Calculation of Wall Shear Stress


I want to calculate the Wall Shear Stress in a 3D case. Although I found this, I have some questions. Firstly, I followed the instructions and I finally I was not able to take the dot product of Gradient and Velocity because the Gradient was analyzed into 9 components. Secondly I cannot understand why we need the normals on the surface and not the tangentials because I want to calculate the Shear stress and not the Normal one.