Opposite sign for pitch moment and moment coefficient
 

Hi,

I'm trying to simulate flow past a pitching up wing.

The orientation is aligned such that in flow is moving in positive x dir, lift upwards is positive y dir, and from root to tip of wing is positive z dir.

Wing is doing a pitch up motion, hence, by right hand rule, it is doing negative rotation in z dir.

I got the actual moment values and also the moment coefficients.

However, I wonder why my pitch moment is positive but pitch coefficient is negative. The other two (yaw, roll) follow the same sign.

Does anyone know why?

Someone gave the reasoning below, but I'm not sure if it applies:

https://www.cfd-online.com/Forums/op...nt-coeffs.html

Hi,

I tried to run another case, with wing, body and horizontal tail.

The forces / moments are calculated separately for each body.

However, the tail value seems a bit strange.

CG (coR) is near the quarter chord of the wing

My tail drag, lift and pitch moment are 6e-3 N, -4.5e-1 N and -1.5e-1 Nm.

The CoR is around the mid body region while the tail is about 0.3m behind.

So pitch = lift * dist = -4.5e-1 * 0.3 ~ -1.5e-1 Nm.

However, the pitch coefficient is +0.045. If I do the conversion, I get the same magnitude but the sign is different. Why is the coeff + while the actual moment -?

In another posting:

https://www.cfd-online.com/Forums/op...nt-coeffs.html

It is mentioned that pitch up is force positive but in this case, it's also not true.

Why can't I find the defn or code in OF?

Thanks.

I was checking my coefficient.dat and forces files.

In forces,

rho rhoInf;
rhoInf 1.;

CofR (0. 0. 0.);//(0.07 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0 1);
magUInf 1.;
lRef 1.;
Aref 0.01;
}

However, in coefficient.dat:

# Force coefficients
# dragDir : (1.000000e+00 0.000000e+00 0.000000e+00)
# sideDir : (0.000000e+00 0.000000e+00 -1.000000e+00)
# liftDir : (0.000000e+00 1.000000e+00 0.000000e+00)
# rollAxis : (1.000000e+00 0.000000e+00 0.000000e+00)
# pitchAxis : (0.000000e+00 0.000000e+00 -1.000000e+00)
# yawAxis : (0.000000e+00 1.000000e+00 0.000000e+00)
# magUInf : 1.389000e+01
# lRef : 1.930000e-01
# Aref : 1.450000e-01
# CofR : (0.000000e+00 0.000000e+00 0.000000e+00)
#

The sideDir and pitchAxis are both -1, which explains the opposite sign.

But so why is this so? I thought everything is defined positive in forces file.