Overlap and gap between cell zone for sliding mesh

cs12345 · December 12, 2017, 06:25

Hi all, I am new to using the sliding mesh function in FLUENT, so I have some questions regarding this.

I am trying to do a simulation for turbomachinery hence I will be using sliding mesh for this. I created 2 different mesh files and appended them in FLUENT. While the meshes were created with the same geometrical considerations (same radius for the fan), but when after appending them into FLUENT and by zooming in, I noticed that there is a slight shift in the fan position (probably due to meshing). This shift causes some parts of the mesh to overlap and some gap to form.

Now, I am aware that overlap is generally bad for the sliding mesh. As I used the same radius for the CAD file for the 2 cell zones, the slight shift only occurred after I meshed it in the meshing tool in ANSYS (I suspect this happen due to the close proximity between the blades and the fan, causing distortion in the mesh). Therefore, how do I ensure that the 2 cell zones actually match?

In addition, I previously did a moving reference frame solution to this exact mesh and there isnt any error message to the problem. Also, I was able to create interface between the 2 meshes, So do you all think if it is just a display error after zooming in or this is actually a problem?

I have attached some pictures of what I mean. The first file shows the 2 cell zones, and the second image shows how they look like after they are zoomed in. Apparently the internal cell zone seems to "eat" into the external zone.

fan 1.jpg

fan 2.jpg

Thanks in advance if any of you have any answers to this

johny0688 · December 13, 2017, 02:11

In my opinion it is not a displaying problem and meshes are physically overlapping. However I have met this problem before in my calculations and I did nothing about it. I have calculated cases with this issue and didn't observed any influence on results. As far as meshes are connected by non conformal interface, the pressure and vectors are interpolated between them. Further more Fluent works with geometrical tolerance. In some cases it is possible to connect non conforlam meshes with interior between them.
Do the mesh check, if fluent will not give any warnings I will not care about it.
This is my experience and my opinion, if someone else could give opinion or advice it would be helpful.

Best,
Jan

cs12345 · December 14, 2017, 08:41

Hey Johny, thanks for the reply, seems like not many people have an answer to this problem.

Yes indeed, I did not run into any errors and it seems that the tolerance by FLUENT covers such a small "overlap". Afterall, this overlap can only be seen at 10^-7 m which is probably too small to matter.

Either way thanks!

December 12, 2017, 06:25	Overlap and gap between cell zone for sliding mesh	#1
cs12345 New Member Join Date: Dec 2017 Posts: 3 Rep Power: 8	Hi all, I am new to using the sliding mesh function in FLUENT, so I have some questions regarding this. I am trying to do a simulation for turbomachinery hence I will be using sliding mesh for this. I created 2 different mesh files and appended them in FLUENT. While the meshes were created with the same geometrical considerations (same radius for the fan), but when after appending them into FLUENT and by zooming in, I noticed that there is a slight shift in the fan position (probably due to meshing). This shift causes some parts of the mesh to overlap and some gap to form. Now, I am aware that overlap is generally bad for the sliding mesh. As I used the same radius for the CAD file for the 2 cell zones, the slight shift only occurred after I meshed it in the meshing tool in ANSYS (I suspect this happen due to the close proximity between the blades and the fan, causing distortion in the mesh). Therefore, how do I ensure that the 2 cell zones actually match? In addition, I previously did a moving reference frame solution to this exact mesh and there isnt any error message to the problem. Also, I was able to create interface between the 2 meshes, So do you all think if it is just a display error after zooming in or this is actually a problem? I have attached some pictures of what I mean. The first file shows the 2 cell zones, and the second image shows how they look like after they are zoomed in. Apparently the internal cell zone seems to "eat" into the external zone. fan 1.jpg fan 2.jpg Thanks in advance if any of you have any answers to this

December 13, 2017, 02:11		#2
johny0688 Member Jan Surwiło Join Date: Feb 2017 Posts: 31 Rep Power: 9	In my opinion it is not a displaying problem and meshes are physically overlapping. However I have met this problem before in my calculations and I did nothing about it. I have calculated cases with this issue and didn't observed any influence on results. As far as meshes are connected by non conformal interface, the pressure and vectors are interpolated between them. Further more Fluent works with geometrical tolerance. In some cases it is possible to connect non conforlam meshes with interior between them. Do the mesh check, if fluent will not give any warnings I will not care about it. This is my experience and my opinion, if someone else could give opinion or advice it would be helpful. Best, Jan

December 14, 2017, 08:41		#3
cs12345 New Member Join Date: Dec 2017 Posts: 3 Rep Power: 8	Hey Johny, thanks for the reply, seems like not many people have an answer to this problem. Yes indeed, I did not run into any errors and it seems that the tolerance by FLUENT covers such a small "overlap". Afterall, this overlap can only be seen at 10^-7 m which is probably too small to matter. Either way thanks!