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December 24, 2014, 12:16 
Validation SimpleFoam sphere Cd

#1 
New Member
H25E
Join Date: Jul 2014
Posts: 27
Rep Power: 3 
Hello,
I was trying to simulate a body car in simpleFoam but i had a output Cd slightly high so I tried to validate my "virtual wind tunnel" simulating an smooth sphere that its Cd is known and tabulated depending on the number of Reynolds, something like that: But, unfortunately I get that: I have compressed the simulation directory of 20m/s and I uploaded to dropbox, I erased the 100 200 300 and 400 intermediate folders and the stl file to save space. If u need them i can upload them. I tried with a more accurate mesh (with 2.8 million cells) and with pisoFoam but I get the same results and I'm stucked here, I need some help. Thanks for your time, greetings. 

December 25, 2014, 17:06 

#2 
New Member
H25E
Join Date: Jul 2014
Posts: 27
Rep Power: 3 
I'm working really hard but I'm totally stucked here. It's an smooth sphere with 5cm of diameter and I tried everything I could tried but I always get the same Cd, so low. What could be failling? I'm using the kOmegaSST turbulent model.
Also I can upload the simulation files in another server if it's necessary. Thanks again. 

December 25, 2014, 17:56 

#3 
Senior Member
anonymous
Join Date: Aug 2014
Posts: 145
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Could it be that the value of the first graph uses Radius as the dimensional parameter instead of Diameter?
Which relaxationFactors are you using? Relaxing help a lot convergence, but can give some garbage when we are talking about fine tuning. Have you tried running a transient solver? Mesh photo? 

December 25, 2014, 22:33 

#4 
New Member
H25E
Join Date: Jul 2014
Posts: 27
Rep Power: 3 
Hello ssss thanks for the answer,
To the first question, I have found in several places and Re seems to be calculated everywhere with the diameter. Anyway, this only would displace the graph in the horizontal axis where it's well placed approximately. Where I have a big error is in the vertical axis with the Cd values. To the second one, I have used 0.3 for pressure and 0.7 for U, k, and omega. To the third one, I tried running pisoFoam at the simulation of 20m/s and i have practically the same Cd (0.114345 with simpleFoam VS 0.108134 with pisoFoam) And to the last one I have simulated at all the speeds with the following mesh, that has 250.000 cells: Then, and only for the case of 20m/s I have tried with a more accurate mesh with 2.800.000 cells: But I get the same error, Cd of 0.124017 for the accurate case VS the 0.114345 of the coarse case. (The 20m/s is supposed to to give a Cd between 0.45 and 0.5) Thanks for your time. 

December 26, 2014, 08:22 

#5 
Senior Member
anonymous
Join Date: Aug 2014
Posts: 145
Rep Power: 2 
I'm sure your problem is related to your mesh. Why don't you try to mesh a 2D case and see what happens? What does checkMesh say about your mesh?
Did you try to run it without relaxationFactors? Did you try a transient solver like pimpleFoam icoFoam, pisoFoam,etc? Could you also post your fvSchemes? 

