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A wall has been placed at portion(s) of an OUTLET

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Old   November 9, 2011, 17:14
Default A wall has been placed at portion(s) of an OUTLET
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Hey Guys,

I'm trying to simulate a Car Frontwing in CFX. My air volume is about 1000mm long, I read that it could be a problem if Inlet and Outlet are close together but I guess 1m is far enough.

Anyway, I get this "Notice" when I'm trying to solve:

| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 30.0% of the faces, 30.7% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: Outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+----------------------------------------------------

The "Outlet" is my only outlet, so switching to Opening results in an "overflow".

The percentage "of the faces" and "of the area" changes with every iteration, sometimes it is bigger, sometimes it's smaller.

In some cases the number gets so high, that the solver exits.

I just tested and it did all 100 iterations and said it "stopped normally".

My RMS P-Mass gets very low, but all other curves stay at about 10e-4.


Can you help me with my problem?


Thank you very much!

Greetings!
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Old   November 9, 2011, 19:14
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I should write an FAQ on this, it has been asked a thousand times.

You have a big separation off the back of your body which intersects the outlet. An outlet does not allow backflow so it puts false walls on it to stop backflow. When you switch to an opening it allows backflow but then you are assuming the pressure is constant over the boundary which is not correct.

You need to do a sensitivity analysis of the proximity of your exit boundary to the body. Double the distance and see if the parameters you care about (lift? drag?) change. They probably will, so keep doubling the distance until the parameters of importance have converged to a tolerance you are happy to accept.
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