[fish man]

We use OpenFOAM v1806.
The "force" function is used to calculate the force.
When performing the "force" function, what is the equation used to determine the frictional drag force?

[Tobermory]

Check member function calcForcesMoments() in forces.C (https://www.openfoam.com/documentati...8C_source.html) ... it's all in there.

[fish man]

Thank you.
I have done the analysis with a laminar flow model.
Does the equation for viscous drag then depend on line 253 of force.c?

[Tobermory]

Very close - it's the section before that is used when a laminar turbulence model is selected:

    else if (foundObject<transportModel>("transportProperties"))
    {
        const auto& laminarT =
            lookupObject<transportModel>("transportProperties");
 
        const auto& U = lookupObject<volVectorField>(UName_);
 
        return -rho()*laminarT.nu()*dev(twoSymm(fvc::grad(U)));

... i.e. line 242 (same expression though!). This calculates the viscous stress, and then this is dot producted with the boundary face area vector on line 721:

const vectorField fV(Sfb[patchi] & devRhoReffb[patchi]);

to give the vector force.

[fish man]

Thank you very much.
I now have a better understanding of "force.c".