# Forces and drag coeffs in simpleFoam

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 November 11, 2009, 09:10 Forces and drag coeffs in simpleFoam #1 New Member   Milos Stanic Join Date: Mar 2009 Location: Novi Sad, Serbia Posts: 29 Rep Power: 10 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

 December 15, 2009, 10:19 #2 New Member   Kai-Jia Han Join Date: Nov 2009 Posts: 4 Rep Power: 9 functions ( forces { type forces; functionObjectLibs ("libforces.so"); patches (PARALELL); rhoInf 1000.0; CofR (0 0 0); } );

 December 15, 2009, 10:21 #3 New Member   Kai-Jia Han Join Date: Nov 2009 Posts: 4 Rep Power: 9 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)?

 December 16, 2009, 05:06 #4 New Member   Milos Stanic Join Date: Mar 2009 Location: Novi Sad, Serbia Posts: 29 Rep Power: 10 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.

 December 16, 2009, 05:32 #5 Senior Member   Anonymous Join Date: Mar 2009 Posts: 110 Rep Power: 10 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.

 December 16, 2009, 05:35 #6 New Member   Milos Stanic Join Date: Mar 2009 Location: Novi Sad, Serbia Posts: 29 Rep Power: 10 I didn't know that there was a way to compute forces in ParaView... How do you do that? Thanks!

 December 16, 2009, 10:46 #7 New Member   Kai-Jia Han Join Date: Nov 2009 Posts: 4 Rep Power: 9 Till Milos: http://www.cfd-online.com/Forums/ope...rces-of15.html This page might be helpful for you.

December 16, 2009, 10:49
#8
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Kai-Jia Han
Join Date: Nov 2009
Posts: 4
Rep Power: 9
Quote:
 Originally Posted by madad2005 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?

Thank you for any suggestions and comments in advance!

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