Can someone explain me this error?

kbaker · May 21, 2012, 12:24

Hi when I run my simulation the error below appeared after nearly 15 iterations:

A wall has been placed at portion(s) of an OUTLET boundary condition (at 100.0% of the faces, 100.0% of the area) to prevent fluid from flowing into the domain.
The boundary condition name is: outflow.
The fluid name is: Water.
If this situation persists, consider switching
to an Opening type boundary condition instead.

may somebody tell me what its reason?

Thanks
kbaker

Lance · May 21, 2012, 12:41

Use the forum search function, this is one of the most common questions asked...

kbaker · May 21, 2012, 13:00

ِAs I saw from the other posts about this error some suggests to solve the problem with moving the exit boundary further away from the inlet to prevent reverse flows I need to say my pipe length is 25 m and diameter is 98.5 mm so I think the inlet is located in a far distance from the outlet ? I am right or I need really to make the pipe longer?

ghorrocks · May 21, 2012, 19:36

Your warning is saying 100% of the outlet has backflow. In this case the problem is more fundamental as none of the outlet is working as expected. You need to look into why the entire outlet wants to flow backwards.

Danial Q · May 21, 2012, 19:41

Try this, it might work for you;
Expert parameters > Discretization > Miscellaneous > Build artifical wall > set to " f ". By default its "t". F will not build any artifical wall and try to reduce your time steps also.

cheers

ghorrocks · May 21, 2012, 19:43

Sorry Danial, this is a bad idea. Something is wrong with the simulation and that is making the flow want to go backwards. Rather than letting it go backwards you should fix the root cause of the problem, and make the flow want to go forwards.

piyo · May 22, 2012, 11:38

In CFX, when the outlet condition is used, the solver will build an artifitial at outflow position to allow for outflow but prevent backflow. As been recommanded, this setting should not be switched off in expert parameters in most time. If your position has backflow in real case, you'd better use opening condition, and of course, requiring more time to converge.

kbaker · May 22, 2012, 12:00

The message disappeared when I use double precision and switch to segregated treatment for the volume fraction coupling and reduce time step to 0.001 s and I still use outlet boundary but the results shows higher liquid level which I define it at inlet and outlet as the same with value 5.5 mm (where pipe diameter is 98.5 mm) and it seems more higher than expected with unknown reason?

ghorrocks · May 22, 2012, 20:07

So it seems the outlet back flow was due to problems converging. This confirms you should fix the root cause of the problem and not just allow back flow to occur.

Your question about accuracy is an FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

May 21, 2012, 12:24	Can someone explain me this error?	#1
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	Hi when I run my simulation the error below appeared after nearly 15 iterations: A wall has been placed at portion(s) of an OUTLET boundary condition (at 100.0% of the faces, 100.0% of the area) to prevent fluid from flowing into the domain. The boundary condition name is: outflow. The fluid name is: Water. If this situation persists, consider switching to an Opening type boundary condition instead. may somebody tell me what its reason? Thanks kbaker

May 21, 2012, 13:00		#3
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	ِAs I saw from the other posts about this error some suggests to solve the problem with moving the exit boundary further away from the inlet to prevent reverse flows I need to say my pipe length is 25 m and diameter is 98.5 mm so I think the inlet is located in a far distance from the outlet ? I am right or I need really to make the pipe longer? Last edited by kbaker; May 21, 2012 at 15:49.

May 22, 2012, 12:00		#8
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	The message disappeared when I use double precision and switch to segregated treatment for the volume fraction coupling and reduce time step to 0.001 s and I still use outlet boundary but the results shows higher liquid level which I define it at inlet and outlet as the same with value 5.5 mm (where pipe diameter is 98.5 mm) and it seems more higher than expected with unknown reason? Last edited by kbaker; May 22, 2012 at 16:07.

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May 21, 2012, 12:41		#2
Lance Senior Member Lance Join Date: Mar 2009 Posts: 669 Rep Power: 22	Use the forum search function, this is one of the most common questions asked...

May 21, 2012, 19:36		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Your warning is saying 100% of the outlet has backflow. In this case the problem is more fundamental as none of the outlet is working as expected. You need to look into why the entire outlet wants to flow backwards.

May 21, 2012, 19:41	Hi	#5
Danial Q Senior Member Danial Join Date: Nov 2011 Posts: 179 Rep Power: 15	Try this, it might work for you; Expert parameters > Discretization > Miscellaneous > Build artifical wall > set to " f ". By default its "t". F will not build any artifical wall and try to reduce your time steps also. cheers

May 21, 2012, 19:43		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Sorry Danial, this is a bad idea. Something is wrong with the simulation and that is making the flow want to go backwards. Rather than letting it go backwards you should fix the root cause of the problem, and make the flow want to go forwards.

May 22, 2012, 11:38		#7
piyo New Member Join Date: May 2012 Posts: 6 Rep Power: 14	In CFX, when the outlet condition is used, the solver will build an artifitial at outflow position to allow for outflow but prevent backflow. As been recommanded, this setting should not be switched off in expert parameters in most time. If your position has backflow in real case, you'd better use opening condition, and of course, requiring more time to converge.

May 22, 2012, 20:07		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	So it seems the outlet back flow was due to problems converging. This confirms you should fix the root cause of the problem and not just allow back flow to occur. Your question about accuracy is an FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F