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Airfoil with simpleFoam and kOmegaSST: high drag values? |
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April 7, 2014, 15:02 |
Airfoil with simpleFoam and kOmegaSST: high drag values?
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#1 |
New Member
Marco C.
Join Date: Nov 2013
Posts: 4
Rep Power: 12 |
Hello FOAMers,
I am a beginner in OpenFOAM: I’ve learned the basics through some tutorials and I’ve been recently trying to obtain some good results from a 2D airfoil case. I’m using simpleFoam with kOmegaSST turbulence model to simulate the flow around a NACA 2415 airfoil (Reynolds number approximately 3*10^6 and 8 degrees angle of attack) and I’m currently struggling to achieve nice results in terms of drag coefficient. I can’t really understand where the error lies, so I’m asking for some help. MESH: Could this be the problem?: I built it using a Matlab code from https://www.hpc.ntnu.no/display/hpc/...l+Calculations This script generates the blockMeshdict file you can find at the end of this post. This is how the mesh looks like in Paraview: BOUNDARY CONDITIONS: As in the airfoil2d tutorial, I’m using freestream BC for U both at inlet and outlet, and obviously zero velocity condition on airfoil patch. For p, freestreampressure at inlet/outlet and zerogradient on airfoil. For k, freestream BC at inlet/outlet and kqRWallFunction on airfoil. For omega, freestream BC at inlet/outlet and omegaWallFunction on airfoil. For nut, calculated BC at inlet/outlet and nutkWallFunction on airfoil. Boundary values for k and omega were taken from http://www.cfd-online.com/Wiki/Turbu...ary_conditions, using a turbulence intensity I=0.005 and turbulent length scale 0.05 (5% of airfoil chord). FINALLY, THE RESULTS: After 2000 iterations, i get the following results (this were the best I could get after many little changes in terms of BC and mesh): lift coefficient Cl = 1.0307 drag coefficient Cd = 0.01679 I have the following experimental data from the famous Abbott-Von Doenhoff database: Cl approximately 1 Cd around 0.010 As you can see, lift coefficient is good while drag coefficient is really high. These values were the best I could get after some mesh/BC tuning: usally Cd values wander between 0.018 and 0.022. Regarding y+, I’ve run yPlusRAS after the simulation and these are the results for the last timestep: Patch 2 named airfoil y+ : min: 14.2581 max: 162.504 average: 107.005 Is this bad/good? I can't really say as I haven't found (yet) any advice on good y+ for kOmegaSST. Thank you for your patience and for your help. Down here you’ll find the complete case directory with all the files (it's just 120 kB) https://www.dropbox.com/s/sm99dt8lq8...ACA2415_KW.tar Thanks in advance, Marco |
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April 7, 2014, 15:27 |
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#2 |
Senior Member
Pete Bachant
Join Date: Jun 2012
Location: Boston, MA
Posts: 173
Rep Power: 14 |
Your BCs at the foil surface seem good for the mesh resolution, but for accurate drag prediction, you may want to get down to a y+ ~ 1. Then I believe you'd use these BCs:
k: Code:
fixedValue; uniform 1e-12; Code:
nutUSpaldingWallFunction; uniform $internalField; // or similar |
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April 7, 2014, 17:44 |
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#3 | |
New Member
Marco C.
Join Date: Nov 2013
Posts: 4
Rep Power: 12 |
Quote:
I've just made a (very) quick test changing the mesh expansion rates to reach an average y+ ~ 3 and setting BCs as you suggested, but results don't seem to improve very much... |
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April 18, 2014, 19:54 |
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#4 |
New Member
Join Date: Apr 2014
Posts: 1
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I don't have an answer about improving Cd values, but in the .tar file you posted, the 0/U file as well as the liftDir and dragDir in the system/forceCoeffs dictionary are pointing in the wrong directions.
The grid generated by blockMesh puts the airfoil in the x-z plane, while the velocity and lift/drag vectors are in the x-y plane. Last edited by whophil; April 18, 2014 at 22:13. |
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April 18, 2014, 21:39 |
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#5 |
Senior Member
Dongyue Li
Join Date: Jun 2012
Location: Beijing, China
Posts: 845
Rep Power: 18 |
Sorry to hijack this thread. Could you tell me how to insert your pic in your main passage. I just can attach pic. Thanks
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April 19, 2014, 03:47 |
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#6 | |
New Member
Marco C.
Join Date: Nov 2013
Posts: 4
Rep Power: 12 |
Quote:
rotateMesh "(0 0 1)" "(0 1 0)" After doing this the airfoil lies in the xy plane. |
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November 21, 2018, 05:56 |
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#7 |
New Member
CFD
Join Date: Jul 2016
Posts: 2
Rep Power: 0 |
Hii,
Maybe, the Grid which you are using is not orthogonal to the airfoil surface. So make it orthogonal and give it a try, it may help you. |
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Tags |
airfoil, komegasst, simplefoam |
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