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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
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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 (current status: http://albertopassalacqua.com/?p=1541) 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 (current status: http://albertopassalacqua.com/?p=1541) 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 
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July 30, 2011, 15:32 

#7 
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Alberto Passalacqua
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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 (current status: http://albertopassalacqua.com/?p=1541) 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 (current status: http://albertopassalacqua.com/?p=1541) 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,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) 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 (current status: http://albertopassalacqua.com/?p=1541) 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
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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 (current status: http://albertopassalacqua.com/?p=1541) 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,
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Amir 

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