# Lift & Drag calculation help

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 November 27, 2018, 10:19 Lift & Drag calculation help #1 New Member   kevin Join Date: Oct 2018 Posts: 6 Rep Power: 2 Hi, everyone I am a new man of openfoam. Recently day I was running the naca0012 airfoil case by openfoam. I realized the lift & drag calculations in the system/controlDict file. The details are as follows Howerever, these calculations can not get correct results of Cl (lift) and Cd (drag). They are about 1e-7, too small. Is there any error in the folllowing functions? Could anyone help me? Thanks. __________________________________________________ _______________ // *** calculate force coefficients *** // functions { forceCoeffs { type forceCoeffs; functionObjectLibs ("libforces.so"); patches (wall); log true; rho rhoInf; rhoInf 1.29; CofR (0 0 0); liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1); magUInf 250; lRef 1; Aref 1; writeControl timeStep; writeInteval 100; } };

 November 28, 2018, 03:04 #2 Member   Zander Meiring Join Date: Jul 2018 Posts: 87 Rep Power: 2 Force calculation is correct. Any error is more likely a result of improper modelling/solving. 1e-7 is a very small error though

November 28, 2018, 09:59
#3
New Member

kevin
Join Date: Oct 2018
Posts: 6
Rep Power: 2
Quote:
 Originally Posted by yambanshee Force calculation is correct. Any error is more likely a result of improper modelling/solving. 1e-7 is a very small error though
Thank you. But the values of Cl and Cd are as small as 1e-7. They are unphysical.

 November 29, 2018, 01:40 #4 Member   Zander Meiring Join Date: Jul 2018 Posts: 87 Rep Power: 2 It would be helpful if you can include at what conditions you are testing. See this sticky post on the forum. Based on the coefficient function attached, I would assume that your inlet velocity is 250 m/s and that your blade chord length 1 m. Is this correct? Are you running a 3d case; and in which case, what is the thickness of the domain? What is your viscosity? What is your mesh like? What is the rough dimensions of your domain? How many iterations have you done (and how converged is your flow field