# buoyantSimpleFoam velocity oszillation 1.6-ext

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May 12, 2011, 03:56
buoyantSimpleFoam velocity oszillation 1.6-ext
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Stefan
Join Date: Jun 2009
Posts: 67
Rep Power: 9
Hallo user ans developer of OpenFOAM 1.6-ext,

I want to simulate a room without any inlets and outlets only with a heat source on the ground. I choose the buoyantSimpleFoam solver. The steps I had done:
1. build and mesh the domain with Gambit
2. checkMesh OK (nonOrthogonality max = 63)
3. buoyantSimpleFoam: after 200 Iterations there are nonphysical oszillations (+-0.04 m/s) in the velocity fild (picture)
4. simpleFoam: same mesh the oszilations are very low (+- 1e-7) (picture)

Is this a problem with the mesh ore with buoyantSimpleFoam?

Preibie
Attached Images
 buoyantSimpleFoam.jpg (92.7 KB, 32 views) simpleFoam.jpg (87.2 KB, 25 views)

 May 13, 2011, 02:54 #2 Member   Stefan Join Date: Jun 2009 Posts: 67 Rep Power: 9 Some more informations about the last hours: We tryed something more. We changed the mesh (only the mesh) and used a hex grid. What I can say all was fine, no oscillations. Why I'm confused: The first mesh passed checkMesh! Next step: we convert the hex mesh with tetDecomposition into a tet Mesh. This mesh passed also checkMesh (nonOrthogonality max = 44 average = 18). It seems that this mesh is good enough, no oszillations! Is the threshold (70) of nonOrthogonality to high?

 May 13, 2011, 03:49 #3 Senior Member   Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,910 Rep Power: 27 Hi, you should also list the schemes you are using. On tet meshes you often need to turn on limiters which are off by default in OpenFOAM. __________________ 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 open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

 May 13, 2011, 03:58 #4 Member   Stefan Join Date: Jun 2009 Posts: 67 Rep Power: 9 fvSchemes: Code: ```ddtSchemes { default Euler; } gradSchemes { default Gauss linear; } divSchemes { div(phiU,p) Gauss upwind; //div(phi,U) Gauss upwind; div(phi,U) Gauss reconCentral cellLimited leastSquares 1.0; div(phi,h) Gauss upwind; div(phi,k) Gauss upwind; div(phi,epsilon) Gauss upwind; div((muEff*dev2(grad(U).T()))) Gauss linear; } laplacianSchemes { default Gauss linear corrected; } interpolationSchemes { default linear; interpolate(U) reconCentral phi leastSquares; //für tetraeder Gitter von Jasak } snGradSchemes { default corrected; } fluxRequired { default no; p ; }```

 May 13, 2011, 04:09 #5 Senior Member   Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,910 Rep Power: 27 I would try: Code: ```gradSchemes { default cellLimited leastSquares 1; } divSchemes { div(phi,U) Gauss linearUpwindV cellLimited leastSquares 1; div(phiU,p) Gauss upwind; div(phi,h) Gauss upwind; div(phi,k) Gauss upwind; div(phi,epsilon) Gauss upwind; div((muEff*dev2(grad(U).T()))) Gauss linear; }``` Note that in your schemes, you are using inconsistent gradient formulations. Additionally, if leastSquares gives you problems, replace it with Gauss linear, which will reduce the accuracy, but tends to be more robust. I hope this helps. __________________ 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 open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

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