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-   -   SimpleFoam k and epsilon bounded (http://www.cfd-online.com/Forums/openfoam-solving/58221-simplefoam-k-epsilon-bounded.html)

nedved November 25, 2008 10:53

Hello everyone. I have been
 
Hello everyone.

I have been working on the nlf-0414 laminar airfoil. I have calculated the drag and lift coefficients for various angles and have got decent results. I have been using the steady state solver - simpleFoam - for unstructured grids. My simulations are two-dimensional.

I am now working on an ice accreted nlf-0414 airfoil. I am using the same solver settings as the non-ice nlf-0414 airfoil but my solution blows up after a few iterations. Here are some details about the case:

I created the mesh in ICEMCFD. I converted it into a fluent mesh which i then converted into foam format using fluent3dMeshTofoam.

As the case is 2D, i am using empty boundaries on the left and right sides of the domain. I am using inlet and outlet boundaries and walls for the airfoil, top and bottom regions.

I am using the k-epsilon turbulence model. I estimated k to be 20.23 and epsilon to be 166.12. I used these same settings for the non-ice cases which converged fine. My Reynolds number based on airfoil chord (0.914m) is 4.6x10^6 and inlet velocity is 73.45m/s.

I am attaching my case file.



When i run my case this is the problem i am having:

Create time

Create mesh for time = 0

Reading field p

Reading field U

Reading/calculating face flux field phi

Selecting incompressible transport model Newtonian
Selecting RAS turbulence model kEpsilon
kEpsilonCoeffs
{
Cmu 0.09;
C1 1.44;
C2 1.92;
alphaEps 0.76923;
}


Starting time loop

Time = 1

DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 0.0868802, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 1, Final residual = 0.0384705, No Iterations 1
ICE default IO error handler doing an exit(), pid = 25139, errno = 0
DICPCG: Solving for p, Initial residual = 1, Final residual = 0.000974596, No Iterations 453
time step continuity errors : sum local = 0.0443247, global = -0.000256781, cumulative = -0.000256781
DILUPBiCG: Solving for epsilon, Initial residual = 0.177983, Final residual = 0.0134905, No Iterations 1
bounding epsilon, min: -5124.54 max: 2.59707e+06 average: 23725.9
DILUPBiCG: Solving for k, Initial residual = 1, Final residual = 0.0221967, No Iterations 1
ExecutionTime = 75.45 s ClockTime = 77 s

Time = 2

DILUPBiCG: Solving for Ux, Initial residual = 0.00022456, Final residual = 1.72182e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.171213, Final residual = 6.76872e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.199818, Final residual = 0.00173239, No Iterations 1001
time step continuity errors : sum local = 0.25953, global = 0.000524615, cumulative = 0.000267833
DILUPBiCG: Solving for epsilon, Initial residual = 0.380199, Final residual = 3.54249e-13, No Iterations 1
bounding epsilon, min: -62.2232 max: 6.84878e+06 average: 26467.3
DILUPBiCG: Solving for k, Initial residual = 0.00031994, Final residual = 5.93409e-06, No Iterations 1
ExecutionTime = 211.38 s ClockTime = 223 s

Time = 3

DILUPBiCG: Solving for Ux, Initial residual = 0.999831, Final residual = 0.0334756, No Iterations 2
DILUPBiCG: Solving for Uy, Initial residual = 0.999976, Final residual = 0.0290285, No Iterations 2
DICPCG: Solving for p, Initial residual = 0.783759, Final residual = 0.000762911, No Iterations 635
time step continuity errors : sum local = 5126.09, global = -115.949, cumulative = -115.949
DILUPBiCG: Solving for epsilon, Initial residual = 1, Final residual = 0.0740209, No Iterations 2
DILUPBiCG: Solving for k, Initial residual = 1, Final residual = 0.000545361, No Iterations 1
ExecutionTime = 304.56 s ClockTime = 319 s

Time = 4

DILUPBiCG: Solving for Ux, Initial residual = 0.637303, Final residual = 0.0534082, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.392361, Final residual = 0.000317455, No Iterations 2
DICPCG: Solving for p, Initial residual = 0.949414, Final residual = 0.00213447, No Iterations 1001
time step continuity errors : sum local = 4384.93, global = -1118.18, cumulative = -1234.13
DILUPBiCG: Solving for epsilon, Initial residual = 0.255896, Final residual = 0.0139967, No Iterations 1
bounding epsilon, min: -1.31507e+11 max: 2.87675e+27 average: 9.48764e+21
DILUPBiCG: Solving for k, Initial residual = 0.999977, Final residual = 0.092381, No Iterations 1
ExecutionTime = 441.92 s ClockTime = 466 s

