# Divergence in AMG solver!

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 December 14, 2010, 14:49 Divergence in AMG solver! #1 New Member   marina Join Date: Nov 2010 Posts: 2 Rep Power: 0 Hi everyone! I'm trying to simulate in Fluent a 2D compressible flow around the NACA 0012, in order to study the velocity profiles in the boundary layer. Reynolds=500000 and M=0.22 Boundary conditions: - inlet: pressure-inlet (direction x) - outlet: pressure-outlet - top & bottom: symmetry Turbulence: K-Epsilon. Intensity of turbulence: 3.1% and Lenght scale: 0.0976m Solution method: Pressure-velocity coupling, scheme: coupled. Second order After 15 iterations, three errors appear: # Divergence detected in AMG solver: pressure coupled --> Decreasing coarsening group size! # Divergence detected in AMG solver: pressure coupled --> Increasing relaxation sweeps! # Divergence detected in AMG solver: pressure coupled --> Changing to W-cycle! It also gives me some messages saying: turbulent viscosity limited to viscosity ratio of 10^5 in 1325 cells I don't know what it means and how to solve it... I'm really desperated! :S Any ideas?? Thanks in advance!

 December 20, 2010, 16:04 #2 Member   jonathan Join Date: Jun 2009 Posts: 47 Rep Power: 10 Have you used the FMG initializer at all to get a good approximate starting solution for your case? The FLUENT documentation describes how to use it. Also, try reducing your Courant number. The default is 200, try lowering that to between 50-100.

 December 20, 2010, 16:37 #3 New Member   marina Join Date: Nov 2010 Posts: 2 Rep Power: 0 I've already solve it!! I ran it in transient and it converged very well, then I changed to steady. Anyway, thank you for your advice

 December 21, 2010, 09:30 #4 Member   jonathan Join Date: Jun 2009 Posts: 47 Rep Power: 10 Glad to hear you solved the problem! Your solution does work but I imagine it took a while for you to wait until the solution settled out. I would suggest trying the solutions I've mentioned above. They should help you get a converged steady state solution much quicker than performing a transient analysis.

March 25, 2013, 06:15
#5
New Member

peter
Join Date: Mar 2013
Posts: 1
Rep Power: 0
Quote:
 Originally Posted by marina Hi everyone! I'm trying to simulate in Fluent a 2D compressible flow around the NACA 0012, in order to study the velocity profiles in the boundary layer. Reynolds=500000 and M=0.22 Boundary conditions: - inlet: pressure-inlet (direction x) - outlet: pressure-outlet - top & bottom: symmetry Turbulence: K-Epsilon. Intensity of turbulence: 3.1% and Lenght scale: 0.0976m Solution method: Pressure-velocity coupling, scheme: coupled. Second order After 15 iterations, three errors appear: # Divergence detected in AMG solver: pressure coupled --> Decreasing coarsening group size! # Divergence detected in AMG solver: pressure coupled --> Increasing relaxation sweeps! # Divergence detected in AMG solver: pressure coupled --> Changing to W-cycle! It also gives me some messages saying: turbulent viscosity limited to viscosity ratio of 10^5 in 1325 cells I don't know what it means and how to solve it... I'm really desperated! :S Any ideas?? Thanks in advance!
Hi,I'm sorry to disturb you.I met the same problem while I'm learning CFD.How do you solve it?

January 4, 2014, 13:29
#6
Senior Member

Tanjina Afrin
Join Date: May 2013
Location: South Carolina
Posts: 168
Rep Power: 5
Hello everyone,

Hope everyone is doing fine. I am modeling a 3D model of underdrain pipe flow. Details about the model is :

Inlet: pressure inlet with hydrostatic pressure
outlet: pressure outlet
wall and symmetry

Solver: Pressure based
Model: Standard k-e model
Solution Method: coupled
Least square cell based gradient and 2nd order upwind momentum

Time : transient

URF was all 1 except k and e. Those were 0.8

I started with 0.0015 time step. After running 5 second ( almost 35000 iteration) it gave the error " divergence detected in AMG solver: k"

so I reduced all URF to 0.7 . Then it gave the error " divergence detected in AMG solver: epsilon ". I reduced the URF to 0.5~0.6. Then it gave several error after 2.7 sec run time.

# Divergence detected in AMG solver: pressure coupled --> Decreasing coarsening group size!
# Divergence detected in AMG solver: pressure coupled --> Increasing relaxation sweeps!
# Divergence detected in AMG solver: pressure coupled --> Changing to W-cycle!

turbulent viscosity limited to viscosity ratio of 10^5 in 93000 cells.

