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Old   March 10, 2011, 12:25
Default CFX 12.1 Notice
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Roland Rakos
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Dear CFX Users,

As far as I know, you have already run into the next problem:

****** Notice ****** |
| The non-dimensional near wall temperature (T+) has been clipped |
| for calculation of Wall Heat Transfer Coefficient. |
| |
| Boundary Condition : WALL_MINI_B |
| T+ clip value = 1.0000E-10 |
| |
| If this situation persists and you are using the High Speed Model, |
| consider enabling Mach number based blending between low speed and |
| high speed wall functions. You can do so by specifying a Mach |
| number threshold as follows: |
| |
| highspeed wf mach threshold = 0.1 # default=0.0 (off) |

- What does it mean?
- How large numerical error can it cause in the results?
- And where can be found "highspeed wf mach threshold" ? (I haven't found it in the expert parameters.)

Thanks in advance;
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Old   March 10, 2011, 14:17
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Edmund Singer P.E.
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I usually get this when I try to hammer start a model with high speed compressible flow, by being hasty or lazy.

For example, steady state high speed external aerodynamics, M>1, starting from a clean def file, usually IC's set to inlet speed, and temps etc. Timescale factor set at 1.0.

Alot of times, CFX will work through the nasty startup, but more often than not, it fails, with above warnings, and those are usaully accompanied by very high, unrealistic mach numbers.

When that happens, I back up, put a little more effort in, either interpolate a lower mach, or similar run results onto new analysis, or reduce the timescale way down, or do both or those, and slowly ramp the timescale back up to 1.

Not knowing what your model is or looks like, I would put forward the uninformed decision that it is a bad thing, you should get rid of it, and it will cause large numerical errors.

If you wish to follow the expert parameters advice (which I would think is not your problem), just add that one in via command (it might not be seen in gui).
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