Time = 5

DILUPBiCG: Solving for Ux, Initial residual = 0.565411, Final residual = 0.000464199, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.361508, Final residual = 0.000488244, No Iterations 1
DICPCG: Solving for p, Initial residual = 5.22619e-06, Final residual = 1.98698e-08, No Iterations 1
time step continuity errors : sum local = 1.87592e+10, global = -390.727, cumulative = -1624.85
DILUPBiCG: Solving for epsilon, Initial residual = 0.178086, Final residual = 0.0001416, No Iterations 1
bounding epsilon, min: -3.05682e+26 max: 2.8755e+33 average: 5.0373e+27
DILUPBiCG: Solving for k, Initial residual = 0.0246705, Final residual = 0.000480176, No Iterations 1
bounding k, min: -341683 max: 5.29681e+24 average: 2.90027e+19
ExecutionTime = 452.27 s ClockTime = 477 s

Time = 6

DILUPBiCG: Solving for Ux, Initial residual = 0.00134352, Final residual = 1.02079e-05, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.0010846, Final residual = 5.96071e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 3.86649e-06, Final residual = 8.28192e-08, No Iterations 3
time step continuity errors : sum local = 1.84347e+19, global = 11386.1, cumulative = 9761.24
DILUPBiCG: Solving for epsilon, Initial residual = 2.93311e-10, Final residual = 2.93311e-10, No Iterations 0
DILUPBiCG: Solving for k, Initial residual = 0.114498, Final residual = 0.00215299, No Iterations 1
bounding k, min: -332156 max: 1.09647e+40 average: 8.20484e+34
ExecutionTime = 462.33 s ClockTime = 488 s

Time = 7

DILUPBiCG: Solving for Ux, Initial residual = 0.0498993, Final residual = 0.000172662, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.0784033, Final residual = 0.000290499, No Iterations 1
DICPCG: Solving for p, Initial residual = 2.31156e-23, Final residual = 2.31156e-23, No Iterations 0
time step continuity errors : sum local = 1.52759e+30, global = 1.7678e+14, cumulative = 1.7678e+14
DILUPBiCG: Solving for epsilon, Initial residual = 0.197579, Final residual = 0.00838687, No Iterations 1
bounding epsilon, min: -7.90723e+56 max: 8.65931e+63 average: 4.08564e+58
DILUPBiCG: Solving for k, Initial residual = 0.999368, Final residual = 0.0632189, No Iterations 1
bounding k, min: -4.54562e+29 max: 1.42308e+47 average: 1.79576e+42
ExecutionTime = 471.52 s ClockTime = 500 s

Time = 8

DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 7.10734e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.646717, Final residual = 9.62027e-09, No Iterations 1
DICPCG: Solving for p, Initial residual = 4.36332e-21, Final residual = 4.36332e-21, No Iterations 0
time step continuity errors : sum local = 1.27322e+32, global = 8.48049e+15, cumulative = 8.65727e+15
DILUPBiCG: Solving for epsilon, Initial residual = 0.997213, Final residual = 2.00069e-08, No Iterations 1
bounding epsilon, min: -1.51347e+71 max: 1.90819e+89 average: 7.36319e+83
DILUPBiCG: Solving for k, Initial residual = 0.203853, Final residual = 7.00189e-08, No Iterations 1
bounding k, min: -9.21274e+43 max: 3.56366e+57 average: 1.11523e+52
ExecutionTime = 480.72 s ClockTime = 509 s

Time = 9

DILUPBiCG: Solving for Ux, Initial residual = 4.80891e-09, Final residual = 4.80891e-09, No Iterations 0
DILUPBiCG: Solving for Uy, Initial residual = 4.77966e-09, Final residual = 4.77966e-09, No Iterations 0
DICPCG: Solving for p, Initial residual = 1.15667e-20, Final residual = 1.15667e-20, No Iterations 0
time step continuity errors : sum local = 2.88697e+32, global = 1.34591e+17, cumulative = 1.43248e+17
DILUPBiCG: Solving for epsilon, Initial residual = 9.2793e-12, Final residual = 9.2793e-12, No Iterations 0
bounding epsilon, min: 4.97127e-19 max: 1.90819e+89 average: 7.36319e+83
DILUPBiCG: Solving for k, Initial residual = 2.00211e-06, Final residual = 2.97586e-13, No Iterations 1
bounding k, min: -6.9942e+51 max: 1.64917e+75 average: 4.51736e+69
ExecutionTime = 490.87 s ClockTime = 521 s

Time = 10

DILUPBiCG: Solving for Ux, Initial residual = 4.50988e-06, Final residual = 3.0425e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 2.41985e-05, Final residual = 1.47112e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 3.28653e-20, Final residual = 3.28653e-20, No Iterations 0
time step continuity errors : sum local = 5.24766e+32, global = -6.73284e+15, cumulative = 1.36515e+17
DILUPBiCG: Solving for epsilon, Initial residual = 7.52866e-30, Final residual = 7.52866e-30, No Iterations 0
DILUPBiCG: Solving for k, Initial residual = 3.68813e-06, Final residual = 1.75885e-07, No Iterations 1
bounding k, min: -6.74138e+68 max: 1.08664e+75 average: 2.03979e+70
ExecutionTime = 500.85 s ClockTime = 533 s


K and epsilon keep on being bounded which causes the case to diverge. I have tried to adjust the inlet values of k and epsilon but nothing changes. I have looked through the forum for similar problems but still cannot fix my problem. I would be very greatful for any suggestions to improve my case.

