Symmetry boundary condition problem

jhtcten · June 23, 2016, 10:28

Hi all,

I am running a simulation in CFX with a symmetry boundary condition on a side. The first mesh I created worked fine - no problems. However, as soon as I make the mesh finer, I get errors with the symmetry boundary condition.

It turns out one or more nodes are not directly on the face ie not parallel, and therefore the symmetry boundary condition cannot be applied. I have tried increasing the vector parallel tolerance up to 5 degrees - no luck.

I can identify where the problem nodes are, but I have no idea what is causing this to happen. My questions are as follows:

1) Why does this happen when I have a finer mesh?

2) How can I fix this in ANSYS mesher?

Thank you all in advance.

ghorrocks · June 23, 2016, 19:45

In CFD Post or CFX Pre make the problem face visible and everything else invisible. Then align the view angle so you are looking at the face edge on. Then any element faces which are off the plane should be obvious.

jhtcten · June 23, 2016, 21:44

Quote:

Originally Posted by ghorrocks

In CFD Post or CFX Pre make the problem face visible and everything else invisible. Then align the view angle so you are looking at the face edge on. Then any element faces which are off the plane should be obvious.

The scale is so small that I couldn't do that - I'm talking 1e-3 mm. But in any case, I HAVE identified the problem nodes, and know where it is occurring.

I still have no idea how to fix it though.

ghorrocks · June 24, 2016, 06:30

ICEM meshing does this when you set up the edges wrong, or it sometimes happens when you have a small wedge angle. In ICEM you can either fix the problem or manually edit the mesh to get it right.

In ANSYS Mesh you are more limited. Don't forget the patch independent mesher is using ICEM technology so suffers the same problem. But fixing it is ANSYS Mesh is challenging as you don't have the low-level control over the mesh.

Tingyun YIN · April 20, 2017, 00:00

Hello jhtcten

Did you solve your symmetry problem?
Now, I have the same problem: CFX can run 6 million nodes (first lawyer setting: 0.1) successfully, while when I increased the nodes up to 10 million nodes in ICEM (also I changed the first lawyer setting: 0.01), It failed to solve and symmetry error occurred. By the way, the 2*2*2 quality is 0.9 and angle is more than 36 for 10 million.

I tried to convert to the unstructured mesh by selecting "no projection" in ICEM. CFX ran a few step and stopped with error "overflow".

I didn't change the expert parameters suggested by CFX help. Because I believe it will result in a bad result.

My model is the Twisted-11 foil.

Many thanks!

Sent from my iPhone using CFD Online Forum mobile app

June 23, 2016, 10:28	Symmetry boundary condition problem	#1
jhtcten New Member J Join Date: Feb 2016 Posts: 7 Rep Power: 11	Hi all, I am running a simulation in CFX with a symmetry boundary condition on a side. The first mesh I created worked fine - no problems. However, as soon as I make the mesh finer, I get errors with the symmetry boundary condition. It turns out one or more nodes are not directly on the face ie not parallel, and therefore the symmetry boundary condition cannot be applied. I have tried increasing the vector parallel tolerance up to 5 degrees - no luck. I can identify where the problem nodes are, but I have no idea what is causing this to happen. My questions are as follows: 1) Why does this happen when I have a finer mesh? 2) How can I fix this in ANSYS mesher? Thank you all in advance.

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June 23, 2016, 19:45		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,937 Rep Power: 145	In CFD Post or CFX Pre make the problem face visible and everything else invisible. Then align the view angle so you are looking at the face edge on. Then any element faces which are off the plane should be obvious.

June 24, 2016, 06:30		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,937 Rep Power: 145	ICEM meshing does this when you set up the edges wrong, or it sometimes happens when you have a small wedge angle. In ICEM you can either fix the problem or manually edit the mesh to get it right. In ANSYS Mesh you are more limited. Don't forget the patch independent mesher is using ICEM technology so suffers the same problem. But fixing it is ANSYS Mesh is challenging as you don't have the low-level control over the mesh.

April 20, 2017, 00:00		#5
Tingyun YIN Member Tingyun YIN Join Date: Apr 2017 Posts: 31 Rep Power: 10	Hello jhtcten Did you solve your symmetry problem? Now, I have the same problem: CFX can run 6 million nodes (first lawyer setting: 0.1) successfully, while when I increased the nodes up to 10 million nodes in ICEM (also I changed the first lawyer setting: 0.01), It failed to solve and symmetry error occurred. By the way, the 222 quality is 0.9 and angle is more than 36 for 10 million. I tried to convert to the unstructured mesh by selecting "no projection" in ICEM. CFX ran a few step and stopped with error "overflow". I didn't change the expert parameters suggested by CFX help. Because I believe it will result in a bad result. My model is the Twisted-11 foil. Many thanks! Sent from my iPhone using CFD Online Forum mobile app