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-   -   icoFoam VS pisoFoam-Laminar (http://www.cfd-online.com/Forums/openfoam-solving/94785-icofoam-vs-pisofoam-laminar.html)

M3hdi November 25, 2011 07:31

icoFoam VS pisoFoam-Laminar
 
Hi every body,

I have a simple question: I run two simulations (flow over a circular cylinder Re = 150 : laminar unsteady case). The first one using icoFoam, and the second one using pisoFoam with turbulence turned off (laminar). The two setups are exactly identical.

Results : with icoFoam I got a correct Strouhal number (Sr = 0.156) by plotting the Cl, wheras pisoFoam gives a quite lower value (Sr = 0.0527) !!

I am a bit puzzling since I expected that: (pisoFoam + laminar) = icoFoam.

Any help please ??

Mehdi

M3hdi November 28, 2011 04:59

None??

none has experienced this issue?

:confused:

M3hdi December 12, 2011 06:46

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akidess December 13, 2011 04:44

Compare the outputs of " turbulence->divDevReff(U)" and "- fvm::laplacian(nu, U)", and try to find out why they are not equal?

M3hdi December 13, 2011 05:17

Thank you very much for your answer :)

Where and how can I compare these outputs ?

Sorry for my ignorance

M.

juho December 13, 2011 06:11

Make sure the turbulence model is set to "laminar". Turning turbulence "off" just means that the turbulence model is not corrected during the solution. Depending on your initial k and epsilon fields, they can still affect the effective viscosity and thus the simulation results.

M3hdi December 13, 2011 10:17

I set in turbulenceProperties dictionary :

simulationType laminar;

:confused:

In addition, I have only p and U files in 0/ directory.

M

xuan8908 January 4, 2012 19:04

Mehdi

Have you already solved your issue?

Basically, I also have a similar problem. I run a case (flow over a square cylinder with Re=22000) by both pisoFoam and pimpleFoam. The Strouhal number in pimpleFoam matches the experimental data (0.132), but the one in pisoFoam (approximately 0.05) does not. It's really like what you got.

Both of the two simulations use RANS K-OmegaSST turbulent model. The only differences for setup were 'application' in system/controlDict (the former use pisoFoam, the latter use pimpleFoam) and the algorithms in system/fvSolution (the former use PISO, the latter use PIMPLE).

Xuan

M3hdi January 5, 2012 04:37

Quote:

Originally Posted by xuan8908 (Post 337917)
Mehdi

Have you already solved your issue?

Basically, I also have a similar problem. I run a case (flow over a square cylinder with Re=22000) by both pisoFoam and pimpleFoam. The Strouhal number in pimpleFoam matches the experimental data (0.132), but the one in pisoFoam (approximately 0.05) does not. It's really like what you got.

Both of the two simulations use RANS K-OmegaSST turbulent model. The only differences for setup were 'application' in system/controlDict (the former use pisoFoam, the latter use pimpleFoam) and the algorithms in system/fvSolution (the former use PISO, the latter use PIMPLE).

Xuan

Dear Xuan,

Your problem seems like mine indeed ! Unfortunately, I have not solved it.
For the time being, I concluded that pisoFoam is adapted to turbulent unsteady cases and icoFoam to laminar unsteady ones... but I still worried about this strange behavior..

Any hints would be welcome.

Regards.
M.

xuan8908 January 5, 2012 11:44

Thank you for your reply.

Your conclusion is correct. But based on my results, pisoFoam does not work in unsteady turbulent case also.

Best,

Xuan

M3hdi January 5, 2012 12:19

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xuan8908 January 9, 2012 17:34

Hi, Mehdi,

I have already figured out the reason why I got a wrong Strouhal number in this unsteady case. Actually, I used relaxationFactors in fvSoltuion dictionary because I just copied what I had in my steady case. But generally, relaxation factors are particularly used for steady case to improve the stability of computing. After I removed those things, I got good solutions and correct Strouhal number as well.

Hopefully, it will be helpful to you.

Xuan

M3hdi January 10, 2012 04:33

Thank you Xuan for sharing your ideas :)

I checked my fvSolution files for icoFoam and pisoFoam simulations and in both cases I found relaxation factors. For you, I have to remove them for both simulations since they are unsteady cases ? (I will check both cases without these relaxation factors and I will tell you if this solves my problem ..)

An other question : I have run both simulations (icoFoam and pisoFoam) with relaxation factors and icoFoam gives good Strouhal number wheras pisoFoam has failed. If relaxation factors are the only cause of false results, can we conclude that icoFoam does not use them where pisoFoam does ?

Thank you again for your remark, I will dig on that in the next weeks ..

regards
M.

ngj January 10, 2012 04:48

Hi Mehdi

The reason that you see different results when using icoFoam/pisoFoam with the same relaxation parameter is very simple: The relaxation parameters does not have any effect what so ever in icoFoam, as the line:

Code:

UEqn.relax();
is not present in the code, whereas, you can find it is pisoFoam.

Kind regards,

Niels

M3hdi January 10, 2012 06:03

Thank you, Niel, for this brilliant remark !

Indeed, this can explain the difference in results.

I feel more relaxed now

V

xuan8908 January 10, 2012 11:45

Thanks, Niel, for your explanation, too.

Best,
Xuan

kkpal May 17, 2013 12:34

this is a great thread!!
I've also been trapped by the same problem for half a month, now it is solved!!

I'm using pimpleFoam and relaxation is allowed in this. as long as we are doing unsteady cases, should we never use relaxation for future simulations?


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