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Airfoil NACA 0006 with alpha -4° is not going as expected, why? |
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October 7, 2017, 06:06 |
Airfoil NACA 0006 with alpha -4° is not going as expected, why?
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#1 |
New Member
Edoardo B.
Join Date: Oct 2017
Posts: 4
Rep Power: 8 |
hi everyone,
I'm focusing on solving an AIRFOIL NACA 0006 in 2D, I have to find the correct Coefficient of DRUG and LIFT, but after several times I haven't get the clue yet. I've got a section angle of attack about -4 degrees, which means according to the tables a value for Cl=-0.4,-0.5 and Cd=0.010,0.014 i suppose?! (CORRECT ME IF IS WRONG, PLEASE) I'm using this function objects for the calculation of the Coeffs:: functions { forces { type forceCoeffs; functionObjectLibs ("libforces.so"); outputControl timeStep; outputInterval 1; patches (profiloalare); pName p; UName U; rhoName rhoInf; log true; rhoInf 1.2; liftDir (0.069756474 0.99756405 0); dragDir (0.99756405 -0.069756474 0); //liftDir (0 1 0); //dragDir (1 0 0); CofR (1.0375 0 0); pitchAxis (0 0 -1); magUInf 21.6867; lRef 4.15; Aref 4.15; } } I think I'm using a good mesh, (i used youtube videos to create the best mesh possible) I settled all the named section, wall for the airfoil, inlet and outlet for the edges. I had post as system settings: divSchemes { default none; div(phi,U) bounded Gauss linearUpwind grad(U); div(phi,nuTilda) bounded Gauss linearUpwind grad(nuTilda); div((nuEff*dev2(T(grad(U))))) Gauss linear; } relaxationFactors { fields { p 0.3; } equations { U 0.7; nuTilda 0.7; } } simulationType RAS; RAS { RASModel SpalartAllmaras; turbulence on; printCoeffs on; } transportModel Newtonian; rho [1 -3 0 0 0 0 0] 1.2; nu [0 2 -1 0 0 0 0] 1.5e-05; I launch the simpleFoam, but in the end my coefficient are SO WRONG! and i don't know why! I said the should be Cl=-0.4,-0.5 and Cd=0.010,0.014, but mine values are about Cl=-0.83 and Cd=0.027. Anyone has any suggestion, please?? |
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October 7, 2017, 22:45 |
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#2 |
Member
Join Date: Nov 2014
Posts: 92
Rep Power: 11 |
Have you tried other turbulence model or numerical scheme? kOmegaSST should be better model.
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October 8, 2017, 03:59 |
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#3 |
Senior Member
Oskar
Join Date: Nov 2015
Location: Poland
Posts: 184
Rep Power: 10 |
Hello.
I think Your Aref value may be wrong. Is Your 2d mesh 1000mm in z direction? As I remember Aref should be equal to lRef multiply by depth of Your mesh. How about Your y+ values? |
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October 8, 2017, 06:24 |
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#4 | |
New Member
Edoardo B.
Join Date: Oct 2017
Posts: 4
Rep Power: 8 |
Quote:
Thanks man, what i said was wrong, you were right, i used a Mesh about 2000mm because i didn't considered both ways, plus and less!! thank you!!! /*------------i'm dope-------------------------*\ Hi, Yes my mesh is depht 1 m, i had used transformPoints -scale '(1 1 1.33662678)' to convert my initial depth into 1m. Also yes, Aref is equal to Iref multiply by depth, so in this case i think it's right, and Iref and Aref are the same because my depth is 1m. my value of y+ are about between 50 and 120. that's why i used a high RE model. any other suggestion? /*------------------------------------------------------------------*\ Last edited by SideshowED; October 12, 2017 at 12:10. |
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October 8, 2017, 06:31 |
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#5 | |
New Member
Edoardo B.
Join Date: Oct 2017
Posts: 4
Rep Power: 8 |
Quote:
bounded Gauss linear Upwind bounded Gauss linear ddtSchemes i only used default steadystate but now that i think about it maybe should use a Euler o CrackNikolson? gradSchemes only used Gauss linear and because i had low orthogonality (<10) i used laplacianSchemes and snGradSchemes orthogonal thanks, i'm gonna try a kOmega SST then. |
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October 10, 2017, 06:21 |
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#6 |
Senior Member
Oskar
Join Date: Nov 2015
Location: Poland
Posts: 184
Rep Power: 10 |
Hello again.
How Your simulation is going? I just finished my naca airfoil calculations and my forces output isn't right for both - kEpsilon and kOmegaSST. I'm using sonicFoam because my case is compressible and getting wired results with rhoPimpleFoam. I'm getting acceptable (for now) values of lift (CL = 0.37 while there should be CL = 0.3) but drag values are way to high (CD=0.014 while there should be CD=0.004) I think if Your case is incompressible then rho should be set to 1 not 1.2. Check for examples: tutorials\incompressible\simpleFoam\motorBike\syst em\forceCoeffs tutorials\incompressible\pisoFoam\les\motorBike\mo torBike\system\forceCoeffs Isn't Your liftDir and dragDir set for angle of attack +4degree? |
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October 10, 2017, 22:30 |
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#7 | |
Member
Join Date: Nov 2014
Posts: 92
Rep Power: 11 |
Quote:
Are your result include the wall shear stress as well? For airfoil, slender body, wall shear stress should have contributed a portion of the forces. |
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October 12, 2017, 12:08 |
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#8 | |
New Member
Edoardo B.
Join Date: Oct 2017
Posts: 4
Rep Power: 8 |
Quote:
I was Meshing a deltaZ equals to 2, because the mesh move from -1 to +1 in Z. I was not considering that and i used deltaZ equals to 1 for my calculation of Aref, and in this way obviously i had Coeffs who were the double they should be. now I fixed it and the values are in the ranges, but i would like to improve my residuals, how can i do now? |
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airfoil 2d |
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