[Jeanp]

Hello everybody,

I've been working on a 2D validation case for a laminar airfoil. The problem I have, is that my lift and drag coefficients are wrong (the ones calculated in controlDict).

I'm using komegaSST and I know that the drag coefficient will be different from the experimental one but the lift coefficient should be good. To know if my case is ok I ploted Cp around the airfoil and it matches the wind tunnel data. So I think my problem is on post-processing.

I suspect that my reference area and length are wrong. I've read some topics where is written that I should take the chord for the length and the area since I'm working on a 2D case. Is this ok? I don't really understand why I have to take the chord for the ref. Area. For me it will be more logical to take the frontal area (airfoil heigth times cell width). Hope somebody helps me.

Also I was wondering if it is necessary to multiply the coefficients results by rho. To plot Cp I had to multiply pressure by rho (1.225). Should I do the same for the coefficients?

In summary, I don't understand why my pressure distribution is ok while my coefficients are totally wrong..

Thanks for your help!

Jean

[Jeanp]

Hi all again!

I think I may have found the problem. I went to forceCoeffs.C and I found that coefficients are calculated like this :

forcesMoments fm = forces::calcForcesMoment(); 
scalar pDyn = 0.5*rhoRef_*magUInf_*magUInf_; 

vector totForce = fm.first().first() + fm.first().second(); 
vector totMoment = fm.second().first() + fm.second().second(); 

scalar liftForce = totForce & liftDir_; 
scalar dragForce = totForce & dragDir_; 
scalar pitchMoment = totMoment & pitchAxis_; 

scalar Cl = liftForce/(Aref_*pDyn); 
scalar Cd = dragForce/(Aref_*pDyn); 
scalar Cm = pitchMoment/(Aref_*lRef_*pDyn);

So I tried to change rhoRef from 1 to 2 and I obtain the same results for Cl and Cd. Is this normal? I should have coefficients two time smaller.

Thanks for your reply!

[mhd_mrt]

Originally Posted by Jeanp

Hi all again!

I think I may have found the problem. I went to forceCoeffs.C and I found that coefficients are calculated like this :

forcesMoments fm = forces::calcForcesMoment(); 
scalar pDyn = 0.5*rhoRef_*magUInf_*magUInf_; 

vector totForce = fm.first().first() + fm.first().second(); 
vector totMoment = fm.second().first() + fm.second().second(); 

scalar liftForce = totForce & liftDir_; 
scalar dragForce = totForce & dragDir_; 
scalar pitchMoment = totMoment & pitchAxis_; 

scalar Cl = liftForce/(Aref_*pDyn); 
scalar Cd = dragForce/(Aref_*pDyn); 
scalar Cm = pitchMoment/(Aref_*lRef_*pDyn);

So I tried to change rhoRef from 1 to 2 and I obtain the same results for Cl and Cd. Is this normal? I should have coefficients two time smaller.

Thanks for your reply!

Hello JeanP,

I did the airfoil2D tutorial and added the forceCoeff to system folder as well as making the right modifications to liftDir and DragDir, I get results that are far from experimental, I was wondering if you know what the issue with OF calculating these coefficients are?

iter:350
Cm:-1.528235e+01
CD:2.858215e-02
CL: 1.871433e+00

Thanks,
Mehdi