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Top Wall Opening Behaves as No Slip?

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Old   April 14, 2021, 01:35
Unhappy Top Wall Opening Behaves as No Slip?
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Hello,

I ran a simulation of air passing through a rectangular box where the top wall was set as an opening. When I look at the velocity in the wall normal direction, the velocity at the top goes to zero.

Any ideas as to why this might be happening? I have laminar flow and no heat transfer, so not sure what would be slowing down the flow near the top.
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Old   April 14, 2021, 05:34
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The default opening option does not allow flow parallel to the boundary, this appears to be what you are seeing. The Entrainment option allows flow parallel to the boundary, but can be harder to converge. This is all described in the CFX documentation.
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Old   April 14, 2021, 11:56
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Quote:
Originally Posted by ghorrocks View Post
The default opening option does not allow flow parallel to the boundary, this appears to be what you are seeing. The Entrainment option allows flow parallel to the boundary, but can be harder to converge. This is all described in the CFX documentation.
Oh, sorry, I should have mentioned that I was using an opening with Static Pressure and Direction, normal to the boundary, 0 Pa. Should I consider switching to Entrainment?
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Old   April 14, 2021, 14:46
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If your real opening boundary has velocity tangent to the opening plane, then most likely yes. You want your boundary condition to be as close to realistic as possible right?

A sensitivity analysis would show whether it makes a difference of not to your result of interest.
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Old   April 14, 2021, 19:38
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Thank you Glenn and Erik for those points. You are both correct; it does not make much sense to have specified the flow normal to the boundary. When solving flat plate problems, we have the freestream velocity parallel to the boundary.

I will try the other boundary condition and report back.
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