Help with calculation of wall velocity gradients

Hi,

I’m trying to get the wall velocity gradients for the three_fin_3d example. In the user guide for 2106, in page 131, we were given the code to add into the three_fin_flow_solver.py.

I modified and ran the code.
I got the WallGradients.vtu file in the inference dir. When I opened it using Paraview, I saw the variables like, normal x/y/z, u/v/w etc. However there is no du/dx (u__x) etc.

Did I make a mistake somewhere? Can someone help?