| Friendly |
October 17, 2017 09:29 |
Source Code: Calculation of Drag
Hi,
I try to understand how OpenFOAM calculates the drag forces and coefficients. In the sourceCode of forceCoeffs.C I have found the following exression:
Code:
bool Foam::functionObjects::forceCoeffs::write()
{
forces::calcForcesMoment();
if (Pstream::master())
{
writeFiles::write();
scalar pDyn = 0.5*rhoRef_*magUInf_*magUInf_;
Field<vector> totForce(force_[0] + force_[1] + force_[2]);
Field<vector> totMoment(moment_[0] + moment_[1] + moment_[2]);
List<Field<scalar>> coeffs(3);
coeffs[0].setSize(nBin_);
coeffs[1].setSize(nBin_);
coeffs[2].setSize(nBin_);
// lift, drag and moment
coeffs[0] = (totForce & liftDir_)/(Aref_*pDyn);
coeffs[1] = (totForce & dragDir_)/(Aref_*pDyn);
coeffs[2] = (totMoment & pitchAxis_)/(Aref_*lRef_*pDyn);
scalar Cl = sum(coeffs[0]);
scalar Cd = sum(coeffs[1]);
scalar Cm = sum(coeffs[2]);
Nothing special here, everything is more or less as expected. I just need to find out how the force [0],[1] and [2] are calculated. I have found out that 0 is the normal, 1 the tangential and 2 the porous force. But what is a porous force? I have never read such thing before.
Looking in forces.C I have found:
Code:
if (directForceDensity_)
{
const volVectorField& fD = obr_.lookupObject<volVectorField>(fDName_);
const fvMesh& mesh = fD.mesh();
const surfaceVectorField::Boundary& Sfb =
mesh.Sf().boundaryField();
forAllConstIter(labelHashSet, patchSet_, iter)
{
label patchi = iter.key();
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);
applyBins(Md, fN, fT, fP, mesh.C().boundaryField()[patchi]);
}
}
So forces are calculated via a forceDensity, but how does the calculation of the forceDensity work?
Does anyone know how OpenFOAM calculates the forces and coefficients? I would be very thankfull for any help.
Greetings,
Friendly |
