# lift, drag, moment coefficients using forceCoeffs

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 April 1, 2016, 14:50 lift, drag, moment coefficients using forceCoeffs #1 New Member   Join Date: Mar 2016 Posts: 15 Rep Power: 10 Hello everyone, I am simulating the aerodynamic behavior of a 2D airfoil and trying to find the lift, drag and moment coefficients using forceCoeffs. I found a relevant code online as follows: functions { forces { type forceCoeffs; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( wing ); pName p; UName U; rhoName rhoInf; log true; rhoInf 1; CofR ( 0 0 0 ); liftDir ( 0.087 0.996 0 ); dragDir ( -0.996 0.087 0 ); pitchAxis ( 0 0 1 ); magUInf 75.00; lRef 1; Aref 1; } } I had some questions about this code: 1- what is CofR (0 0 0) ? is this the origin of the coordinate system of the whole domain (global coordinate system)? or is it the center of mass of the airfoil !? 2- I know liftDir and dragDir are the direction of the lift and drag forces. How about pitchAxis ( 0 0 1 )? is it the direction of the pitch axis? or is it the position of the pitch axis (if this the position, should I use the global coordinate system)? 3- can anyone please explain lRef 1; and Aref 1; lines for me? what are these lines for? and what should be the value of them? can I leave them as 1? Thank you all and really need your help! Tjcross31 likes this.

 April 1, 2016, 15:04 #2 Senior Member   Fabian Roesler Join Date: Mar 2009 Location: Germany Posts: 213 Rep Power: 18 Hi when using this with vehicles your Aref is the reference surface of the car, so the frontal surface. lref is important to calculate rear and front lift of the car and it is the wheel base, so the length between the two axis. You are right concerning lift and drag direction. The pitch axis is only the normal vector of the axis. The position of the axis is the CofR, so the center of reference. This could be the center of mass or for cars the half distance between the two axis. Hope this helps Cheers Fabian brucecumt and Mike_star like this.

April 1, 2016, 15:19
#3
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Quote:
 Originally Posted by fabian_roesler Hi when using this with vehicles your Aref is the reference surface of the car, so the frontal surface. lref is important to calculate rear and front lift of the car and it is the wheel base, so the length between the two axis. You are right concerning lift and drag direction. The pitch axis is only the normal vector of the axis. The position of the axis is the CofR, so the center of reference. This could be the center of mass or for cars the half distance between the two axis. Hope this helps Cheers Fabian
Thank you for your quick reply Fabian. As I mentioned I am using this code for a 2D airfoil, so how should I define the Aref and Iref parameters for a 2D airfoil? also the code is giving me cl(f) and cl(r) and cl. I wonder what the front and rear lift coefficients would mean for a 2D airfoil.

Thank you.

May 21, 2021, 22:01
#4
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Thomas Cross
Join Date: Mar 2021
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Hi Mike,

I know this is a few years back but thought this would be useful in case others came across this. With regards to pitching moment definition it came up in the following reply and was most clear:
Quote:
 Originally Posted by gfoam The moment coefficient in libforces.so, is possitive under the rule of the right hand, while in aeronautics, the pitching moment is possitive nose up. I think this may the cause of your differences. Gonzalo
And your other points: Aref for a 2D section set to 1, then quote your calculations as 'per unit wing area' (this is the drag or lift reference area), likewise set lref to 1 (if your simulation is a reference chord).

cl is your net lift coefficient, I think the f and r are the front and rear lift coefficients (they both add to cl) i.e. how much of lift comes from forward or rear of your reference point - if your wing is balanced and your reference point is the aerodynamic centre then with no pitching moment cl(f)=cl(r)=0.5*cl

Best years later,
T

September 30, 2022, 06:24
#5
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Marius
Join Date: Sep 2022
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Quote:
 Originally Posted by fabian_roesler Hi when using this with vehicles your Aref is the reference surface of the car, so the frontal surface. lref is important to calculate rear and front lift of the car and it is the wheel base, so the length between the two axis. You are right concerning lift and drag direction. The pitch axis is only the normal vector of the axis. The position of the axis is the CofR, so the center of reference. This could be the center of mass or for cars the half distance between the two axis. Hope this helps Cheers Fabian

Hi Fabian,

I have a question about the position of CofR. Is this point measured from the global coordinate system? Or really just the center between Iref? For example 2.0 wheelbase is a CofR of 1.0.

 Tags 2d airfoil, drag, lift, moment