Numerical instability in one mesh case, very simple case

[gforney]

Please complete the following lines...

Application Version: FDS 6.0.0
SVN Revision Number: 8906
Compile Date: Fri, 19 Aug 2011
Operating System: win32 exe on a win64 machine

Describe details of the issue below:

Something with the pressure solver or velocity.
Case is just a room (one mesh, walls are the mesh
boundaries) with two OPEN vents as "doors" and there
is a simple burner (OBST + SURF on top face) with
HRRPUA input (1000 kW/m2). The resolution is a little
bit bad, dx=dy=20cm, the burner is 1m x 1m, but
I use this as a verification/example case for
evacuation stuff (well, with some additional inputs,
but the file is now "cleaned").

For example, at t=48 s the pressure does something
strange, see the png figures. Similar stuff hapens
once or twice in some other times also. At t=48 s
you get really large velocities at the burner. And
this is seen also for t=189.30 s, i.e., the 
numerical instability:

Time Step:  19400,    Simulation Time:    186.68 s
Time Step:  19500,    Simulation Time:    187.38 s
Time Step:  19600,    Simulation Time:    188.42 s
Time Step:  19700,    Simulation Time:    189.03 s
Time Step:  19741,    Simulation Time:    189.30 s

STOP: Numerical Instability

I have been lazy and I have not set any time step
parameters for FDS, these might help, but the
defaults should be working for this case, because
it is so simple.

Or am I doing something silly with the READ line:

&REAC ID         = 'POLYURETHANE',
      FUEL       = 'myPOLYURETHANE',
      FYI        = 'C_6.3 H_7.1 N O_2.1, NFPA Handbook, Babrauskas'
      SOOT_YIELD = 0.10,
      CO_YIELD   = 0.05,
      N          = 1.0,
      C          = 6.3,
      H          = 7.1,
      O          = 2.1  /

Timo

Original issue reported on code.google.com by tkorhon1 on 2011-08-24 10:03:45

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• _Attachment: evac_example1d.fds_
• _Attachment: evac_example1d_0047.png
  _
• _Attachment: evac_example1d_0048.png
  _
• _Attachment: evac_example1d_0049.png
  _

[gforney]

This appears to involve the molecular weight of the fuel, which in your case is about
125 g/mol. I've seen similar problems with instability for fuels on the order of 100
g/mol. When I replace the FUEL with PROPANE in your case, I do not see the large fluctuations
in the pressure.

Original issue reported on code.google.com by mcgratta on 2011-08-25 14:51:26

[gforney]

The problem is the BAROCLINIC term. It will take us some time to fix. As a work-around,
do one of the following:

1. BAROCLINIC=.FALSE. on the MISC line
2. Give the burner a temperature, like TMP_FRONT=500. on the SURF line
3. Reduce the fuel molecular weight.

Original issue reported on code.google.com by mcgratta on 2011-08-26 15:33:10

[gforney]

Hi Kevin,

I tested my case (a little bit more OBSTs etc + evacuation
than in this simplified test case) and it runs:

 1) BAROCLINIC=.FALSE. + TMP_FRONT=500. and original (heavy) fuel

 2) just reduced the fuel molecular weight (0.2*original fulel)
    about 36 g/mol compared to 178 g/mol originally.

So, I can live with this. I just need to change my example
inputs. These have the fire just to produce some smoke,
I could even use propane as the fuel with some co and soot
yields. Well, it might be better to change the example
inputs, so nobody will copy my bad fuels, you know the
effect of the fds-database file...

TimoK

Original issue reported on code.google.com by tkorhon1 on 2011-08-29 13:27:28

[gforney]

Randy has looked at this case and we also noticed that the problem is solved when using
a different turbulence model. The current DYNSMAG=.TRUE. default model creates alot
of gas movement near the burner which appears to have triggered the bad behavior. We've
noticed this in other cases. We'll let you know when you no longer need the work-around.

Original issue reported on code.google.com by mcgratta on 2011-08-29 13:34:43

[gforney]

Timo, could you run this case again when you have a chance with the latest version.

Original issue reported on code.google.com by mcgratta on 2011-12-22 22:19:38

[gforney]

Kevin,

I tested it with the latest version:

Fire Dynamics Simulator
Compilation Date : Thu, 22 Dec 2011
Version          : 6.0.0 Serial
OpenMP Disabled
SVN Revision No. : 9685

And the file seems to be running nicely and pressures
seem to be fine, -2 Pa ... +8 Pa about.

So, you could change this issue to Verified.

TimoK

Original issue reported on code.google.com by tkorhon1 on 2011-12-23 13:19:12

[gforney]

Thanks for checking. K

Original issue reported on code.google.com by mcgratta on 2011-12-28 17:10:58