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Drag and lift force using OpenFoam2.1.1

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Old   October 26, 2012, 10:09
Default Drag and lift force using OpenFoam2.1.1
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R. P.
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Location: Brazil
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Hi everyone,

I am trying to calculate the drag, lift and pitching-moment coefficients for a flow over a probe. The code structure below works fine when the flow is parallel to the probe. However, when I changed the angle of attack I got a rude difference for lift and pitching-moment when compared with experimental data (the drag values is OK).

I guess, this difference is related to the vectors (liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1)). I this this vectors is just valid for a parallel flow, isn't it ?

Anybody knows how to change this vectors according to a certain angle of attack ?

Thank you very much.

Rophys

forces
{
type forces;
enabled true;
functionObjectLibs ( "libforces.so" );
outputControl outputTime;
patches (probe);
directForceDensity true;
fDName fDMean;
CofR (0.02 0 0);
log on;
}

forceCoeffs
{
type forceCoeffs;
functionObjectLibs ("libforces.so");
patches (probe);
outputControl outputTime;
fDName fDMean;
rhoInf 1.73e-5;
CofR (0.02 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0 1);
magUInf 1503.1;
lRef 0.05;
Aref 9.817e-4;
}
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Old   February 4, 2014, 08:16
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Hello Rophys,

I hope your problem is up to date or did you fix it since october 2012?

You're right. The vectors for lift, drag and pitch axis are only valid for an angle of attack of 0. So if you have different angle of attacks your values for clift and even for cdrag will not accord to the values from your experimental data.

The difference is based on the different coordinate systems(COS) which are used. For angle of attack = 0 the body fixed COS and the aerodynamic COS are the same, so the coefficients are right. But if you have an angle of attack, they are not longer the same. The easiest way is to convert the coefficients of the bodyfixed COS (which you get in OpenFOAM) to the aerodynamic COS. If you do this with some sinus/cosinus functions the values should be the same as in your experimental data.

Best regards,
CFDNewbie147
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