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Transient Simulation: Boundary Condition Problem

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Old   December 1, 2010, 14:58
Default Transient Simulation: Boundary Condition Problem
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sam13
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Hi everybody,

I'm running a transient simulation of a marine propeller in CFX 11.0.
But I'm getting a notice after some 100 time steps which is as follows-

***** Notice *****
A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 6.8% of the faces, 3.6% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Water. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+-------------------------------------------------------

I set "Mass Flow Rate " for the outlet BC, Cylindrical velocity components for both outlet and outer BC. I also tried with setting "Avg Static Pressure" to zero for outlet BC, but same thing happened!

Just wondering, is there anybody can explian me whats wrong in the simulation?

Thanks in advance for your help.

Shafi
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Old   December 1, 2010, 16:30
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Edmund Singer P.E.
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Move your outlet farther away
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Old   December 9, 2010, 11:55
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sam13
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Thanks Singer for the suggestion.

I tried a new simulation setting the outlet farther away(1.5 times of the previous) but it did not work!! I'm getting the same message again. Infact, in this case, the notice appeared a bit earlier(after 60 time steps!!).

I should mention that the propeller is inclined by some degrees to the horizontal to genarate the wake. My setup was as follows:

1. Inlet: Cylindrical velocity component (axial, radial & theta)
2. Outer: opening, Cylindrical velocity component(values are same as the inlet),
3. Outlet: Mass flow rate

I'm so confuesd where I'm doing wrong!! Please help me to figure it out. I badly need this.

Thanks,
Shafi
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Old   December 9, 2010, 12:06
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Edmund Singer P.E.
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I have no idea what your simulation looks like but you indicate it is a propellar simulation and I have no idea what your initial solution looks like but I would guess at:

let the transient run out further, and perhaps it is a startup vortex that needs to translate out of the domain and the wall will stop being placed at the exit

-or-

Move your exit further away
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Old   December 9, 2010, 14:04
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sam13
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Thanks a lot Singer for your advice. It makes sense, it could be a startup vortex problem.

I just can't share you any image as it is highly confidential but I can assure that my initial solution looks pretty good, the RMS residuals are resonable, levelled out at roughly 2e-4, momentum residuals near 1.1e-3, mass residuals are 1.1e-5. I'm continuing the simulation further and let you know what happen.

Thanks again.

Shafi
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Old   January 11, 2011, 16:25
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Hi there,

I got something and feel like an idiot. I have been using "General" mode option of CFX-Pre for my rotating propeller simulation instead of "Turbomachinery" mode. Do you think it might be the problem?

Please reply asap. I already wasted too much time on it!

Thanks in advance,

Shafi
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Old   January 11, 2011, 16:35
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Edmund Singer P.E.
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No. That just custom tailors the GUI.
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Old   January 11, 2011, 17:40
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Hi Edmund,

This inlet/outlet backflow question gets asked far too often. Can you write a FAQ on it? (http://www.cfd-online.com/Wiki/Ansys_FAQ)
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