December 26, 2014, 08:56 

#6 
New Member
H25E
Join Date: Jul 2014
Posts: 27
Rep Power: 3 
Hello ssss,
Yes, I have written in the previous post over the mesh pictures info about the relaxation factors and pisoFoam. I give you the chechMesh logs for the coarse case: Code:
/**\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: 2.1.1   \\ / A nd  Web: www.OpenFOAM.org   \\/ M anipulation   \**/ Build : 2.1.1221db2718bbb Exec : checkMesh Date : Dec 26 2014 Time : 13:48:00 Host : "pc" PID : 2703 Case : /home/hector/Desktop/TFG/corr/20 nProcs : 1 sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE). fileModificationChecking : Monitoring runtime modified files using timeStampMaster allowSystemOperations : Disallowing usersupplied system call operations // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // Create time Create polyMesh for time = 0 Time = 0 Mesh stats points: 271861 faces: 772008 internal faces: 745264 cells: 250752 boundary patches: 6 point zones: 0 face zones: 0 cell zones: 0 Overall number of cells of each type: hexahedra: 242170 prisms: 888 wedges: 0 pyramids: 0 tet wedges: 4 tetrahedra: 0 polyhedra: 7690 Checking topology... Boundary definition OK. Cell to face addressing OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Surface topology frontAndBack 9600 9922 ok (nonclosed singly connected) outlet 1600 1681 ok (nonclosed singly connected) inlet 1600 1681 ok (nonclosed singly connected) lowerWall 4800 4961 ok (nonclosed singly connected) upperWall 4800 4961 ok (nonclosed singly connected) PTri_esfera 4344 5486 ok (closed singly connected) Checking geometry... Overall domain bounding box (0.1 0.1 0.1) (0.1 0.1 0.5) Mesh (nonempty, nonwedge) directions (1 1 1) Mesh (nonempty) directions (1 1 1) Boundary openness (3.44367e17 5.83811e17 8.31597e19) OK. Max cell openness = 2.70701e16 OK. Max aspect ratio = 10 OK. Minumum face area = 2.48988e07. Maximum face area = 2.57476e05. Face area magnitudes OK. Min volume = 3.76546e10. Max volume = 1.26596e07. Total volume = 0.0239346. Cell volumes OK. Mesh nonorthogonality Max: 30.9833 average: 4.52373 Nonorthogonality check OK. Face pyramids OK. Max skewness = 0.488181 OK. Coupled point location match (average 0) OK. Mesh OK. End Code:
/**\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: 2.1.1   \\ / A nd  Web: www.OpenFOAM.org   \\/ M anipulation   \**/ Build : 2.1.1221db2718bbb Exec : checkMesh Date : Dec 26 2014 Time : 13:48:22 Host : "pc" PID : 2710 Case : /home/hector/Desktop/TFG/corr/20MallaFina nProcs : 1 sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE). fileModificationChecking : Monitoring runtime modified files using timeStampMaster allowSystemOperations : Disallowing usersupplied system call operations // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // Create time Create polyMesh for time = 0 Time = 0 Mesh stats points: 2978827 faces: 8569572 internal faces: 8496572 cells: 2804124 boundary patches: 6 point zones: 0 face zones: 0 cell zones: 0 Overall number of cells of each type: hexahedra: 2657316 prisms: 10760 wedges: 0 pyramids: 0 tet wedges: 4 tetrahedra: 0 polyhedra: 136044 Checking topology... Boundary definition OK. Cell to face addressing OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Surface topology frontAndBack 2400 2562 ok (nonclosed singly connected) outlet 400 441 ok (nonclosed singly connected) inlet 400 441 ok (nonclosed singly connected) lowerWall 1200 1281 ok (nonclosed singly connected) upperWall 1200 1281 ok (nonclosed singly connected) PTri_esfera 67400 84426 ok (closed singly connected) Checking geometry... Overall domain bounding box (0.1 0.1 0.1) (0.1 0.1 0.5) Mesh (nonempty, nonwedge) directions (1 1 1) Mesh (nonempty) directions (1 1 1) Boundary openness (2.02228e17 2.26277e17 2.10272e18) OK. Max cell openness = 3.13147e16 OK. Max aspect ratio = 9.04486 OK. Minumum face area = 1.43276e08. Maximum face area = 0.00010299. Face area magnitudes OK. Min volume = 5.70459e12. Max volume = 1.00377e06. Total volume = 0.0239346. Cell volumes OK. Mesh nonorthogonality Max: 48.6495 average: 5.63905 Nonorthogonality check OK. Face pyramids OK. Max skewness = 0.542972 OK. Coupled point location match (average 0) OK. Mesh OK. End Code:
/** C++ **\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: 2.1.1   \\ / A nd  Web: www.OpenFOAM.org   \\/ M anipulation   \**/ FoamFile { version 2.0; format ascii; class dictionary; object fvSolution; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // solvers { p { solver GAMG; tolerance 1e7; relTol 0.1; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; cacheAgglomeration on; agglomerator faceAreaPair; nCellsInCoarsestLevel 10; mergeLevels 1; } U { solver smoothSolver; smoother GaussSeidel; tolerance 1e8; relTol 0.1; nSweeps 1; } k { solver smoothSolver; smoother GaussSeidel; tolerance 1e8; relTol 0.1; nSweeps 1; } omega { solver smoothSolver; smoother GaussSeidel; tolerance 1e8; relTol 0.1; nSweeps 1; } } SIMPLE { nNonOrthogonalCorrectors 0; } potentialFlow { nNonOrthogonalCorrectors 10; } relaxationFactors { fields { p 0.3; } equations { U 0.7; k 0.7; omega 0.7; } } cache { grad(U); } // ************************************************************************* // Code:
/** C++ **\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: 2.1.1   \\ / A nd  Web: www.OpenFOAM.org   \\/ M anipulation   \**/ FoamFile { version 2.0; format ascii; class dictionary; object fvSchemes; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // ddtSchemes { default steadyState; } gradSchemes { default Gauss linear; } divSchemes { default none; div(phi,U) Gauss linearUpwindV grad(U); div(phi,k) Gauss upwind; div(phi,omega) Gauss upwind; div((nuEff*dev(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear corrected; } interpolationSchemes { default linear; } snGradSchemes { default corrected; } fluxRequired { default no; p; } // ************************************************************************* // UPDATE: I tried to simulate with the relax factors =1 but the continuity diverges completely. UPDATE2: With the relaxing factors at 0.1 I get the same Cd, so maybe the error is in the mesh but I don't know where... Last edited by H25E; December 26, 2014 at 22:06. 

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