calculate wall share stress 3D mesh

Hello,
I am looking into how to calculate the wall shear stress in my fluid simulation. going around the forum i finished in this two links:
https://www.paraview.org/pipermail/paraview/2016-October/038305.html

but not sure about how to calculate it, the first link suggest:

  • Gradient of Unstructured Data Set
  • Extract Surface
  • Generate Surface Normals
  • Calculator filter to compute the wall shear stress from the gradient
    and the surface normals.
    but if I generate the gradient of U i finish with 9 components (different fields) and not a vector/tensor. therefore I am not sure how to do the last step with the calculator, if someone could enlight me with this would be appreciated.