AMG solver error (axi symmetric error)

asaha1 · May 16, 2019, 03:10

Hi
I am solving a diffuser flow. For that I made the 2D (half) geometry and selected the axisymmetric option (symmetry has been chosen as axis i.e x- axis). But as soon as I stared solving fluent is showing an error which is

# Divergence detected in AMG solver: x-momentum -> Increasing relaxation sweeps!
# You may try the enhanced divergence recovery with (rpsetvar 'amg/protective-enhanced? #t)

I tried making different meshing, different under-relaxation factors but this problem is not getting solved. This error is still coming.

But, When I tried with planer instead of axisymmetric this error is not coming. But the planer one is not my concern. What to do now...!
Can any body help please...!!!!!

LuckyTran · May 16, 2019, 11:42

Quote:

Originally Posted by asaha1

Hi
I am solving a diffuser flow. For that I made the 2D (half) geometry and selected the axisymmetric option (symmetry has been chosen as axis i.e x- axis). But as soon as I stared solving fluent is showing an error which is

You should be using the axis BC not symmetry. By x-axis you mean the y=0 line right? Only the y=0 line can be the axis in 2d axisymmetric cases.

asaha1 · May 16, 2019, 12:39

I am doing like that. Still the errors are coming.

racing_student · May 17, 2019, 02:29

Have you choosen the xy-plane for the geometry? If you made it on xz-plane it doesn't work.

asaha1 · May 17, 2019, 03:59

Yes, I have chosen X-Y plane.

LuckyTran · May 17, 2019, 12:36

Do a mesh check. Does your mesh extend into the negative y range?

For debugging to see if it might just be a wonky mesh:

Try shifting the mesh slightly upwards in y-direction and using a wall instead of axis BC.
Try scaling the mesh by factor of 1000 and running it.

asaha1 · May 18, 2019, 09:32

Dear Lucky,
I think you have correctly pointed out the problem. In the fluent interface while I was doing scaling of the geometry in GENERAL section I found that a very small negative y component is there.
As per your advise if I take wall instead of axis at the symmetry line will it affect the flow field...!
Is there any option of removing negative part instead of assuming it as wall..!
It will be highly helpful if you kindly elaborate it..

Thank You

LuckyTran · May 19, 2019, 14:47

You found a negative y-component but haven't confirmed whether it is a problem...

If it is the problem, then it is a meshing problem. The solution is to make a correct mesh. But since you already tried multiple meshes.... All I can say is learn to mesh better. You have to figure out what geometry building / meshing habits you have that results in this type of mesh. Good Luck.

May 16, 2019, 03:10	AMG solver error (axi symmetric error)	#1
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	Hi I am solving a diffuser flow. For that I made the 2D (half) geometry and selected the axisymmetric option (symmetry has been chosen as axis i.e x- axis). But as soon as I stared solving fluent is showing an error which is # Divergence detected in AMG solver: x-momentum -> Increasing relaxation sweeps! # You may try the enhanced divergence recovery with (rpsetvar 'amg/protective-enhanced? #t) I tried making different meshing, different under-relaxation factors but this problem is not getting solved. This error is still coming. But, When I tried with planer instead of axisymmetric this error is not coming. But the planer one is not my concern. What to do now...! Can any body help please...!!!!! mei1018786279 likes this.

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May 16, 2019, 12:39		#3
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	I am doing like that. Still the errors are coming.

May 17, 2019, 02:29		#4
racing_student New Member Marius Join Date: Nov 2018 Posts: 17 Rep Power: 7	Have you choosen the xy-plane for the geometry? If you made it on xz-plane it doesn't work.

May 17, 2019, 03:59		#5
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	Yes, I have chosen X-Y plane.

May 17, 2019, 12:36		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,677 Rep Power: 66	Do a mesh check. Does your mesh extend into the negative y range? For debugging to see if it might just be a wonky mesh: Try shifting the mesh slightly upwards in y-direction and using a wall instead of axis BC. Try scaling the mesh by factor of 1000 and running it.

May 18, 2019, 09:32		#7
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	Dear Lucky, I think you have correctly pointed out the problem. In the fluent interface while I was doing scaling of the geometry in GENERAL section I found that a very small negative y component is there. As per your advise if I take wall instead of axis at the symmetry line will it affect the flow field...! Is there any option of removing negative part instead of assuming it as wall..! It will be highly helpful if you kindly elaborate it.. Thank You

May 19, 2019, 14:47		#8
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,677 Rep Power: 66	You found a negative y-component but haven't confirmed whether it is a problem... If it is the problem, then it is a meshing problem. The solution is to make a correct mesh. But since you already tried multiple meshes.... All I can say is learn to mesh better. You have to figure out what geometry building / meshing habits you have that results in this type of mesh. Good Luck.