Top Wall Opening Behaves as No Slip?

aero_head · April 14, 2021, 01:35

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.

ghorrocks · April 14, 2021, 05:34

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.

aero_head · April 14, 2021, 11:56

Quote:

Originally Posted by ghorrocks

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?

evcelica · April 14, 2021, 14:46

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.

aero_head · April 14, 2021, 19:38

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.

April 14, 2021, 01:35	Top Wall Opening Behaves as No Slip?	#1
aero_head Senior Member Kira Join Date: Nov 2020 Location: Canada Posts: 435 Rep Power: 9	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.

April 14, 2021, 05:34		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	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. aero_head likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

April 14, 2021, 14:46		#4
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	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. aero_head likes this.

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April 14, 2021, 19:38		#5
aero_head Senior Member Kira Join Date: Nov 2020 Location: Canada Posts: 435 Rep Power: 9	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.