# 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? #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

 April 7, 2014, 15:27 #2 Senior Member   Pete Bachant Join Date: Jun 2012 Location: Boston, MA Posts: 173 Rep Power: 13 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;``` nut: Code: ```nutUSpaldingWallFunction; uniform \$internalField; // or similar```

April 7, 2014, 17:44
#3
New Member

Marco C.
Join Date: Nov 2013
Posts: 4
Rep Power: 12
Quote:
 Originally Posted by pbachant 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;``` nut: Code: ```nutUSpaldingWallFunction; uniform \$internalField; // or similar```
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...

 April 18, 2014, 19:54 #4 New Member   Join Date: Apr 2014 Posts: 1 Rep Power: 0 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.

 April 18, 2014, 21:39 #5 Senior Member   Dongyue Li Join Date: Jun 2012 Location: Beijing, China Posts: 841 Rep Power: 17 Sorry to hijack this thread. Could you tell me how to insert your pic in your main passage. I just can attach pic. Thanks

April 19, 2014, 03:47
#6
New Member

Marco C.
Join Date: Nov 2013
Posts: 4
Rep Power: 12
Quote:
 Originally Posted by whophil 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.
Sorry, you're right. I forgot to mention that after building the mesh I rotate it using the command:

rotateMesh "(0 0 1)" "(0 1 0)"

After doing this the airfoil lies in the xy plane.

 November 21, 2018, 05:56 #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.

 Tags airfoil, komegasst, simplefoam