[nedved]

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]

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

[ivanyao]

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]

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]

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]

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]

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]

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

[nimasam]

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

[sima_op]

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]

@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]

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]

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]

Originally Posted by sima_op

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.

[quarkz]

Hi all,

I am looking for the airfoil script and all the necessary files to run the airfoil e.g. but the files are no longer available online (http://www-rocq.inria.fr/macs/spip.php?rubrique69). Can anyone send them to me?

Thanks alot!

[Deagle]

Hi,

I also had problems with epsilon and k diverging quite quickly (using simpleFoam), but as mentioned lowering the relaxation factors fixed the whole problem for me.

[ErikRtvl]

I ran into the same problems, simpleFoam diverging after a few iterations.

What finally solved it for me - after checking BC's and everything multiple times - is to change the ddt scheme to "steadyState". Actually quite logical...

Furthermore, as stated above, I use the bounded Gauss divergence schemes as well. However that is actually something simpleFoam warns you about; if the 'normal' Gauss schemes are used, it tells me to change to bounded divergence schemes.

The explanation for that is actually in the open foam user guide I found here:

https://cfd.direct/openfoam/user-gui...19-1400004.4.3

Finally, some key words to help others find this thread that includes valuable solutions (just registered to post my solution, so I don't know if it helps....)

openFoam kEpsilon divergence simpleFoam 2D