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Dimone January 21, 2011 06:22

Solution variables goes outside upper limit -how to localize fatal overflow occurance
Hello to everyone.

I'm trying to solve transient problem with 2-way FSI (coupled with ANSYS APDL Structural). The working substance is gas (R.-K. model of real gas - one from CFX library). No turbulence presents. Thermal energy is used to account heat dynamics.

At some point of solution (close to its finish) linear solver overflow happens, which is preceded with number of messages, telling that while eveluating some variables (density, enthalpy etc.), the static temperature goes outside upper limit and will be clipped (example message is given bellow).


 |                        ****** Notice ******                      |
 | While evaluating Density,                                          |
 | Static Temperature on domain Default Domain                        |
 | went outside of its upper limit.  Its maximum value was            |
 |  7.3263E+02.  The bounds error was handled by clipping.            |
 | If this situation persists, consider increasing the table range.  |

Noticeable, that value given (7.3e+2) is about two times greater, than I expected it to be (and it actually appears with normal values in those timesteps, which completed normally).

Moreover, the overflow happens at the final stage of solution, which represents relaxation. This means, that no high external forces (or other excitations) should present at that time and all velocities and gradients should go down (no occasional jumps are expected).

I thought, that the problem arise due to poor mesh quality induced by mesh morphing (actually, I'm still thinking so), but neither changing initial mesh nor changing timestep resolved the issue. Although of course changing the mesh changes the time, when overflow happens. And I do not know, where (geometrically) the problem arises so I cannot concentrate my efforts on that region. (All I can do is look at latest successful timestep results mesh and think: here mesh looks OK, but there it is not so good... possibly, the problem is there...).

If anyone can suggest some way to localize (geometrically or in some other way) the source of solution overflow to become able to resolve this issue, I would appreciate it so much.

Thanks in advance.

ghorrocks January 21, 2011 07:26

In the output panel of CFX-Pre add residuals to the output file. Then you should be able to see where the residuals are growing.

Also, the problem seems to be in temperature and density. Can't you look for regions of inexplicably growing density or temperature?

Dimone January 21, 2011 07:35

Thanks for reply.

I'll try to enable residuals in CFX output.

Related to second question: No, I do not see any areas of the groing temperature (global temperature scale limits are ok).

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