icoFoam VS pisoFoam-Laminar

 Register Blogs Members List Search Today's Posts Mark Forums Read

 November 25, 2011, 07:31 icoFoam VS pisoFoam-Laminar #1 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 Sponsored Links 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

 November 28, 2011, 04:59 #2 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 None?? none has experienced this issue?

 December 12, 2011, 06:46 #3 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 ______ | o /|\ /\ _______________

 December 13, 2011, 04:44 #4 Senior Member     Anton Kidess Join Date: May 2009 Location: Germany Posts: 1,244 Rep Power: 23 Compare the outputs of " turbulence->divDevReff(U)" and "- fvm::laplacian(nu, U)", and try to find out why they are not equal? __________________ *On twitter @akidTwit *Spend as much time formulating your questions as you expect people to spend on their answer.

 December 13, 2011, 05:17 #5 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 Thank you very much for your answer Where and how can I compare these outputs ? Sorry for my ignorance M.

 December 13, 2011, 06:11 #6 Member   Juho Peltola Join Date: Mar 2009 Location: Finland Posts: 89 Rep Power: 10 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.

 December 13, 2011, 10:17 #7 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 I set in turbulenceProperties dictionary : simulationType laminar; In addition, I have only p and U files in 0/ directory. M

 January 4, 2012, 19:04 #8 New Member   Xuan Ge Join Date: Oct 2011 Location: Ames, IA, US Posts: 6 Rep Power: 7 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

January 5, 2012, 04:37
#9
New Member

Join Date: Aug 2011
Location: Paris
Posts: 20
Rep Power: 7
Quote:
 Originally Posted by xuan8908 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.

Any hints would be welcome.

Regards.
M.

 January 5, 2012, 11:44 #10 New Member   Xuan Ge Join Date: Oct 2011 Location: Ames, IA, US Posts: 6 Rep Power: 7 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

 January 5, 2012, 12:19 #11 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 _____ | o /|\ /\ _______________No idea

 January 9, 2012, 17:34 #12 New Member   Xuan Ge Join Date: Oct 2011 Location: Ames, IA, US Posts: 6 Rep Power: 7 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 lynncheng likes this.

 January 10, 2012, 04:33 #13 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 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.

 January 10, 2012, 04:48 #14 Senior Member   Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Deltares, Delft, The Netherlands Posts: 1,747 Rep Power: 29 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 hua1015 likes this.

 January 10, 2012, 06:03 #15 New Member   Join Date: Aug 2011 Location: Paris Posts: 20 Rep Power: 7 Thank you, Niel, for this brilliant remark ! Indeed, this can explain the difference in results. I feel more relaxed now V

 January 10, 2012, 11:45 #16 New Member   Xuan Ge Join Date: Oct 2011 Location: Ames, IA, US Posts: 6 Rep Power: 7 Thanks, Niel, for your explanation, too. Best, Xuan

 May 17, 2013, 12:34 #17 Senior Member   Join Date: Jan 2013 Posts: 121 Rep Power: 6 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?

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post phsieh2005 OpenFOAM Running, Solving & CFD 38 July 7, 2017 01:09 Jade M CFX 14 June 15, 2016 09:36 titio OpenFOAM Running, Solving & CFD 2 March 8, 2013 05:44 Greg Givogue OpenFOAM Programming & Development 3 March 4, 2011 18:18 platopus OpenFOAM Bugs 8 April 15, 2008 07:52