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External vehicle aero - cd and cl oscillations (simpleFoam) |
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January 6, 2015, 09:04 |
External vehicle aero - cd and cl oscillations (simpleFoam)
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#1 |
Member
Petr Furmanek
Join Date: Jan 2012
Location: Faenza, Italy
Posts: 66
Rep Power: 14 |
Nice afternoon to all!
what is your experience with cl and cd behaviour during steady (simpleFoam) simulation of external vehicle aerodynamics? The problem I face is that all the residuals are converged (almost don't change in time for many iterations) but cl and cd coefficients oscillate. And quite a lot - cl in <0.11. 0.195> and cd in <0.305, 0.355> (details in the attached pictures). I would expect some oscillations with k-omega SST model I use, but in 3rd or 4th decimal place, not in 2nd... I'm using snappyHexMesh with 10 prism layers on the car body and wheels, 8 on the chassis and 5 on mudguards with the following checkMesh output Code:
Overall domain bounding box (-20 -18 -0.2616) (60 1.03886e-32 17.7384) Mesh (non-empty, non-wedge) directions (1 1 1) Mesh (non-empty) directions (1 1 1) Boundary openness (-3.26662e-16 -4.62421e-16 -8.6276e-14) OK. Max cell openness = 4.55546e-15 OK. Max aspect ratio = 257.513 OK. Minimum face area = 5.435e-10. Maximum face area = 0.453756. Face area magnitudes OK. Min volume = 2.4974e-11. Max volume = 0.302519. Total volume = 25916.9. Cell volumes OK. Mesh non-orthogonality Max: 83.8692 average: 7.62046 *Number of severely non-orthogonal (> 70 degrees) faces: 695. Non-orthogonality check OK. <<Writing 695 non-orthogonal faces to set nonOrthoFaces Face pyramids OK. ***Max skewness = 7.92292, 99 highly skew faces detected which may impair the quality of the results <<Writing 99 skew faces to set skewFaces Coupled point location match (average 0) OK. Code:
ddtSchemes { default steadyState; } gradSchemes { default cellLimited leastSquares 1.0; grad(U) cellLimited leastSquares 1.0; } divSchemes { default none; div(phi,U) bounded Gauss linearUpwindV grad(U); div(phi,k) bounded Gauss upwind; div(phi,omega) bounded Gauss upwind; div((nuEff*dev(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear limited 0.5; } interpolationSchemes { default linear; } snGradSchemes { default limited 0.5; } fluxRequired { default no; p; } Code:
solvers { p { solver GAMG; tolerance 1e-7; relTol 1e-3; // def. 1e-2 smoother DICGaussSeidel; nPreSweeps 0; nPostSweeps 2; cacheAgglomeration on; agglomerator faceAreaPair; nCellsInCoarsestLevel 10; mergeLevels 1; minIter 2; maxIter 50; } "(U|k|omega).*" { solver smoothSolver; smoother DILUGaussSeidel; tolerance 1e-8; relTol 0;//0.01;// def. 0.1; nSweeps 2; } } SIMPLE { nNonOrthogonalCorrectors 0; residualControl { p 1e-3; U 1e-3; "(k|omega)" 1e-4; } } potentialFlow { nNonOrthogonalCorrectors 20; } relaxationFactors { fields { p 0.15; } equations { U 0.5; k 0.5; omega 0.5; } } cache { grad(U); } |
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January 12, 2015, 10:37 |
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#2 |
Senior Member
Aurelien Thinat
Join Date: Jul 2010
Posts: 165
Rep Power: 15 |
Hi petr.f.,
Your mesh presents high value of cell's non-orthogonality. You should try adding some nNonOrthogonalCorrectors (something like 2 or 3 instead of 0). You can also try to decrease a bit your schemes : limited 0,333 instead of limited 0,5. Then if it fails, averaging your solution over few hundreds of iteration should do the trick. If you kept the output log file, post it on the forum. It would be easier to spot any numerical problem (continuity error...). |
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January 14, 2015, 06:11 |
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#3 |
Member
Petr Furmanek
Join Date: Jan 2012
Location: Faenza, Italy
Posts: 66
Rep Power: 14 |
Hi Aurelien,
thanks for the advice. I've tried the averaging and I got a reasonable result with standard deviation 0.006, which is not that bad. However, if I try to simulate the same case with StarCCM+, the oscillations are in the magnitude of 1e-3 (which is one order lower than what happens with simpleFoam). I'll try the non-ortho correctors and decreasing the schemes as well. The continuity error is quite OK (around 2e-7 in the last 5000 iterations). Regards, P. |
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January 14, 2015, 06:53 |
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#4 |
Senior Member
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 26 |
I guess that shows that StarCCM introduces ridiculous amounts of numerical dissipation.
__________________
The skeleton ran out of shampoo in the shower. |
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January 14, 2015, 07:52 |
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#5 |
Senior Member
anonymous
Join Date: Aug 2014
Posts: 205
Rep Power: 12 |
Maybe your simulation is purely unsteady, so even if you use steady solvers you will have some sort of oscillations.
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January 14, 2015, 08:43 |
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#6 |
Senior Member
Blanco
Join Date: Mar 2009
Location: Torino, Italy
Posts: 193
Rep Power: 17 |
Isn't the pressure residual a little bit too high?
Other residuals looks good, but the pressure res seems high in my opinion...can you say it is converged? I had similar problems with external aerodyn. cases and I ofted had numerical oscillations in Cd and Cl when the nonOrthogonality of cells was high...and in those cases, I've always obtained high pressure residual. |
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