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-   -   Forces and drag coeffs in simpleFoam (https://www.cfd-online.com/Forums/openfoam-post-processing/69988-forces-drag-coeffs-simplefoam.html)

 milos November 11, 2009 09:10

Forces and drag coeffs in simpleFoam

Dear FOAM-ers,

I've searched through the forum and I couldn't find an appropriate answer to some basic things on how to obtain forces and/or drag coefficients in simpleFoam solver.

As far as I've seen, people usually used forceCoeffs functions, but I was unable to modify them in order to work in simpleFoam.

Please, if anyone has some tutorial-like instructions, I would seriously appreciate it.

Thanks!

Milos

 Kaja December 15, 2009 10:19

functions
(
forces
{
type forces;
functionObjectLibs ("libforces.so");
patches (PARALELL);
rhoInf 1000.0;
CofR (0 0 0);
}
);

 Kaja December 15, 2009 10:21

I am running turbFoam for a series 60 hull and outputting viscous drags. My question is what is the unit in forces.dat? Are they in unit of Newton (both pressure forces and viscous forces)?

 milos December 16, 2009 05:06

That was my question as well... I get some huge numbers for the forces and I am not sure whether it's Newtons or sth else.

If you set rhoInf = 1, Area = 1, and velocity = 1, then I believe libforces.so will give you the 2*Force in Newtons as it is integrating the pressure and, I believe, the shear stresses on the surface (the 2 comes from the half in the dynamic head). If in doubt, compute the forces in ParaView and compare them to Foam.

 milos December 16, 2009 05:35

I didn't know that there was a way to compute forces in ParaView... How do you do that?

Thanks!

 Kaja December 16, 2009 10:46

Till Milos: http://www.cfd-online.com/Forums/ope...rces-of15.html

 Kaja December 16, 2009 10:49

Quote:
 Originally Posted by madad2005 (Post 240170) If you set rhoInf = 1, Area = 1, and velocity = 1, then I believe libforces.so will give you the 2*Force in Newtons as it is integrating the pressure and, I believe, the shear stresses on the surface (the 2 comes from the half in the dynamic head). If in doubt, compute the forces in ParaView and compare them to Foam.
It seems you are talking about definition of force coefficients. They should be dimensionless.

However, for force output, are the viscous forces in Newton? Or are they in fact calculated by (real viscous force)/ rho?

The other question is related with symmetry. If I have symmetry patch defined in boudanry condition (for example, I only calculate half of this ship), then should I manually multiple 2 for the forces I get from forces.dat?