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Normal and tangential force on a structure

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Old   May 7, 2020, 06:16
Default Normal and tangential force on a structure
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Constance Clément
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Hi everyone,

I am looking for a way to determine the normal and tangential force to a structure in OF. With libforces, one can get Fx, Fy, Fz (pressure and viscous components) in the coordinate system of the mesh. I want to determine the forces in the coordinate system of the structure (a cylinder).

These forces are thus normal and tangential to the cylinder. I saw that in the code forces.C, the normal and tangential components are computed. Unfortunately, I am not sure to understand what "forcedensity" refers to.

Quote:
void Foam::functionObjects::forces::calcForcesMoment()
{
initialise();

resetFields();

if (directForceDensity_)
{
const volVectorField& fD = lookupObject<volVectorField>(fDName_);

const surfaceVectorField::Boundary& Sfb = mesh_.Sf().boundaryField();

for (const label patchi : patchSet_)
{
vectorField Md
(
mesh_.C().boundaryField()[patchi] - coordSys_.origin()
);

scalarField sA(mag(Sfb[patchi]));

// Normal force = surfaceUnitNormal*(surfaceNormal & forceDensity)
vectorField fN
(
Sfb[patchi]/sA
*(
Sfb[patchi] & fD.boundaryField()[patchi]
)
);

// Tangential force (total force minus normal fN)
vectorField fT(sA*fD.boundaryField()[patchi] - fN);

// Porous force
vectorField fP(Md.size(), Zero);

addToFields(patchi, Md, fN, fT, fP);

applyBins(Md, fN, fT, fP, mesh_.C().boundaryField()[patchi]);
}
}
Thanks a lot for your help,

Constance
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Old   May 9, 2020, 12:46
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HPE
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Hi,

>> I am looking for a way to determine the normal and tangential force to a structure in OF. With libforces, one can get Fx, Fy, Fz (pressure and viscous components) in the coordinate system of the mesh. I want to determine the forces in the coordinate system of the structure (a cylinder).

You can change the default coordinate system to a local coordinate system for a given force function object without modifying the code itself.

Having said that I can't write a set of settings out of my head. So, please do search for the forum for an example (a doc: https://www.openfoam.com/documentati...es-forces.html).

Hope it helps.
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Old   May 11, 2020, 03:05
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Constance Clément
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Hi,

Thanks a lot for your reply, it helped me a lot.

Constance
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Old   May 11, 2020, 18:33
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HPE
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pleasure - and good luck!
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