# Find where force is applied

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 September 25, 2012, 20:16 Find where force is applied #1 Senior Member   Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 230 Rep Power: 14 Hi, I am using simpleFoam (of v2.1.1). I am trying to find where the forces are applied and have been outputing the forces and moments using the following in the controlDict: functions ( forces { type forces; functionObjectLibs ( "libforces.so" ); outputControl timeStep; outputInterval 1; patches ( "Volume_0_Wheel-1" "Volume_0_Wheel-2" "Volume_0_Wheel-3" "Volume_0_Wheel-4" "Volume_0_Car" ); rhoName rhoInf; rhoInf 1.205; pName p; UName U; log true; CofR (0.0 0.0 0.0); } ); It worked great and I got all the outputs in the forces\0\forces.dat file: # Time forces( pressure, viscous ) moment( pressure, viscous ) 997 ( ( ( -25.6521 1575.07 -2489 ) ( 0.0440792 73.7995 8.74441 ) ) ( ( -5215.72 15.0189 86.2846 ) ( -15.0971 -0.0198099 -0.124274 ) ) ) 998 ( ( ( -25.6488 1575.07 -2488.99 ) ( 0.0442406 73.8003 8.74387 ) ) ( ( -5215.69 15.0191 86.2908 ) ( -15.0984 -0.0198765 -0.124476 ) ) ) 999 ( ( ( -25.6506 1575.07 -2489.01 ) ( 0.0442343 73.8004 8.74362 ) ) ( ( -5215.72 15.0171 86.289 ) ( -15.0978 -0.02023 -0.124946 ) ) ) My problem is that I can't work out how to derive the x,y,z coordinates where the forces are applied to obtain the moment. I have been struggling for a couple of hours and have to admit that geometry is one of my many weak points... Any help would be appreciated. Thanks in advance Julien __________________ --- Julien de Charentenay

September 26, 2012, 02:53
#2
Senior Member

Join Date: Aug 2010
Location: Groningen, The Netherlands
Posts: 216
Rep Power: 15
Dear Julien,

I hope I got your question right.

Quote:
 ... CofR (0.0 0.0 0.0); ...
In my 'old' files (from OF 1.7.x) this was the reference point
used for the calculations of the moments
So in your case you chose the origin.

I hope I could contribute
regards
Colin

 September 26, 2012, 06:46 #3 Senior Member   Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 230 Rep Power: 14 Hi Colin, Thanks a lot for your answer. I probably need to clarify a bit. I am trying to know the distance - or vector - that define where the force is applied to generate the moment - I am just after the distance in the y-direction. Let me know if it is unclear. julien __________________ --- Julien de Charentenay

 September 28, 2012, 08:38 #4 Senior Member     Vieri Abolaffio Join Date: Jul 2010 Location: Always on the move. Posts: 308 Rep Power: 13 you have the moments referred to a defined point, in this case 0 0 0. moment is force time distance. to find the distance from your reference ponint do moment/force __________________ http://www.leadingedge.it/ Naval architecture and CFD consultancy

 September 30, 2012, 20:53 #5 Senior Member   Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 230 Rep Power: 14 Hi, Thanks a lot for your answer and comments. My understanding is that: - Forces are the integral over surface of p*Face_normal; - Moment are the integral over the surface of p*Vector(CofR,Centroid)^Face_normal. Based on the above, I was trying to inverse a system to get the y coordinate: Mx = yFz - zFy My = zFx - xFz Mz = xFy - yFx Just doing y = Mx/Fz was neglecting the effect of drag on the x-component of moment. I am unsure if the above is correct as it implies that MxFx + MyFy + MzFz = 0 (ie the moment is to be perpendicular to the Force), but when I calculate the expression based on the output from openFoam this is not true. So I am assuming that my interpretation is incorrect. I am using a "work-around" using ParaView (http://www.cfd-online.com/Forums/ope...-parafoam.html) where I construct two expressions: 1) The forces: surface integral of p*Normals 2) The x moment of the z force: surface integral of coordsY*Forces_Z The center of application of the z force is calculated as (2)/(1). I noticed some differences in the forces calculated (order of 0.2% so negligeable). This seems to be doing what I need. Kind regards, Julien __________________ --- Julien de Charentenay Last edited by julien.decharentenay; September 30, 2012 at 21:20. Reason: Just wanted to clarify.