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May 25, 2010, 04:35 
strange behaviour of rhoPisoFoam, circular cylinder

#1 
Senior Member

Hi Foamers,
I'm doing some compressible CFD on a 2D circular cylinder, Mach 0.12, Re 1.4x10^5, komega SST turbulence. First I ran a wallmodeled simulations, y+ around 30, wall functions, (mutSpalartAllmarasWallFunction for mut, as suggested in other threads), and the run converged. I get an error of about 20% on St with respect 3D LES and experiments found in litterature, so I switched to lowRe modeling. I remeshed up to y+ max of 1.2, and I set al turbulent variables at 1x10^12 at wall, excepting for omega (omegaWallFunction should work also for lowRe in the 1.6.x version). I mapped from the previous run and I started the simulation, but now I can't obtain a stable result, the calculation blows up. I did something wrong in the setup? Did someone experienced similar problems? Thanks, Ivan 

May 25, 2010, 04:37 

#2 
Senior Member

For completeness, I use this schemes setup:
ddtSchemes { default backward; } gradSchemes { default cellMDLimited Gauss linear 1; } divSchemes { div(U,p) Gauss linearUpwind cellLimited Gauss linear 1; div(phi,U) Gauss linearUpwind cellLimited Gauss linear 1; div(phiU,p) Gauss linearUpwind cellLimited Gauss linear 1; div((muEff*dev2(grad(U).T()))) Gauss linear; div(phi,h) Gauss linearUpwind cellLimited Gauss linear 1; div(phi,omega) Gauss linearUpwind cellLimited Gauss linear 1; div(phi,k) Gauss linearUpwind cellLimited Gauss linear 1; div(phid,p) Gauss linearUpwind cellLimited Gauss linear 1; } laplacianSchemes { laplacian(muEff,U) Gauss linear corrected; laplacian(alphaEff,h) Gauss linear corrected; laplacian((rho*rAU),p) Gauss linear corrected; laplacian(DomegaEff,omega) Gauss linear corrected; laplacian(DkEff,k) Gauss linear corrected; laplacian(1,p) Gauss linear corrected; laplacian((rho*(1A(U))),p) Gauss linear corrected; } interpolationSchemes { default linear; } snGradSchemes { default corrected; } fluxRequired { default no; p ; } 

May 28, 2010, 05:21 

#3 
Senior Member

The story becomes more intricate:
I tried to do a longer run with the wallmodeled case, that up to 0.5 sec goes well. After a certain number of timesteps, it goes crazy like the lowRe model! I post some pictures of the problem. First, when everything was ok: Cl and Cd versus time of my cylinder turbulent kinetic energy: Omega: log of the calculation: Then, when the calculation go crazy Cl and Cd versus time: turbulent kinetic energy: Omega: log of the wrong calculation: It seems that the dissipation of the turbulence model go crazy, destroying all the k in the simulation. The stranger thing is that I did not change anything between the two calculations, I just let the run go on with the same setup. I have exactly the same problem with the lowRe mesh, the only difference is that this phenomenon appears in a fewer number of timesteps. Please OpenFOAM gurus, give me some hints! Have a nice day, Ivan 

May 28, 2010, 08:46 

#4 
New Member
David Huckaby
Join Date: Jul 2009
Posts: 21
Rep Power: 9 
Ivan,
Have you tried reducing your Courant number or using another timeintegration scheme such as Euler or boundedBackward ? Dave 

May 28, 2010, 08:50 

#5  
Senior Member

Quote:
no I didn't, but my Max Courant in the whole calculation is below 0.9... I have to try with boundedBackward... what's the difference between it and backward? Is more diffusive? 

May 28, 2010, 09:30 

#6 
New Member
David Huckaby
Join Date: Jul 2009
Posts: 21
Rep Power: 9 
Ivan,
The general guidelines from various posts on the message board has been Co < 0.5 for stability Co < 0.2 for accurracy. I have also found that some simulations require a fixed timestep for stability as opposed to a Courant number bound. The thread below briefly mentions the "boundedBackward" scheme http://www.cfdonline.com/Forums/ope...calarles.html, I would assume it is locally more diffusive. 

May 28, 2010, 09:57 

#7  
Senior Member

Quote:
Mmm... I'm not so experienced in unsteady simulations in OF, I used more frequently steady state, but Co < 0.5 for stability seems to me quite a severe limitation. But, I will try to limit my Co to less than 0.5... 

December 19, 2010, 08:20 

#8 
Senior Member
Guilherme da Silva
Join Date: Aug 2010
Location: Sao Paulo  Brazil
Posts: 104
Rep Power: 8 
Hello Cozza,
have you managed to converge? what was your fvSchemes? I am trying to use the linearUpwind in OF16ext and OF171 and they are giving me errors saying those schemes are not acceptable. Did they change the name in new versions? Regards, Guilherme 

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