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lift, drag, moment coefficients using forceCoeffs

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Old   April 1, 2016, 15:50
Default lift, drag, moment coefficients using forceCoeffs
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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!
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Old   April 1, 2016, 16:04
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Fabian Roesler
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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
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Old   April 1, 2016, 16:19
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Quote:
Originally Posted by fabian_roesler View Post
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.
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2d airfoil, drag, lift, moment

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