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-   -   Divergence in AMG solver! (http://www.cfd-online.com/Forums/fluent/83103-divergence-amg-solver.html)

marina December 14, 2010 14:49

Divergence in AMG solver!
 
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!

jonny_b December 20, 2010 16:04

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.

marina December 20, 2010 16:37

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 ;)

jonny_b December 21, 2010 09:30

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.

songlike163 March 25, 2013 06:15

Quote:

Originally Posted by marina (Post 287440)
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?
Thanks in advance!

Tanjina January 4, 2014 13:29

2 Attachment(s)
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

nnvoro January 31, 2014 11:26

Hey Tanjina!
Did you manage to find solution to you problem? I have similar problem right now.

Quote:

Originally Posted by Tanjina (Post 468598)
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


zomayabssa June 5, 2014 11:30

That's the solution for # Divergence detected in AMG solver
 
Hi Guys!

It may come too late but it could help someone else in the future:
When you get the following error message: :rolleyes:

# 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

mohsenhiva July 3, 2014 06:26

# Divergence detected in AMG solver: temperature -> Increasing relaxation sweeps! #
 
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?

sawa25 July 18, 2014 01:48

Divergence detected
 
Quote:

Originally Posted by zomayabssa (Post 495749)
Hi Guys!

It may come too late but it could help someone else in the future:
When you get the following error message: :rolleyes:

# 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"?

findtheinvisiblecow July 29, 2014 09:48

Divergence detected in AMG-solver: pressure coupled
 
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?

findtheinvisiblecow July 30, 2014 10:41

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?

kanmaniraja September 19, 2014 06:32

Using Multiphase
 
Hi, I am analysis injector in multiphase, during the analysis I am getting error as" Divergence detected in AMG solver: pressure coupled "

Please help me

Models
------

Model Settings
-----------------------------------------------------------------
Space Axisymmetric
Time Unsteady, 2nd-Order Implicit
Viscous Standard k-epsilon turbulence model
Wall Treatment Standard Wall Functions
Multiphase k-epsilon Models Dispersed Approach
Heat Transfer Enabled
Solidification and Melting Disabled
Radiation None
Species Disabled
Coupled Dispersed Phase Disabled
NOx Pollutants Disabled
SOx Pollutants Disabled
Soot Disabled
Mercury Pollutants Disabled

Models
------

Model Settings
-----------------------------------------------------------------
Space Axisymmetric
Time Unsteady, 2nd-Order Implicit
Viscous Standard k-epsilon turbulence model
Wall Treatment Standard Wall Functions
Multiphase k-epsilon Models Dispersed Approach
Heat Transfer Enabled
Solidification and Melting Disabled
Radiation None
Species Disabled
Coupled Dispersed Phase Disabled
NOx Pollutants Disabled
SOx Pollutants Disabled
Soot Disabled
Mercury Pollutants Disabled

Boundary Conditions
-------------------

Zones

name id type
----------------------------------------
fuel_inlet 5 velocity-inlet
oxidizer_inlet 6 velocity-inlet
wall_ 7 wall
pressure_outlet 8 pressure-outlet
axis 9 axis
wall-surface_body 10 wall

Setup Conditions

fuel_inlet

Condition Value
--------------------------------------------------
Supersonic/Initial Gauge Pressure (pascal) 0
is zone used in mixing-plane model? no

oxidizer_inlet

Condition Value
--------------------------------------------------
Supersonic/Initial Gauge Pressure (pascal) 0
is zone used in mixing-plane model? no

wall_

Condition Value
-------------------------------------------------------------
Wall Thickness (mm) 0
Heat Generation Rate (w/m3) 0
Material Name aluminum
Thermal BC Type 1
Temperature (k) 300
Heat Flux (w/m2) 0
Convective Heat Transfer Coefficient (w/m2-k) 0
Free Stream Temperature (k) 300
Wall Motion 0
Shear Boundary Condition 0
Define wall motion relative to adjacent cell zone? yes
Apply a rotational velocity to this wall? no
Velocity Magnitude (m/s) 0
X-Component of Wall Translation 1
Y-Component of Wall Translation 0
Define wall velocity components? no
X-Component of Wall Translation (m/s) 0
Y-Component of Wall Translation (m/s) 0
External Emissivity 1
External Radiation Temperature (k) 300
Wall Roughness Height (mm) 0
Wall Roughness Constant 0.5
Rotation Speed (rad/s) 0
X-component of shear stress (pascal) 0
Y-component of shear stress (pascal) 0
Fslip constant 0
Eslip constant 0
Surface tension gradient (n/m-k) 0
Specularity Coefficient 0
Convective Augmentation Factor 1