Please find the following image for error and residuals. I tried with Flow courant number 100, but no improvement.

Before showing error it gives same mass flow rate in outlet for long time, which I assume as converged.

I am not an expert in CFD field. Trying to learn these things for my thesis. Hope someone can help me how can I get rid of these error. Thanks in advance

Regards,
Tanjina
Attached Images
 error.jpg (66.8 KB, 199 views) residuals.jpg (88.0 KB, 264 views)

January 31, 2014, 11:26
#7
New Member

Join Date: Apr 2013
Posts: 17
Rep Power: 5
Hey Tanjina!
Did you manage to find solution to you problem? I have similar problem right now.

Quote:
 Originally Posted by Tanjina Hello everyone, Hope everyone is doing fine. I am modeling a 3D model of underdrain pipe flow. Details about the model is : Inlet: pressure inlet with hydrostatic pressure outlet: pressure outlet wall and symmetry Solver: Pressure based Model: Standard k-e model Solution Method: coupled Least square cell based gradient and 2nd order upwind momentum Time : transient URF was all 1 except k and e. Those were 0.8 I started with 0.0015 time step. After running 5 second ( almost 35000 iteration) it gave the error " divergence detected in AMG solver: k" so I reduced all URF to 0.7 . Then it gave the error " divergence detected in AMG solver: epsilon ". I reduced the URF to 0.5~0.6. Then it gave several error after 2.7 sec run time. # Divergence detected in AMG solver: pressure coupled --> Decreasing coarsening group size! # Divergence detected in AMG solver: pressure coupled --> Increasing relaxation sweeps! # Divergence detected in AMG solver: pressure coupled --> Changing to W-cycle! turbulent viscosity limited to viscosity ratio of 10^5 in 93000 cells. Please find the following image for error and residuals. I tried with Flow courant number 100, but no improvement. Before showing error it gives same mass flow rate in outlet for long time, which I assume as converged. I am not an expert in CFD field. Trying to learn these things for my thesis. Hope someone can help me how can I get rid of these error. Thanks in advance Regards, Tanjina

 June 5, 2014, 11:30 That's the solution for # Divergence detected in AMG solver #8 New Member   Walder Ruis Join Date: Jan 2014 Location: Canada Posts: 23 Rep Power: 4 Hi Guys! It may come too late but it could help someone else in the future: When you get the following error message: # Divergence detected in AMG solver: pressure coupled --> Decreasing coarsening group size! # Divergence detected in AMG solver: pressure coupled --> Increasing relaxation sweeps! # Divergence detected in AMG solver: pressure coupled --> Changing to W-cycle! It actually means that your interfaces are not well configured; please check them on “Mesh Interfaces” and select “coupled wall” on “interface option” Please let us know if it works Thanks Loffy jumaah likes this.

 July 3, 2014, 06:26 # Divergence detected in AMG solver: temperature -> Increasing relaxation sweeps! # #9 New Member     Mohsen Besharat Join Date: Jan 2014 Location: Lisbon, Portugal Posts: 3 Rep Power: 4 Hi, I am modelling 2 hydro-pneumatic tanks in transient 2 phase condition. My model is planar. I have a ball valve in my model. I made the geometry by distinct faces. It works when I use coarse mesh size, but when I make my mesh finer, or change it to tri mesh even when I add inflation this error appears during first iteration! "# Divergence detected in AMG solver: temperature -> Increasing relaxation sweeps!" I tried hybrid initialization and defining my interfaces in ball valve as matching but no success. I think maybe my mesh is not joining well in fine mesh case at interfaces. But I do not know how to solve it. It is very urgent. Can anybody kindly help me to find the problem?

July 18, 2014, 01:48
Divergence detected
#10
New Member

Moscow
Join Date: Dec 2013
Posts: 4
Rep Power: 4
Quote:
 Originally Posted by zomayabssa Hi Guys! It may come too late but it could help someone else in the future: When you get the following error message: # Divergence detected in AMG solver: pressure coupled --> Decreasing coarsening group size! # Divergence detected in AMG solver: pressure coupled --> Increasing relaxation sweeps! # Divergence detected in AMG solver: pressure coupled --> Changing to W-cycle! It actually means that your interfaces are not well configured; please check them on “Mesh Interfaces” and select “coupled wall” on “interface option” Please let us know if it works Thanks Loffy
Could you clarify, zomayabssa, what if not enabled "Mesh Interfaces"?

