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July 28, 2011, 03:53 
moving reference frame  weird results!

#1 
Senior Member

Hi experts,
I'm a newbie in OF and I have tried to change simpleFoam solver to achieve oscillatory reference frame, it seems that it's not a big deal but I faced weird results. I did these changes: Code:
scalar Pi = mathematicalConstant::pi; dimensionedVector omega ( "omega", dimensionSet(0, 0, 1, 0, 0, 0, 0), vector (0.0, 0.0, (amp*(2.0*Pi*freq)*Foam::cos((2.0*Pi*freq)*runTime.value()))) ); dimensionedVector alpha ( "alpha", dimensionSet(0, 0, 2, 0, 0, 0, 0), vector (0.0, 0.0, (amp*pow(2.0*Pi*freq,2)*Foam::sin((2.0*Pi*freq)*runTime.value()))) ); volVectorField source = (2.0*omega ^ U) + (omega ^ (omega ^ mesh.C())) + (alpha ^ mesh.C()); Code:
tmp<fvVectorMatrix> UEqn ( fvm::ddt(U) + fvm::div(phi, U)  fvm::laplacian(nu, U) == source ); Any suggestion or experience in this field is really appreciated. Amir Last edited by Amir; July 28, 2011 at 04:45. 

July 30, 2011, 03:21 

#3 
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Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
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Oscillatory system (=unsteady) in steady state solver (simpleFoam)?
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

July 30, 2011, 04:23 

#4  
Senior Member

Quote:
I have added the transient term to UEqn., fvm::ddt(U); I have used unsteady simpleFoam instead of icoFoam because it's more stable and I could use larger time steps. Do you mean this procedure is incorrect? Amir 

July 30, 2011, 04:34 

#5 
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Alberto Passalacqua
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Are you doing subinterations inside each time step? You can find it already done in pimpleFoam.
Best,
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

July 30, 2011, 04:44 

#6 
Senior Member


July 30, 2011, 15:32 

#7 
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Alberto Passalacqua
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Location: Ames, Iowa, United States
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Hi,
to answer your question you should know if your system reaches a steady state or not. Since it is intrinsically unsteady, it might never reach one. Anyway, to use SIMPLE for unsteady calculations you should:
Best,
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 3, 2011, 07:29 

#9 
Senior Member

Dear Alberto,
I did your recommandations, but now, I have a problem in convergency. I change both pimpleFoam and also pisoFoam but convergency issues exist. I couldn't reduce initail residuals in this case, consequently, there are some differences between results and analytic solution. Do you have any recommendation about another schemes or others? I used different grids and also schemes but I couldn't achieve better convergency. 

August 3, 2011, 10:17 

#10 
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Alberto Passalacqua
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The residual behaviour seems to show the solution is oscillating, which might indicate also a problem in the case setup.
However, you are using inconsistent numerical schemes:
Best,
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 3, 2011, 11:39 

#11  
Senior Member

Quote:
Although the results are better know but there are some differences between this results and analytic one. Anyway, I just want to ensure that whether the solution is converged or not; you said before that in unsteady cases, initial residuals should be around 0.001. Is this criteria acceptable in such cases (oscillatory nature)? (if your answer is No; how can we ensure that the solution is converged?) Thanks again 

August 3, 2011, 11:53 

#12 
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Alberto Passalacqua
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Yes, if you perform an unsteady calculation, the residuals should go down at each time step, until they reach the specified tolerance.
Residuals clearly won't do that in a steadystate solver, simply because your system does not have a steady state solution. Best,
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 3, 2011, 14:35 

#13  
Senior Member

Quote:


August 6, 2011, 16:33 

#15 
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Alberto Passalacqua
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Were you having problems with pimpleFoam? If yes, what kind of problems? There should be little difference compared to a "transient SIMPLE".
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 7, 2011, 03:15 

#16  
Senior Member

Quote:
As I said before, my problem in both pimpleFoam and icoFoam was reducing initial residuals of p and U, I tried different schemes and solution controls and also time steps but I didn't achieve acceptable result particularly peak values in analytic solution; on the other hand, by transientSimpleFoam and with the same time step and schemes I achieved nice results matched with analytic one and both residuals reduced to about 0.01; I know that PISO algorithm is more precise that SIMPLE one but maybe my management for this algorithm was not efficient! Can you explain the reason of that?
__________________
Amir 

August 7, 2011, 03:29 

#17 
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Alberto Passalacqua
Join Date: Mar 2009
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Are you using the same relaxation factors in pimpleFoam and transient SIMPLE?
If the setup is the same, they should give the same result.
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 7, 2011, 04:22 

#18  
Senior Member

Quote:
Thanks,
__________________
Amir 

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mrf, oscillatory 
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