pressure_outlet

Condition Value
-----------------------------------------------
Gauge Pressure (pascal) 0
Backflow Direction Specification Method 1
is zone used in mixing-plane model? no

axis

Condition Value
-----------------

wall-surface_body

Condition Value
-------------------------------------------------------------
Wall Thickness (mm) 0
Heat Generation Rate (w/m3) 0
Material Name aluminum
Thermal BC Type 1
Temperature (k) 300
Heat Flux (w/m2) 0
Convective Heat Transfer Coefficient (w/m2-k) 0
Free Stream Temperature (k) 300
Wall Motion 0
Shear Boundary Condition 0
Define wall motion relative to adjacent cell zone? yes
Apply a rotational velocity to this wall? no
Velocity Magnitude (m/s) 0
X-Component of Wall Translation 1
Y-Component of Wall Translation 0
Define wall velocity components? no
X-Component of Wall Translation (m/s) 0
Y-Component of Wall Translation (m/s) 0
External Emissivity 1
External Radiation Temperature (k) 300
Wall Roughness Height (mm) 0
Wall Roughness Constant 0.5
Rotation Speed (rad/s) 0
X-component of shear stress (pascal) 0
Y-component of shear stress (pascal) 0
Fslip constant 0
Eslip constant 0
Surface tension gradient (n/m-k) 0
Specularity Coefficient 0
Convective Augmentation Factor 1


Solver Settings
---------------

Equations

Equation Solved
------------------------
Flow yes
Volume Fraction yes
Turbulence yes
Energy yes

Numerics

Numeric Enabled
---------------------------------------
Absolute Velocity Formulation yes

Unsteady Calculation Parameters


-------------------------------------
Time Step (s) 1e-06
Max. Iterations Per Time Step 20

Relaxation

Variable Relaxation Factor
----------------------------------------------
Density 1
Body Forces 1
Volume Fraction 1
Granular Temperature 1
Turbulent Kinetic Energy 1
Turbulent Dissipation Rate 1
Turbulent Viscosity 1
Energy 1

Linear Solver

Solver Termination Residual Reduction
Variable Type Criterion Tolerance
------------------------------------------------------------------------
Flow F-Cycle 0.1
Turbulent Kinetic Energy Flexible 0.1 0.7
Turbulent Dissipation Rate Flexible 0.1 0.7
Energy Flexible 0.1 0.7

Pressure-Velocity Coupling

Parameter Value
--------------------------------------------
Type Coupled
Pseudo Transient no
Flow Courant Number 1
Explicit momentum under-relaxation 1
Explicit pressure under-relaxation 1

Discretization Scheme

Variable Scheme
------------------------------------------------
Density Second Order Upwind
Momentum Second Order Upwind
Volume Fraction Modified HRIC
Turbulent Kinetic Energy Second Order Upwind
Turbulent Dissipation Rate Second Order Upwind
Energy Second Order Upwind

Solution Limits

Quantity Limit
---------------------------------------
Minimum Absolute Pressure 1
Maximum Absolute Pressure 5e+10
Minimum Temperature 1
Maximum Temperature 5000
Minimum Turb. Kinetic Energy 1e-14
Minimum Turb. Dissipation Rate 1e-20
Maximum Turb. Viscosity Ratio 100000

Mesh Quality:
Orthogonal Quality ranges from 0 to 1, where values close to 0 correspond to low quality.
Minimum Orthogonal Quality = 1.51819e-01
Maximum Aspect Ratio = 2.58088e+01


Mesh Size

Level Cells Faces Nodes Partitions
0 29209 59553 30345 1

1 cell zone, 7 face zones.

findtheinvisiblecow September 22, 2014 04:52

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.

Alex07 June 17, 2015 15:31

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


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