 3.8E-04 OK|
+----------------------+------+---------+---------+------------------+
----------------------------------------------------------------------
COEFFICIENT LOOP ITERATION = 2 CPU SECONDS = 1.244E+02
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.00 | 1.3E-13 | 1.8E-12 | 3.2E+10 * |
| V-Mom | 0.00 | 1.3E-13 | 2.9E-12 | 3.1E+10 * |
| W-Mom | 0.00 | 4.1E-13 | 3.0E-12 | 9.6E+09 F |
| P-Mass | 0.00 | 0.0E+00 | 2.9E-24 | 9.7 9.1E+10 * |
+----------------------+------+---------+---------+------------------+
| Contaminant | 3.85 | 7.4E-03 | 2.2E+00 | 6.0 3.2E-02 OK|
+----------------------+------+---------+---------+------------------+

+--------------------------------------------------------------------+
| ERROR #004100018 has occurred in subroutine FINMES. |
| Message: |
| Fatal overflow in linear solver. |
+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
| An error has occurred in cfx5solve: |
| |
| The ANSYS CFX solver has terminated without writing a results |
| file. Command on host phipc (PHI-PC) exited with return code 0. |
+--------------------------------------------------------------------+

End of solution stage.

+--------------------------------------------------------------------+
| The following transient and backup files written by the ANSYS CFX |
| solver have been saved in the directory C:\Users\Phi\Documents\New |
| folder\Single\Standrews8_transient_013: |
| |
| 777_full.trn |
+--------------------------------------------------------------------+

shogologo January 4, 2010 08:30

Which Timestep did you chose?

kingjewel1 January 4, 2010 08:32

Quote:

Originally Posted by shogologo (Post 241435)
Which Timestep did you chose?

Timestep is 10s for total of 30mins. Also gives the same problem with adaptive timestepping and regardless of initial value file.
I'm worried about the simulation always giving RMS Courrant Number=0.00 and then jumping suddenly 999 after first iteration then crashing.

shogologo January 4, 2010 09:26

I managed a similar type of simulation.
My suggestion is to reduce timestep so to get Courant between 2 and 10 (or at least 30).

So I would try reduce to 1e-2 / 1e-3 sec...

kingjewel1 January 4, 2010 12:45

Quote:

Originally Posted by shogologo (Post 241441)
I managed a similar type of simulation.
My suggestion is to reduce timestep so to get Courant between 2 and 10 (or at least 30).

So I would try reduce to 1e-2 / 1e-3 sec...

Thank you. Its running with a Courant number at 21 with a 0.1 second timestep. Why does it require such a small timestep for this type of simulation? I don't need this much precise detail, as the code will take, I calculate, 60 days to run 30 mins of simulation.

What was your simulation?

shogologo January 5, 2010 05:51

You can try to speed up the timestep "on the fly" to understand which is the characteristic timescale.

It should depend mainly by grid size and Reynolds number.

Attesz January 5, 2010 08:17

Hi,

why are you doing the simulation so long (i'm just interested)? Doing a steady simulation first, and after that a transient for 5min for example, will not be better? (just an idea, i don't know your goals).

Regards,
Attesz

kingjewel1 January 5, 2010 12:17

I'm looking at the movement of particles/pathogens in a hospital room. The idea is to investigate different scenarios including, coughing, sneezing, bed making etc. In the end I'd also want to look at the way displacement ventilation suspends certain particles too. What do you think?

How do you change the timestep on the fly without going back into pre?

Attesz January 5, 2010 12:31

1 Attachment(s)
I'm not experienced in transient simulation, but perhaps the value of timesteps can be adjusted by clicking on the "Edit run in progress..." button (I've never tryed it). In my centrifugal compressor simulation I've used adaptive timestep, to reach always the desired Courrant number (it was very important at me), and the timestep will be set "automatically" by the solver.

However, your simulation is very interesting!

Good luck,
Attesz
http://www.cfd-online.com/Forums/att...1&d=1262709203
http://www.cfd-online.com/Forums/att...1&d=1262709392

kingjewel1 January 5, 2010 14:53

Thanks! I've just tried it and it works great.


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