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March 6, 2016, 18:57 
doubt about drag coefficient (cylinder)

#1 
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Catalina Valencia
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Hello
I started recently with OpenFoam 2.4 simulations (2D) based on Vortex Induced Vibration around circular cylinder. I'm using pimpleDyFoam for dynamic mesh. I'm trying to calculate drag coefficient using "libforce.so" but I can't obtain good results. I'm using differents Reynolds values (40, 150, 500, 1000) Information: D=0.01, rho=1000, nu=1e06 Re=40, v=0,004 Re=150, v=0,015 Re=500, v=0,05 Re=1000, v=0,1. For now I'm working with Re=150 and try to adjust the data. My controlDict: Code:
application pimpleDyFoam; startFrom startTime; startTime 0; stopAt endTime; endTime 50; deltaT 0.00005; writeControl adjustableRunTime; writeInterval 1; purgeWrite 0; writeFormat ascii; writePrecision 12; writeCompression uncompressed;//off; timeFormat general; timePrecision 12; runTimeModifiable true; adjustTimeStep yes; maxCo 0.5; maxDeltaT 1.0; libs ("libforces.so"); functions { forceCoeffs { type forceCoeffs; functionObjectLibs ("libforces.so"); patches (cylinder); directForceDensity no; pName p; UName U; rhoName rhoInf; rhoInf 1000; CofR (0 0 0); liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1); magUInf 0.015;// Free stream velocity lRef 0.01;// Diameter of cylinder? Aref 0.000078;// Ref. Area = cross sectional area? outputControl timeStep; outputInterval 1; } Code:
solvers { pcorr { solver GAMG; tolerance 0.001; relTol 0; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; nFinestSweeps 2; cacheAgglomeration false; agglomerator faceAreaPair; nCellsInCoarsestLevel 10; mergeLevels 1; } p { $pcorr tolerance 1e7; relTol 0.01; } pFinal { $p; tolerance 1e7; relTol 0; } p_rgh { solver GAMG; tolerance 1e08; relTol 0.05; smoother GaussSeidel; nPreSweeps 0; nPostSeeps 2; nFinestSweeps 2; cacheAgglomeration false; agglomerator faceAreaPair; nCellsInCoarsestLevel 10; mergeLevels 1; } p_rghFinal { $p_rgh; tolerance 1e8; relTol 0; } U { solver smoothSolver; smoother GaussSeidel; tolerance 1e06; relTol 0; nSweeps 1; } UFinal { $U; tolerance 1e05; relTol 0; } cellDisplacement { solver GAMG; tolerance 1e5; relTol 0; smoother GaussSeidel; cacheAgglomeration true; nCellsInCoarsestLevel 10; agglomerator faceAreaPair; mergeLevels 1; } } PIMPLE { n0uterCorrector 10; nCorrectors 3; nNonOrthogonalCorrectors 0; pRefCell 0; pRefValue 0; residualControl { "(Up)" { tolerance 1e4; relTol 0; } } } Code:
ddtSchemes { default Euler; } gradSchemes { default Gauss linear; grad(p) Gauss linear; grad(U) Gauss linear; } divSchemes { //default none; div(phi,U) Gauss upwind; div((nuEff*dev(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear limited corrected 0.5; } interpolationSchemes { default linear; } snGradSchemes { default corrected; } fluxRequired { default no; p_rgh ; pcorr ; p; } 

March 7, 2016, 00:10 

#2 
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Alex
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I am not exactly sure what your setup looks like but if it's like a standard flowovercylinder setup you have calculated your reference area wrong. You take the cross sectional area which would be correct if the freestream would 'see' the cylinder from the top.
If the flow is around the cylinder it is supposed to be diameter x length (whatever height of mesh you have  I guess it's 1. If not, I recommend to change this to 1m ) so it would simply be aRef = l. In case this is not the solution to the problem, your mesh, meshCheck output and BCs would be of importance. 

March 7, 2016, 10:01 

#3  
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Quote:
Thanks for your answer. The flow is around the cylinder, so I shanged the reference area as you say (D=0.01, length=0.01). Also I checked the mesh but itīs fine. I have experimental data: Re=40 Cd=1.8, Re=150 Cd=1.5, Re=500 Cd=1.2, Re=1000 Cd=0.9. As you can see in the attached the results are very strange. I dodn't know where are my errors Please, if you need more information about my model feel free to ask. Thanks again 

March 10, 2016, 01:09 

#4 
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Alex
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Sorry, I didn't have much time in the recent days. I don't see any specifications, relaxation factors, and schemes for turbulence in your case. What turbulence model are you running?
Cheers Alex 

March 10, 2016, 08:28 

#5 
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Hi Alex
Many thanks for your reply Please check the information below RAS Properties Code:
RASModel laminar; turbulence off; printCoeffs on; Code:
transportModel Newtonian; nu nu [0 2 1 0 0 0 0] 1e06; rho rho [1 3 0 0 0 0 0] 1000; Code:
simulationType laminar; Code:
ddtSchemes { default CrankNicolson 1; } gradSchemes { default Gauss linear; grad(U) Gauss linear; grad(p) Gauss linear; } divSchemes { default bounded Gauss upwind; div(phi,U) bounded 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; } If you can help me I'd be super grateful. 

March 10, 2016, 09:08 

#6 
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Alex
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Okay, maybe you should read into turbulence modelling a bit.
Your aim is to investigate vortex shedding at a cylinder but you are simulating laminar flow. That doesn't make much sense. Turn turbulence on, use the kOmegaSST or kkLOmega model and restart the simulation. Then the results should look much better. Maybe DES is also an option. Cheers Alex 

March 10, 2016, 10:23 

#7 
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Hi
I will explain in detail my study case. I wanted to describe the fluidinduced problema of unsteady 2D flow past around fixed cylinder at diferents Reynols (low Reynolds, 401000). As I know this regime could be regarded as not turbulent, so I applied icoFoam and all went well. Then I needed to apply one degree of freedom, to considere only the lateral motion of the cylinder. That's why I changed icoFoam to pimpleDyFoam. Now I need to validate the model to continue with the next step, oscillating flow past around cylinder. I have tried to consult the documentation and to my knowledge the turbulent model does not behave well with that regime. However, I will follow your advice and I'll post the results later Thanks and best regards 

March 10, 2016, 10:54 

#8  
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Alex
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Quote:
Cheers Alex 

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