# BC for measuring force on blunt body

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 December 8, 2013, 19:36 BC for measuring force on blunt body #1 New Member   Join Date: Dec 2013 Posts: 4 Rep Power: 6 I'm trying to measure force exerted on a blunt body by a constant flow using "forces" function but the forces are always with a very low value (0.0005). 0/U: body { type fixedValue; value uniform (0 0 0); } 0/p: body { type zeroGradient; } This is a 3D case.

 December 9, 2013, 16:27 #2 New Member   W. Schuyler Hinman Join Date: Apr 2013 Location: Calgary, Alberta, Canada Posts: 26 Rep Power: 6 Did you make sure you have everything defined correctly in the forceCoeffs dictionary? S.

December 9, 2013, 18:43
#3
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Quote:
 Originally Posted by schuyler Did you make sure you have everything defined correctly in the forceCoeffs dictionary? S.
You mean in the forces dictionary, right?

forces
{
type forces;
functionObjectLibs ("libforces.so");
outputControl timeStep;
outputInterval 5;
patches ( "blunt_object" );
pName p;
UName U;
rhoName rhoInf;
log true;
CofR (0 0 0);
rhoInf 1000;
}

December 9, 2013, 19:15
#4
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W. Schuyler Hinman
Join Date: Apr 2013
Posts: 26
Rep Power: 6
Quote:
 Originally Posted by letthe You mean in the forces dictionary, right? forces { type forces; functionObjectLibs ("libforces.so"); outputControl timeStep; outputInterval 5; patches ( "blunt_object" ); pName p; UName U; rhoName rhoInf; log true; CofR (0 0 0); rhoInf 1000; }
Looks right to me... Your BCs look right too. If your geometry isnt complicated you can check the force calculation yourself manually with the output pressure (and shear if its significant). It could just be something such as a unit conversion problem for your geometry. Like if you have a multiplier in blockmesh and didn't realize it etc. Double check the size of your blunt object. If it is a small object in the simulation it will have small force.

Other than that I'm out of ideas sorry! Maybe someone else will be more helpful.

December 9, 2013, 19:20
#5
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Quote:
 Originally Posted by schuyler Looks right to me... Your BCs look right too. If your geometry isnt complicated you can check the force calculation yourself manually with the output pressure (and shear if its significant). It could just be something such as a unit conversion problem for your geometry. Like if you have a multiplier in blockmesh and didn't realize it etc. Double check the size of your blunt object. If it is a small object in the simulation it will have small force. Other than that I'm out of ideas sorry! Maybe someone else will be more helpful.
You have been very helpful, thank you! I am now convinced that this is a problem of units ... fingers crossed I find it

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