How to get normal vectors on slices?

Hi everyone,

I am trying to find forces on a slice that taken over a surface and the total closed area in this slice. I got some scalar quantities over the surface. I need normal vectors to be multiplied to get forces.

I can obtain normal vectors on surface and I can obtain these scalar variables on the slice. However, I couldn’t get normal vectors on the slice. I think an interpolation is needed.

My question is that is there any command I can use to interpolate these surface normal vectors to the slice. If there is no such a command, can you suggest any way to do that?

I am using vtk library in python and I used cutter command to obtain a slice. Thank you for your answers.