# lift and drag coefficient

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 October 28, 2012, 20:00 Drag and lift force using OpenFoam2.1.1 #41 Member   R. P. Join Date: Jul 2010 Posts: 71 Rep Power: 8 Hi everyone, I am trying to calculate the drag, lift and pitching-moment coefficients for a flow over a probe. The code structure below works fine when the flow is parallel to the probe. However, when I changed the angle of attack I got a rude difference for lift and pitching-moment when compared with experimental data (the drag values is OK). I guess, this difference is related to the vectors (liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1)). I this this vectors is just valid for a parallel flow, isn't it ? Anybody knows how to change this vectors according to a certain angle of attack ? Thank you very much. Rophys forces { type forces; enabled true; functionObjectLibs ( "libforces.so" ); outputControl outputTime; patches (probe); directForceDensity true; fDName fDMean; CofR (0.02 0 0); log on; } forceCoeffs { type forceCoeffs; functionObjectLibs ("libforces.so"); patches (probe); outputControl outputTime; fDName fDMean; rhoInf 1.73e-5; CofR (0.02 0 0); liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1); magUInf 1503.1; lRef 0.05; Aref 9.817e-4; }

 October 29, 2012, 04:29 #42 Member   Join Date: Apr 2012 Location: France Posts: 72 Rep Power: 6 Hi, We have two cases : If you change the components of the velocity (and don't rotate the geometry), the lift and drag direction must change. If you rotate the geometry, the lift and drag direction don't change. For exemple, if you change your flow direction (angle of attack) like this : (V*cos(alpha) 0 V*sin(alpha)). You must have to change the direction like this : liftDir(-sin(alpha) 0 cos(alpha)) dragDir (cos(alpha) 0 sin(alpha)) I hope that I have answered to your question

 October 29, 2012, 05:20 #43 Member   R. P. Join Date: Jul 2010 Posts: 71 Rep Power: 8 Hi Rider, Thanks for your response. It was clear enough. In my case, the mesh still the same and I just rotate the geometry (the liftDir and dragDir remained the same, liftDir (0 1 0); dragDir (1 0 0)). Even doing this the lift and pitching-moment is not matching with experimental results. It is something around 20-30% difference. The drag force is matching quite well, as well the Ca and Cn. If anybody have some suggestions in how to solve this, I'll appreciate. Cheers.

 October 29, 2012, 06:10 #44 Member   Join Date: Apr 2012 Location: France Posts: 72 Rep Power: 6 Your domain is enough biggest to capture all the tail? What is your y+ value ?

 October 29, 2012, 06:34 #45 Member   R. P. Join Date: Jul 2010 Posts: 71 Rep Power: 8 I am using the OpenFOAM but I am not using a Navier-Stokes solver. It is called DSMC and for it, I don't have problems with y+

 October 29, 2012, 06:37 #46 Member   Join Date: Apr 2012 Location: France Posts: 72 Rep Power: 6 Ok, it's your company solver or a free solver ?

 October 29, 2012, 06:46 #47 Member   R. P. Join Date: Jul 2010 Posts: 71 Rep Power: 8 Yes it is free....DSMC solver is part of the OpenFoam. It is used, for example, to simulate re-entry problems in very high altitudes where the N-S is no long valid (is too rarefied to solve NS equations). It take into account the molecular movement instead to solve differential equation but when used for "low altitude" the results should be quite close to CFD results.

 October 29, 2012, 07:06 #48 Member   Join Date: Apr 2012 Location: France Posts: 72 Rep Power: 6 Ok. Thanks.

 May 18, 2013, 07:52 why do you remove Uname and pName ,and add rhoName rhoInf? #49 New Member   jiujiumin Join Date: May 2013 Posts: 9 Rep Power: 5 i am a new user in openFoam ,I want to know why it do not use the U and p in caculating the forces and forcecoeffs? And i also want to know why it always has the error :From function void forces::read(const dictionary& dict) in file forces/forces.C at line 277 Could not find U, p or rho in database. De-activating forces. Because I have the U and p files in the 0 file,so i am so confused about it .

 January 3, 2014, 04:38 lift and drag coeffs in paraview #50 Member   arash mahboubi Join Date: Jun 2013 Location: Iran Posts: 40 Rep Power: 5 Hi How can I calculate lift and drag coefficient in paraview? I've run an oscillating airfoil with pitching motion.I've calculated lift and drag coefficient through libforces in controlDict, but the values are wrong. I want to calculate them in paraview and plot them over the time or angle. Please tell me how to do this? alirezahd likes this.

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