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-   -   wallShearStress vs. force functionObject (http://www.cfd-online.com/Forums/openfoam-post-processing/122181-wallshearstress-vs-force-functionobject.html)

pad August 14, 2013 02:43

wallShearStress vs. force functionObject
 
Hello,

I have been simulating an impeller and extracting the power by using both wallShearStress utility (for moment calc. in paraview) and the force functionObject for gettting the moment directly.

Comparing the data I realized that the moment from viscous forces are in opposite direction, i.e. with wallShearStress utility and paraview I get the following moment from viscous forces

(0.300199 0.191866 1.55875)

while the force function object gives me

(-0.3002453 -0.1916713 -1.556391)

Looking at the code for wallShearStress and forces.c it seems that wallShearStress.C uses

Code:

forAll(wallShearStress.boundaryField(), patchI)
    {
        wallShearStress.boundaryField()[patchI] =
        (
          -mesh.Sf().boundaryField()[patchI]
          /mesh.magSf().boundaryField()[patchI]
        ) & Reff.boundaryField()[patchI];
    }

while forces.C uses

Code:

forAllConstIter(labelHashSet, patchSet_, iter)
        {
            label patchi = iter.key();

            vectorField Md
            (
                mesh.C().boundaryField()[patchi] - coordSys_.origin()
            );

            vectorField pf(Sfb[patchi]*(p.boundaryField()[patchi] - pRef));

            fm.first().first() += rho(p)*sum(pf);
            fm.second().first() += rho(p)*sum(Md ^ pf);

            vectorField vf(Sfb[patchi] & devRhoReffb[patchi]);

            fm.first().second() += sum(vf);
            fm.second().second() += sum(Md ^ vf);
        }

Why does wallShearStress.C incorporate the neg. sign while forces.C does not? Which approach is the correct one to use when trying to calculate the pump / impeller power?

louvel December 8, 2015 13:30

Hallo

do you already konw the answer?
i am not also have the same questiones
I donnot understand the difference to calculate visocus forces between wall shear stress and force utilities .the code are different but the result should be the same

mkiewat March 8, 2016 12:24

I would guess that wallShearStress gives you the shear stress forces acting on the fluid while forces returns the total force (pressure AND shear stress forces) acting on a specified patch (the wall of an object). Because actio = reactio, the sign is opposite, the force vectors point in oopposite direction. So if you want to compute the moments acting on your impeller or whatever you should go with the "forces" function object.


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