CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   FLUENT (http://www.cfd-online.com/Forums/fluent/)
-   -   Problem with divergence (http://www.cfd-online.com/Forums/fluent/106674-problem-divergence.html)

TDK September 5, 2012 05:30

Problem with divergence
 
I'm trying to model a single wheel in free stream air within fluent.
I'm using the realizable k-epsilon turbulence model but when I run my calculation for around 20 iterations, at around the 6th one it says:
'Divergence detected in AMG solver: epsilon'

Anyone got any ideas on what this means?
I'd much appreciate the help!

stuart23 September 5, 2012 07:41

If you are sure your case is set up right, try the following:

Do a mesh check. Is your skewness acceptable?

Boundary Conditions. Are your k & e values suitable. http://support.esi-cfd.com/esi-users/turb_parameters/

Initialization. Try the hybrid (FMG) initialization.

Underrelaxation Factors. try using 0.5 for both k & e, then ramp them up to 0.9 after 50~100 iterations. Also decrease timestep or Courant Number (dependent on solver)


Hopefully one of these will work

Stu

TDK September 5, 2012 09:40

Thanks alot Stu, it worked :D
but now, after around 12 iterations, i get the message,
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in (a number) cells

And that message just keeps appearing for every iteration after the 12th one or so.

stuart23 September 5, 2012 10:17

Does the number of cells it occurs in decrease? Also, which of the above solutions caused your simulation to run?

Maybe try turning on wall functions. You will find this in your turbulence settings

TDK September 5, 2012 10:26

erm my skewness and mesh was fine so I just tried out the initialization, because im following up some work that was done before and I am trying to keep the boundary conditions the same as the work done before.

Also the number of cells it occurs in increases.

stuart23 September 5, 2012 10:47

Maybe try decreasing the Turbulent Viscosity UnderRelaxation Factor. Also could try higher order schemes for turbulence terms.

What p-v scheme are you using?

Stu

yonchong September 5, 2012 11:35

In material, what are you using for the air? If you are running incompressible then are you using incompressible ideal gas for the density?

pghoseju September 6, 2012 01:57

# Divergence detected in AMG solver: w-swirl -> Increasing relaxation sweeps!

Divergence detected in AMG solver: w-swirl
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: w-swirl
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: w-swirl
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: w-swirl
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Primitive Error at Node 0: floating point exception

Primitive Error at Node 1: floating point exception

Primitive Error at Node 2: floating point exception

Primitive Error at Node 3: floating point exception

Error: floating point exception
Error Object: #f
any body help me?

TDK September 6, 2012 04:32

when i tried turnin the wall function on, after 95 iterations or so i get this message again..
Error: Divergence detected in AMG solver: epsilon
Error Object: #f

and the p-v scheme is the simplec method

TDK September 6, 2012 05:59

Calculating the solution...
iter continuity x-velocity y-velocity z-velocity k epsilon cl cd time/iter
10 3.2734e-01 3.0660e-04 1.1532e-03 1.9049e-03 4.5918e-02 3.2611e-01 -3.1384e-03 -7.0838e-03 0:26:06 40
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 75 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 37 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 44 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 55 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 70 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 76 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 84 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 90 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 100 cells
20 1.8165e-01 3.1788e-04 9.4947e-04 1.2923e-03 6.6196e-02 9.0918e-02 -5.8485e-03 6.6261e-04 0:18:44 30
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 113 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 87 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 106 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 110 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 110 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 110 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 119 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 116 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 115 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 108 cells
30 1.6222e-01 1.1946e-03 1.1032e-03 1.3504e-03 2.6384e-01 3.9193e-01 -9.1864e-03 4.6980e-03 0:12:29 20
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 103 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 107 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 108 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 114 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 117 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 116 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 116 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 116 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 113 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 112 cells
40 1.7292e-01 6.6118e-04 9.4381e-04 1.1887e-03 6.4812e-02 1.8510e-01 -9.7351e-03 1.0745e-02 0:06:10 10
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 112 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 117 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 116 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 115 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 117 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 117 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 116 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 116 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 117 cells
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 114 cells
50 1.7279e-01 6.0444e-04 8.8136e-04 1.1025e-03 3.0306e-02 6.1610e-02 -9.8257e-03 2.0159e-02 0:00:00 0
Calculation complete.
Interrupting...
Done.

I'm still getting this same error...any ideas on what it could be :S

stuart23 September 8, 2012 01:11

Could be due to huge gradients across adjacent cells. Try refining around your body and using higher order numerics for turbulence.

Stu


All times are GMT -4. The time now is 23:26.