# Force on a patch

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March 14, 2012, 01:33
Force on a patch
#1
Senior Member

Daniele Vicario
Join Date: Mar 2009
Location: Novara, Italy
Posts: 142
Rep Power: 8
I need to calculate the force caused by the flow on a patch.

In the first picture you can see the result of the simpleFoam solver. In the second there's the patch I want to calculate Fx on.

I tried in Parafoam to integrate pressure on the surface but maybe I miss something (BTW, is there a tutorial regarding this managements ?)

Another simple question: p for incompressible solver is actually divided by density.
That means that the real pressure at the end has to be multiplied by density, right ?

Daniele
Attached Images
 sfoam-p.jpg (9.4 KB, 26 views) sfoam-pisto.jpg (7.2 KB, 21 views)

 March 14, 2012, 04:04 #2 Senior Member   Daniele Vicario Join Date: Mar 2009 Location: Novara, Italy Posts: 142 Rep Power: 8 Probably I should have posted this under the parafoam forum. Is there any way to move it by me ? Anyway, I tried the following in Parafoam: 1. Select the above patch. 2. Generate surface normals. 3. Use calculator to define a new var called Px defined as p*N_x*1000. (where 1000 is density of water). 4. Integrate Px on the surface. Is it correct ? The obtained value is in Newton, right ?

 March 15, 2012, 02:42 #3 Senior Member   Daniele Vicario Join Date: Mar 2009 Location: Novara, Italy Posts: 142 Rep Power: 8 Another way, more flexible is to use the libforces library as follow: Code: ```functions { forcespisto { type forces; functionObjectLibs ("libforces.so"); //Lib to load patches (pisto); // change to your patch name pName p; Uname U; rhoName rhoInf; rhoInf 1000; CofR (0 0 0); //Origin for moment calculations outputControl timeStep; outputInterval 5; } }``` I'm interested in the Fx value and it seems quite the same, compared with the one obtained by parafoam (see prev post). Just a confirmation: is it right ? Daniele

 March 15, 2012, 05:54 #4 Senior Member   Aurelien Thinat Join Date: Jul 2010 Posts: 154 Rep Power: 7 Hi Daniele, Yes it is. Just modify the CoR (center of reference) if you want the values of Mx My and Mz. There is also a "forceCoeffs" function for Cx Cy and Cz calculations. Aurélien

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