I am trying to calculate the wall shear stress within a fluid in a cylindrical pipe. I have defined WSS as a volVectorField in a createFields.H file, so that I can look at the wall shear stress in dxFoam. The code I have been using for the wall shear stress is:

label patchA = mesh.boundaryMesh().findPatchID("wall");

fixedValueFvPatchVectorField& wallshearstress =
refCast(WSS.boundaryField()[patchA]);

forAll(wallshearstress, patchFaceA)
{
volTensorField gradU = fvc::grad(U);

volTensorField shear = mu*(gradU + gradU.T());

shear[patchFaceA].component(tensor::XX) = 0;
shear[patchFaceA].component(tensor::YY) = 0;
shear[patchFaceA].component(tensor::ZZ) = 0;

wallshearstress[patchFaceA].component(vector::X) = shear[patchFaceA].component(tensor::XY) + shear[patchFaceA].component(tensor::XZ));

wallshearstress[patchFaceA].component(vector::Y) = shear[patchFaceA].component(tensor::YX) + shear[patchFaceA].component(tensor::YZ)

wallshearstress[patchFaceA].component(vector::Z) = shear[patchFaceA].component(tensor::ZX) + shear[patchFaceA].component(tensor::ZY);

}

So, I have been able to get the stress tensor and set the 3 normal components to zero. I have then added the 2 X components to get the overall X component of the shear stress, added the 2 Y components etc.
However, looking at my results in dxFoam, the WSS vectors are not aligned correctly along the wall of the cylinder.
Do I have to project my stress vector onto my surface and does anyone have any idea how to do this? Has anyone written a code to calculate the wall shear stress of a fluid already? Thanks,

Sinead

You might find the application

applications/utilities/postProcessing/wall/wallShearStress

useful. Simply replace R in the expressions with your shear field and it should do what you want.

[T.D.]

hi henry,
what do you mean by "R" in the expressions where?
thanks