subtracting line data from a plane

I have a channel flow, I would like to subtract the turbulent intensity at the side wall in the whole domain.

How can I do that with paraview calculator?

I want to the the contour plot of this value

I = Iw^2 - Io^2;

Iw is the total turbulence

I would like to subtract the turbulence intensity
Io is the turbulence at the lateral surface;

I attached figure for clarity

Probably can be done with the python calculator, if not with a ProgrammableFilter

Thank you Mathieu,

How can I extract the turbulence data along the lateral line (Iamb) in python calculator?

Sorry am a newbie