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Bounds error message with no observable effect on simulation

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Old   March 19, 2012, 11:46
Default Bounds error message with no observable effect on simulation
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Hi,

In some of my conjugate heat transfer simulations a bounds error is reported at fluid outlet boundary for absolute pressure. However, the simulation converges nicely.

================================================== ====================
OUTER LOOP ITERATION = 288 ( 99) CPU SECONDS = 3.486E+05 (1.191E+05)
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.95 | 8.7E-09 | 2.5E-07 | 3.8E-02 OK|
| V-Mom | 0.95 | 4.1E-09 | 3.9E-07 | 4.2E-02 OK|
| W-Mom | 0.94 | 1.7E-09 | 2.0E-07 | 4.4E-02 OK|
| P-Mass | 0.93 | 2.1E-10 | 1.8E-08 | 9.7 7.5E-02 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| While evaluating |
| Total Enthalpy |
| on boundary "Outlet fluid", |
| the variable |
| Absolute Pressure |
| went outside of its upper limit. Its maximum value was |
| 2.6667E+15. The bounds error was handled by clipping. |
| If this situation persists, consider increasing the table range. |
+--------------------------------------------------------------------+
| H-Energy | 0.95 | 9.6E-07 | 6.6E-05 | 9.5E-02 OK|
| T-Energy-Manifold so | 0.99 | 3.0E-09 | 3.6E-08 | 9.5E-02 OK|
| T-Energy-Bottom plat | 0.99 | 2.7E-09 | 2.1E-08 | 6.2 9.5E-02 OK|
+----------------------+------+---------+---------+------------------+

CFD Solver finished: Mon Mar 19 01:08:37 2012
CFD Solver wall clock seconds: 3.1119E+04

================================================== ====================
Termination and Interrupt Condition Summary
================================================== ====================

CFD Solver: All target criteria reached
(Equation residuals AND global imbalances)

================================================== ====================


As I show above the bounds error is reported in even the last iteration before convergence. Now when I plot the absolute pressure on the fluid outlet boundary, everything seems alright ...there are no values of order of 1e15.
Any idea why CFX would throw out this message? The mesh is also ok.
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Old   March 19, 2012, 19:37
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There will be a value of 1e15 somewhere, that is why it is being reported. It is pobably a single node on an outlet, next to the wall or something.
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Old   March 20, 2012, 07:44
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I agree.
But since I cannot spot that value when I analyze the results in CFXpost (even the range of absolute pressure over the entire computational domain is physical i.e. the range of absolute pressures spans in the order of 1e5), so I think my results are OK.Please correct me if I am wrong.
But why this value is reported is curious. Sometimes, by changing the time-step the Bounds error message stops. Simulation converges anyhow though to a steady state solution, with or without changing time-step and final results are also same.
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Old   March 20, 2012, 17:13
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It might be a transitory numerical thing. A small numerical instability is causing that small area of the mesh to go hay-wire, but it eventually gets over it and converges. If the final solution is properly converged and bounded then I would ignore it. But it does suggest your simulation's numerical stability is marginal.
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