# "libforces.so" reporting too low values for forces on airfoil.

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 March 8, 2013, 10:43 "libforces.so" reporting too low values for forces on airfoil. #1 New Member   Håkon Bartnes Line Join Date: Mar 2013 Posts: 27 Rep Power: 5 Hi everyone! My name is Håkon Line, and I'm a mechanical engineering student at the NTNU, Trondheim. I started using OpenFOAM about half a year ago, and am now using it for my Master's thesis. I'm posting here because I've recently hit upon a problem which at first I thought would be trivial, but which has me completely stumped. Any help would be immensely appreciated, as I still have a long way to go and my deadline is fast approaching =) I'm simulating a classical benchmark case, the naca0012 foil at Ma = 0.15, Re = 6*10⁶ (http://turbmodels.larc.nasa.gov/naca0012_val.html). I'm using simpleFoam with the SA-model and 2nd order upwind convection schemes. The mesh is generated with SHM. The airfoil is generated from the equation given at the previously linked-to webpage, and converted to stl-format by the utility "points2stl" developed by Alejandro Roger. Now, my problem is that the forces on the wing, as calculated by the utilities in 'libforces.so' are way too low: (Cl = 0, 0.104 and 0.193 for alpha = 0, 5 and 10 degrees, respectively) (Cd = 0.0019, 0.0023 and 0.0041 for alpha = 0, 5 and 10 degrees, respectively) As these values are several times smaller than they should be, I suspect a multiplying factor is off somewhere, but I've no idea where that may be. The values for the lift and drag coefficients are consistent with the forces reported in forces.dat, so the error doesn't lie with the non-dimensionalization. Furthermore, when I retrieve the surface normals and the surface pressure from paraView, and use that to find the pressure distribution and lift on the wing, my results closely match the validation data. Since my pressure distribution is ok, I guess I could ignore the bad data produced by libforces, but I'm going to simulate this and similar shapes in pimpleDyMFoam, and I want to make sure that the forces "seen" by the wing are correct before I proceed. Where should I start looking? best regards, Håkon Line I'm attaching a dropbox-link to one of my run cases: http://dl.dropbox.com/u/21618777/12a10simpleSA.zip

 March 16, 2013, 07:03 #3 New Member   Håkon Bartnes Line Join Date: Mar 2013 Posts: 27 Rep Power: 5 Thanks a lot! Man, I can't believe I overlooked that bit. It really helps with a fresh pair of eyes when you've stared yourself blind at a problem. =) EDIT: I've also been made aware of a bug in 2.2.0 - if the body on which you'd like to measure the forces consists of several patches, only the force on the last patch will be written to file (http://www.openfoam.org/mantisbt/view.php?id=773). This bug has been ironed out in the latest git release (http://www.openfoam.org/download/git.php). Last edited by hakonbar; March 18, 2013 at 09:48.

 July 12, 2013, 06:42 #4 New Member   Julio Silveira Join Date: Feb 2013 Location: London Posts: 15 Rep Power: 5 You should change this lines on your controDict for every angle of attack liftDir (0 1 0); dragDir (1 0 0); e.g. liftDir (0.05233 0.99863 0) dragDir (0.99863 0.05233) 0)

 July 16, 2013, 13:16 #5 New Member   Håkon Bartnes Line Join Date: Mar 2013 Posts: 27 Rep Power: 5 Hi biau, In this case, the foil was at an angle, and the incoming flow was in the positive x-direction, so there was no need to change the reference directions away from the default. s.m likes this.

August 13, 2013, 14:59
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Quote:
 Originally Posted by hakonbar Hi biau, In this case, the foil was at an angle, and the incoming flow was in the positive x-direction, so there was no need to change the reference directions away from the default.
Dear hakonbar, for applying the angle of attack to our simulation, is it true to write the velocity of flow over an airfoil e.g (50 0 0),and use the "transformPoints" utility to rotate the domain and the airfoil e.g 15 deg?
instead of writing the velocity (50cos(alpha=15) 50sin(alpha=15)) we write the velocity (50 0 0) and rotate the domain 15 degree by "transformPoints" utility.
i attach the pictures clarifying my question.
Thank you
Attached Images
 angle-of-attack-15.jpg (10.1 KB, 30 views) angle-of-attack-0.jpg (8.8 KB, 24 views)

 August 14, 2013, 06:12 #7 New Member   Håkon Bartnes Line Join Date: Mar 2013 Posts: 27 Rep Power: 5 Yep, that's correct! If your foil is a 4-digit NACA foil, you can use Håkon Strandenes' foil generator (link further up in the thread). This one lets you set the angle of attack of the foil when you create it, so you don't have to rotate the whole domain. s.m likes this.

August 14, 2013, 07:30
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Quote:
 Originally Posted by hakonbar Yep, that's correct! If your foil is a 4-digit NACA foil, you can use Håkon Strandenes' foil generator (link further up in the thread). This one lets you set the angle of attack of the foil when you create it, so you don't have to rotate the whole domain.
Thank you very much, actually i have multi element airfoil for my analysis, now i am working on one element airfoil.
any way, i understand from your explanation that there is no difference between rotating domain and applying the alpha to our initial velocity,
do i get it right?

 August 15, 2013, 08:38 #9 New Member   Håkon Bartnes Line Join Date: Mar 2013 Posts: 27 Rep Power: 5 Exactly. The drag component of the force is parallel with the flow direction, and the lift component is normal to it, pointing upwards for positive lift, and downwards for negative.

 Tags airfoil2d, lift and drag

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