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- - **strange behaviour of rhoPisoFoam, circular cylinder**
(*http://www.cfd-online.com/Forums/openfoam-solving/76398-strange-behaviour-rhopisofoam-circular-cylinder.html*)

strange behaviour of rhoPisoFoam, circular cylinderHi Foamers,
I'm doing some compressible CFD on a 2D circular cylinder, Mach 0.12, Re 1.4x10^5, k-omega SST turbulence. First I ran a wall-modeled 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 low-Re 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 low-Re 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 set-up? Did someone experienced similar problems? Thanks, Ivan |

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*(1|A(U))),p) Gauss linear corrected; } interpolationSchemes { default linear; } snGradSchemes { default corrected; } fluxRequired { default no; p ; } |

The story becomes more intricate:
I tried to do a longer run with the wall-modeled case, that up to 0.5 sec goes well. After a certain number of timesteps, it goes crazy like the low-Re model! I post some pictures of the problem. First, when everything was ok: Cl and Cd versus time of my cylinder http://img99.imageshack.us/img99/2900/correctcl.png turbulent kinetic energy: http://img191.imageshack.us/img191/781/correctk.png Omega: http://img198.imageshack.us/img198/3444/correctom.png log of the calculation: http://img38.imageshack.us/img38/245/correctlog.png Then, when the calculation go crazy Cl and Cd versus time: http://img27.imageshack.us/img27/3152/strangecl.png turbulent kinetic energy: http://img25.imageshack.us/img25/2433/wrongk.png Omega: http://img571.imageshack.us/img571/7307/wrongom.png log of the wrong calculation: http://img99.imageshack.us/img99/6364/wronglog.png 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 low-Re 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 |

Ivan,
Have you tried reducing your Courant number or using another time-integration scheme such as Euler or boundedBackward ? Dave |

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? |

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 time-step for stability as opposed to a Courant number bound. The thread below briefly mentions the "boundedBackward" scheme http://www.cfd-online.com/Forums/ope...calar-les.html, I would assume it is locally more diffusive. |

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... |

Hello Cozza,
have you managed to converge? what was your fvSchemes? I am trying to use the linearUpwind in OF16-ext 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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