Wall skin friction

Hello,I ask a question about post-processing.

In a 2D case, when I calculate the wall skin friction through paraview, I use ‘Gradient’ and 'generate surface normal’ filter, the normal_x and normal_y are zero, normal_z is 1. then use Calculator (Density*Velocity_X/Re)*GradientV_XY, the last step uses ‘plot over line’ on the wall to get the data. But the result is close to zero. Is anything wrong?