Also, to change from the k-epsilon to spalartallmaras turbulence model, what changes do i have to make in my case?

ameya December 6, 2008 05:36

Dear Naveed even i have faced
 
Dear Naveed
even i have faced such a problem.
The only solution i came across was to use hex mesh

ivanyao December 9, 2008 22:04

hi, I meet the same problem
 
hi,
I meet the same problem ever, but when i am change the BC,i fix the problem.
you should check your BC again.
ivan

lord_kossity December 10, 2008 04:45

dear ivan/yao, what do you
 
dear ivan/yao,

what do you change your BC to? And which BC do you modify, only k and eps or also U?

Kind regards,
Andreas

wolle1982 December 11, 2008 05:27

try the komegaSST solver. this
 
try the komegaSST solver. this works better. the epsilon model calculates freestream vorticities very good. the komegaSST is better used on airfoils where the freestream vorticities are not so important.

DLC April 8, 2010 11:15

Hi Foamers,
I know I'm entering the conversation quite late, but i'm having more or less the same problems...

I'm new to simpleFoam, and i'm testing it on a NACA0012. (2D problem)
I use hex mesh generated using a fortran scripts found here (http://www-rocq.inria.fr/macs/spip.php?rubrique69)... i have to say they generate a very nice mesh...

I'm having some issues understanding which turbulence mode I must use...
the best seems to be the k-e with the same values as nedved.
I tested the realizable k-e but the solution never reaches a steady state..
I tried the SST, but the solution blows up after few iterations... can anyone tell me the values for the SST coefficients? I used the ones I used for an interDyMFoam case and in that case they were fine...

which BC are you using? I'm using more or less the same as nedved..

last, and very important thing.... how can i find out forces on the airfoil? someone told me that it was possible while post-processing in paraview, but I couldn't figure out how.. does anyone know? the code doesn't calculate forces anywhere, right?

answers would result quite important for me....
thanks a lot...

DLC

visakhmg October 25, 2013 03:30

Has anyone got the solution to this problem yet? About bounding of epsilon values? please help. I am faving the same proble. Thanks in advance.
\vmg

sima_op October 28, 2013 05:25

hallo, i'm facing this probleme,too. need for help.

nimasam October 28, 2013 05:56

first try to reach steady-state solution in laminar condition, then turn on turbulence

sima_op October 28, 2013 06:39

yes, thank you very much for your reply.
in fact, i have tried to off the turbulence in RASProperties,and it works very well. But if i turn on turbulence after 400 iterations(i'm not sure when it will reach the steady-state), there will be the same wrong message.

visakhmg October 29, 2013 04:27

@nimasam; thankyou.
Yeah, i had tried with running laminar for some time and switching on turbulence later (at once). Still it has got the same problem of unbounding epsilon (it shows "bounding epsilon, min: -0.000470716658863 max: 454112.158676 average: 133.216441665"). The geometry is little heavy, with porous media and i have noticed unbounding of epsilon happens even if i give bounding limits. Is it like it bounds the value of epsilon after printing? The velocity and pressure fields are comparable to fluent results though, but i am not satisfied. Can you tell me which is the best option:
running pottentialFoam to get inviscid flow pattern followed by porousSimpleFoam; or running laminar followed by switching turbulence at once; or still gradually increasing the turbulence intensity? Or is it the boundary conditions and initial conditions of k and epsilon? Thanks in advance..
\vmg

nimasam October 29, 2013 04:54

well, all options are on the table :)

running potentialFlow in the begining of simulation will reduce the time of reaching steady-state, if you face sever condition then you may want to turn off turbulence and give a laminar result which can be an initial for lateral turbulence modeling.
if you still face the difficulty you may want to:
1- check your BC
2-check your fvScheme
3- use relaxation in fvSolution

sima_op October 29, 2013 06:19

hallo, thanks very much for your reply to this question, my simulation was converge after i made changes in FvScheme, i changed the 'gausse linear' to 'bounded Gauss upwind' for div(phi,U)div(phi,k)div(phi,k)and div(phi,epsilon), i hope these can help you a litte,too.
However, in fact, i still don't understand why i should changes them....it will be very kind if anyone can explaine that for me..

kreddy November 4, 2013 15:13

Quote:

Originally Posted by sima_op (Post 459603)
hallo, thanks very much for your reply to this question, my simulation was converge after i made changes in FvScheme, i changed the 'gausse linear' to 'bounded Gauss upwind' for div(phi,U)div(phi,k)div(phi,k)and div(phi,epsilon), i hope these can help you a litte,too.
However, in fact, i still don't understand why i should changes them....it will be very kind if anyone can explaine that for me..

In general, upwind schemes tend to suppress numerical oscillations and would be more stable than linear schemes.
This means that they also tend to suppress the turbulent fluctuations. So if you are only interested in computing the mean flow (and hence using a RANS model like k-epsilon), then upwind schemes are a good option. For cases where you want to capture the unsteady flow (say, in LES) you will have to use linear schemes.


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