Help with force coefficient from fluent imported multi element airfoil 2d mesh
 

Hi,

I got a mesh from fluent. It's a 2D multi element airfoil (front + back). I'm trying to get the force coefficient. I search thru cfd-online but I'm still confused.

I checked that my 2d "wingspan width" is 0.65m. Full multi element airfoil chord is 1m.

The force result I got is ~0.0869 times of my force coefficient. But how is the force coefficient calculated?

1. Is the value of rhoInf important? I read somewhere in the forum that it's not impt for compressible flow, is that so? But I think my result changes when I change its value.

2. Are lRef and ARef impt? Especially in my 2D case?

Btw, my forces file in the system directory is:

forces
{
type forces;
libs (forces);

writeControl timeStep;
writeInterval 500;
log false;

patches
(
"(front|back)"
);
rho rhoInf;
rhoInf 1.2;
CofR (0.25 0 0);
}

ForceCoeffs
{
type forceCoeffs;
libs (forces);
//writeControl writeTime;

writeControl timeStep;
writeInterval 500;
log false;

patches
(
"(front|back)"
);

rho rhoInf;
rhoInf 1.2;

CofR (0 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0 1);
magUInf 4.38;
lRef 1;
Aref 1;
}

I tried to do some experiments using different values. So what I found is that the coefficients are calculated using cl = L / 0.5 / rho / U^2 / wing area.

For my case, I have to know the thickness of the z axis, which is 0.65m. So I have to calculate the area and insert into the force file as Aref. Then I will get the correct value.

Hope it will help others.