Last edited by sawa25; July 18, 2014 at 05:00.

 July 29, 2014, 09:48 Divergence detected in AMG-solver: pressure coupled #11 New Member   Robert Join Date: Jul 2014 Location: Delft, The Netherlands Posts: 12 Rep Power: 4 Hi guys, being relatively new to CFD, I'm trying to analyse a model of a 3D-wing in FLUENT 14.5. I've put a link below where you can find my mesh, a little over 1,6*10^6 cells octree mesh from ICEM. Inviscid flow, Energy equations ON, all models turned off. I've chosen for the simplest options available to simply analyse lift on the wing. Materials: Fluid is air, ideal gas, default settings Boundary conditions Inlet: pressure-farfield, M=0.15, Gauge pressure=101325 Pa Farfield: pressure-farfield, M=0.15, Gauge pressure=101325 Pa Outlet: pressure-farfield, M=0.15, Gauge pressure=101325 Pa Symmetry: symmetry Wing: wall Flap: wall Solution methods Coupled scheme, default settings, Pseudo Transient Further more I've run a FMG-initialization with default settings, except for: Number of cycles on level 1: 100 Eventually my simulation will not start due to the error: Divergence detected in AMG-solver: pressure coupled I've tried using a SIMPLE scheme for the solution methods which let again to Divergence detected in AMG-solver: x-momentum. Also I've changed the combination of Boundary conditions (inlet - farfield - outlet): - pressure inlet - farfield - pressure outlet - velocity inlet - farfield - outflow - pressure inlet - wall - pressure outlet Below I've added links to my case and data as well. Mesh: https://www.dropbox.com/s/qdt066dkij...ACA23012-4.msh Case: https://www.dropbox.com/s/5tn6zyr2mp...ACA23012-4.cas Data: https://www.dropbox.com/s/dao5536kpc...ACA23012-4.dat My questions are: 1. Is my combination of Boundary Conditions feasible? 2. What do I have to change to get this going?

 July 30, 2014, 10:41 #12 New Member   Robert Join Date: Jul 2014 Location: Delft, The Netherlands Posts: 12 Rep Power: 4 I have some additional information concerning my error in the previous post: In the command window the following was displayed: Divergence detected in AMG solver: pressure coupled -> Decreasing coarsening group size! Divergence detected in AMG solver: pressure coupled -> Increasing relaxation sweeps! Divergence detected in AMG solver: pressure coupled -> Changing to W-cycle! Do I have to dig deeper into the AMG-solver or is there something wrong with my general settings? In the meantime - I have tried a different Mach number with the Boundary Conditions, being M=0.6. - Also I have changed the Relaxation Factor for Density from 1.0 to 0.5. - Furthermore I have changed the Convergence Criterion of the Residuals Monitors from 'absolute' to 'none'. - Finally I've changed the FMG Courant Number from 0.75 to 0.1 The latter two were attempts to solve the problem that I've read elsewhere here on the forum. However none of the above measures have solved the issue. Is there anyone who knows how to solve this and share his or her wisdom? chenhuismile likes this.

 September 22, 2014, 04:52 #14 New Member   Robert Join Date: Jul 2014 Location: Delft, The Netherlands Posts: 12 Rep Power: 4 Hi Kanmaniraja, Could please provide some additional information about your case? What is the size of your mesh, what does the quality report say, which 'Boundary Conditions' and 'Solution Controls' do you use? Your error can be caused by various issues, so the more information the better.

 June 17, 2015, 15:31 #15 New Member   Alexandru C Join Date: Jun 2015 Posts: 3 Rep Power: 3 Hi, Guys I am trying to simulate the fulfilling of a bottle with compressed air in Fluent. I set as boundary conditions Inlet_pressure - is a constant value, and mass flow inlet also a constant value. Models: viscous-laminar, energy on Now when I run the simulation I receive the following errors iter continuity x-velocity y-velocity energy time/iter # Divergence detected in AMG solver: temperature -> Increasing relaxation sweeps! Error: Divergence detected in AMG solver: temperature Error: Divergence detected in AMG solver: temperature Error Object: #f Any suggestions? 10x

 August 9, 2016, 20:15 #16 New Member   Omar jumaah Join Date: Jan 2016 Posts: 14 Rep Power: 2 thank zomayabssa, it works for my case Last edited by jumaah; August 9, 2016 at 20:18. Reason: editting

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